Current standardized testing on air-permeable multilayer cladding is inaccurate since one of the primary parameters for setting the peak net wind loads across the cladding is the external pressure gradient, which is removed from all standardized testing. After implementing a latex-barrier system and examining the control strategy for the pressure loading actuators, a multichamber airbox system for testing air-permeable multilayer cladding is developed and compared to a benchmark study performed by the Insurance Research Lab for Better Homes. The results indicate that using multichamber, pressure-based testing to obtain wind loads for air-permeable, multilayer wall systems with flexible cladding is sufficiently accurate. This method also shows that eliminating temporal variations from the external pressures for easier standardized testing would provide fairly accurate results.

Keywords: wind loads, full-scale test methods, air-permeable cladding, pressure equalization, pressure loading

# INTRODUCTION

#### Background

actuators

Severe wind storms around the world continue to cause significant economic losses. While there have been significant advances and successes with building codes, design standards, and construction methods, which have reduced major structural failures (e.g., Gurley and Masters, 2011), losses caused by extreme wind storms continue to grow. These tend to be due to failures of the building envelope and connections, e.g., cladding failures caused by either net pressure loads or wind-borne debris impacts, failures of nailed connections, etc. Entry of rain water through the resulting openings is a significant source of damage (e.g., Sparks et al., 1994; Gurley and Masters, 2011). This contrasts significantly with earthquakes where a substantial portion of the losses is due to failures of major structure members. Where structural members fail in wind storms, they are often due to failure of an envelope component first, such as a window or door that allows internal pressurization. Thus, mitigation of economic losses in wind storms is strongly dependent on enhanced performance of cladding systems and their connections.

The performance of cladding and connections is usually evaluated by component tests, normally in compliance with standardized tests. Examples of standardized tests that are used to evaluate product performance under wind loads include ASTM D5206 (2013) for vinyl siding,

#### *Edited by:*

*Matthew Mason, University of Queensland, Australia*

#### *Reviewed by:*

*Aly Mousaad Aly, Louisiana State University, USA Ioannis Zisis, Florida International University, USA*

*\*Correspondence:*

*Gregory A. Kopp<EMAIL_ADDRESS>
#### *Specialty section:*

*This article was submitted to Wind Engineering and Science, a section of the journal Frontiers in Built Environment*

*Received: 30 September 2016 Accepted: 18 January 2017 Published: 10 February 2017*

#### *Citation:*

*Miller CS, Kopp GA, Morrison MJ, Kemp G and Drought N (2017) A Multichamber, Pressure-Based Test Method to Determine Wind Loads on Air-Permeable, Multilayer Cladding Systems. Front. Built Environ. 3:7. doi: 10.3389/fbuil.2017.00007*

ASTM E1592 (2012) for metal roofing, and ASTM E1300 (2012) for glazing and glass. Most of these standardized tests make use of a sealed airbox system,1 where the pressure required to fail the particular product is obtained. Many of these standardized tests use uniform, static pressures or slowly increasing pressures, such as those listed above. For components that have low-cycle fatigue as a failure mechanism, sinusoidally varying pressures at various frequencies and amplitudes are applied (Henderson et al., 2013a). In all cases, the pressures applied by these standard tests are uniform over the component. This, of course, provides applied loads that are significantly different than real wind loads, but which are relatively simple and inexpensive to apply (Kopp et al., 2012).

While there are standardized tests to assess the capacity of cladding systems, wind loads are much less well-defined, particularly for cladding and component systems with air-permeable, multilayer assemblies. ASCE 7-05 (American Society for Civil Engineering, 2006) acknowledges this (in section C<IP_ADDRESS> of ASCE 7-05 or section 30.1.5 of ASCE 7-10), indicating that

The designer may elect either to use the loads derived from Section 6.5 [single-layer pressures] or to use loads derived by an approved alternative method. If the designer desires to determine the pressure differential across the air-permeable cladding element, appropriate full-scale pressure measurements should be made on the applicable cladding element, or reference be made to recognized literature for documentation pertaining to wind loads.

For such systems, net wind load coefficients are not generally known, and standard methods for determining wind loads, i.e., scale-model wind tunnel tests, cannot be used. While ASCE 7 indicates that "full-scale pressure measurements should be made," there are limited methods that have been developed. For example, the test standard for vinyl siding, which is a type of relatively low-cost cladding used in North American residential wood-frame wall assemblies, is ASTM D5206 (2013). In this standard, pressure equalization is eliminated by placing an airtight barrier between the siding layer and internal sheathing layer or insulating layer. Uniform, static pressures are then applied *via* an airbox in order to determine the capacity of the siding. This yields the net pressure at the limit state (assuming that the failure mechanism and capacity are not substantially altered by the approach). The limit-state pressure is related to the wind loads in ASTM D3679 (2013). This standard requires that the design external pressure in ASCE 7 be multiplied by a "pressure equalization factor" (PEF), which in this case has a value of 0.36. Thus, ASTM D3679 (2013) assumes that the net pressure across the siding is 64% lower than the external pressure acting on the wall (i.e., 64% lower than the net pressure across the entire wall, assuming the building internal pressure is 0 or that there is a sealed internal layer). The PEF in ASTM D3679 (2013) was obtained from static, uniform airbox tests conducted by Architectural Testing, Inc. (2002); with similar values found by uniform, but time-varying, pressures in Gavanski and Kopp (2011a). Recently, using full-scale wind tunnel tests from the Insurance Institute for Business and Home Safety (IBHS) Research Center, Morrison and Cope (2015) found a PEF of closer to 0.8, indicating load coefficients that are more than double those of ASTM D3679 (2013), all else being equal. As discussed in detail below, the external pressure gradient is a primary parameter in setting the net wind load on air-permeable, multilayer cladding. Clearly, there is a need to develop appropriate methodologies for assessing net pressures for air-permeable, multilayer assembles, which is the objective of the current study.

## Methods for Determining Wind Loads on Air-Permeable, Multilayer Cladding

There are at least four possible methods for determining the net pressure coefficients on air-permeable, multilayer cladding systems: (i) analytical models or computational fluid dynamics, (ii) large-scale or full-scale wind tunnel tests, (iii) full-scale field measurements, and (iv) airbox tests with applied external pressures.

Bienkiewicz and Sun (1997) have shown that the wind loads for air-permeable multilayer systems depend on the external pressure gradient, the gap between panels, the thickness of the panels, the cavity depth between the panels and the interior surface, and the loss coefficients for the flows through the various openings. Because of the prominent dependence on the external pressure gradient, compartmentalization of the cavity is extremely effective at reducing the net wind loads (Kumar, 2000) by limiting the effects of the spatial gradients, leaving only the second order dependence on the temporal gradient. Rain screen walls make extensive use of this concept (Kumar, 2000). Oh and Kopp (2014) found that, because of the external pressure gradient, there is a neutral pressure line, which causes the net, area-averaged pressure coefficients to reduce with area faster than the external pressure coefficients do. These authors' analytical model shows that spatial and temporal variations of the cavity pressures can be modeled analytically for one-dimensional cavity pressure fields using the unsteady Bernoulli equation and appropriate loss models. Sun and Bienkiewicz (1993) used a similar, but steady-flow, model to model the mean two-dimensional cavity pressure field. However, a full spatial and temporal analytical model of cavity pressures has not yet been developed. Computational fluid dynamics-based solutions are also possible, but these have not been conducted to the authors' knowledge.

Large-scale wind tunnel tests have been used to study pressure equalization for roof pavers. Mooneghi et al. (2014) showed that sufficient tap resolution is required to ascertain wind loads on these systems, using their 1/2-scale building model in the wall of wind facility. Considering the small gaps involved, it seems unlikely the more typical boundary layer wind tunnel model scales can be used since the full-scale gaps are often on the order of 3 mm and smaller. For roof-mounted photovoltaic arrays, larger gaps mean that standard boundary layer wind tunnels can be used at scales of 1/20 to 1/30, although there has not yet been a full-scale validation of these relatively large-model-scale

1We will use the terms "airbox" and "pressure chamber" synonymously to indicate the enclosed volume where pressure is applied to a specimen's surface.

results (compared to typical model scales of 1/300 to 1/500 in most boundary layer wind tunnels). For vinyl siding, with gaps between panels that are largely undefined (because of how the panels clip together), model-scale wind tunnel methods cannot be used at all (even at 1/10). For this reason, Morrison and Cope (2015) examined such systems at full scale in the IBHS Research Center wind tunnel. The main disadvantage of this approach is that it is not cost effective for routine product testing and one can only test the products on a limited set of building sizes, limiting the generality of the findings, as discussed by Kopp et al. (2012). Full-scale field measurements are useful for validation of other test methods, but the cost, limited pressure tap resolutions, and limited numbers of building configurations mean that this approach cannot be practically used for determining design loads.

Kopp et al. (2012) have shown that full-scale multichamber airbox testing can be used to test overall building responses to wind loads. However, it has not been established whether such approaches can be used with air-permeable, multilayer systems. Given the potential advantages of a relatively low-cost and straightforward test methodology, the objective of the current study is to determine whether multichamber airbox testing is feasible for assessing wind loads on air-permeable, multilayer cladding systems. To this end, the multichamber, PLA approach of Kopp et al. (2010, 2012) is modified, as described in Section "The Multichamber, PLA Approach," with a test case of vinyl siding examined in Section "Test Case for Air-Permeable, Multilayer Cladding."

## THE MULTICHAMBER, PLA APPROACH

#### Background

One of the first systems to replicate realistic fluctuating pressures on a scale suitable for representative sections of the building envelope is the Building Research Establishment's real-time wind uniform load follower (BRERWULF; Cook et al., 1988). While this test method was a significant advance, the main disadvantage of the BRERWULF system is that it could not accommodate airflow through the specimen, which is common in many standard building materials. As such, a multichamber BRERWULF system for testing air-permeable cladding would not function well because of the necessity to allow airflow between chambers.

A loading system capable of applying spatially and temporally varying loads on standing seam metal roof cladding was developed at Mississippi State University (Sinno et al., 2003; Surry et al., 2007). The test rig used large capacity electromagnets to apply an array of quickly varying uplift loads on top of a uniform positive pressure applied from an airbox underneath the cladding. Importantly, the MSU system brings the application of realistic (temporally and spatially varying) wind pressures to the test specimen. The disadvantages of this technique are that it only works with metal cladding elements and there is a significant amount of tuning to the system required before a specific loading trace could be applied to a specimen.

Kopp et al. (2010, 2012) presented a loading system based on "pressure loading actuators" (PLAs), which overcame the limitations of the BRERWULF and MSU loading systems. The PLA system uses multiple pressure chambers to capture the spatial variations of the wind loads while each individual PLA is able to capture pressure fluctuations up to about 10 Hz with peak pressures of up to about 23 kPa in pressure and −20 kPa in suction (Kopp et al., 2010). Thus, very large loads, at the limit states of wood-frame houses, could be applied (Morrison et al., 2012; Henderson et al., 2013b) to buildings with significant leakage through the building envelope *via* a system of flexible, independent airboxes.

#### Pressure Chambers

All pressure-based loading systems require a chamber in which to apply the pressures. Usually, these are nominally airtight and of fixed volume, i.e., they have solid side walls. Thus, the PLAs also require pressure chambers in order to apply the load to a building surface. However, for applying loads to the surface of a house, Morrison et al. (2012) developed a system of multiple, independent, flexible airbags with rigid supporting frames. These pressure chambers consisted of a rigid modular lid, which incorporated a molded inlet duct with air filter, the surface of the building (test specimen), which forms one surface of the chamber, and a flexible vinyl skirt that encloses the space between the lid and building surface, which forms the walls of the chamber. This system was required to be at least nominally airtight so that the pressure traces could be controlled reliably with leakage coming only through the building surface (such as bricks or cracks). The lids of the pressure chambers were connected to a rigid reaction frame. The skirt or membrane was required to be flexible since the building or component could deflect (on the order of 15 cm or more). Each pressure chamber was independent of the others because they were separated by approximately 2″ (5 cm) with no physical communication between them.

