Results of these simulations are summarized in Table 14. For this simulation, the two houses saved an average of 9.0% in energy use. Exceedance is higher than Case 1 with an average of 35.3 Kh. Total occupied hours for both houses for the week was 185 h, with House 1 being occupied for 100 h and House 2 being occupied for 85 h. With the average exceedance of 35.3 Kh, an average distribution of thermal discomfort would yield 0.4 ◦C above the ideal temperatures.

**Table 13.** Settings used for MPC optimization in Case 2A.

**Table 14.** Results for Case 2A.

Figure 12 shows temperatures for House 1 for two days of the simulation. Chosen temperatures ranged from 19 ◦C to 27 ◦C, with the average setpoint temperature at 25.4 ◦C and 26.2 ◦C for House 1 and 2, respectively. With the expanded temperature range, temperature values selected did not always conform to setpoint temperatures as seen in Case 1. While setpoint values allowed an 8 ◦C range, ambient air temperatures occurring within the building had a 4.5 ◦C to 4.7 ◦C range. More extreme setpoint temperatures only lasted for an hour, preventing temperature within the building from reaching the setpoint and maintaining a comfortable space despite the setpoints used.

**Figure 12.** Case 2A temperatures for House 1 using individualized hybrid prediction model.

Figure 13 shows the electricity consumption resulting from the setpoint temperatures used. Due to changing setpoint temperatures, electricity consumption jumped in hours using low setpoint temperatures

as more cooling occurred. In other hours, however, electricity was significantly less than the conventional constant temperature. Over the week simulated, electricity consumption was reduced to allow 7.5% and 10.4% energy savings for House 1 and 2, respectively.

**Figure 13.** Case 2A electricity consumption for House 1 using individualized hybrid prediction model.

Figure 14 shows the duration curve of operative temperatures for House 1. The figure shows that allowed temperature deviation was higher for unoccupied hours. In hours that the house was occupied, operative temperature was kept closer to the center point temperature of 23.5 ◦C. With the chosen cost function, some temperature deviation was allowed to achieve higher energy savings. Unlike Case 1, which had tight occupied temperature constraints, temperature deviation in Case 2A is higher. Changes to the comfort penalty slope would change how much deviation is allowed and, in result, how much energy was saved.

**Figure 14.** Case 2A duration curve of deviation from 24.5 ◦C operative temperatures.

### 5.4.3. MPC Case 2B: Atlanta with Perfect Occupancy Forecasting

In a third scenario, all settings used from Case 2A were kept the same except for the occupancy prediction model. In this scenario, actual occupancy data was used in Houses 1 and 2 to imitate perfect occupancy forecasting. This allows for an exploration of how imperfections in the occupancy prediction impact MPC results. A summary of all used settings are shown in Table 15.

Results from the MPC optimization with perfect occupancy predictions are shown in Table 16. Energy savings for Houses 1 and 2 increased by 5.4% and 2.9%, respectively, while comfort exceedance in both homes decreased (Figure 15). Like Case 2A, setpoint temperatures range from 19 ◦C to 27 ◦C. However, unlike Case 2A, low setpoint temperatures are used less often to achieve a quick temperature change. The resulting internal air temperature ranged from 22 ◦C to 27 ◦C.

**Table 15.** Settings used for MPC optimization in Case 2B.

This experiment shows that with an accurate occupancy forecast, MPC optimization is able to allow less energy-intensive temperatures during vacant periods, without the penalty from discomfort when an occupant unexpectedly returns. This allows improvements in both energy savings and thermal comfort. Thus, with accurate occupancy forecasting, energy savings above 10% are possible with the occupancy patterns recorded. Accurate occupancy prediction, therefore, is essential in improving HVAC control.

**Figure 15.** Case 2B duration curve of deviation from 24.5 ◦C operative temperatures.

### **6. Summary and Conclusions**

Residential heating and cooling accounts for a large portion of annual energy consumption in the United States. Reducing energy use can contribute to furthering the goals of the Paris Agreement by

reducing the burning of fossil fuels and thus reducing CO2 emissions. Within the residential energy sector, energy savings can be realized by accounting for occupancy in HVAC control. This allows energy reduction without negatively impacting thermal comfort of occupants, which is essential to the widespread adoption of new HVAC control technologies.

A literature review revealed that non-probabilistic models historically have performed best for short-term occupancy prediction. By collecting real occupancy data from six different homes, individual non-probabilistic models were created and evaluated. Prediction inaccuracy in the models, termed state-matching error in the study, ranged from 8.0% to 48.7%. Model training data that used a moving, multi-week training set worked the best for all homes, with differences in occupancy patterns being the highest contributor to prediction inaccuracy. An examination of increased training time indicates that models can improve over time as more data is collected and included into the prediction model. Once the occupancy prediction models were generated, they were then incorporated in occupancy-based setpoint control and occupancy-based model predictive control in a building performance simulation. Five EnergyPlus home models were used to simulate the energy use and indoor temperatures for two-week periods in summer and in winter. Occupancy-based setpoint control showed possible energy savings from 0% to 50.0% over control methods that used a constant setpoint temperature depending on climate, occupancy pattern, and control strategy. Non-probabilistic prediction models achieved the highest energy savings, with an average of 10.0%, but with the disadvantage of high thermal discomfort for the occupants. By including an override, in which the occupancy prediction model can sense the actual occupant presence and react to it, thermal discomfort was reduced. In these hybrid occupancy models, the energy savings averaged 5.0%, while the number of hours that the space was deemed uncomfortable were low.

Model predictive control showed that energy savings is highly dependent on how the cost function and constraints are parameterized. In Case 1, where the temperature constraints were much stricter during occupied hours, little to no energy savings was achieved. However, in Case 2A, where temperature constraints were relaxed during occupied hours, energy savings increased with only a slight impact on discomfort. In Case 2B, where occupancy was perfectly predicted, both energy savings and thermal comfort improved, leading to two conclusions: First, a cost function that combines both energy consumption and thermal discomfort allows for flexibility for the user to determine what trade-off between energy savings and discomfort is appropriate for the them. Second, accurate occupancy prediction improves both performance aspects in the cost function. This shows that, as occupancy prediction improves, the ability for occupancy-aware HVAC control to maintain comfort and increase energy savings improves.

**Author Contributions:** Conceptualization, G.H.; methodology, G.H. and C.T.; software, C.T. and G.P.; validation, C.T., M.J., G.P. and G.H.; formal analysis, C.T.; investigation, C.T.; resources, M.J.; data curation, C.T. and M.J.; writing–original draft preparation, C.T.; writing–review and editing, C.T., M.J., G.P. and G.H.; visualization, C.T.; supervision, G.H.; project administration, G.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research and the APC was funded by DOE ARPA-E grant number DE-AR0000938.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

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### *Article* **Efficiency of Energy Consumption between Reinforced Concrete Structure and Cross-Laminated Timber Based Hybrid Structure in East Asian Cities**

**Meng-Ting Tsai \* and Wei-Ting Lin**

Department of Architecture, National Taiwan University of Science and Technology, Taipei 10608, Taiwan<EMAIL_ADDRESS>
**\*** Correspondence<EMAIL_ADDRESS>Tel.: +886-2-27303212

**Abstract:** From the environmental perspective, wooden structures are favorable insulators that are suitable for carbon fixation and wooden-related products are considered the most sustainable material. Research has indicated that wooden structures have superior energy-saving performance compared to reinforced concrete (RC) structures. In this study, a CLT-based hybrid structure system that potentially improves the efficiency of energy consumption is proposed. The proposed hybrid structure system, which preserved original RC beams, columns and replaced CLT floors and walls, has less building weight compared to the original RC building. Additionally, less energy required for the manufacturing of building materials in the renovation of the aged building is achieved, compared to building a new CLT building. The energy consumptions for buildings with heights of 10 stories were compared. CLT and RC were selected as benchmark building materials to compare the energy-saving efficiencies with the proposed hybrid structure system. In addition, to examine the energy consumption differences at different latitudes, the energy consumptions in Taipei, Tokyo, Harbin, and Singapore were compared as well. The simulation results indicate the proposed hybrid structure system, which comprises RC beams and columns and CLT floors and walls, and has an energy-saving efficiency close to that of a CLT structure, by approximately 3–5% higher, however, had a superior energy consumption performance to the RC structure. In general, the proposed hybrid structure system can be effectively used for old building renewal in the selected Asian cities.

**Keywords:** energy consumption; carbon emission; Green Building-Rating System (GBS); cross laminated timber (CLT); hybrid structure system

### **1. Introduction**

Due to the environmental problems caused by global warming, energy consumption and carbon emissions must be reduced, and it is the only solution to stop the climate crisis [1]. The topic of environmental protection has drawn the attention of numerous countries. The building density in an area rapidly increases with increasing population. A report of the United Nations Environment Programme [2] indicates that the energy consumption of the construction sector accounts for 40% of the global energy consumption. In addition, the greenhouse gas emissions of the construction sector account for over 33% of the total global emissions, and the construction sector is considered the largest emission source. Consequently, to reduce their environmental impact, construction-related industries should aim to reduce their energy consumption and carbon dioxide emissions in the usage stage of the building life cycle.

The design and construction of wooden structures completely conform to the global trend of green building [3–7]. The material characteristics of cross-laminated timber (CLT), which has been rapidly developed recently, are different from those of traditional gluelaminated timber. CLT can provide high strength when used in vertical and horizontal combinations [8–11]. Theoretically, CLT can be used as the structural material for wooden

**Citation:** Tsai, M.-T.; Lin, W.-T. Efficiency of Energy Consumption between Reinforced Concrete Structure and Cross-Laminated Timber Based Hybrid Structure in East Asian Cities. *Energies* **2022**, *15*, 165. https://doi.org/10.3390/ en15010165

Academic Editor: Audrius Banaitis

Received: 28 October 2021 Accepted: 24 December 2021 Published: 27 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

structures over 10 stories. The fast construction of CLT structures has resulted in the rapid development of CLT structures in recent years [4–6]. From the environmental perspective, wooden structures are favorable insulators that are suitable for carbon fixation and wooden-related products are considered the most sustainable material [12]. From the Building Envelop point of view, the thermal conductivity of concrete is 1.4 w/m◦k, which is 10 times the conductivity of wood. In addition, concrete has high thermal storage. In countries in cold zones, concrete walls are often placed in the direction of sunlight for thermal storage due to their high thermal storage characteristics. At night, the heat stored in concrete structures is released indoors; thus, the heating load in winter can be reduced [13]. However, in subtropical and tropical regions, the high thermal storage of concrete is one of the major reasons for increased air-conditioning consumption in the summer. Wood is a favorable insulator, and for the same thickness, the insulation value of wood is approximately 10 times that of concrete. Thus, wood can effectively block thermal conduction. Research has indicated that wooden structures have superior energy-saving performance to reinforced concrete (RC) structures [14–18]. Based on the analyses performed, it was indicated that wood-based buildings have a favorable energy balance in comparison to traditional buildings based on traditional materials [19]. Recent research concluded that CLT generally provides a significant improvement on energy efficiency, and studies on occupants' comfort in buildings have been examined [20,21]. However, its energy performance efficiency and occupants' comfort can be affected by weather, building size, internal loading, and HVAC control. Generally, CLT buildings provide sound insulation against cold weather as when the temperature outside is cold, the occupants can still find comfort in the house. The daily energy consumption of structures, such as their electricity generation and fuel consumption, influences the overall development direction of energy consumption for a country. Consequently, reducing energy consumption or adopting recyclable energy and thus reducing the overall energy consumption cost is a goal of many countries [22–25]. However, previous research has mostly focused on comparing the energy-saving efficiency between wooden and RC structures, as well as in high-latitude regions, energy-saving efficiency between hybrid structure systems and CLT or RC structures [14–18], as well as the performance of energy consumption in low-latitude regions, has not been focused.

For most East Asian cities, taking Taipei City as an example, RC is the primary building material for most structures. According to statistics, close to 400,000 residences were constructed between 1971 and 1990 in Taipei City [26]. These structures have a life expectancy of 30–50 years and currently require reconstruction or renovation. Thus, for urban renewal, a crucial aspect of this study is to understand how to use the advantages of wooden structures to develop efficient renewal models of energy consumption for the daily usage stage of the building life cycle. To propose a solution for the renovation of aged RC buildings, a new hybrid structure system that potentially improves the efficiency of energy consumption is proposed in this study. Hybrid structure systems by adopting CLT panels outside of original wall systems as a building's additional envelope has been studied [27–30]. However, adding the additional CLT panel increased the total weight of the building, potentially increasing the seismic load of the aged buildings, which had reduced seismic resistance ability, due to the decay of the materials. In this study, a renovation method that preserves the RC beams and columns in old buildings and uses CLT floors and walls for renewal was proposed. Replacing the original RC wall and floor with a CLT panel reduces the total weight of the building, hence helping reduce the potential seismic load [31,32]. Furthermore, the energy required for the manufacturing of building materials could represent in the near future, almost 400% of operational energy [33], it could cause less energy consumption if only part of the elements, such as wall and floor were replaced in the aged building instead of rebuilding a new building. The other objective of this study was to compare the energy-saving trends of these structures of different heights from high-latitude regions to low-latitude regions. The energy consumptions for buildings with heights of 4 and 10 stories were compared. CLT and RC were selected as benchmark building materials to compare the energy-saving efficiencies with hybrid structure systems. Consequently, the Revit program was used with Green Building Studio (GBS) for simulating and analyzing the energy consumption of structures. The common residence structure type in Taipei City was selected, and its floor plan was used as the standard floor. The energy consumption efficiency of the hybrid structure system was compared with those of the other structure systems. Thus, a system with superior energy consumption efficiency could be identified for future urban renewal. In addition, to examine the energy consumption differences at different latitudes, the energy consumptions in Taipei City, Taiwan (taken as the standard); Tokyo, Japan (to the north of Taipei); Harbin city, China (to the north of Taipei), and Singapore (to the south of Taipei), which is close to the equator, were compared. Thus, the energy-saving efficiencies of wooden and RC structures in regions with different environmental conditions were compared.

### **2. Methodology**

### *2.1. Assessment of Energy Consumption*

Autodesk Green Building-rating System (GBS) was used for simulating the structure energy consumption in this study. GBS can be used as an independent cloud-service-based program or a plug-in component of the Revit program for energy analysis. GBS comprises the DOE-2.2 analysis core and can provide extremely detailed analysis. As a cloud-based tool, GBS can facilitate rapid computation on the Autodesk server. In general, the DOE-2.2 analysis core requires extremely detailed information on the building envelope and electromechanical system for computation. However, GBS presets numerous building envelope and electromechanical system parameters according to the ASHRAE standard. Thus, architects can focus more on the design factors that have decisive influences on the overall energy consumption of buildings and can ignore technical details. In addition to the building energy consumption, electricity consumption, and annual carbon emissions, GBS can calculate the Energy Star score of buildings. It can also evaluate the glass property and water usage efficiency scores according to the LEED evaluation system published by the U.S. Green Building Council. GBS can even determine the solar energy usage potential. In this study, a plug-in component of the Revit program 2019 version for energy analysis was used for simulation.

### *2.2. Assessment of Carbon Dioxide Emission*

The Guidelines for National Greenhouse Gas Inventories published by the Intergovernmental Panel on Climate Change were used for the assessment of carbon dioxide emissions. In this study, the calculation method for the emissions of carbon dioxide, methane, and nitrous oxide was adopted to calculate the carbon emissions of different countries. GBS calculates the energy consumption of buildings according to two major parameters: electricity consumption and fuel consumption. The calculated carbon emission of electricity consumption differs according to the electricity carbon emission coefficients of different countries. The carbon emissions can be calculated using Equation (1). The emission coefficients of different countries are listed in Table 1. For calculating the carbon emission of fuel consumption, the fuel volume (m3) is first converted into energy units. An energy of 38 MJ or 10.6 kWh can be generated by burning 1 m<sup>3</sup> of natural gas. The carbon emission of fuel consumption can then be calculated using Equation (2). The carbon emissions coefficient of fuel is listed in Table 2, and all the compared countries have the same carbon emissions coefficient of fuel. The carbon emissions of the total energy consumption of a building can be obtained by summing the carbon emissions of electricity and fuel.

**Table 1.** Electricity carbon emission coefficients of different countries.

**Table 2.** Fuel carbon emission coefficient of all the compared countries.

(A) Electricity consumption is converted into carbon emissions by using the following formula:

Electricity usage (kWh) × electricity emissions coefficient (kg CO2e/kWh) = electricity carbon dioxide emissions (kg) (1)

(B) Fuel consumption is converted into carbon emissions by using the following equation:

Fuel usage (m3) × natural gas emissions coefficient (kg CO2e/ m3) = fuel carbon dioxide emissions (kg), (2)

(C) The carbon emission of total energy consumption is calculated as follows:

Carbon emission of total energy consumption (kg) = electricity carbon dioxide emissions (kg)+ fuel carbon dioxide emissions (kg) (3)

### **3. Simulation Modeling**

To determine the energy usage efficiencies under different conditions, the energy consumption and carbon emission were compared for different numbers of stories (4 and 10), different construction materials (RC and CLT), and cities with different latitudes (from north to south, Harbin, Tokyo, Taipei, and Singapore). The related structure usage situations and air admission timing of the air-conditioning system were set. In addition, the energy simulation was only conducted for the daily usage stage in the structure life cycle. When GBS was used for structure energy simulation, the basic settings, simulation parameters, weather data, electromechanical system, indoor load, and operation schedules had to be input in the simulation process. The basic parameters and settings of this study are as follows:

Because this study was a preliminary study, a 24/7 operation schedule was set. Thus, simulations were conducted 7 days a week and 24 h a day annually. Then, GBS was used to analyze the simulated building energy consumption. In the comparison of the energy consumptions of different building materials (RC and CLT) in cities at different latitudes, only the energy consumption in the operation stage was considered. In addition, the following assumptions were made in the simulation process for the energy consumption:

### *3.1. Target Building*

To determine the energy usage efficiencies for different numbers of stories and different weather conditions, the standard floor of social housing in Taipei City was used as the standard floor in this study. The layouts and basic information of a four-story building and a 10-story building are listed in Table 3 and displayed in Figure 1.

**Table 3.** Basic information on the target buildings.

**Figure 1.** Standard floor plans of the target buildings.

### *3.2. Building Materials*

The targeted building that was selected was a typical social housing building surrounded by buildings in central Taipei city, and the orientation of the building is with the main entrance facing southward. This building type is adopted and simulated in other selected cities, and the surrounding condition of the building is assumed the same. The simulated building was an RC structure with fixed floor plans on different floors. In this study, the building height, building direction, total area, and opening, such as windows and doors were fixed for the building simulation. The material of major structures, such as beams and walls, was replaced with CLT. Thus, the influence of the floor height on energy-saving efficiency was examined. In addition, the energy consumption efficiencies of the RC and CLT structures were compared. Moreover, only the energy consumption in the daily life stage of the building was examined; the structural properties of the structure were not determined.