This pressure-chamber system was adequate for testing the structural response of buildings, but there is a significant issue that needs to be addressed for cladding tests. The surface areas outside of the chambers, but part of the test sample, are problematic for multilayer cladding tests where the intent is to determine the net pressures across layers of the cladding. The reason for this issue is that the cavity pressures depend significantly on the external pressure gradients, as discussed above. Thus, these non-pressurized areas need to be eliminated (or only located at non-porous areas of the cladding surface) because they artificially alter the pressure gradients. For vinyl siding, where there may be continuous leakage, the only solution is to eliminate the unloaded areas on the external surface of the cladding by having common "skirts" for adjacent pressure chambers. This poses a challenge for the PLA control system because there are two forms of communication between adjacent chambers: (i) net flows through the interior volume of the cavity between layers from one chamber to another and (ii) the changes in chamber volume due to the movement of the skirt because of the time-varying differences in pressure between adjacent chambers. Details of the implemented pressure-chamber solution for testing vinyl siding are provided in Section "Test Case for Air-Permeable, Multilayer Cladding." To understand the control solution, the operation of the PLAs is described next.

#### Flow-Reversing Valves and the PLAs

Conceptually, the control of the pressure in an airbox can be done in two different ways. One can vary the supply pressure into the airbox by varying the fan speed and direction using a variable speed drive. Alternatively, one can use a flow-reversing valve to control the pressure inside a pressure chamber. The first option provides the simplest and cheapest solution; however, it is not technically feasible to change the fan speed fast enough to be able to meet both the pressure magnitudes and frequencies required to simulate realistic wind loads on cladding elements. As such, the conceptual design consists of a fan or blower to produce the pressures, a valve system to regulate the applied pressures, and a feedback control system in order to monitor the applied pressure and adjust the valve system accordingly.

Different valve configurations could be employed to quickly reverse airflow to and from a fan operating at constant speed. The minimum number of ports required would be four, with one into the low-pressure side of the fan, one out of the highpressure side of the fan, one connected to atmosphere, and one to the airbox. However, for the current, as well as for the previous BRERWULF design, a five-port design was employed, with the additional port also connected to atmosphere. The advantage of a five-port design is that it avoids a closed loop between the fan inlet and outlet which would result in heat building up in the system. It should be noted that individual valves could be used on the different ports to change the flow direction but to minimize the number of components, moving parts, and control complexity, a rotating disk within the valve was chosen. This has the advantage of one moving part, which is on the axis of rotation of a servomotor.

**Figure 1** shows a schematic of a five-port valve and rotating disk concept along with a flow schematic showing its operation. The valve is made in two halves (i.e., two castings), one with two ports and the other with three, with a slotted disk between them. A schematic representation of these parts is presented in **Figure 1** while **Figure 2** depicts an assembly drawing of all the parts making up the valve. The two ports in the two-port half of the valve are connected to the fan with "i" (**Figure 1A**) representing the connection to the inlet (low-pressure side) of the fan and "o" representing the connection to the outlet (high-pressure side) of the fan. The direction of flow through these ports is also shown in **Figure 1A**. For the three-port half of the valve, there is inflow from the atmosphere into port "a" and outflow to the atmosphere from port "c." Port "b" is connected to the pressure chamber, and flow can be either into or out of this chamber, depending on the position of the slotted disk. It is for this reason that this device is called a flow-reversing valve.

**Figure 1B** shows the three limiting states of the valve: (i) neutral, with no flow into or out of the pressure chamber, (i) full flow out of the pressure chamber, and (iii) full flow into the pressure chamber. The position of the slotted disk, which is controlled by a servomotor, determines which state occurs. For example, when the disk is in the position such that port "a" is entirely blocked, there is no inflow from the atmosphere, and the flow is from the pressure chamber through port "b" into the fan inlet, through the fan, and out to atmosphere. This state leads to lower than atmospheric pressure (i.e., suction) in the pressure chamber. This is depicted in the middle schematic of **Figure 1B**, while the other two schematics in **Figure 1B** depict the other two limit states discussed above.

As noted above, the slotted disk is positioned by a servomotor, which can move the slotted disk to any angle. Depending on the disk angle, a range of positive and negative pressures can be achieved, which is determined by the fan characteristics, flow losses (i.e., pressure drops) in the ducts, valve, and other flow elements, and leakage into (or out of) the pressure chamber. **Figure 2** presents the final assembly of the different valve components, along with the symmetric slotted disk design. The final shape of the valve was determined by using computational

fluid dynamics to minimize the losses, improving the overall performance of the system (i.e., reducing power requirements to achieve particular pressures at particular leakage flow rates). The next section discusses the development of the valve in detail, and how it controls the pressure under different leakage conditions.

## System Performance and Control under Variable Leakage Flows

One of the critical aspects of simplifying the computer control system was the linearization of the valve, i.e., ensuring a nearly linear relationship between airbox pressure and valve position. **Figure 3** depicts the "valve map" (the static-flow relationship between pressure and velocity) for the perfectly symmetric valve/ disk arrangement (which is shown schematically in **Figure 1**) in a pressure chamber with fixed volume and very small leakage. The gradient of pressure with respect to valve position at high magnitudes of suctions and pressures is roughly 30 times the gradient in regions closer to 0 pressure (neutral position). This non-linearity would result in increased complexity of an already complex control system, significantly increasing the time critical computing resources required, particularly when operating multiple PLAs. In order to improve the linearity of the valve map with the symmetric disk, stationary tabs with a suitable profile were introduced to the valve housing. The improved linearity of the modified system can be seen in **Figure 3**. Overall performance of the PLA in fixed volumes and high peak pressures is discussed in Kopp et al. (2010). At much lower pressures, with interconnected pressure chambers, the PLAs still function with high accuracy, as discussed below. **Figure 4** shows the pressure versus valve position of the symmetric disk under different leakage conditions as indicated. While the pressures at a given valve position reduce with increasing leakage in the airbox, it decreases linearly with increasing leakage. The linearity of pressure versus leakage flow at each valve position allows the control system to automatically adjust to different initial leakage conditions or to adapt to changing leakage conditions during a test.

As can be inferred from the valve maps presented in **Figures 3** and **4**, the PLA is a single-input–single-output system with the valve position as the input and the required pressure as the output. Thus, the requested valve position and the measured pressure are an input–output pair for use in a proportional–integral–derivative

(PID) control system. The control system is effectively approximated to a linear first-order model as the required pressure trace, whether fluctuating or a simple ramp, is preloaded as a feedforward term. The feedback PID term is based on the tracking error which is simply the difference between the required and measured pressures. The valve map is used to determine what valve position is required to meet the applied pressure under steady-state conditions resulting in a known system time constant, τ, for the PID control. In calculating τ from the requested pressure and achieved pressure in real time, the adjacent pressures from the current and previous time steps are used.

Changes in leakage and air-chamber volume cause the time constant, τ, to vary enormously. The solution for this issue was to use gain scheduling; the real-time estimation of τ is used to determine the system gain from a suite of preloaded lookup tables, which are generated for known leakage and volumes. However, the system gain (*K*) increases with increasing τ, which can lead to instabilities with an overestimation of τ, particularly for very small volumes relative to power or input traces that have minimal changes in time (i.e., static pressures or ramp loads). This is only an issue at start up when the control system has no information on τ, that is, the pressure chamber and test element properties. Therefore, a median value of τ and a limit on the increase in *K* is set for the first 10 s of program control. The system learns the chamber's characteristics within this time frame. In cases where absolute control is required in the first 10 s, a user defined value of τ can be used. However, this necessitates additional experiments to determine the correct system time constant. With the lookup tables and the linearization of the valve chamber with disk position, the PID control system then can adapt rapidly to changing chamber conditions. The control system was designed this way so that that it could adapt to any pressure chamber and input trace (within certain bounds), and still run effectively without any user input into the PID.

The lookup tables scale by a constant factor over the fan speed range of 30–70 Hz. (Note that the lower fan speeds are used to avoid operating in a narrow region of the valve control curve, to maintain accuracy in tests that require small excursions around neutral pressure.) The control system predicts the required position of the valve up to 1 s into the future based on the current estimations of air-chamber leakage, user requested pressures, and system gain. If the control system anticipates the valve will be unable to meet the pressure requirements of the user defined input pressure time history, referred to as valve saturation, the fan speed will be increased to meet the demands of the experiment (when possible). This avoids the need for the user to predefine the required fan speed time history in addition to the input pressure time history for the airbox.

Adaptation of the control system is also required when sudden leakage, volume change, or flow change through the cavity occurs. **Figure 5** shows results from a sinusoidal load applied to a section of metal roof cladding using a 1.8 m × 0.9 m airbox. The achieved pressure trace deviated from the requested trace at approximately 882.2 s. This occurred due to a sudden increase in the leakage and the volume of the air chamber. As can be seen, the system adapts within a tenth of a second. This fast response in adaptation of the PLA control system is important since it is able to maintain the correct pressure loading even as onset of failure of the test specimen occurs.

#### Airbox Leakage and Fan Performance

Airbox leakage is an extremely important system parameter and has two possible sources: the test specimen and leakages through the valve and pressure chamber. The former can be a combination of material porosity and cracks or holes in the test specimen. The latter can be a combination of the pressure-chamber fit to the test specimen, the chamber, or the pipes and fittings

connected to the pressure chamber. The overall performance of the system, i.e., maximum pressure and frequency response, is highly dependent on the amount of leakage. Ultimately, the amount of allowable leakage in the system is a function of size of fan or blower used and, as such, significantly influences the power required to operate the system.

While it may be desirable to purchase the largest fan possible to provide the system with the maximum flow rate possible, there are several practical constraints of why this is not the optimal solution for the current loading system. Such a large fan would require high power consumption even for small, nominally sealed boxes; moreover, the physical size of the fan would be large and make applying spatial gradients, with many pressure chambers in close proximity, more difficult. Finally, in order to accommodate the large flow rates, the size of the valve would also need to be increased so that the air velocity through the valve is kept to a minimum, and consequently the losses. The performance of the entire fan–valve–airbox (nominally sealed) was evaluated by using an orifice plate to measure the flow rate going into the airbox and measuring the achieved pressure. Leakage flow rates were obtained by creating controlled openings within the airbox. In addition, these tests were repeated at numerous fan frequencies ranging from 30 to 70 Hz. The results of these tests are shown in **Figure 6**. In addition to meeting other requirements such as physical size, the manufacturer's fan curve for the chosen fan is shown in **Figure 6**. The fan combined with the tuned valve is capable of providing flow rates of up to 300 CFM at ~10 kPa and 100 CFM at 20 kPa at a fan speed of 60 Hz. However, as can be seen from the figure, the system does not need to run the fan at full speed, which leads to consequent changes in maximum pressures and flow rates. This, in turn, allows the total power consumption to be reduced in aggregate for an array of PLAs that are running different pressure traces.

To allow greater flexibility, the PLA loading system can be run in three different configurations depending on the application. The first configuration is the simplest where a single PLA unit is connected to a single chamber. The second configuration uses a single fan/blower connected to two independent valve and servo motors. Each valve is connected to its own box, with its own feedback pressure transducer allowing two airboxes to be controlled independently of each other using only a single fan. This configuration is ideal for small airboxes with minimal leakage and has the advantage of reducing the power consumption of the system, along with reducing the per airbox capital cost of the entire system. The third PLA configuration uses one airbox with multiple individual PLA units. Under this configuration, only the first PLA (leader unit) has a pressure transducer, and the remaining units mimic the valve movements of the leader exactly. Under this configuration, significantly higher flow rates can be realized while maintaining the performance characteristics required to simulate the real wind loads. This is the approach used for the vinyl siding tests discussed in Section "Test Case for Air-Permeable, Multilayer Cladding."

# TEST CASE FOR AIR-PERMEABLE, MULTILAYER CLADDING

## Full-Scale Pressure Measurements on Vinyl Siding

Recent tests at the IBHS examined the loads on various siding systems using their full-scale wind tunnel (Cope et al., 2012, 2014; Morrison and Cope, 2015). Pressures were measured on the walls of a full-scale, single story wood-frame house with plan dimensions of 9.1 m (30 ft.) × 12.2 m (40 ft.), a mean roof height of approximately 3.7 m (12 ft.), and a roof slope of 4-on-12. The walls of the test building were 2.4 m (8 ft.) high and clad with 11.1 mm (7/16″) OSB. In the study done by Morrison and Cope (2015), a total of four siding products were tested: vinyl siding, foam backed vinyl siding, wood siding, and hardy board. In order to test all four products simultaneously the walls of the building were divided into eight sub-wall assemblies. The long walls were divided in half resulting in a test wall 6.1 m (20 ft.) long, while the shorter walls extended 3.7 m (12 ft.) from the corner leaving 6 ft. in the center of the short wall to access the interior of the building. All siding products were installed following the

manufacturers guidelines; drywall was installed on the interior of the walls so that the wall assembly matched, as accurately as possible, real construction. Along the 6.1 m (20 ft.) walls, windinduced pressures were measured at 16 locations, while along the 3.7 m (12 ft.) walls, they were measured at 12 locations. At each measurement location, the external pressure, the net pressure across the siding, and the net pressure across the OSB were measured simultaneously. Experiments were conducted in an open exposure at four different wind speeds (Morrison et al., 2012). The orientation of the building to the wind was varied over a full 360° in 10° increments. The results of these tests indicate that pressure equalization has a significant effect on the outer layer of these systems. For example, results have shown that vinyl siding must carry a net load of up to 75–80% of the peak net load across the entire wall assembly. Given this 20–25% reduction in loads, the IBHS results are substantially different than the results obtained using a single-partitioned pressure chamber or considered through ASTM D3679 (2013). This study is used to provide benchmark data for validating the multichamber pressure-loading approach using PLAs. In particular, **Figure 7** shows segments of the measured external pressure time histories

history for each individual airbox.

at five locations along the wall from the IBHS experiments—it is these time histories that are applied in the multichamber tests described below.

#### Multichamber Pressure Test Setup

As described in Section "The Multichamber, PLA Approach," a new pressure-chamber system is required for testing airpermeable multilayer cladding systems. The main task is to ensure that the unloaded surfaces are minimized because of the importance of the external pressure gradients on setting the cavity pressure. In addition, the chamber walls should be (i) nominally airtight, so as to minimize the direct flows between chambers, (ii) flexible, so that the barriers have limited effects on the performance or deformations of the flexible cladding, (iii) strong enough to be robust through a range of test conditions, and (iv) easily installed and repeatable. After taking the material properties into consideration, a latex-barrier system was constructed. Small-scale testing showed that latex worked well in all of its required functions and, therefore, was implemented for the full-scale testing.

Since the objective was to replicate the IBHS results, test specimen was built with the same siding. These walls were 12 ft. long by 8 ft. high, with 2-by-4s used as the studs. The sheathing was 3/4″ plywood; however, a polyurethane sheet was placed in between the studs and the plywood to seal the pressure chamber, which is slightly different in setup compared to the IBHS tests, but which ensures that leakage between the pressure chamber to the atmosphere was minimized. House wrap was then placed over the plywood to replicate typical construction practice, since the focus of the study is the net loads on the outer (i.e., vinyl siding) layer. Pressure taps were then installed through the plywood and house wrap at the same locations as for the IBHS wall. Vinyl siding was installed on to the wall using appropriate nails at 16″ intervals along the length of the wall. All pieces of vinyl siding were cut to be 12 ft. long in order to not have any lap splices in the test chamber. Starter strips, ending strips, and utility trim were also used to model typical construction practice. The test wall could then be placed in a rigid-sided chamber of slightly larger dimensions than the test wall, which in this case is the same chamber used by Gavanski and Kopp (2011b).

Five chambers were created within the airbox for this test by placing four latex barriers within the larger rigid box. These five chambers coincided with the pressure tap layout from the IBHS tests: four of the chambers were 2 ft. long, with one being 4 ft. long. All latex barriers ran vertically across the siding, as can be seen in **Figure 8**, which presumes that the primary external gradient was along the length of the wall. This is a reasonable approximation for the side walls when they are under suction, although further work is needed to examine the effects of these gradients. In the current case, with five pressure time histories available, five pressure chambers are used. These data were obtained from the IBHS experiments, as depicted in **Figure 7**. [Regarding the effects of pressure gradients and the use of one set of pressure time histories, as discussed in Bienkiewicz and Sun (1997) and Oh and Kopp (2014), and shown below, the pressure gradient is clearly one of the governing parameters of the

net wind loads on air-permeable cladding elements, with high net loads being associated with high-pressure gradients. Due to this, further work is required to determine both appropriate design pressure gradients and the resulting number of required pressure chambers to obtain sufficient accuracy in the tests. This will involve additional wind tunnel testing to examine how the pressure gradients and pressure time histories change with overall building geometry and terrain conditions, compared to the single test case from IBHS.]

#### RESULTS AND DISCUSSION

The primary objective of the current tests is to determine whether multichamber pressure tests are capable of assessing net pressures on an air-permeable cladding system. This is done by applying the IBHS measurements of external pressures through the PLA system and measuring the resulting PEF. **Figure 9** shows a plot of a segment of the resulting time histories of the differential pressures across the cladding layer (i.e., the vinyl siding) for two cases: one with the external pressures from IBHS data (see **Figure 7** above) and one with spatially uniform pressures at each location. For the latter case, the pressure time history with the largest peak external pressure from the IBHS data was repeated identically in all five chambers. The different colors represent the differential pressures, *pd*, in each separate airbox,

$$p\_d = p\_e - p\_e,$$

where *pe* is the external pressure coefficient and *pc* is the cavity pressure coefficient. The five curves are overlaid on top of each other to determine the minimum (i.e., largest magnitude) observed differential pressure. The PEF is defined as

$$\text{PEF}\left(\boldsymbol{\omega}\right) = \frac{P\_{\text{external}}\left(\boldsymbol{\omega}\right) - P\_{\text{cavity}}\left(\boldsymbol{\omega}\right)}{\widehat{P}\_{\text{external}}}\_{\text{s}}$$

where *P x* external( ) and *P x* cavity ( ) represent the external and cavity pressures, respectively, at the location on the wall and *P* external is

the peak external pressure across the entire wall. [Note that in these experiments there should be negligible flow or pressure transferred from the interior of the wall because the house wrap provides a sealing layer. Thus, the net pressure over the assembly from the outside (siding) layer to the sealed layer is equal to the external pressure. For this reason, we use the external pressure instead of the net wall pressure in the definition of the PEF.] The data in **Figure 9** indicate that the peak differential pressures across the siding layer are about 350 Pa, while the peak external pressures are about 400 Pa. Thus, under fluctuating, spatially varying pressures, the reduction of the largest net pressure to the largest external pressure is about 12%, leading to an approximate peak PEF of 0.88. This result is substantially different when compared with the data obtained from uniform, but time-varying, external pressures, where the largest peak differential pressure across the siding layer is about 40 Pa. Thus, under fluctuating uniform pressures, the net reduction relative to the peak external pressure is about 90%. This result is consistent with the findings of Gavanski and Kopp (2012) but leads to an even larger reduction than that used in ASTM D3679-13 [although it is reasonably consistent with the data obtained from the study performed by Architectural Testing, Inc. (2002) who obtained PEFs between 0.03 and 0.18]. These results definitively show that the spatial pressure gradient is a critical parameter for the process of pressure equalization and neglecting it leads to highly erroneous loads for these types of cladding systems.

Figure 9 | Measured differential pressure time histories under

(A) uniform and (B) gradient external pressures.

**Figure 10** presents a comparison of the PEFs from the IBHS full-scale wind tunnel experiments and the current multichamber pressure-based experiments. As with the previous plots, the different colors represent the PEF in each chamber, with the colors from the two experiments matched. Between the two results, the multichamber airbox results seem to be deviating less from the mean. This is most likely because, although this new method of multichamber airbox testing is a step forward in the accuracy of the net pressure on cladding elements due to the application of spatial gradients, the spatial gradients being applied are still highly simplified. Due to limited tap resolution from IBHS, there may be subtle differences in the spatial distribution between tap locations that were not captured in original testing and which are made uniform within each airbox in the current tests.

The PLA system was able to simulate the applied external pressure time histories from IBHS results, with minor smoothing of the peak pressures. Recent model scale testing of multilayer roof systems suggests that the short duration, localized peak pressures control the cavity pressure, and the resulting net wind loads, to a significant extent (Oh and Kopp, 2014). This will have some effect on the results, but as shown in **Figure 10**, the distributions of the PEF values are similar, even with the minor smoothing of peak pressures. In fact, the PEF values are higher during the multichamber airbox testing, when compared to the IBHS wind tunnel experiments. In addition, further research is needed to

define the number of pressure chambers actually required, along with analysis of data to determine design pressure gradients to go with the peak pressures.

To determine whether a static, multichamber pressure test may be useful for a test standard, the external pressures at the moment that caused the peak PEF in **Figure 10** were applied (i.e., all temporal variations were eliminated except for the rapid change in external pressure from 0 to the target values). **Figure 11** shows the resultant PEF value, plotted against time. The results of this test show that the largest steady-state PEF is slightly larger than that obtained in the dynamic tests (**Figure 9**), although it is only about 5% higher. This suggests that static, but multichamber, pressure tests are feasible for a test standard so that the complexity of using the PLA system may not be required in standardized product tests. Interestingly, the temporal gradient of the rapidly changing pressure in the test caused a temporary increase of the maximum PEF in two of the five pressure chambers, which, considering that the PEF in the dynamic test is due to peak pressures that also had rapid variations, indicates there is a complexity in the temporal aspects of pressure equalization, as found by Oh and Kopp (2014). In the present case, having the pressures jump from 0 to their final values, the magnitude of the load was only altered by about 10% compared to the final steady values.

# CONCLUSION

An experimental study was conducted to determine whether multichamber airbox testing is feasible for assessing wind loads on air-permeable, multilayer cladding systems. Although there is airflow between individual airboxes and uncontrolled volume changes caused by shared barriers for the pressure chambers creating complications for the PLA control system, the concept of creating a multichamber, pressure-based, testing apparatus that can accurately obtain wind loads for air-permeable, multilayer wall systems with flexible cladding has been shown to work. This was accomplished by (i) the creation of multiple flexible latex barriers to enable the application of multiple, discreet, time-varying loads across a test specimen, (ii) the development of linearized five-port, flow-reversing valve, in conjunction with (iii) an adaptive PID control strategy. Using this multichamber pressure loading system together with external pressure data obtained at the full-scale IBHS wind tunnel, cavity pressures and PEFs were found to match the full-scale results reasonably well. The results confirm the conclusion of Cope et al., (2012) that the PEF in ASTM D3679-13 is unconservative. The good news is that static multichamber-based pressure tests are able to capture the important features of pressure equalization. Further work is required to create an improved test standard, particularly with respect to the minimum number of pressure chambers which may be required and setting the design pressure gradient or pressure pattern, neither of which were examined herein. Future work will examine these issues.

# AUTHOR CONTRIBUTIONS

CM is a Ph.D. student under the supervision of GAK, and this research is a portion of what will become CM's doctoral thesis. The experiments and approach were designed collaboratively

# REFERENCES

by the authors. The manuscript was written collaboratively; CM wrote the first draft. The experiments and the direct analysis of the experimental data were conducted by CM, with direct input from GAK. MM contributed information from the full-scale testing at IBHS. GK and ND contributed information about the development of the PLA system.

#### ACKNOWLEDGMENTS

This work is dedicated to the memory of Mr. Geoffrey Robson, who made significant contributions to development of the PLA control system. Funding provided by the Institute for Catastrophic Loss Reduction (ICLR) and the Natural Sciences and Engineering Research Council (NSERC) of Canada under the Collaborative Research and Developed Program is gratefully acknowledged. The ongoing support and encouragement for our work provided by Mr. Paul Kovacs of ICLR is gratefully acknowledged. Some equipment was developed under funding from the Canada Foundation for Innovation. The authors also wish to acknowledge the many contributions to the development of the PLAs and PLA-based testing methodologies by Drs. Marek Myszko, David Henderson, and Eri Gavanski.