The physical properties of RC and CLT, including their heat transfer coefficient, specific heat, and density, were also determined for complete analysis. As presented in Table 4, RC has a higher heat transfer coefficient, specific heat, and density than CLT does. Thermal resistance and heat loss are inversely correlated. Thus, an increase in the thermal resistance of the wall material between the interior and exterior of a building can reduce the heat loss of the building (Equation (4)). As presented in Table 4, CLT walls have a higher thermal resistance than RC walls do. The thermal resistance of a 300-mm-thick external CLT wall is up to 3.3 m2·K/W (U-Value = 0.3 W/m2·K), while with a variable thickness of insulation applied, the U-Value ranges from 0.13 to 0.45 W/m2·K [41]. The thermal resistance of a 300-mm-thick RC external wall is only 0.25 m2·K/W, which is approximately 1/13th of the thermal resistance of a 300-mm-thick CLT external wall. Table 5 lists the physical properties of RC and CLT walls of different thicknesses. Figure 2 displays the models of numerical analysis. Four- and Ten-story CLT and RC buildings were constructed for comparing the energy consumptions in their daily life stages. In this paper, the buildings in different cities with different latitude were studied, to emphasize the influence of the building envelope on total energy consumption, the reflected U-Values of different cities for their roof and floor are assumed as follow, based on the different climate zone located. U-value of the roof was 0.28, 0.45, 0.50, 0.50 for Harbin, Tokyo, Taipei, and Singapore respectively. Moreover, the U-value of the floor was 0.38, 0.50, 0.80, 1.50 for Harbin, Tokyo, Taipei, and Singapore respectively. In order to simplify the simulation, the U-value of the window and door were set to be the same in order to clarify the major influence of the exterior wall.

$$\text{Heat loss} = \text{(A/R)} \times \text{(Tirdoor} - \text{Toutdoor)} \tag{4}$$

A: external surface area of the building. R: thermal resistance (R value). Tindoor: indoor air temperature. Toutdoor: outdoor air temperature.

**Table 4.** Physical properties of different walls.

### *3.3. Cities in Different Latitude*

Cities at different latitudes have different climates, which influences the energy consumption of buildings. The climate includes the highest and lowest outdoor temperatures and humidity. In this study, the energy consumption of buildings in cities at different latitudes was also simulated to determine how the overall energy usage efficiency varied with the latitude. As depicted in Figure 3, the latitude of Taipei was selected as the standard. The other cities selected for the comparison were Tokyo and Harbin, which are located to the north of Taipei, as well as Singapore, which is located close to the equator and to the south of Taipei. These cities were selected for comparing the energy-saving efficiencies of RC and CLT buildings of different heights in different environmental conditions.

The monthly average temperatures of the aforementioned cities in 2019 are presented in Table 6 and Figure 4. The lowest monthly average temperature in Harbin, which is located at a high latitude, was −17.2 ◦C in January, and the highest monthly average temperature in Harbin was 23.6 ◦C in July. Thus, the largest monthly average temperature difference was 40.8 ◦C in Harbin. The lowest monthly average temperatures in Singapore, which is located at a low latitude, were 26.8 ◦C and 27 ◦C in December, and the highest monthly average temperatures in Singapore were 30.6 ◦C and 28.9 ◦C in April and March, respectively. The highest monthly average temperature difference in Singapore was only 3.8 ◦C. The highest monthly average temperatures of Tokyo and Taipei, which are located in the temperate zone, were 28.6 ◦C in August and 30.6 ◦C in July, respectively. These temperatures were comparable to the high temperatures in Singapore, which is located in the tropics.

**Table 6.** Weather information in the considered cities.

**RC structure**

**CLT structure** (**a**) Analysis models of the 10-story RC and CLT structures

**RC structure**

**CLT structure**  (**b**) Analysis models of the four-story RC and CLT structures

**Figure 2.** Analysis models of numerical simulation.

**Figure 3.** Latitudes of the considered cities.

**Figure 4.** *Cont*.

**Figure 4.** Annual temperatures of the considered cities.

### **4. Results and Discussion**

*4.1. Energy Consumption*

In the simulation, electricity was used by the HVAC system, illumination equipment, and other equipment. Fuel was used by the HVAC and domestic water heating systems. Table 7 presents the simulation results for the 4- and 10-story structures made of different building materials and located in cities at different latitudes.

**Table 7.** Energy consumption comparison of structures with different heights in different cities.

For Harbin, which is located at the highest latitude among the considered cities, the annual electricity consumptions of the RC and CLT structures were 244 and 148 kWh/ m2, respectively. For Singapore, which is at the lowest latitude, the annual electricity consumptions of the RC and CLT structures were 232 and 227 kWh/ m2, respectively. For both 4 and 10-story structures, the simulation results indicated that the electricity consumption of RC structures was considerably higher than that of CLT structures at high and low latitudes. However, at low latitudes, the difference in the electricity consumption of RC and CLT structures was relatively small (Figure 5). The same trend was observed for fuel energy usage. Thus, the energy-saving efficiencies of CLT structures were higher than those of RC structures. The differences in the energy-saving efficiencies of CLT and RC structures were higher at higher latitudes (Figure 6).

**Figure 5.** Energy consumption of 4 story building in different cities.

**Figure 6.** Energy consumption of 10 story buildings in different cities.

A comparison of the total energy consumptions of RC and CLT structures is presented in Table 8 and Figure 7. For four-story structures, the total energy consumptions of the CLT structures were approximately 57%, 71%, and 88% of those of the RC structures in Harbin, Tokyo, and Taipei, respectively. No significant difference was observed in the total energy consumptions of the four-story CLT and RC structures in Singapore. For 10-story structures, the total energy consumptions of the CLT structures were approximately 61%, 73%, and 89% of those of the RC structures in Harbin, Tokyo, and Taipei, respectively. For Singapore, the difference between the total energy consumptions of the 10-story CLT and RC structures was marginally higher than that of the four-story CLT and RC structures.

**Table 8.** Energy consumption Ratio between RC Building and CLT Building.

**Figure 7.** Comparison of different types of energy consumption in different cities.

The electricity and fuel energy consumption ratios of the CLT and RC structures were compared. Electricity consumption mainly originates from rooms with air-conditioning. Thus, a higher number of stories caused a lower ratio of the floor area being directly heated by sunlight. Consequently, the electricity-energy-saving efficiency of the 10-story structures was higher than that of the four-story structures (Figure 7). However, this energy-saving efficiency decreased with decreasing latitude. The consumption of fuel energy mainly originates from rooms with heating. Thus, the larger the surface area of a structure, the higher is the heat exchange. Table 8 and Figure 7 indicate that all the fuel energy consumption ratios of 10-story structures were higher than those of four-story structures in the different cities. In addition, CLT structures at high altitudes (Harbin) had a high fuel energy consumption efficiency. The energy consumption ratios of 4- and 10-story CLT structures were approximately 56% and 62% of those of 4- and 10-story RC structures, respectively. For Singapore, which is at a low latitude, indoor heating demands were low. Thus, the fuel energy consumption ratios did not exhibit a significant difference.

### *4.2. CO2 Emission*

Table 9, Figures 8 and 9 present the carbon dioxide emissions per unit area for 4- and 10-story RC and CLT structures in cities at different latitudes. The carbon emissions of electricity energy consumption mainly originate from air-conditioning systems, illumination systems, and basic facilities of the structure. The carbon emissions of fuel energy consumption mainly originate from the use of heating systems. Electricity carbon emissions were calculated according to the electricity emission coefficients of the countries in which the considered cities are located (Tables 1 and 2). Thus, the carbon emissions from the same electricity energy consumption were different in different countries.

The electricity carbon emissions of four-story RC and CLT structures were 276 and 167 kg/ m2·yr, respectively, in Harbin; thus, the difference in the electricity carbon emissions of the two types of four-story structures was 109 kg/ m2·yr in Harbin. In Singapore, the electricity carbon emissions of four-story RC and CLT structures were 96 and 93 kg/ m2·yr, respectively, which represents an electricity carbon emission difference of 3 kg/ m2·yr. The difference in carbon emissions was small because the monthly average temperature in every month in Singapore was higher than 26 ◦C, which was the temperature set in this study for the air-conditioning system to be turned on. The air-conditioning system demands were high. Thus, the demands for carbon emissions from electricity energy consumption were high for both RC and CLT structures. For Harbin, in addition to the air-conditioning system demands, the heating system demands were high. Thus, significant differences were observed in the carbon emissions of electric energy consumption for RC and CLT structures in Harbin. In addition, the electricity emissions of Harbin were higher than those in other cities due to the higher electricity carbon emission coefficient in Harbin (1.13 kg CO2e/kWh). The carbon emissions of fuel energy consumption for four-story RC and CLT structures in Harbin were 199 and 111 kg/ m2·yr, respectively, which represents a fuel carbon emission difference of 88 kg/ m2·yr. In Singapore, the fuel carbon emissions of four-story RC and CLT structures were both 12 kg/ m2·yr. In cities at higher latitudes, the heating system needs were higher. Thus, the fuel carbon emissions and fuel carbon emission differences increased considerably with the latitude.

**Table 9.** Carbon emissions comparison of structures with different heights in different cities.

Figure 10 indicates that the 10-story structures had superior carbon emission reduction effects to those of the four-story structures for electricity and fuel energy consumption. For regions at high latitudes (Harbin), the differences in electricity carbon emissions of 10- and 4-story structures were not significant. However, the carbon emissions reduction efficiencies of structures with more stories could be shown in the differences in fuel carbon emissions.

### *4.3. Potential Hybrid Structure System for Renovation in Asian Cities*

Previous results of this study indicate that CLT structures have higher energy-saving and carbon reduction efficiencies than RC structures do at different latitudes. These efficiencies increase with the number of floors. Moreover, the differences in the energy consumption efficiencies of CLT and RC structures increase with latitude. In this section, CLT and RC structures that were studied as benchmark building materials were furthermore compared with the energy-saving efficiencies of a hybrid structure system.

**Figure 10.** Comparison of different type of Carbon emissions in different cities.

In Taipei, most buildings are RC structures that are approximately 30–50 years old. These structures are still usable; however, their overall energy consumption is high because of the RC building material. Consequently, a strategy is proposed for the renewal of these buildings in the future. The proposed strategy mainly involves preserving existing RC beams and columns and renewing the floors and walls with CLT. On the basis of this concept, a hybrid structure analysis model was established in this study. Figure 11 illustrates the model of preserving the RC beams and columns and replacing the floors and walls with CLT. The energy consumptions of CLT and RC structures of the same size were also compared. The analysis and simulation conditions were the same as stated previously as shown in Section 3.

The analysis results presented in Table 10 indicate that the 10-story hybrid structure system had a superior electricity energy consumption performance to the RC structure. The electricity energy consumption performance of the hybrid structure was only marginally worse (3–5% lower) than that of the CLT structure. Thus, the proposed hybrid structure system is close to a CLT structure in terms of electrical energy consumption. For fuel energy consumption, no significant difference was observed between the different structures in Singapore, which is located at a low latitude. In Taipei, Tokyo, and Harbin, which are located at relatively high latitudes, the fuel energy consumption of the hybrid structures was higher than that of the RC structures and not significantly different from that of the CLT structures. The CLT and hybrid structures exhibited no significant difference in their total energy consumption. The aforementioned structures exhibited lower total energy consumptions than the RC structures did. The energy-saving efficiency of the proposed hybrid structure system, which comprises RC beam structures and CLT floors and walls, is close to that of CLT structures. Taking energy consumption in RC structure as 100%, the relative consumption ratio is illustrated and as shown in Table 11, indicating that using CLT as building skin, such as hybrid structure proposes in this study, performs as well as the CLT structure. For all the East Asian cities selected in this study, the proposed hybrid structure system can be used effectively for old building renewal.

**Figure 11.** Analysis models of hybrid structure system.

**Table 10.** Energy consumption comparison of hybrid structures with RC and CLT structures in different cities.

**Table 11.** Comparison of energy consumption ratio of hybrid structures with RC and CLT structures in different cities.

**Table 11.** *Cont.*

### **5. Conclusions**

In this study, the energy consumption efficiency of the proposed hybrid system was compared with those of the RC and CLT structure systems. Detailed energy consumption and carbon emissions were compared for different numbers of floors (4 and 10), different building materials (RC and CLT), and cities at different latitudes (from north to south, Harbin, Tokyo, Taipei, and Singapore) to understand the energy usage efficiencies under different conditions. In order to clarify the major influence of exterior walls on energy consumption, part of the parameters for the simulation model are simplified, such as doors and windows, further detailed study needs to be conducted in order to obtain a deep understanding. For the preliminary study, the following conclusion is drawn, and worthy to provide to local authorities for the policymaking regarding urban renewal issues. For the proposed hybrid structure system, the electricity consumption performance of 10-story hybrid structures was superior to that of RC structures but marginally inferior to that of CLT structures (by approximately 3–5%). Thus, the electrical energy consumption of the hybrid structures was close to that of the CLT structures. No significant differences were observed in the fuel energy consumptions of the different structures in Singapore. In Taipei, Tokyo, and Harbin, which are located at higher latitudes than Singapore is, the fuel energy consumption of the hybrid structures was higher than that of the RC structures. No significant difference was observed in the fuel energy and total energy consumptions of the hybrid structure system and CLT structures. The fuel energy and total energy consumptions of the aforementioned structures were lower than those of the RC structures, indicating the advantage of the hybrid structure system used for old building renovation. In conclusion, the proposed hybrid structure system, which comprises RC beams, columns and CLT floors and walls, has less building weight compared to the original RC building, and less energy required for the manufacturing of building materials [31] in the renovation of the aged building, has an energy-saving efficiency close to that of a CLT structure. For all the East Asian cities selected in this study, the proposed hybrid structure system can be effectively used for old building renewal.

**Author Contributions:** Conceptualization, M.-T.T.; methodology, M.-T.T.; software, W.-T.L.; validation, W.-T.L. and M.-T.T.; formal analysis, W.-T.L. and M.-T.T.; investigation, W.-T.L. and M.-T.T.; resources, W.-T.L. and M.-T.T.; data curation, W.-T.L. and M.-T.T.; writing—original draft preparation, W.-T.L.; writing—review and editing, M.-T.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the Ministry of Science and Technology of Taiwan, R.O.C. under Grant No. MOST 109-2221-E-011-032—and the National Taiwan University of Science and Technology.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

### *Article* **Impact of Enclosure Boundary Patterns and Lift-Up Design on Optimization of Summer Pedestrian Wind Environment in High-Density Residential Districts**

**Zhengrong Jiang 1,\* and Weijun Gao <sup>2</sup>**

**Abstract:** A comfortable wind environment favors the sustainable development of urban residential districts and public health. However, the rapid growth of high-rise urban residential districts leads to low wind velocity environments in summer. This study examines the influence of enclosure boundary patterns and lift-up design on the wind environment and proposes an optimization strategy to improve the low wind velocity environment in residential districts in summer. A typical residential district in Hangzhou was selected; the average wind velocity, calm wind zone ratio and comfortable wind zone ratio were selected as the evaluation indexes. The wind environment for different enclosure boundary patterns and lift-up designs were obtained via computational fluid dynamics (CFD) simulations. The results indicate that the pedestrian wind environment is greatly improved in residential districts by reducing the height/width of the enclosure boundary, increasing the permeability rate and adopting a lift-up design in all buildings within residential districts. A combination of permeable railings and lift-up design is recommended; this can increase the average wind velocity and the ratio of comfortable wind zones by 70% and 200%, respectively. This study provides practical guidelines for the optimization of a low wind velocity environment in Chinese high-density residential districts in summer.

**Keywords:** high-density residential district; pedestrian wind environment; computational fluid dynamics; enclosure boundary; lift-up design

### **1. Introduction**

With the rapid development of urbanization in China, land resources are becoming increasingly scarce, and building density and height in urban residential districts are rapidly increasing [1,2]. High-density residential districts exhibit reduced ventilation and heat dissipation conditions, thus leading to low wind velocity environments and poor thermal comfort in residential districts [3]. This problem is particularly serious in summer, especially in the densely populated eastern coastal areas of China [4]. These related phenomena may negatively affect the sustainable development of urban residential districts. For instance, air stagnation due to a low wind velocity environment may prevent the evacuation of airborne pathogens [5,6]. During epidemic periods of infectious diseases, this issue is particularly prominent, and the concentration of pollutants poses a clear threat to public health safety [7]. Especially during the development of the COVID-19 epidemic, it is essential to improve the pedestrian-height wind environment in urban high-density residential districts for public health safety purposes [8].

A series of studies has been conducted to investigate effective methods for improving the pedestrian wind environment. Field measurements, wind tunnel experiments, and computational fluid dynamics (CFD) simulations are often employed to analyze outdoor wind environments [9,10]. Du et al. adopted the overall mean velocity rate (OMVR) as an

**Citation:** Jiang, Z.; Gao, W. Impact of Enclosure Boundary Patterns and Lift-Up Design on Optimization of Summer Pedestrian Wind Environment in High-Density Residential Districts. *Energies* **2021**, *14*, 3199. https://doi.org/10.3390/ en14113199

Academic Editors: Giorgio Ficco, Mark Luther, Igor Martek and Mehdi Amirkhani

Received: 8 April 2021 Accepted: 26 May 2021 Published: 30 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

evaluation criterion, and through a wind tunnel experiment on the campus of the Hong Kong Polytechnic University, they demonstrated that the area of acceptable wind comfort increases from 20% to 50% via the adoption of building opening and lift-up design [4]. Tsanga et al. performed wind tunnel experiments to study the influence of building separation on pedestrian wind environments under low and high wind conditions, and found that when the building separation is smaller than half of the building width, it imposes adverse effects on the natural ventilation conditions for pedestrians [11].

Although wind tunnel experiments are reliable, they are relatively expensive and insufficiently flexible to be applied to architecture design on a large scale. Instead, the CFD simulation method has the advantages of high speed, easy implementation, high accuracy and effectiveness, and low cost [12,13]. Therefore, with the development of computer technology and grid generation techniques, increasing numbers of researchers have performed CFD simulations to study urban wind environments [14–16]. For instance, Hang et al. used CFD simulations to numerically investigate the air age and air exchange efficiency of an ideal high-density building complex under the effect of parallel winds, and proposed that wide streets and small building array lengths could help improve ventilation [17]. Based on a highly simplified model, Li and Chen adopted the CFD technique to simulate the wind environment around buildings and found that when the area of the opening remains the same, the larger the number of openings, the better the distribution of the vertical wind velocity behind the building [18].

In recent years, an increasing number of studies have focused on the pedestrian wind environment in residential districts. For example, Wang et al. introduced dimensionless layout parameters of rank-and-file building groups through wind tunnel experiments, and they proposed that changing the building layout and orientation can help produce a good wind environment [19]. Hadavi and Pasdarshahri quantified the effects of the wind velocity, urban planning district density, and urban form on the infiltration rate via CFD simulations, and concluded that building tightness enhancement could reduce the infiltration rate and augment the exfiltration rate [20]. Hu et al. analyzed the relationship between the residential building density and wind environment and reported that when the residential building density increases from 0.18 to 0.32, the outdoor average wind velocity ratio decreases by 0.18 and the average air age increases by 58.63 s [21]. Hong and Lin compared the wind environments of several residential areas with different layout patterns and tree arrangements and concluded that orienting the long facades of buildings parallel to the prevailing wind direction can yield satisfactory thermal comfort [22]. Ghadikolaei et al. analyzed the effects of wing wall at the balcony on the natural ventilation performance in medium-rise residential buildings [23]. Besides, Tes et al. explored the impact of liftup building models with different core heights and widths, and found that the lift-up core height is the most influential parameter and governs the area and magnitude of high and low wind velocity zones around such buildings [24]. Guo et al. compared the natural ventilation performance of different building morphologies via CFD simulation, and indicated that enclosed city blocks are unfavorable to natural ventilation [25]. Kubota et al. carried out wind tunnel tests on 22 residential neighborhood districts selected from actual Japanese cities, and concluded that an increase in the gross building coverage ratio decreases the mean wind velocity ratio [26].