Surry, D., Sinno, R. R., Nail, B., Ho, T. C., Farquhar, S., and Kopp, G. A. (2007). Structurally effective static wind loads for roof panels. *J. Struct. Eng.* 133, 871–885. doi:10.1061/(asce)0733-9445(2007)133:6(871)

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2017 Miller, Kopp, Morrison, Kemp and Drought. This is an openaccess article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# Development of a Full-Scale Structural Testing Program to Evaluate the Resistance of Australian Houses to Wind Loads

*Geoffrey Neville Boughton, Korah Parackal, Navaratnam Satheeskumar and David James Henderson\**

*Cyclone Testing Station, James Cook University, Townsville, QLD, Australia*

Extensive damage to houses during severe tropical cyclones in the 1970s in Australia highlighted the need for research-based structural engineering principles to be applied in design and construction of houses. Houses have structural redundancies and complex load paths, so the analysis of even simple houses was complicated. In order to evaluate the structural performance of these parallel systems, full-scale house testing commenced at the Cyclone Testing Station (CTS) in the early 1980s with the static multipoint loading of an old house destined for demolition. Following that test program, nine full-scale houses were tested under static and cyclic loading to evaluate load paths in different types of houses with different building practices and materials. Results have been incorporated into amendments of house construction codes, standards, and manuals. Advances in computer modeling and instrumentation have led to more sophisticated full-scale studies. Data on real houses under construction have informed this work and enabled analytical models to incorporate variability in strength of connections in a way that a single test cannot. Progressive failure in the structural systems of timber-framed housing can now be studied to differentiate between houses that have significant damage and seemingly identical ones that have limited damage in the same wind event. These studies involve wind tunnel investigations to determine temporal pressure distributions; full-scale multiple tests on connections (with and without construction defects) to determine statistical distributions of strength and load/deflection relationships; and full-scale tests on houses or portions of houses to determine load sharing mechanisms between parallel structural and non-structural elements. The results of the test programs were used to calibrate the analytical models, which can be used for reliability studies. The paper presents a summary of the methodology and findings from previous CTS full-scale house tests. The results of the most recent research using full-scale tests on a portion of a house and its companion numerical models are discussed and the benefits and limitations of the process outlined.

Keywords: cyclone, full-scale, testing, wind, house, model, failure

# INTRODUCTION

Destructive cyclones including Cyclone Althea (Trollope, 1972) and Cyclone Tracy (Walker, 1975) in Australia in the 1970s highlighted the poor performance of housing in comparison to engineered buildings (e.g., office buildings, commercial premises). Investigations of damage concluded that engineering design was required to improve the safety of house construction. House construction at

*Edited by:*

*Wei Zhang, University of Connecticut, USA*

#### *Reviewed by:*

*Ilaria Venanzi, University of Perugia, Italy Peng Huang, Tongji University, China*

*\*Correspondence: David James Henderson<EMAIL_ADDRESS>
#### *Specialty section:*

*This article was submitted to Wind Engineering and Science, a section of the journal Frontiers in Built Environment*

*Received: 06 October 2016 Accepted: 22 March 2017 Published: 07 April 2017*

#### *Citation:*

*Boughton GN, Parackal K, Satheeskumar N and Henderson DJ (2017) Development of a Full-Scale Structural Testing Program to Evaluate the Resistance of Australian Houses to Wind Loads. Front. Built Environ. 3:21. doi: 10.3389/fbuil.2017.00021*

that time was largely based on traditional building methods that only evolve slowly in response to infrequent but severe events. Since then, engineering principles have been applied to develop Australian Standards used in house design and construction that provide rationally based guidance on resisting wind loads.

Light-framed timber construction is the most common form of residential construction in Australia and North America. This type of construction is characterized by the use of multiple small cross section timber members to create walls, roof, and floors. Light-framed construction developed in the early 1800s with the advent of industrial saw mills that were able to mass produce standard timber sections.

Due to the large numbers of repetitive structural members, a light-framed house is easy to construct but is an extremely complex structure to analyze. The interaction of structural elements result in load sharing, multiple load paths, partial composite action, and under severe wind loading—the non-linear behavior of connections results in load redistribution. **Figure 1** shows the multitude of structural elements that can compose a light-framed house, each of these is connected to others with various fasteners each with different structural behaviors.

Construction techniques for light-framed construction were largely developed by builders' and carpenters' traditions with codified structural engineering principles being implemented in only the last 30 years in Australia. This lack of engineering input along with the high complexity of the structure has meant that houses have been especially vulnerable to severe wind events compared to engineered structures.

Extensive research programs were required to support the rapid evolution of Codes and Standards and required structural models of complete house behavior that had to be verified. The Cyclone Testing Station (CTS) undertook testing on full-scale houses to establish models of load paths and the structural behavior of houses under wind loads. The testing program was used to verify details proposed in a range of deemed-to-satisfy solutions for houses in high-wind areas.

While the initial tests replicated static and repeated applications of patch loads derived from wind-loading standards, the testing program has evolved to include replication of spatial and temporal variations in wind pressures on house surfaces and the combination of full-scale tests and computer models of behavior to model a number of different variations of normal construction practice.

# EARLY FULL-SCALE STRUCTURAL TESTS ON WHOLE HOUSES AT CTS

#### Test Programs

By 1980, CTS had undertaken wind load tests on many components of houses in a laboratory setting. However, it was recognized that testing of structural components under simulated wind loads in the correct structural context required full-scale *in situ* testing. The most appropriate context could be delivered by testing a complete house.

A 40-year-old house that had survived Tropical Cyclone Althea in 1972 was used to test the concept of full-scale tests on houses in 1981. The house was located only 1,500 m from the anemometer that had recorded 52 m/s during the event (Trollope, 1972). The simulated wind loads were applied to the house on site and

intended to replicate the loads calculated from the anemometer data. The damage was compared with damage observed on similar houses in the area during TC Althea (Boughton and Reardon, 1982a,b). These tests demonstrated the applicability of full-scale house testing for assessing the interaction of the houses of many interconnected components.

The concept was further developed in 1982 and 1983 when full-scale tests were performed on a new house that had been built to comply with the planned deemed-to-satisfy provisions for suburban houses in tropical cyclone prone areas of Queensland. The house was a rectangular plan 12.8 m × 6.4 m with 10° roof pitch and gable ends. The house was subjected to wind loads calculated from the wind loading standard at the time, AS 1170.2 (Standards Australia, 1983) equivalent to a site wind speed of 42 m/s (Boughton and Reardon, 1983). **Figure 2** illustrates the application of:

Where loads incorporating full internal pressure were applied to the underside of the roof, an equivalent downward load on the floor was created using drums of water to ensure that global loads on the house were representative. Cyclic load simulations were applied to the structure by regulating flow to the hydraulic rams based on the load cell signals. In the early tests, repeated loads at different percentages of the ultimate design wind loads were applied with thousands of uplift cycles and hundreds of lateral load cycles.

Wind forces do not reduce as the house deflects, so wind is very flexible compared with the structures it loads. Therefore, the loading system was designed to be very flexible compared with the house under test by incorporating flexible cables. The measured flexibility of the full-scale loading system was around three times that of the first house constructed for tests. (It was around 10 times the flexibility of later brick veneer houses.)

Displacement transducers that measured vertical and horizontal movement at a number of locations monitored the response of the house. The data were logged on computers. The limitations of early data logging equipment meant that it took 5 s to read all load cells and transducers. The combination of flexible loading systems and slow read times meant that it was difficult to obtain data on loads and deflections near to and immediately after failures. As the full-scale test house testing program developed, the equipment to control loads and monitor deformations became more sophisticated and could achieve more variability in loading regimens and faster response time for measurements.

The tests aimed to identify load paths through the structure under wind loads, identify any vulnerabilities in typical houses built to current standards and develop improved details, and confirm design methods for calculating wind resistance of houses. Each house was loaded with simulated wind loads evaluated from design wind speeds (Standards Australia, 1983) and appropriate for each house. (The houses were designed for a range of wind speeds.)

The following is a summary of complete houses tested in the program during the 1980s and 1990s using the same test equipment:

• The "Hyne house"—a high-set (elevated) house (single storey with floor level 2.4 m above ground), with rectangular floor plan 12.8 m × 6.4 m, a steel sheet roof on hardwood trusses with a 10° pitch and gable ends (Boughton and Reardon, 1983). The house included sawn hardwood timber wall and floor framing, timber floorboards, 6 mm thick fiber cement external cladding, and 10 mm plasterboard internal linings. The design wind speed at eaves level was 42 m/s.

Figure 3 | Tongan house after low-cycle failure of truss-to-wall connection.

channel as part of the foundation. The design wind speed at eaves height was 50 m/s.

• Brick veneer split-level house for construction in non-tropical cyclone areas (Reardon and Henderson, 1996). The single storey section had a plan of 8 m × 9 m connected to the twostorey plan of 10 m × 6 m. The test house replicated the new trend of large open plan living areas. The design wind speed at eaves height was 27 m/s.

# Results and Outcomes from the CTS House Testing

Each test series provided tens of thousands of data, which were analyzed to establish load paths for wind forces through the structure of the house, identify vulnerable elements, and provide feedback to designers and Australian Codes and Standards about the resilience and vulnerabilities of housing.

#### Vulnerable Elements

Leicester and Reardon (2010) postulate that all houses contain one or more elements with significantly less strength than the desired design capacity, through for example, an error in construction or defect material. The full-scale tests confirmed that at least one element/connection in the houses failed at less than the design load. In fact, for each full-scale cyclonic region test building, a loss of strength from low cycle fatigue of a major connection along the load path occurred resulting in the house not being able to achieve its original target design load. After minor changes were made to the vulnerable elements, the modified houses were able to resist the design wind speeds with an appropriate factor of safety:

Figure 4 | Loading trees attached to roof battens of single storey house (the Hyne high-set house in background showing the damage to exterior wall cladding from simulated wind driven debris to evaluate loss of racking resistance in wall lining).

Figure 5 | View of light-gage steel-frame house with details of anchorage failures (Reardon, 1996).

structure and wall were insufficient, and there was tearing around small washers under bolts between wall panels. After redesigning those details, the house was able to resist lateral loads that were 160% of the design load and uplift loads that were 130% of the design load.

resulting in the failure. A redesign of the connection to remove the eccentricity allowed the building to achieve its design load requirements.

The recommended changes to the vulnerable details were given to designers to implement in houses constructed for use in the community. The revised details were also incorporated into relevant Codes and Standards. For example, thousands of the Tongan house design were built during the 1980s and have since survived several tropical cyclones. The success of the Tongan house design through their long-term performance has resulted in a refinement of the design and the construction of more of these houses in 2014.

The tests also showed that, in general, bracing walls in fullscale houses were more vulnerable and less ductile than laboratory tests indicated. In laboratory testing, the reaction frame and the loading system are typically very stiff. First failure of a connection in a bracing wall during laboratory tests typically causes an immediate reduction in load and an effective redistribution to other fasteners. Subsequent increases in load show very ductile behavior of the wall with progressive yielding of the fasteners and an increase in strength of the wall following first fastener failure. However, tests on a house with a flexible loading system and a flexible suspended timber floor system produced complete failure of the bracing walls when the first fastener failed. The energy stored in the floor and the flexible loading prevented release of load and redistribution to the whole panel following first fastener failure. Instead, the adjacent fasteners were rapidly overloaded. The full-scale test bracing capacity of the house was 60% of the capacity taken from the sum of individual bracing walls tested in laboratory conditions (Boughton, 1988).

#### Load Paths

Load paths for both lateral and uplift loads through the structure were identified by comparing results of each house loaded at a number of stages during construction. The increase in stiffness at each stage of construction gave an indication of the change in load path with the addition of each structural and non-structural element. (An example is plotted in **Figure 6** for the two-storey section of the brick veneer split level house.) The results of tests on all houses showed that some non-structural elements were effective in transferring wind loads through the structure. For example, on most houses, without the cornice, the lateral load transfer between the ceiling diaphragm and the top of bracing walls followed a circuitous path: ceiling to ceiling screws, to ceiling battens, to lower chords of trusses, to truss-to-wall connections, to the top of exterior walls, to frame-to-frame nailed connections, to internal bracing wall frames, and then through cladding fasteners to the bracing panels. After the installation of cornices, the load path was from the ceiling cladding through the cornice directly to the bracing panels; a very direct and stiff load path. Therefore, where glued cornices had been installed, almost no lateral load was transferred through the structural connections until the cornice cement had broken.

Many load paths included a number of parallel elements that had the capacity to share loads if one element failed. Therefore, weaker elements in load paths that incorporated redundancy did not pose a risk of significant structural damage. The vulnerable elements identified in Section "Vulnerable Elements" were not part

upper storey with each additional construction stage (Reardon and Henderson, 1996).

of a system that allowed alternate load paths or redundancies. This highlighted the need for further research into the structural redundancy of elements in houses and the progression of failure following first failure of a vulnerable element.