However, most of these studies were concerned with the effects of different building layout patterns, densities and heights on the wind environment in residential districts. The influence of different enclosure boundary patterns on residential wind environments has rarely been considered in studies to date. Several studies have analyzed the influence of the lift-up building model on the wind environment, but few have analyzed the influence of the combination of the enclosure boundary and lift-up design on the wind comfort of a residential district. The research objects in most of the literature are highly simplified, ideal models such that planning and architectural practice constraints are neglected. The results of related studies do not reflect the complexity and diversity of urban construction projects.

Chinese residential districts exhibit their own characteristics due to local urban guidelines, customs and culture [22]. They are distinguished by enclosed boundaries, lift-up designs and regular layout patterns. Closed boundaries are mainly divided into four patterns: low-rise podiums, main buildings, solid walls and permeable railings. Appropriate types of enclosure boundaries and the combined design of boundaries and lift-up options have been noted as being highly important for achieving a comfortable outdoor environment in residential districts. However, existing studies generally neglect these aspects. Moreover, there remains a lack of quantitative analysis and research on the effect of different combined designs on the wind environment in residential areas. It is essential for architects to develop a design method to determine the optimum arrangement for creating a comfortable wind environment.

In this paper, the characteristics of 16 high-density residential districts in the Yangtze River Delta region of China were analyzed. Then, a typical residential district in Hangzhou was selected to investigate the low wind velocity environment problem in summer. With regard to the CFD simulation method, the average wind velocity, calm wind zone ratio and comfortable wind zone ratio were selected as the evaluation indexes to quantify the results of the wind environment under different enclosure boundaries and lift-up forms, and the impacts of different improvement measures on the low wind velocity environment were determined. The research framework is shown in Figure 1. The results of this study provide practical guidelines for the optimization of low wind velocity environments in high-density residential districts in summer.

**Figure 1.** Framework and workflow of this study.

### **2. Methodology**

*2.1. Case Selection*

2.1.1. Survey of Residential Districts in the Yangtze River Delta

The Yangtze River Delta urban agglomeration is one of the most economically developed and urbanized areas in China. According to Chinese specifications [27], in this study, a high-density residential district is defined as a site area ranging from 2–10 ha containing 11 layers or more with a floor area ratio higher than 2. Sixteen high-density residential districts are randomly selected in typical cities. Figure 2 shows the locations of these residential districts, and Figure 3 illustrates the details of these residential districts with the depicted information sourced from Google Earth images. Due to the geographic environmental constraints, sunlight norms, and urban planning requirements in the Yangtze River Delta region, certain common characteristics are observed in these residential districts.

**Figure 3.** Site layouts of 16 high-density districts in the Yangtze River Delta region of China.

2.1.2. Common Characteristics of High-Density Districts

**Figure 4.** Characteristics of high-density residential districts in the Yangtze River Delta region of China.

**Figure 5.** Four typical patterns of enclosed boundaries in residential districts.

2.1.3. Selection of a Typical Case

Based on the common characteristics of residential districts, Case 1 (i.e., a typical high-density residential district in the central urban area of Hangzhou city) was chosen as the research object. The residential district mainly consists of ten high-rise buildings A–J (refer to Figure 6a), among which buildings A–E contain lift-up areas with a height of 3 m. The main structure of the lift-up buildings is elevated from the ground by a combination of columns, shear walls and a central core. Figure 6a shows the locations of the lift up areas and the vertical structural elements. Figure 6b shows the schematic diagrams of the lift-up building E. The core area percentage is expressed as the percentage of area covered by the core of the plan area of the building at the ground level, which is termed the "area percentage" [24]. The area percentage value in this study was 18.1–27.4%.

**Figure 6.** Schematic diagram of lift-up buildings: (**a**) The locations of the lift-up areas and the vertical structural elements, (**b**) schematic diagrams of the lift-up building E.

City roads border the western and southern sides of the residential district. The western side is enclosed by a low-rise podium that is 8 m in height and 11 m in depth, while the northern and eastern sides are separated from the neighboring areas via permeable railings. The dimensions of the area are 240 m at its longest point and 120 m at its widest point, with a total area of 2.7 ha and a maximum building height of 54 m.

### *2.2. CFD Simulation Setup*

### 2.2.1. Geometric Model and Domain Size

According to existing research [29,30], to accurately reproduce the flow field in this residential district, the geometric model considers the building information in the target area (i.e., the blue area shown in Figure 7a), and the surrounding area is three times larger than the target area (see Figure 7a). Detailed information was retrieved from field measurements and Google Earth images.

The CFD calculation area in the present study satisfies a distance of 4D from the entrance to the nearest building boundary, a distance of 6D to the outflow boundary, a distance of 4W to the side boundary and a distance of 5H to the top boundary, where D is the total length of the building complex (D = 720 m), H is the maximum building height of the target complex (H = 72 m), W is the total width of the building complex (W = 360 m) and

the obstruction rate is 2.4%, which is lower than the recommended maximum obstruction ratio of 3% [31].

**Figure 7.** CFD model: (**a**) typical residential district and surrounding buildings, (**b**) measurement point locations, (**c**) grid arrangement.

### 2.2.2. CFD Model and Grid Arrangement

A commercial CFD program, ANSYS Fluent 19.0, was adopted in the present study. The unsteady Reynolds-averaged Navier–Stokes (RANS) equations with the RNG k-e turbulence model were used to solve the turbulent wind flow in the computational domain [32]. This model was verified by simulating wind flows at the urban scale, with an average relative error of 12% with respect to the measured data [33].

A polygonal grid scheme was adopted in this study, which is adaptable to different geometric shapes. The grid spacing was set to 20 m in the surrounding open area, which was gradually reduced to 3 m in the object model. To accurately represent key simulation areas (e.g., lift-up floors and pedestrian heights), the grid size was thus refined to 1 m. This resulted in a total grid number of 3 million, and the corresponding local grid division is shown in Figure 7c.

### 2.2.3. Boundary Conditions

As the research target is located in the city, it was difficult to obtain the boundary data in the city by field measurement. Moreover, it was hard to gain enough meteorological data in the large area, and difficult to conduct field measurement to verify the large-scale simulation results of the wind environment. Hence, the wind data from the Hangzhou Meteorological Bureau were used as the inflow boundary conditions. The Hangzhou Meteorological Bureau used the interpolation method to get the wind field based on the real-time data of each meteorological monitoring point. The average wind velocity near the inflow boundary from 10:40 a.m. to 11:40 a.m. on 22 July 2020 was 1.7 m/s and the wind direction was south-southwest. The upstream vertical boundary in the calculation domain was set as the inflow boundary, and the outflow boundary was set as the pressure outlet. The vertical velocity profile along the inflow boundary was modeled as a power law based on the structural building code [34].

$$V\_h = V\_0 \left(\frac{h}{h\_0}\right)^a \tag{1}$$

where *Vh* is the wind velocity at a height of *h*, m/s; *V*<sup>0</sup> is the wind velocity at height *h*<sup>0</sup> m/s; *h*<sup>0</sup> = 10 m; and *α* is the roughness index. The target area is located in a dense urban area of a large city with a ground topography of category C, and the exponent of the power law *α* = 0.22 [34]. The computations were performed on an 8-core workstation (Intel Core i7-9700, 3.0 GHz) with 32 GB DDR of system memory. When the scaled residuals reached 10−<sup>4</sup> for mass conservation, U, V, W, k, ε, the solution was considered to be converged.

### *2.3. CFD Validation*

To verify the reliability of the CFD simulation results, this study carried out field measurements in the target residential district. The field measurements were conducted from 10:40 a.m. to 11:40 a.m. on 22 July 2020, with seven anemometers (405i, Testo, Testo AG). The field measurement net covered seven locations for pedestrian activities in the district, as shown in Figure 7b. Velocity measurement points 1–6 were located 1.5 m above the ground, measurement points 2–4 were situated in the lift-up area, and measurement point 7 was located 1.5 m above the roof.

The wind velocity results obtained with the CFD simulation method at a height of 1.5 m were compared to the measured results, as shown in Figure 8. The average error between the CFD model simulation data and the measured data is 20.4%. The trends at the different positions are the same, which verifies the reliability of the CFD simulation results [29]. Therefore, the CFD model established in this study and the obtained simulation results are credible.

**Figure 8.** Comparison of the average measured and simulated wind velocity values at points 1–7.

### *2.4. Case Design*

Considering engineering practices, on the basis of not altering the layout of residential districts, this study adopted the enclosure boundaries and the lift-up design as the parameter design, and 6 cases were obtained (refer to Figure 9).

Case O represents the original state of the research target. The northern and eastern enclosure boundaries are permeable iron railings, and the southern enclosure boundary is the main building. The dominant wind direction in Hangzhou in summer is southsouthwest (SSW), and the changes in enclosure boundary patterns largely involve the optimization of the western boundary.

Case A1, as shown in Figure 9, indicates that the existing lift-up areas of buildings A–E are blocked. Cases B1, B2, and B3, as shown in Figure 9, reveal the changes in the western enclosure boundary pattern. Case B1 reduces the enclosure boundary height and changes the podium height from 8 to 4 m. In Case B2, the enclosure height and depth are reduced. The podium is changed to a solid wall with a height of 3 m and a thickness of 0.3 m. In Case B3, the degree of the enclosure boundary is changed, and the podium is replaced by a permeable railing with a height of 3 m. Case C1 (see Figure 9) considers the combined design of a permeable railing and lift-up, i.e., the western podium is changed to a 3-m-high permeable railing. All high-rise buildings A–J contain a lift-up area that is 3 m high.

**Figure 9.** Design of the six cases.

### *2.5. Wind Environment Evaluation Index*

According to the study of Du et al., summer wind comfort criteria are typically classified into five groups [35]: (i) an unfavorable wind environment for pedestrian activities due to low wind conditions; (ii) a favorable wind environment for pedestrian activities, including light, gentle and moderate breezes; (iii) a tolerable wind environment for any activity; (iv) an intolerable wind environment for any activity; and (v) a dangerous outdoor environment.