#### Models of Structural Load Transfer in Houses

The results from each series of tests on a house type were able to confirm or refine accepted models of load transfer between elements. For example:

## Limitations of Early Tests on Full-Scale Houses

In many of the full-scale tests, particular care was taken to ensure that correct construction practices were used. However, slight variation in design, workmanship, and materials has been observed in inspections of houses in the community and is one of the factors accounting for differences in the performance of similar houses in the same wind event. The cost of building fullscale test houses prevents using this method to test variations in construction practices; however, damage investigations show that this is still a major cause of house failures under wind loads.

The CTS-simulated wind loading across the building is an approximation of the pressure gradients derived from Standards or wind tunnel models. The patch loads and line loads are applied to the structure such that higher loads can be applied to edge regions compared to the field (middle) of the roof. The loads can also be cycled in a sinusoidal application. Although this simulated loading can be repeatedly applied to the test house following change of materials/components, it cannot replicate the spatial and temporal variations from aerodynamic loading, such as the full-scale testing discussed by Huang et al. (2009).

No new complete full-scale test houses have been built at the CTS since the late 1990s with the intervening two decades of research and development targeting the performance of components within the structural models identified in the full-scale house testing program. Recent research has extended the original concept of full-scale testing so that a range of component characteristics can be investigated in tests of full-scale sections of houses.

# FULL-SCALE TESTS ON HOUSE ROOFS

Recently, several other full-scale house tests have been carried out to evaluate the structural response and load sharing of the North American and Canadian residential houses (Datin and Prevatt, 2007; Morrison, 2010; Canino et al., 2011; Doudak et al., 2012; Henderson et al., 2013). However, the usefulness of these experimental results to assess the structural response and load sharing of Australian residential house is limited, as their construction types are different. These variations in construction result in differences in their structural response and stiffness of roof-to-wall connections. Thus, it can be expected that the load sharing and reaction influence coefficients will be different in the Australian timber-framed structure compared to that of other countries' houses.

Based on the applied loads and displacements, the full-scale tests at the CTS (Boughton and Reardon, 1982a,b, 1983, 1984a,b; Reardon, 1986, 1990; Reardon and Mahendran, 1988; Reardon and Henderson, 1996) qualitatively showed the strength, stiffness, and load transfer of the house system subjected to wind load. These tests did not investigate the significance of reaction influence coefficients on the truss-hold down force. The reaction influence coefficient is used with wind pressure distribution to evaluate the truss hold-down forces of timber-framed houses (Ginger et al., 2000). The study presented in this section describes a full-scale test conducted to identify the load sharing and reaction force influence functions of timber-framed houses in Australia. A combination of; full-scale tests on sections of a house, numeric models, characterization of the variability of components, and detailed wind tunnel loading data has been used to investigate structural behavior of houses with a variety of construction practices, determine statistical distributions of strength and load/deflection relationships and load sharing mechanisms between parallel structural and nonstructural elements (Satheeskumar et al., 2016a,b).

Data on houses under construction (see Investigation and Testing of Contemporary House Construction) have been incorporated into analytical models to represent variability in the strength of connections in a way that a single full-scale test cannot. The focus of these studies is the investigation of the effects of defects in individual connections and the mechanics of progressive failure in the complex structural systems in timber-framed housing. The results of the test programs were used to calibrate the analytical models, which can be used for reliability studies.

This section discusses research projects that extend the previous CTS full-scale testing programs to evaluate the performance of full-scale roof structures under wind load using a combination of wind tunnel pressure records and full-scale testing of components and assemblies.

# Investigation and Testing of Contemporary House Construction

As a collaborator in the Climate Adaptation Engineering for Extreme Events Cluster (CAEx) (Stewart et al., 2016), CTS undertook surveys of houses under construction in three Australian capital cities to document current design and construction practices. The surveys recorded data on structural members (size, grade, and span), connections (fasteners, spacing, and configuration), and identified inadequate design and installation of some elements. Incorrect site classifications for wind, errors in connections for tie-down of roof structural elements, over-driven nails on bracing elements, and incorrect window and door installation were observed. The plans of the inspected houses showed that their roof structure was more complex than the roofs of houses used in previous CTS tests on full-scale houses or in wind tunnel studies to derive peak wind pressures.

Cyclone Testing Station used these data from the surveys to develop a design of a representative house with specified dimensions, roof shapes and pitch, plan footprints, materials, and structural systems as shown in **Figure 7**. It included a nonrectangular floor plan with hips, valleys, and short ridges. Wind tunnel studies on this representative house were used to establish base-line peak pressure coefficients on all areas of the roof, as illustrated in **Figure 8**. Pressure taps were installed on the

models in a 12 mm × 18 mm grid-pattern (representative of the 600 mm × 900 mm truss-batten spacings in full-scale) to enable cladding loads, batten-truss loads, and the resulting wind load effects on the trusses to be determined.

Cyclone Testing Station has also conducted investigations of damage to houses following most major wind events in Australia since the mid 1980s. The data from these investigations have been used to highlight/identify deficiencies in design and construction practices that have contributed to changes in Codes and Standards and initiated research projects to improve building resilience. Many of the deficiencies in structural details that contributed to damage in houses during severe wind events were also observed in the CAEx surveys. These deficiencies have been incorporated into recent full-scale tests in order to establish their role in progressive failure and investigate whether redundancies in some elements provide alternative load paths around the deficiencies.

#### Behavior of Truss-to-Wall Connections

A 3.3 m-long section of house including five 6.6 m-long roof trusses, shown in **Figure 9**, was constructed for full-scale testing to evaluate the capacity of normal house components such as wall linings and cornices to carry loads (Satheeskumar et al., 2016a) around weak truss-to-wall connections. Vertical load transfer through the structural connections was measured at the top plate and the bottom plate. The full-scale test structure included five standard prefabricated nail plate trusses, metal battens, corrugated steel roof cladding, timber wall frames, plasterboard wall and ceiling lining, and cemented ceiling cornices.

Hand-nailed metal framing anchors that complied with recommended construction practices ("ideal") were initially installed to connect the trusses to the wall frames. Some of the connections were altered (e.g., missing nails) for subsequent tests to reflect poor construction practices observed in some houses as discussed in the Section "Investigation and Testing of Contemporary House Construction." The results showed that the load distribution of uplift forces was a function of the stiffness of the roof-to-wall connection. The lining elements (i.e., ceiling, ceiling cornice, and wall lining) contributed significantly to vertical load sharing.

Figure 9 | Full-scale tests on house cross-section.

A 3D finite element model (FEM) was also developed using ABAQUS (6.12-3) (ABAQUS, 2013) to further assess the load sharing and structural response of the timber-framed structure to different construction defects in roof-to-wall connections. The connections were represented in the FEM as either linear or non-linear spring elements. The connection forces in the FEM analysis were compared with measurements from full-scale structural testing using uniformly distributed loads. The results were similar, with a maximum variation of about 15% (Satheeskumar et al., 2016b). The validated FEM provided confidence to investigate the response of roof-to-wall connections to more realistic uplift forces that vary across a house roof.

The FEM was developed and analyzed for three different configurations: case 1 using "Ideal" connections; case 2 with a single nail missing in one of the truss tie-downs; case 3 with two nails missing from one truss-to-wall connection and one nail missing from an adjacent truss-to-wall connection. The force displacement relationships for these cases were obtained from the experimental tests and high resolution FEM of isolated connections (Satheeskumar et al., 2016c,d).

**Table 1** presents the *CN* (i.e., truss hold-down coefficient) at each connection when 1 kPa uniform suction pressure was applied on the roof. It shows that the loads in connections adjacent to the trusses that have defective tie-down are increased by about 10% where there is only one defective tie-down (case 2). This figure is significantly less than the 50% redistribution from a statically determinate system. There is a higher load transfer if there are two adjacent defective connections. The research project demonstrated that the linings in normal house construction can carry load around defective truss-to-wall connections.

# PROGRESSIVE FAILURE AND LOAD REDISTRIBUTION OF BATTEN-TO-TRUSS CONNECTIONS

Full-scale house testing conducted in the 1980s and 2015 examined the complex load paths, the effects of construction defects, and damage. However it is still largely unknown how loads are redistributed during a failure and how structural failure can propagate through the roof structure. Damage investigations have shown that the failure of a few batten-to-truss connections can trigger a progressive failure resulting in the loss of large sections of the roof structure.

The study presented in this section examines progressive failures of nailed timber batten-to-truss connections. Laboratory tests were conducted to determine the behavior of connections under wind loads. Wind tunnel studies were conducted to

Table 1 | *CN* of truss-to-wall connections with uniform suction pressure 1 kPa applied to the roof with trusses affected by defective fasteners highlighted.

determine the correlation of load time histories at neighboring batten-to-truss connections and its effect on the initiation of a progressive failure. Finally, non-linear structural analysis was undertaken to determine how loads are redistributed as connections fail under uplift loads.

#### Behavior of Batten-to-Truss Connections

Laboratory tests were conducted on new batten-to-truss connections and on connections taken from 50-year-old houses. The tests on 50-year-old connections provided data on the performance of nailed connections in service and enabled deterioration due to age to be quantified.

Static testing of batten-to-truss connections was conducted to determine the mean strength of the connections under slow load rates (2.5 mm/min as specified in Standards Australia, 2001). A representative load/deflection plot of a 50-year-old connection with a strength close to the mean strength is shown in **Figure 10**. The 50-year-old connections showed a much greater variability in strength than new connections.

Dynamic tests of single batten-to-truss connections were undertaken to characterize progressive failure or deterioration of connections under fluctuating wind loads. The connections were tested using a servo hydraulic universal testing machine with a synthetic test load trace based on the wind tunnel study discussed under Section "Investigation and Testing of Contemporary House Construction". Appropriate time scaling was applied to determine the required full-scale frequency.

Dynamic testing applied repeated synthetic peak loads to full-scale specimens until failure. The loading rate and frequency of the time history signal was based on a gust wind speed of around 40 m/s for a nominal tributary area for the connections of 0.36 m2 .

The results of the dynamic tests still showed large variability in connection performance, with some connections able to survive only two or three peaks and some able to survive more than 100

peaks. All showed some ductility as they were able to sustain loads at deformations much higher than their elastic limit.

**Figure 11** shows an example of 50-year-old connection behavior under dynamic loading. There was nail slip with each peak event. In most cases, there was an increase in performance similar to strain hardening behavior in steel; the magnitude of each slip decreased near the center of the plot. After the accumulated nail slip had reduced the depth of embedment to a critical level, the connection rapidly lost strength and failed as shown at the right hand side of the plot.

Preliminary testing also showed that for many of the 50 year-old connections, some nail slip occurred at peak loads below the mean connection strength. The accumulation of nail slip over a number of peak load cycles resulted in lower depth of embedment of the nail into the truss. This observation is compatible with older connections in service that have experienced nail slip in a number of previous wind events failing at loads lower than expected.

The dynamic tests showed that a connection's elastic stiffness, indicated by the gradient of the loading and unloading paths, does not change with accumulated damage through successive peak events, i.e., the elastic behavior of the connection is similar for each peak load event. This indicates that load redistribution to adjacent connections occurs due to total nail slip in a connection rather than to a decrease in elastic stiffness. Once a connection fails completely, load will be rapidly redistributed to adjacent connections.

The next phase of this study will involve the construction of a full-scale portion of a roof with full instrumentation of each batten-to-truss connection to evaluate the redistribution of load during progressive nail slip and complete connection failure of a single batten-to-truss connection.

#### Wind Tunnel Time-History of Loadings on Roof Elements

Previous CTS full-scale testing had focused on demonstrating that deemed-to-satisfy designs could withstand the design wind loads and that models of structural behavior were valid, so loads calculated from contemporary wind loading standards were used. However, the Standard loading model is generalized. In order to use full-scale testing to evaluate progressive failure of elements within a house and demonstrate load sharing among elements close to the cladding, as noted in Section "Early Full-Scale Structural Tests on Whole Houses at CTS," it is important to replicate the temporal and spatial variations in wind loads. Wind tunnel studies were used to create fluctuating load sequences that could be used to simulate wind forces on crucial elements of houses.

Boundary layer wind tunnel studies were conducted on 1/50 scale models to determine pressure distributions on the surface of houses for multiple wind directions (Parackal et al., 2016). Pressures were measured using hundreds of pressure taps on roof and wall surfaces. These pressures were integrated to calculate loads on connections supporting different areas of roof surfaces.

The tributary areas of batten-to-truss connections experienced the highest peak loads at roof corners and the apex of gables. The loads on a single connection were evaluated by summing the pressures over the tributary area of the surface for a single batten-to-truss connection. These loads were characterized by "peak events" of load more than 3.5 SDs from the mean lasting about 0.5–2.0 s for a mean wind speed of 100 km/h at full scale. The peak loads experienced varied with wind directions and were due to different aerodynamic mechanisms such as separation of flow or vortices entrained in the airflow.

In order to determine the factors that may promote failures to progress through a full-scale roof, correlation coefficients of load-time histories at connections were evaluated. The crosscorrelation coefficient (*rij*) defined in Eq. 1 gives a measure of how well correlated the pressures on a specified connection (*i*) are with the pressures on an adjacent connection (*j*). A time lag term (τ)

is incorporated to allow for the time it takes for a gust vortex or eddy to move from one connection to another:

$$r\_{\bar{y}} = \frac{1}{T \times \sigma\_{\rho i} \times \sigma\_{\rho j}} \int\_0^r p\_i'(t) \times p\_j'(t+\tau) \, dt \tag{1}$$

where *pi* and *pj* are the fluctuating components of the pressure at locations *i* and *j*, σ*p* is the SD of fluctuating load, and *T* is the time over which the signal is analyzed.

As shown in **Figure 12**, cross-correlations of time histories show that peak loads are correlated with pressures on adjacent connections. The negative lag times indicate that the pressure is well correlated with pressures on an upwind connection. For this wind direction, it was the connection on the same batten immediately upwind of the connection being evaluated. For other wind directions, the correlations indicated that the peak loads had been moving diagonally across the roof. It was also found that wind directions that caused the highest loads were not necessarily those that caused the highest correlations amongst adjacent connections.

The patterns of correlation can have implications on the initiation of progressive failures, especially if the correlation patterns align with the direction in which loads will be redistributed when a single connection fails or weakens. The direction the loads are redistributed between batten-to-rafter/truss connections is determined by the ratio of the bending stiffness of the battens to that of the cladding in the direction parallel to its corrugations as well as the batten and rafter/truss spacing. For example, the gable roof house studied may be most susceptible to progressive failures for wind direction 270° with the correlation patterns aligning along rafter lines and if the bending stiffness of the corrugated cladding is greater than the battens.

Figure 12 | Cross-correlations of time histories. Different lines within each color refer to different batten-to-truss connections on each truss. Truss 1 is over the gable, then, Truss 2 is one truss spacing from the gable, then Truss 3.

#### Non-Linear "Pull-Up" Analysis

To determine the direction loads are redistributed and to determine how progressive failure may propagate through the roof structure, a non-linear time history analysis using a finite element method structural analysis model was performed. An array of batten-to-truss connections representing the connections within a study area of a gable roof house was modeled as shown in **Figure 13**. Roof cladding is modeled as a thin shell with the thickness selected to give the same flexural rigidity (EI) as a 0.42 BMT "custom orb" cladding profile. A "stiffness modifier" of 0.1 is applied to EI of the sheeting in the direction parallel to the battens to represent the lower bending stiffness in the direction perpendicular to the cladding corrugations. Battens were modeled as frame elements that represent 45 mm × 70 mm MGP10 battens. Batten-to-truss connections were modeled as multi-linear plastic link elements, with their force–displacement behavior determined from laboratory tests. These links are assigned a proportional limit 0.5 kN, plastic deformation for 10 mm at 0.5 kN, and complete failure at 15 mm extension. The roof structure below is not modeled.

A non-linear time history analysis was performed with a quasistatic ramp load (1 kN/min) applied at the location of connection T2-B7 (batten 7 fastened to truss 2 as shown in **Figure 13**). The ramp loads are continued as the first connection

Figure 14 | Color scale plots of batten-to-truss connection loads at five time steps showing percentage of total load borne by each connection. From left to right: (A) at and before yield of loaded connection, (B) at yield of connections along battens, (C) at yield of connections along cladding corrugations, and (D) at failure of loaded connection.

fails and a cascading failure takes place. Selected time steps of color scale plots showing the redistribution of load are shown in **Figure 14**.

As load is applied above T2-B7, the system behaves as a set of springs in parallel, with part of the load being resisted by the connection T2-B7 itself and the remaining resisted by the batten and the cladding in flexure as well as extension of the neighboring connections. Uplift loads are thus shared among neighboring connections even when T2-B7 is undamaged and in the elastic range.

Before connection, T2-B7 yields as shown in **Figure 14A**, a total of 27% of the load is shared among connections immediately to the left and right on the same batten (T3-B7 and T1-B7). Load is also resisted along the corrugations of the cladding, with a total of 21% shared with connections T2-B8 and T2-B6. Finally, 15% is shared among connections diagonally away from the loaded connection. One hundred three percent of the uplift load is resisted by connections in the 3 × 3 grid surrounding the loaded connection with connections outside of this grid having the sheeting pressing down on the batten (the remaining −3% of loads). These proportions of load sharing continue to the point where connection T2-B7 yields.

After connection T2-B7 yields, the proportion of load borne by this connection decreases and the amount of load sharing at all neighboring connections increases. When the connection to the left (T3-B7) yields, a total of 37% of load is taken by connections that are immediate neighbors on the same batten and 28% spans along corrugations. As the connection to the right (T1-B7) yields, the battens have reduced the proportion of load sharing and bear 34% of the load, as shown in **Figure 14B**. Loads are now redistributed along corrugations and connections T2-B8 and T2-B6 take 30% of the load. From this point on, the loads borne by connections on the same batten reduce and load transferred along corrugations increases.

Once all connections adjacent to the loaded connection yield as shown in **Figure 14C**, load is shared equally among all the connections in the 3 × 3 grid including the loaded connection. However, when the loaded connection fails completely as shown in **Figure 14D**, loads are equally shared among battens and along corrugations. At this stage, loads are redistributed to diagonal connections.

The next connection to fail completely is T1-B7, to the right of the loaded connection. Loads are again redistributed to connections along the corrugations. At this stage, a cascading failure commences with loads being rapidly redistributed as connections fail in rapid succession resulting in the failure of all the connections in the study area.

The non-linear analysis has shown that load sharing and redistribution between batten-to-rafter connections is a complex process. Loads are shared among neighboring batten-to-truss connections depending on the ratio of stiffness of the battens and cladding as well as the batten and truss spacing. Additionally, the load path changes whenever a connection yields or fails completely. Connection failure and load redistribution occurs rapidly but in durations similar to pressure fluctuations experienced on the roof surface. Thus, the correlations of wind pressures may play a significant role in the initiation of a progressive failure. In future work, time history analysis with spatial and temporally varying loads can be used to determine the effects of the load correlations. This will lead to a study of the propagation of the failure cascade as well as how the structural system responds under fluctuating dynamic loads. Similar techniques can be applied to other connections in the vertical load path such as roof-to-wall connections discussed in Section "Full-Scale Tests on House Roofs."

# DISCUSSION OF BENEFITS AND LIMITATIONS

The original house testing research program at CTS was able to demonstrate the strengths and weaknesses of contemporary house design and enabled the validation of structural details and prescriptive standards (codes) for Australian housing. Typically, once any weak links in the load path were found and repaired, the test houses were stronger than design expectations. The test program led to a greater understanding of the transfer of load and distribution of forces through the various elements of the house structure—the program repeatedly highlighted the complex non-engineered load path of typical house construction. Also, through the application of repeated load cycles representing the duration of a tropical cyclone, issues of low-cycle fatigue on structural connections of light gage steel were highlighted.

In recent years, full-scale testing of US and Canadian houses subjected to wind loads has been conducted to evaluate structural and component response (Datin and Prevatt, 2007; Morrison, 2010; Canino et al., 2011; Henderson et al., 2013). However, there are only limited data available on the load distributions in inter-component connections, and progressive damage due to connection failure to wind loading.

However, full-scale test programs are expensive and time consuming. In early full-scale tests, the costs were justified by the value of these data and outcomes from each test. The current full-scale test program incorporates detailed FEA modeling during the experimental design to ensure that the load and displacement data are recorded for the locations required to best assess the FEA models. Using a validated model, changes in material properties and construction defects can be analyzed with confidence.

Full-scale studies on houses and parts of houses have identified a number of elements that provide structural redundancies. For example, grid systems in roof structures can bridge over weaknesses in specific elements. Wall systems also incorporate multiple parallel elements that can also share load. In addition, there are many non-structural elements such as plasterboard wall linings and ceiling cornices that can also transmit loads around structural systems if the primary structural system fails. However, that does not mean that there is enhanced resilience for all critical elements in the tie-down chain.

The most recent full-scale tests indicate that battens and roof cladding provide some bridging between batten-to-rafter/truss connections. Battens have the potential to carry the load from a failed batten-to-truss connection to only the adjacent connections along the batten. This study also indicates that the roofing provided a stiffer load transfer mechanism that only transfers load in a direction perpendicular to the ridge of the roof. However, in completed buildings, a failure of an edge batten-to-truss connection at the eaves could transfer load *via* bending in the battens to the fasteners in the same batten or *via* cantilever bending of the roof sheeting to the batten connector on the next batten up the roof. The load transfer from the failed connector significantly increases the load on the adjacent fasteners and can lead to progression of the failure. Where first failure is near a corner of a roof panel, there can be a rapid spread of failure through the roof in both directions.

Connections deeper within the tie-down load path have greater redundancy and resilience than connections immediately under the roof cladding. Recent tests indicate that at lower levels in the house, many more elements have the potential to provide bridging systems that can transfer load to other elements if a connection fails. Roof sheeting, battens, ceiling elements, ceiling cladding, and other roof structural elements can transfer load laterally in wall systems over a significant distance if tie-down elements in walls fail. In addition, non-structural elements such as plasterboard wall linings and ceiling cornices can also contribute to load transfer and make roof-to-wall connections and wall systems less vulnerable to progressive failure.

However, in examining the load path progression from cladding through rafter to wall, the gust pressure correlation over the time domain follows the load sharing path for a number of wind directions. These correlation results indicate that in an overload situation, the peak gust pressures are applied to connections that have already attracted extra load as a result of load sharing from a failed fastener. This effect can also contribute to rapid progression of failure of batten-to-truss connections. Therefore, the limited load sharing at batten-to-truss connections coupled with the pressure correlation does not lead to an improvement in resilience of the structure, so it is important that each and every connection has the capacity to resist its design loads.

As further research on progressive failure in roofs continues, the varying resilience of connections within houses can be quantified.

## AUTHOR CONTRIBUTIONS

DH, GB, KP, and NS: all revised the work critically, gave final approval for publication, agreed to be accountable for all aspects of the work, and have ensured the accuracy, integrity of the work. In addition, they have all contributed to the conception or design of the work and drafted the following sections: DH—Sections "Full-Scale Tests on House Roofs" and "Discussion of Benefits and Limitations," GB—Sections "Introduction" and "Early Full-Scale Structural Tests on Whole Houses at CTS," KP—Sections "Full-Scale Tests on House Roofs" and "Progressive Failure and Load Redistribution of Batten-to-Truss Connections," and NS— Section "Full-Scale Tests on House Roofs."

## ACKNOWLEDGMENTS

This work was supported by Greg Reardon and Debbie Falck.

#### FUNDING

The authors and CTS would like to acknowledge the longstanding support of the Cyclone Testing Station's Supporters https://cyclonetestingstation.com.au/benefactors-links.