According to the research of Lawson [36], the NEN 8100 standard [37], and the air ventilation assessment (AVA) scheme [38] regarding wind comfort indicators, the summer low wind comfort indicators are summarized in Table 1. Based on previous studies [39,40], the following indicators were selected as evaluation indexes for the summer low wind velocity environment:

**Table 1.** Summer low wind comfort criteria in terms of the average wind velocity.

### **3. Results and Discussion**

Wind data for the target residential district were retrieved from the design standards for wind and thermal environmental design of residential buildings [41]. In July, the dominant wind direction in Hangzhou is SSW at a wind velocity of 2.5 m/s. The summer wind rose map is shown in Figure 10. Within the outdoor pedestrian activity range of the residential district, 70 measurement points were selected at a uniform height of 1.5 m (refer to Figure 11). The wind environment results in the different cases were obtained through CFD simulations, and the wind environment evaluation indexes in the different cases were summarized and compared. Moreover, axes X1, X2, and X3, corresponding to the three typical boundaries, were selected to evaluate the change trends in the wind velocity at typical axial velocity measurement points.

**Figure 10.** Wind rose map for Hangzhou in July, information used from reference [41].

**Figure 11.** Location of the 70 velocity measurement points.

### *3.1. Original Model*

Figures 12–14 depict the wind environment simulation results for the six cases in summer. Case O, as shown in Figure 12, indicates that the wind velocities in the western and southern entrance plaza areas are higher than those in the other areas and that the wind velocities in most of the residential district are low. Table 2 shows that the average wind velocity in the whole residential district is 0.63 m/s. Eighty-three percent of the outdoor spaces throughout the entire residential district are found in the calm wind zone, which is not conducive to pedestrian activities, and only 17% of the outdoor spaces are located in the comfort zone. In general, the overall wind comfort in the residential district is unsuitable for pedestrian activities during the hot and humid summer months.

**Figure 12.** Wind velocity simulation results corresponding to the six cases.

**Figure 13.** Wind velocity box plots for the six cases.

**Figure 14.** Streamline patterns of X1, X2, and X3 at pedestrian height for the six cases.

**Table 2.** Results of the wind environment simulation for the six cases.

\* There are no wind velocity measurement points in the lift-up area in Case A1.

### *3.2. Lift-Up Design*

In Figures 12–14, the results of wind environment simulation show that winds beyond the lift-up area are blocked for Case A1. Comparing Case A1 with Case O reveals that the lift-up buildings within the residential district increase the wind velocity in nearby areas. Axis X2 is located 15 m downwind to the north of lift-up building D (refer to Figure 11). Figure 15 shows the wind velocity at the monitoring points along axis X2, i.e., at g2, h2, and i2, and the wind velocities are 0.12, 0.15 and 0.09 m/s higher in Case O than those in Case A1, respectively.

**Figure 15.** X1, X2, and X3 velocity measurement point wind velocities for the six cases.

However, the lift-up design in Case O does not improve the low wind velocity environment throughout the entire residential area. Table 2 indicates that the overall average wind velocity in Case A1 is 0.69 m/s, and the comfortable wind zone ratio is 22%. The overall wind comfort zone in the residential area remains limited. There are no velocity measurement points in the lift-up area in Case A1, so the average wind velocities cannot

be directly compared. When the lift-up area velocity measurement points are removed, the average wind velocity in Case A1 is slightly higher (3%) than that in Case O.

### *3.3. Patterns of Enclosure Boundaries*

Case B1 in Figures 12–14 shows the wind environment simulation results after podium height reduction. The wind velocity behind the podium greatly increases, but the wind velocity change in the rest of the residential district is small. The average wind velocity reaches 0.66 m/s, which is 5% higher than that in the original case. Furthermore, the comfortable wind zone ratio is 17%, and 83% of the outdoor spaces occurring in the calm wind zone are suitable for pedestrian activities. In general, the improvement in residential wind comfort is not obvious.

Case B2 in Figures 12–14 depicts the wind environment simulation results after adoption of a solid wall. Figure 13 and Table 2 demonstrate that the average wind velocity reaches 0.79 m/s, which is 25% higher than that in the original case. The comfortable wind zone ratio is 31%, which is 82% higher than that in the original case. Overall, the wind environment comfort is notably improved.

Case B3 in Figures 12–14 shows the wind environment simulation results after the adoption of a permeable railing. Table 2 indicates that the average wind velocity in the residential district reaches 0.85 m/s, i.e., 35% higher than that in the original case. The comfortable wind zone ratio reaches 34%, and the increase in the ratio of the wind comfort zone is 100%.

According to Cases B1 to B3, the higher the permeability of the enclosure boundary is, the smaller the wind barrier, and the higher the proportion of the wind comfort zone. The wind comfort increases from Cases B1 to B3 and is optimal in Case B3.

### *3.4. Combination of the Permeable Railing and Lift-Up Design*

Figure 12, specifically Case C1, and Figure 14 show the wind environment simulation results based on the permeable railing and lift-up design. All buildings in the residential area adopt the lift-up design. As indicated in Table 2, the average wind velocity in the residential area reaches 1.07 m/s, and the wind comfort zone ratio is 51%. Compared to Case B3, Case C1 also lifts the boundary buildings, and the average wind velocity increases by 26%. The ratio of the calm wind zone decreases by 26%, and the ratio of the comfortable wind zone increases by 50%. The lift-up design is applied to all buildings in the residential district, which notably improves the wind environment.

Compared with Case O, Case C1 has an average wind velocity that increases by 70%; more than half of the outdoor areas are located in the comfort zone, the ratio of the calm wind zone decreases by 41%, and the ratio of the wind comfort zone increases by 200%. The comfortable wind environment in the residential area is greatly improved. Among these six cases, the combined case exhibits the most apparent improvement effects on the wind environment.

Through a wind tunnel experiment on the campus of the Hong Kong Polytechnic University, Du and Mak demonstrated that pedestrian-level wind comfort is greatly improved with the adoption of measures such as opening and lift-up [3]. Tes et al. analyzed the impact of lift-up building models with different core heights and widths on surrounding wind environments [24].

In this study, we confirmed that the lift-up design affects wind comfort on hot summer days and further examined the difference between the partial lift-up design and the total lift-up design. In addition, the effect of the enclosure boundary patterns on wind comfort in residential districts was examined.

### *3.5. Effects of Different Enclosure Boundaries on the Wind Environment along the Horizontal Direction*

To further study the influence of the different enclosure boundary patterns on the surrounding wind velocity environment, three enclosure boundary patterns were selected based on Case O: the low-rise podium, main building, and permeable railing. Along the leeward direction of the boundary, horizontal lines at a height of 1.5 m were selected as axes X1, X2, and X3. Velocity measurement points were selected at 11-m intervals along the horizontal lines. Figure 10 shows the locations of the axes.

As shown in Figures 14 and 15, the wind velocity along the X1 axis is most notably affected by different boundary conditions. In case O, the 8-m podium provides the most obvious barrier to wind flow. In case B1, the 4-m-high podium provides a weaker barrier to wind. In case B2, the solid wall provides a smaller barrier to wind. The wind velocity at the various measurement points exhibits relatively large variations. Compared to that for the original case, the wind velocity at point b3 nearest to the boundary (11 m) indicates the most notable increase, with the maximum increase occurring in Case C1 (i.e., an increase of 1.9 m/s). The wind velocity at point f3 (55 m) also increases, with a maximum increase of 0.8 m/s in Case C1. The wind velocity is directly affected by the conditions in these cases. The main reason for this phenomenon is that the wind velocity of the horizontal boundary corresponding to the X1 axis depends on the enclosure boundary patterns.

The changes in the X2 axis wind velocity vary in different cases. Compared with those in Case O, the maximum increase in the wind velocity occurs at d2 in Case C1, with an increase of 0.4 m/s, while the maximum decrease occurs at f2 in Case C1, decreasing by 0.6 m/s. The horizontal boundary corresponding to the X2 axis is the main building. As shown in Figure 15, the wind velocity is not greatly affected by the change in cases but is affected mainly by the corresponding wind shadow area of the main building.

As shown in Figures 14 and 15, the variation in the range of the wind velocity along the X3 axis is very limited in the different cases. The maximum wind velocity change occurs at h1 in Case C1, and the wind velocity increases by 0.4 m/s. The horizontal boundary corresponding to axis X3 is the permeable fence, which is the residential area entrance. The wind velocity is slightly affected by the change in cases.

In general, the different enclosure boundary patterns directly influence the horizontal wind field along the downwind direction, and the wind gradually decreases with increasing distance from the boundary. This exerts a slight effect on the wind velocity along the nonadjacent axes.

### **4. Conclusions**

The purpose of this study was to analyze the impacts of various enclosure boundary patterns and the lift-up design on the pedestrian wind environment in a high-density residential district and to improve the low wind velocity environment in summer. A typical residential district in Hangzhou was selected, and the wind environment results for different enclosure boundary patterns and the lift-up design were obtained via CFD simulations. The average wind velocity, calm wind zone ratio and comfortable wind zone ratio in different cases were quantitatively analyzed, and optimization strategies for the pedestrian low wind velocity environment in residential districts were determined. The main conclusions are as follows.

4. The enclosure boundary pattern has an obvious influence on the wind field along the horizontal downwind direction but only a slight influence on the wind speed along the nonadjacent axes. Moreover, the influence gradually decreases with increasing distance from the boundary.

In this study, we focus on the influence of the enclosure boundary patterns and the liftup design in the hot summer and cold winter region of China. However, the characteristics of residential districts vary greatly in different areas of China. We will investigate the effects of different enclosures for various climates in future studies. Simulations for larger areas will be performed to gain more accurate CFD inflow boundary conditions. Besides, we will also try to use the Weather Research and Forecasting (WRF) and CFD methods to obtain more complete data for the inflow boundary.