# REFERENCES

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2017 Boughton, Parackal, Satheeskumar and Henderson. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# A Cyber-Based Data-Enabled Virtual Organization for Wind Load Effects on Civil Infrastructures: VORTEX-Winds

#### *Ahsan Kareem\* and Dae Kun Kwon*

*NatHaz Modeling Laboratory, Department of Civil & Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN, United States*

Despite many advances in the area of wind effects on structures in recent decades, research has been traditionally conducted within limited resources scattered geographically. With the trend toward increasingly complex designs of civil infrastructure combined with the escalating potential for losses by extreme wind events, a new culture of research needs to be established based on innovative and collaborative solutions for better management of the impact of extreme wind events. To address this change, this paper presents a new paradigm of a multiscale cyber-based laboratory framework for the analysis/ design, modeling, and simulation of wind load effects based on an ongoing collaborative cyberinfrastructure-based platform, Virtual Organization for Reducing the Toll of EXtreme Winds (VORTEX-Winds, https://vortex-winds.org), and discusses its current status since its inception in 2007 and ongoing developments. This collaborative framework as it evolves would enable a paradigm shift by offering advanced cyber-enabled modules (e-modules) for accelerating advances in research and education to achieve improved understanding and better modeling of wind effects on structures. Accordingly, it will enhance wind community's analysis and design capabilities to address next-generation challenges posed by wind. Through empowering those without computational or experimental resources, the e-modules will encompass a large set of subject areas and topics categorized as Database-enabled design, Full-scale/Field site data repository, Statistical/Stochastic toolboxes, Tele-experimentation, Uncertainty modeling, Damage assessment, and Computational platforms. This prototype will allow access to the individual e-module, while it is envisaged that next level of development in VORTEX-Winds will have the capability for an automated and integrated analysis/design through a nexus of e-modules. A highlight of the e-modules currently completed or in development is presented not only to show the efficacy of the framework to enhance and supplement the limitation of traditional off-line approaches but also to describe architecture and features of e-modules to promote additional cyber-enabled data-driven developments in the field.

Keywords: virtual organization, cyberinfrastructure, extreme winds, wind load effects, modeling, simulation, analysis, design

# INTRODUCTION

Wind-related catastrophes by extreme winds such as hurricanes, tornadoes, thunderstorms/downbursts, etc. inflict enormous devastation on the built environment and result in a staggering number of fatalities. A recent report by Munich Re Group (2012) showed that North America has been most affected by weather-related extreme events in the world in recent decades. To better understand and

#### *Edited by:*

*Gregory Alan Kopp, University of Western Ontario, Canada*

#### *Reviewed by:*

*Franklin Lombardo, University of Illinois at Urbana– Champaign, United States Chris Letchford, Rensselaer Polytechnic Institute, United States*

> *\*Correspondence: Ahsan Kareem<EMAIL_ADDRESS>
#### *Specialty section:*

*This article was submitted to Wind Engineering and Science, a section of the journal Frontiers in Built Environment*

> *Received: 09 January 2017 Accepted: 21 July 2017 Published: 21 August 2017*

#### *Citation:*

*Kareem A and Kwon DK (2017) A Cyber-Based Data-Enabled Virtual Organization for Wind Load Effects on Civil Infrastructures: VORTEX-Winds. Front. Built Environ. 3:48. doi: 10.3389/fbuil.2017.00048*

manage the impact of such extreme wind events, a new culture of research needs to be developed based on innovative solutions. In this regard, a virtual organization (VO) or cyberinfrastructure (CI) has been recognized as a possible means in many disciplines to offer real-time shared access to geographically dispersed resources, which would enable the pooling of resources reliant on Internet and information technology (IT) (Kijewski et al., 2003; Frame et al., 2009). A conceptual example framework for integrated design for wind effects is envisioned in **Figure 1A** in which computational, experimental, and other features are managed and coordinated by a cyber studio using a cloud platform.

Other fields are fast recognizing the potential impact of such a technology. An example is NERC environmental eScience consortia comprising a variety of VOs in the fields of environmental sciences to share resources and perform collaborator (Gurney et al., 2009). Similarly, the structural design field is currently at the dawn of adopting a new IT known as Building Information Modeling, which promises to revolutionize the design and the construction of buildings. Building Information Models are 3-D, smart, parametric e-models of buildings that are shared by a team of designers and builders to facilitate the exchange and interoperability of information in a digital format. However, such a conventional concept of VO or CI that simply shares valuable resources for collaboration has now evolved into the new stage with the rapid IT developments. For example, nanoHUB for nanotechnology offers not only community-contributed resources but also online computational tools based on grid computing (Goasguen et al., 2008). In structural engineering field, the Network for Earthquake Engineering Simulation (NEES) (Van Den Einde et al., 2007) offered a feature of networked facilities for enhancing seismic hazard research through hybrid tests. These facilities have made high-end resources accessible to groups otherwise limited by their personal research tools allowing them to expand the scope of their research to address complex problems. NEES also offered computational tools by adopting the framework of nanoHUB, named NEEShub (Rodgers, 2011). Although those advanced CI frameworks despite their promise were either to computational or experimental/hybrid methods.

Similar to other disciplines that usually include broad research subjects, the topic of wind effects on structures is of an interdisciplinary nature that requires a knowledge base from a large number of subject areas. It includes meteorology, structural engineering, engineering mechanics, probabilistic/statistical methods, fluid dynamics, turbulence, structural dynamics, experimental methods, risk, reliability, etc. (e.g., Kareem, 2005, 2008). For better quantifying the load effects, a new paradigm shift is required to provide efficient and reliable tools or procedures for both research and practice. Certainly, it is a daunting task to develop such innovative solutions by a group or two, thus community-level efforts are necessary. With the recognition of this demand, for example, the National Science Foundation (NSF) has recently launched a new initiative, Natural Hazards Engineering Research Infrastructure (NHERI) [National Science Foundation (NSF), 2014], replacing NEES, which is a distributed, multiuser, national platform to provide the natural hazards engineering community (e.g., earthquake and wind engineering) with access to research infrastructure. A CI underpins and integrates NHERI as a VO that offers researchers a platform for research collaboration and knowledge sharing (DesignSafe-CI1 ). Central to NHERI is the experimental testing, computational modeling and simulation, research data and their integration with theory for generating knowledge, and innovation needed to mitigate the impact of future natural hazards such as earthquakes, windstorms, etc. [National Science Foundation (NSF), 2014].

In anticipation of a CI based data-enabled engineering as a central component of a computational/experimental infrastructure, this study describes the evolution of a cyber-enabled framework for analysis/design, modeling, and simulation of wind load effects, Virtual Organization for Reducing the Toll of EXtreme Winds (VORTEX-Winds) at https://vortex-winds.org, and discusses its current status since its inception in 2007 and ongoing developments that predate the current CI-based initiatives. It aimed not only to facilitate an effective, transformative, and conveniently accessible methodology for the acceleration of advances in research and education in the broad area of wind effects but also offered initial developments in an end-to-end automated and integrated wind design approach for practical application in the end. This framework would enhance analysis and design capabilities to address the challenges of innovative structural systems needed to realize, in a cost effective manner, buildings with ever increasing heights, bridges that span oceans, and offshore platforms tapping hydrocarbons in deeper waters exposed to weather extremes like hurricanes. This challenge is envisaged to be accomplished by cyber-enabled e-modules classified into *Database-enabled design* (DED), *Full-scale/Field site data repository*, *Statistical/Stochastic toolboxes*, *Tele-experimentation*, *Uncertainty modeling*, *Damage assessment*, and *Computational platforms*. The prototype framework would allow access to the modules individually, while the fully functional VORTEX-Winds may also have the capability for automated, integrated analysis and design by interactively stitching multiple modules. Throughout a highlight of ongoing e-module projects encompassing broad wind research areas, the efficacy of the framework to enhance and supplement traditional off-line approaches is demonstrated. In addition, architectures and implementations of e-modules are described to serve as a reference or direction for prompting additional cyber-enabled developments by the community.

## OVERVIEW OF VORTEX-WINDS

The basic vision of VORTEX-Winds is the development of a comprehensive gateway for research and education to achieve improved understanding and modeling of wind effects on structures to counter the escalating loss of property and associated indirect losses and the increase in the sensitivity of emerging structural systems to winds. In response to this vision, the VORTEX-Winds was established in 2007 with the following goals:

(i) to establish and sustain a community contributing to and employing the resources integrated by CI technologies to facilitate the mitigation of escalating damage, loss of life, and disruption of local economies posed by wind;

1https://www.designsafe-ci.org.

VORTEX-Winds envisions offering all aspects of CI capabilities to research and engineering community in the area of wind effects on structures, including (1) communication tools to support collaboration, (2) high-performance computational infrastructure, (3) data storage, data mining, visualizations, and data warehousing, and (4) access to remote sensors and tele-experimentation. The structure of VORTEX-Winds consists of two main branches: the knowledge base and the cyber-enabled e-modules as shown in **Figure 1B** (Kareem et al., 2009). The knowledge base is similar to the functionality of a conventional VO to aggregate and centralize the shared knowledge of the collaboratory. Services in this area include the virtual encyclopedia, *Wind-Wiki*, encompassing basic terminology and concepts pertaining to wind–structure interaction, *Damage database* that is curated archives of postdisaster reconnaissance (e.g., Kareem, 1985, 1986), *Help desk*, where users can submit a question to the collaboratory and where past responses are archived as *FAQ*, *Bulletin boards* hosting open discussions, *Email list servers* for rapidly circulating announcements and other information, and *Curriculum tools* to provide educators a means to formally integrate VO services into their teaching.

A key component of VORTEX-Winds is the cyber-enabled e-modules to not only offer an automated and integrated design approach under CI but also facilitate research activities by researchers and students, which are developed by authors as well as harvested from the independent work of a number of universities (largely supported by federal funding) and government agencies such as NIST, etc. (Kareem et al., 2009). For wind design, a traditional off-line procedure for civil infrastructures as shown in **Figure 2A** relies on a broad-band knowledge and extensive procedures involving a variety of subjects. The integrated design approach proposed in this study mimics the traditional procedure schematically depicted in **Figure 2B** for providing more efficient and accurate assessment of wind effects on structures and design

process. In view of the interdisciplinary nature of wind effects on structures requires knowledge from a large number of subject areas, the e-modules are classified into seven categories: *Databaseenabled design*, *Full-scale/Field site data repository*, *Statistical/ Stochastic toolboxes*, *Tele-experimentation*, *Uncertainty modeling*, *Damage assessment*, and *Computational platforms* (**Figure 1B**)*.* Examples of the modules offered within each category are shown in **Figures 3** and **4**. The modules are currently interrogated independently but the final goal is that modules can be automatically queried and input into an integrated analysis and design approach by stitching together multiple modules. A more advanced framework will be the inclusion of a knowledge-based expert system

in the loop of the integrated design concept (e.g., Kareem and Allen, 1990). Consequently, it is envisaged that VORTEX-Winds will provide access to communication, data, computational, and remote sensing and tele-experimentation services to generate a comprehensive, interactive design aid. Users will be able to execute either individual services or composite services integrated from multiple atomic services for the purpose of research activity or wind design process. This will require format conversion of data and those conversions will be provided as stand-alone services or directly integrated into the prepackaged web-service workflows. It should be noted that the interactive design aid will take the notion of hybrid simulation a step further by utilizing databases,

experiments, full-scale measurements, and analytical models through repeated queries and multistage analyses.

From an educational perspective, VORTEX-Winds' introduction of a knowledge-based framework and cyber-enabled e-modules can indirectly serve as educational tools to countless professional end users as well as K-12 and postsecondary education. This is particularly important considering the general shortage of engineers formally educated in wind effects in this country and requires intervention with K-12 students as early as possible as part of a STEM education. At the postsecondary level, for example, these services can be integrated into an introductory structural engineering course at the freshman level and a vital resource for the wind engineering and structural system courses for graduate students emphasizing the cross-disciplinary nature of modern design subject to winds. Similar efforts using web-based technology have been reported by researchers (e.g., Wang and Adeli, 2012a,b). In addition, educators such as K-12 teachers and college/university professors can use these services to develop curriculum tools. Then, these will be archived in the knowledge base for access by other educators nationwide so that VORTEX-Winds will provide an effective pedagogical tool to learn a highly technical subject, which has its roots in a host of interdisciplinary fields, through the use of cyberspace and visualization capabilities.

These services are hosted on the front end by a single gateway server with computation and analysis conducted on back-end parallel servers. The primary technical hurdle is in achieving e-module developments and their integration, which will require significant effort and coordination, particularly with respect to the integrated design feature. At this stage, VORTEX-Winds has been focusing on the achievement of individual e-modules. An overview of ongoing cyber-enabled modules is shown in **Figure 2C** and their current and ongoing statuses are listed in **Table 1**. More detailed information about the highlights of CI modules for analysis/design, modeling, and simulation modules of wind load effects on structures in the VORTEX-Winds is presented in the following sections. It is anticipated that the architecture and implementation of e-modules demonstrated in the following will serve as a reference or direction for prompting additional cyber-enabled developments by the community.