**Author Contributions:** Z.J. developed the original idea, designed the methodology, and wrote the original draft, which was revised by W.G. Both authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are thankful to anonymous reviewers for their helpful comments.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

### *Article* **Bushfire: Retrofitting Rural and Urban Fringe Structures—Implications of Current Engineering Data**

**Glenn P. Costin**

School of Architecture & Built Environment, Faculty of Science, Engineering and Built Environment, Deakin University, Geelong, VIC 3220, Australia<EMAIL_ADDRESS>
**Abstract:** Since the 2009 Black Saturday bushfires in which 173 lives were lost, two-thirds of whom died in their homes, the question of what a home prepared for bushfire looks like has been repeatedly raised. The 2019/2020 fires saw us not much further advanced. This paper seeks to consolidate what is known about bushfire behavior, its influence upon structures, and, through this data, infer improved standards of practice for retrofitting rural and urban fringe homes. In particular, the prevention of ember and smoke incursion: the data suggesting the prior as the main mechanism of home destruction; the latter as high risk to sheltering occupant health. The article is framed around a comprehensive literature review, and the author's own experiences and observations from fire impacted structures in Victoria's northeast. The article's import lies in demonstrating how embers and smoke may enter homes otherwise seen to be appropriately sealed prior to the fire's approach. Included in the findings are developed hypotheses based on thermal expansion, pressure differentials and backdraft; offering defined paths towards future research. In addition, the work provides practical advice towards mitigating the identified issues using retrofit practices based upon the author's practical experience as a tradesperson and building designer.

**Keywords:** bushfire; retrofit; ember attack; pressure differential; urban fringe; rural housing; pyrotornadogenesis; backdraft

### **1. Introduction**

Prior to the Black Saturday fires of 2009 Australian bushfire policy could be summed up by the phrase 'stay or go'. The premise underlying this approach suggests most homes lost to bush fire succumb to ember attack, not the fire front [1–8]. Received wisdom held, holds today, that ember attack can be defended against, and thus many homes saved. The policy, however, was based upon another premise: that homes were prepared, and residents mentally and physically capable of such defense. The year 2009, and the loss of 173 lives—two-thirds of whom died in their homes—changed that perspective radically [2,3].

Whilst conceptually 'stay or go' still exists, active defense risks to homeowners, discussed by many including state and territory fire authorities [2,3,9–12] have altered the underpinning message. Today, Australians are advised to prepare property before the bushfire season, then leave early should a fire start. In conditions categorized as Catastrophic or Code Red (state dependent categories) the advice is to prepare the home and leave *before* a fire event begins [13,14]. In emergencies, with or without a 'state of emergency' declaration, mandatory evacuations may be ordered, though the legalities of forced removal from a home property are debatable, state specific and unclear [15,16].

Occasionally evacuation is not possible; on others, the indicators of potential fire are low, and communities are taken by surprise. In December of 2015, over 100 homes were lost in the Victorian coastal community of Wye River [6]. On that occasion, the McArthur Forest Fire Danger Index (FFDI)—the measure by which Australia's fire danger levels of High, Very High, Severe, etc., are identified—was only 49 or 'Very High' [6]. Code Red or Catastrophic is 100+. Fortunately, due to the fire's approach direction, no lives were lost.

**Citation:** Costin, G.P. Bushfire: Retrofitting Rural and Urban Fringe Structures—Implications of Current Engineering Data. *Energies* **2021**, *14*, 3526. https://doi.org/10.3390/ en14123526

Academic Editors: Mark Luther, Igor Martek and Mehdi Amirkhani

Received: 19 April 2021 Accepted: 24 May 2021 Published: 14 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Both before [17] and after [18] that event, the FFDI as a sole indicator of extreme bushfire risk has been considered questionable. This aside, what remains clear is that homes were not prepared adequately. The 2019/2020 fires again exposed this truth, with fires raging, and homes lost, in all states and territories of the country except the capital. NSW and Victoria took the brunt of losses, with over 2800 houses destroyed in those two states alone. Sadly, on this occasion, 34 human lives were also directly lost [19]. Indirectly, a further 417 lives were lost and 4456 hospitalized due to smoke inhalation [20].

Such deaths give rise to another purpose behind retrofitting: air quality. Occasionally the house is the only shelter available; though Dengate [9] notes that some people decide to stay, others have no such option [2,3,10,21]. Retrofitting will not turn an older home into a bushfire bunker, but it will improve its potential for occupant survival. Much retrofitting discussion is about sealing the home against embers, however as identified above, smoke is just as important, studies also identifying in utero growth restriction of babies through the mother's bushfire smoke inhalation [22,23]. Examples of new homes purposefully built to withstand bushfire attacks without smoke incursion are promising [24]: improving indoor air quality should likewise be a consideration in retrofitting [25].

So what does a home prepared for bushfire actually look like? Specifically, how might we retrofit a home to improve it, and its resident's, preparedness? When constructing a new home in Australia, performance requirements are found in the National Construction Code (NCC) [26] and the standard *AS3959 Construction of buildings in bushfire-prone areas* [27]. However, that standard's veracity is questioned both in this article and by others [28], due to its currently limited interpretation of ember attack mechanisms. Further advice for new and existing homes is given by state and territory fire agency guides on land and property preparedness. Yet, whilst commentary on defensible Australian homes began as early 1945 [1], only recently has discussion focused on retrofitting.

This paper explores the efficacy of these approaches, and AS3959, by drawing upon extant literature, current engineering modeling, reported experiences of homeowners and the author's own experiences. Improved retrofitting actions are proposed based upon these findings; particularly issues arising from potential pressure differentials in extreme bushfire events.

### **2. Researching Bushfire and Its Influence Upon Structure**

What causes a structure to burn is best studied through careful analysis of burnt buildings [1,4–6,29], practical experimentation [30–32], or, when such is problematic (missing evidence, cost, risk), through computer modeling [33,34]. Much of this work has been, or is being, undertaken in facilities around the globe. This article consolidates that material, combining a comprehensive literature review and anecdotal evidence. In reviewing the literature a broad range of terms and phrases were chased through online search engines, academic libraries, Springer, Scopus, Google Scholar and the like. To this was added news reports, blogs, drilled down for quality evidence such as photographs or events witnessed by multiple parties. The main terms used derived from previously understood retrofitting actions and basic bushfire defense systems such as: bushfire sprinklers; intumescent paints; water tanks and bushfire; glazing; ember attack and the like. These terms were followed by those surrounding the hypothesis of pressure differentials, cyclonic winds, pyrogenic winds and such, prior to turning to the specifics of structural components and materials under temperature extremes.

Within this core of articles, the hypothesis of pressure differential was explored; seeking more information as the data exposed questions, potential answers, new terminology: testing the hypothesis in light of known data and reported experiences. Future, targeted, research was thus developed in pursuit of insights to aid retrofitting of older homes, guide the construction of new homes, improve legislated standards and, perhaps, solve mysteries surrounding why seemingly secure homes are destroyed by bushfire, whilst others, much older and more decrepit, survive. To interpret the insights gained from this exploration, an understanding must first be had of the structure, design and context of the typical Australian home; be it rural, or within the rural-urban fringe interface.

### *The Australian Home: A Structural Description*

The rationale for retrofitting existing dwellings against bushfire attack is clear. Also clear, within the literature, is that ember ingress is the main concern. To understand how ingress occurs requires appreciation of the typical Australian home's structure. Generally, this is a one or two story timber or steel frame building clad with brick veneer, timber weatherboard, cement sheet or rendered panels of autoclaved aerated concrete or polystyrene. Roof cladding is either corrugated metal sheeting or tiles of concrete or glazed terracotta. Windows are usually timber or light aluminum framing, frequently with single panes of glass as little as 3 mm in thickness. More recent homes will have thicker glass (5 mm) and or be double glazed. The whole structure, with rare architectural exceptions, will be constructed upon either timber, concrete or steel stumps or posts, brick piers or a concrete slab. Raised floor structures may have this underfloor space enclosed by a vented brick dwarf wall, be partially enclosed with timber battens, or be fully open.

Common to homes is some form of verandah. In addition, there may be a deck, generally of timber, though on occasions steel-framed, with timber or polymer strip flooring. Eaves of 450–600 mm are typical, though recent trends towards 'eave-less' homes have reduced this to gutters only in many cases. Fascias are frequently timber, or more recently, light steel. Eave soffits are usually cement sheet lined, though older homes may have timber strips venting the roof space. Further roof venting may be available through grills or slats in gables or via static or spinning metal vents. Guttering is usually of galvanized or prepainted steel, though aluminum and PVC systems exist. Generally, gutters feed to above or below ground water tanks made of steel, fiberglass, plastic or concrete. On occasions, water may be stored under raised floor houses.

In older homes doors and windows seldom seal well, particularly door sills. Weatherboard homes commonly have gaps where external architraves overlap boards. Others may not be flashed correctly at the heads, leaving gaps for embers to settle or be driven into the framework. Some windows will simply not close properly, or have openings in bathrooms or toilets to prevent condensation or odors.

The above demonstrates that most homes under consideration provide ample opportunity for ember ingress. At first glance, retrofitting these homes against bushfire is not a complex task. However, even at a basic level, it requires an understanding of their weaknesses and an appreciation of how bushfires compound these weaknesses through dynamic, high velocity, winds, extreme temperatures and pressure differentials.