## CYBER-ENABLED ANALYSIS/DESIGN, MODELING, AND SIMULATION MODULES

#### Damage Database

The Damage database is a digital repository of postdisaster investigation documenting damage to structures during extreme wind events such as hurricane/typhoon/cyclone, tornado, thunderstorm/downburst, extratropical winds, etc. The information provided by this database may be beneficial to a variety of studies, e.g., evaluation of extreme wind effects on structures

#### Table 1 | Current and ongoing statuses of e-modules in the VORTEX-Winds.

*a PBDED-OC involves an uncertainty analysis through a Monte Carlo simulation. CFD, Computational Fluid Dynamics; DEDM-HR, Database-enabled design module for high-rise; DEDM-HRP, Database-enabled design module for high-rise building with pressure data; DEDM-LR, DED module for low-rise buildings; NALD, NatHaz aerodynamic loads database; NGFF, NatHaz gust-front factor; NH-GUST, NatHaz interactive wind tunnel; NOWS, NatHaz online wind simulator; PBDED-OC, performance-based design of high-rise buildings for occupant comfort; VOWS, VORTEX-Winds online wind simulator; VWT, Virtual Wind Tunnel.*

in full-scale, learning lessons from extreme events to improve structural design, postdisaster assessment in a regional area, correlation between wind speed and structural performance in damage aspect, etc. (e.g., Kareem, 1985, 1986). The database was established based on a web-based database-query method and powered by Google Map/Earth API (Google, 2015), which allows users to geographically view submissions of fellow VORTEX-Winds members, sorting by location, event classification, and damage attributes automatically (**Figure 3**). Users can search damage archives either by pointing at a spot in the Google map or by using query-criteria in the right side of the map. Each damage report is comprised of wind event on damage, maximum wind speed, damage location in either addresses or GPS coordinates, structural properties, damage photos, etc. which registered members can upload their own damage reports to the database through a web interface.

It is worth noting that this Damage database is one of the first e-module developed for the VORTEX-Winds in 2009, thus rather old technology in view of current rapid developments in web-based technologies. For this reason, we are in the process of eventually retiring this module and transfer the existing data to newly developed "Data Intake and Discovery" module in CyberEye (Kijewski-Correa et al., 2014) in which authors are also involved. It is envisioned that this new module will be further enriched with the contribution of members over time.

#### DED Frameworks

Most international wind codes and standards traditionally have relied on reductive formats and simplifications, which often lead to tables and plots that describe wind loads on structures, especially, focusing primarily on the alongwind response owing to the introduction of the well-known quasi-steady/ strip theories and gust loading factor approach introduced by Davenport (1967) and later further refined over the years by several researchers (e.g., Solari, 1993a,b; Solari and Kareem, 1998; Zhou and Kareem, 2001; Kareem and Zhou, 2003). However, failure of the basic assumptions in these theories and the gust loading factor approach in the acrosswind and torsional directions has precluded a formulation based on first principles. Some codes and standards do provide limited guidelines on these aerodynamic loads, however, they are restricted to empirical expressions based on measured response of limited number of building models tested in wind tunnels. Although these expressions are still valuable for assessing preliminary estimates of response, their accuracy may not be consistent for different shapes as often small changes in the shape result in major changes in the aerodynamic effects. In addition, the level of accuracy inherent in codification information in this format and the uncertainty associated with interpolation or extrapolation of information may compromise the overall accuracy in code-specified load effects.

To overcome the aforementioned limitation, a DED procedure has been gradually gaining acceptance. The basic concept of DED is to directly use wind tunnel-derived data to better estimate the response instead of relying on code/standard-based procedure. A DED generally offers convenient meshing of a database with analysis software for either low to mid-rise and high-rise buildings. The NatHaz aerodynamic loads database (NALD) ver. 1.0 introduced in 2000 has served an important first step in establishing a DED through online experimental archive of high frequency base balance (HFBB) data for use in the preliminary design of high-rise buildings subjected to wind loads. As a result, NALD ver. 1.0 was introduced in the Commentary of ASCE 7-05 (C6.5.8) [American Society of Civil Engineers (ASCE), 2005] and the latest ASCE 7-10 (C26.9) [American Society of Civil Engineers (ASCE), 2010] as an alternative means of assessing the dynamic wind load effects on high-rise buildings. This includes the web-enabled wind tunnel data dissemination to evaluate the response of tall buildings (Zhou et al., 2003). However, it required manual dynamic analyses based on random vibration theory for the preliminary design.

To overcome the last step in NALD v. 1.0 (Zhou et al., 2003) involving off-line calculations, a more advanced DED framework for estimating the wind-induced response of tall buildings has been proposed with the help of recent developments in IT (Kwon et al., 2008). This new paradigm offers an attractive solution with a concept of a cyber-based online on-the-fly analysis/design *via* user-friendly web interfaces. It is particularly useful for those who may not be very familiar with the details of the random vibration-based dynamic analysis procedure generally used in conjunction with wind tunnel-driven data. Accordingly, NALD evolved into version 2.0 in 2004 using this advanced framework. NALD ver. 2.0 (Kwon et al., 2005, 2008) has integrated the latest advances in data management and mining for interactive queries of aerodynamic load data and an integrated web-based analysis framework for determining the resulting maximum displacements, base moments, and equivalent static wind loads (ESWL) for survivability and peak/root mean square (RMS) accelerations for serviceability (habitability). The key feature of NALD ver. 2.0 is the flexibility that its analysis module offers: users may select not only the data from the online NALD but also may input desired power spectral density (PSD) expressions or wind tunnel-derived PSD data set obtained from HFBB experiment for the evaluation of wind load effects on high-rise buildings. Thus, it also serves as a stand-alone analysis engine. The NALD ver. 2.0 provides a platform that can be readily expanded and supplemented to yield a comprehensive, simplified, and efficient avenue for e-analysis/ design of high-rise buildings. The architecture of NALD ver. 2.0 and the role of various web-based tools such as HTML/ Java Script and PHP are summarized in **Figure 4**. It is basically operated by Apache web server with two main processes, i.e., external (foreground) process and internal (background) process. The external process includes user-friendly web interfaces for the selection of a desired analysis case (interface 1 as shown in **Figure 5A**), additional interfaces for design inputs such as structural parameters of building (interface 2), and display of analysis results for the user-specified building. On the other hand, the internal processes are server-side operations involving MySQL for database operations and MATLAB (Mathworks, 2015) for the computation, which are implicitly utilized in NALD ver. 2.0. The MySQL database server handles data transmissions between interfaces and if necessary transmits information stored in the database. Preprogrammed MATLAB and PHP codes function as a numerical analysis engine for on-the-fly calculations as well as serving as the nexus between interface 2 and the design results. The cyber-based online analysis module performs the requisite calculations and then generates meaningful figures such as the non-dimensional PSD and ESWL, as well as downloadable output file. This architecture became a prototype cyber-based framework, which prompted further development of DED modules such as DEDM-HR, Database-enabled design module for high-rise building with pressure data (DEDM-HRP), DED module for low-rise buildings (DEDM-LR), and performancebased design of high-rise buildings for occupant comfort (PBDED-OC), which will be discussed in the following.

#### Database-Enabled Design Module for High-Rise

Although DED is a promising design procedure for better and more reliable wind-induced responses than conventional codes and standards, it requires a database of wind tunnel data, which are often limited in building shapes and configurations. As more data sets become available, the designers/researchers will have more flexibility. To alleviate this limitation, a DEDM-HR introduced a new concept of DED, which seamlessly pools multiple HFBB measurement databases from geographically dispersed locations and merges them efficiently to expand the number of available building configurations for the preliminary design (Kwon and Kareem, 2013a). This feature not only offers a new direction for the research and professional community that can be utilized to efficiently pool multiple databases, expand capability of individual database and ensures the reliability of design estimates to pool databases in the future. In addition, the DEDM-HR embeds a novel feature that allows the employment of wind characteristics defined differently between codes/standards to supplement the specifications of a specific code or standard, which in many cases lacks guidance on the acrosswind and torsional response estimates (**Figure 5B**). Currently, the DEDM-HR is associated with two databases: one is the NatHaz database from the NatHaz Modeling Laboratory, University of Notre Dame, USA, which is also the one used in the NALD (Zhou et al., 2003; Kwon et al., 2005, 2008), and the other is the Tamkang University database from the Wind Engineering Research Center, Tamkang University, Taiwan (Cheng and Wang, 2004).

#### Database-Enabled Design Module for High-Rise Building with Pressure Data

The aforementioned DED modules such as NALD and DEDM-HR have utilized HFBB database(s) resulting from the HFBB technique, which is a relatively simple and popular experiment to measure base forces/moments. Nonetheless, it involves some inherent drawbacks such as mode shape corrections and the assumption of wind pressure distribution over the building surface. On the other hand, the synchronous pressure measurement (SPM) technique in wind tunnel test is to synchronously measure the pressure fields using a test model with quite a number of pressure taps distributed at a model surface, which has the advantage to directly assess local and global aerodynamic loads acting on the structure without specific assumptions. Thus, the technique offers the direct estimation of the generalized forces including any coupled effects, non-linear and higher mode shapes (e.g., Kwon et al., 2005, 2008; Cluni et al., 2011). A drawback concerning the SMP technique is that a large number of pressure taps are required to accurately estimate wind loads, which may cause an issue of manufacturing test models, especially for complex geometries to locate pressure taps. In addition, the amount of data is generally much larger than HFBB experiments. Furthermore, the treatment and management of SMP datasets with the knowledge of complex background theories and procedures are often difficult. For an effective treatment of a large SPM data set within the concept of cyber-based DED framework, a DEDM-HRP has been developed (Kwon et al., 2014). It addresses the management of synchronously measured pressures and their reduction to global loads, which is often a daunting task due to the large amount of data information involving highly sampled time histories by its experimental nature. DEDM-HRP currently hosts a publicly available Tokyo Polytechnic University (TPU) aerodynamic database for a variety of building configurations for side and aspect ratios and various wind directions for each test model to account for directional effects of winds (Tamura, 2013). However, the e-module can be easily expanded to host multiple SPM databases similar to DEDM-HR. This framework is based on a time domain treatment of the random vibration (e.g., Simiu et al., 2008) and uses a minimal set of easily obtainable parameters as input such as wind speed, natural frequencies, and mode shapes of a target building etc., which the user can interact with user-friendly web interfaces (**Figure 5C**). For efficient calculation to minimize online computational time, this module employed a dynamic analysis based on floor level loads, precalculated and stored within the module. Estimated outputs are maximum base shears and moments, maximum top displacement/acceleration responses for alongwind, acrosswind, and torsional directions, and various oncoming wind directions. In addition, DEDM-HRP also offers the assessment of ESWL based on the formulation in Chen and Kareem (2005) for preliminary design purposes.

#### DED Module for Low-Rise Buildings

This new DED concept has been expanded to address a need for a preliminary design of low-rise buildings, namely, a DEDM-LR, which provides calculation of wind loads for main frames in low-rise buildings that make direct use of pressure time histories measured at a large number of pressure taps over building models in wind tunnel tests (Kwon et al., 2016). Presently, DEDM-LR works for a database constructed at TPU, Japan (Tamura, 2013), which includes various building configurations, but it will be expanded with more available databases. The main analysis/ design engine inside DEDM-LR searches for extreme load effects based on pressure data at multiple locations and wind directions given the influence line coefficient for the load effect. The fundamental theory employed in DEDM-LR have mainly been adopted from a procedure developed at NIST (Main and Fritz, 2006), which utilizes synchronous pressure data sets measured in various wind directions of a test model to evaluate responses in a low-rise building such as base moments at knee, ridge etc. using influence line coefficient-based approaches, computed using a structural analysis program such as SAP2000, etc. The DEDM-LR is basically operated under an Apache web server with two main processes, i.e., foreground and background processes as shown in **Figure 6**, which is similar to the architecture used in other DED modules. Foreground process functions interactive web interfaces for user inputs and analyzed/designed results and background process performs database-query and computational analysis/design.

#### Performance-Based Design of High-Rise Buildings for Occupant Comfort