### **3. What Is Known of How Houses Burn in Bushfires**

Recent research [4–7] strongly supports Barrow's 1945 [1] suggestion that embers were the most significant cause of house destruction. The fires at Tathra, NSW, in March 2018 furthered this understanding. In that instance, only 32 of the 69 homes destroyed were in a decreed bushfire prone area [35]. The rest were within the seaside township where neither flames nor fire front heat flux had influence. Research has suggested that ember penetration into urban or suburban areas is typically less than 700 m [36], however the Canberra fires of 2003 destroyed homes in the suburb of Lyons over 2 km distant from the fire front [29]. Notably, the Australian Institute for Disaster Resilience (AIDR) Canberra Fires Field Report [29] documents bushfire generated winds damaging homes well before flame arrival; leading to an extremely high percentage (91%) of totally destroyed homes: i.e., once fire took hold, invariably the house was lost.

However, stating ember attack alone is insufficient as it fails to account for the randomization of its effect. In research [1,37,38], news reports [39] and blogs [40], there are multiple examples of homes that 'should' have been destroyed, yet survived intact despite abutting seemingly secure but destroyed homes. Whilst chance must be conceded, there are potentially other factors in play.

During a bushfire, residents are advised to seal homes as much as possible: citing Barrow's proposition ' ... a house should be as air-tight as practicable ... ' [1] (p. 1). Windows, doors, vents, everything should be blocked to prevent ember incursion. With modern homes, this is reasonably achievable: potentially to the point of fault. An untested hypothesis held by this author, to some degree suggested by Ghaderi et al.'s modeling [33], is that of pressure differentials in bushfire contexts: Differentials caused by pyrogenic winds—fire generated winds high in both temperature and velocity—impacting homes incapable of rapid pressure equalization. In such instances at least two things may challenge the integrity of a building: extreme uplift forces; and rapid, potentially destructive, pressure equalization.

The first, uplift, is well understood and engineered for, particularly in cyclonic zones, through AS 1684.3 [41] and AS 4055 [42]. Unfortunately, bushfires also occur in non-cyclonic zones where homes are designed for significantly lower wind pressures. That extreme bushfires can create their own weather systems is also well understood. Pyrocumulonimbus clouds and their associated fire thunderstorms bring about fierce winds, downbursts, and in extreme cases, pyro-tornadogenesis—fire tornadoes generated by the rotating convective winds of the cloud [43]. Video evidence of bushfires assaulting Canberra suburbs Kambah and Chapman in 2003 [43,44] show roof sheeting falling kilometers from the fire front, and the fire tornado's enormity—approximated at 500 m base diameter [45].

Pyro-tornadoes aside, Ghaderi et al.'s investigation [33] into wind-driven surface fires demonstrates that the presence of a structure alters the intensity and dynamics of that fire, supporting earlier work by Honey and Rollo [34]. A range of factors deriving from this study have great import into how homes are challenged by fire, and hence, may be defended. One notable outcome [33] (p. 12) shifts our interpretation of fire behavior from steady heat action (the premise behind AS3959) to dynamic pulsation, uplifting vortices, extremely low pressures and high velocity reverse airflows (reflective of Sharples et al. 2012 wind-terrain modeling [46]).

Figures 1–3 below, evidence these key influences upon a standard flat plane structure (effectively a 6 m × 6 m × 6 m box replicating a house) downstream from an oncoming fire. The main points of interest are as follows:

Additionally, radiative heat flux was determined to focus upon upper wall portions facing the fire, whilst the sides, rear and roof are significantly less affected.

**Figure 1.** In subfigures (**a**) and (**e**) the white box instances the presence of a 6 m cubic building. Subfigures (**b**) and (**f**) show the fire passing by the building 9 m distance from its center. I.e., 6 m from the side [33] (p. 9). Colorbar represents velocity in m/s along x axis.

**Figure 2.** Subfigures (**a**,**b**) show the vertical profile of the airflow's changing longitudinal velocity at different longitudinal positions relative to the inlet location. Subfigure (**a**) shows airflow aligned with the building's center line. Subfigure (**b**) shows airflow passing 9 m on either side of center line [33] (p. 10).

Of particular interest is the identified wind velocities. The fire front wind velocity is seen to be significantly greater than the driving input. I.e., the fire is drawn forward by self-generated low pressures, not simply driven forward by high winds. The input wind velocity in this model is only 6 km/h. Thunderstorms commonly generate wind gusts of 90 km/h, more damaging storms, 160 km/h; cyclones may exceed 360 km/h [47]. The velocity the fire itself travels at in such conditions is discussed in Sharples et al.'s study of the Canberra 2003 fires [46]. Such dynamic low-hi-low heat, wind velocity and pressure fluctuations place inordinate stress upon a structure.

**Figure 3.** Subfigure (**a**) shows the instantaneous surface temperature as measured in Degrees Kelvin at 3 points located on the surface 5 m from the building front. P1 is directly on the building's center line, whilst P2 and P3 are located 9 m on either side of the center. Subfigure (**b**) shows the frequency of peak temperature signals at these same sensors [33] (p. 12).

The significance of the above regarding ember attack is multifaceted: particularly ember disposition being the rear or downstream side of the structure, not the surface facing the fire front. This is concerning for homes with verandahs or other semi enclosed spaces on this side. Also, embers attack both the front and rear of the structure at an upwards trajectory, exposing any weakness under eaves and verandahs.

Figure 4 describes high velocity winds over and around a structure with (b) and without fire (a). Figure 4b demonstrates that in the presence of fire, myriads of small vortices initially hug the ground, then buoyancy inducing forces dominate, creating larger vortices that randomly cross hatch the structural zone generating significant uplift: The structure is attacked simultaneously by embers, heat flux, and wind loads from multiple directions, introducing dynamic loading at numerous points and angles.

**Figure 4.** The vertical airflow structures in the presence of fire (**b**) and without (**a**) [33] (p. 11).

A defensively well sealed building now works against itself as rising temperatures and, to a lesser degree, pressures, occur within the home. What the internal temperature and internal-external pressure differential might be is not addressed in this modeling—nor in any modeling to date. However, it may be assumed that this differential at least adds behind surface loading to uplift forces already existent through high velocity winds.

Coupled with the dynamic fire front heat shock inflicted upon windows, this pressure may suffice to fracture or shatter glass. In either case, cladding and windows of the structure may be compromised: creating gaps and openings, allowing ember ingress and potentially an explosive backdraft action.

The temperatures modeled are also concerning for exposed structural steel elements. It is well argued [48] that at around 600 ◦C—steel's yield strength drops by approximately

60%, Young's modulus falling by 70%. Given high wind loads, and Figure 3a showing surface temperatures fluctuating between 400 ◦K and 1000 ◦K (125 ◦C to 725 ◦C), exposed structural steel failure is potentially high. In relation to ember incursion, failure need not involve collapse—columns, beams or lintels need only flex—creating gaps sufficient to allow ember ingress; ingress being considered possible through gaps as little as 2 mm [27].

Understanding the forces affecting structures as dynamic, not stable is therefore crucial to both new homes and retrofitting. Buildings must not be viewed as static entities: rather, as vibrating, pulsating structures attempting to 'breathe', whilst swaying, rocking, being twisted, even if not visibly so.

### *3.1. Inference: Structural and Cladding Integrity; Ember Incursion*

Current modeling does not reflect an actual house with its complexities of form. However, the aforementioned studies provide sufficient data to highlight key areas of risk from ember attack; suggesting useful actions. Rising internal pressures would at first seem a positive defense outcome.  The problem comes, however, with the principles of backdraft and flashover (the latter discussed by Caird Ramsay et al. [37,38]). Flashover occurs when energy trapped within a space cycles upwards such that all materials reach their ignition point—a 'fuel-dependent phenomenon' [49] (p. 55). Backdraft occurs when a closed compartment, bordering on combustion but low on oxygen, is suddenly provided oxygen through an opening—such as broken windows [49]. Add fumes inside a home from furnishings, finishes and fittings [50], frequently volatile organic compounds (VOCs)—Fleischmann et al.'s unburned hydrocarbons [51]—and a rapid combustion event may occur acting explosively outwards.

Not enough is known about VOC off-gassing in homes experiencing extreme bushfire temperatures to state authoritatively that these could amount to an explosive event. Despite multiple anecdotal and media accounts of buildings 'exploding' [52], evidence after the fact is insufficient to support, or refute, such accounts [4,6,37,38]. However, the data above suggests the potential should be conceded. Regardless, the potential for cladding breaches and backdraft remains. Finding means by which to reduce internal air pressures safely, therefore, forms part of the discussion on refitting strategies that follows. Before developing that discussion, an outline of applicable Australian Standards [27,41,42,53] is requisite to understanding contemporary approaches to construction and retrofitting of homes in bushfire zones.

### *3.2. Australian Standards and the Bushfire Attack Level (BAL)*

Whilst not required for all home retrofits, AS3959 [27] remains the preeminent guide for Australian housing in bushfire zones. Integral to this standard is the Bushfire Attack Level (BAL) rating system. Briefly, the 'BAL' is a statement of the likely severity of a site or building's exposure to bushfire and ember attack. A radiant heat flux range statement in kW/m2, it is an evaluation of a structure's context. The evaluation includes:

Derived ratings are either BAL-LOW, 12.5, 19, 29, 40, or FZ (Flame Zone). The higher the BAL, the more threatened a structure. Once identified, guidance on acceptable construction is given to each BAL level through the major sections of the standard. A typical visual guide is given in Figure 5 below:

**Figure 5.** Pictorial description of Bushfire Attack Level (BAL) ratings [54].

AS3959 becomes challenged when dynamic, not static, heat flux is considered. As others have found [55], even these static values are questionable when detailed modeling is applied. Add the previously identified vortices, uplift and negative pressure loadings and the construction provisions lose relevance. This does not negate the standard's total relevance, however, BAL must now be understood as indicative rather than definitive potentially a significant understatement. When retrofitting or designing and constructing new buildings far greater attention to detailing, hold down and material choices should apply.

The BAL system of identifying risk is further challenged when Figure 5 is reflected upon in light of documented home losses; many destroyed despite BAL-LOW ratings. It fails to account for ember attack many kilometers ahead of the fire front, frequently within town or urban fringe areas many streets back from classified vegetation. Yet the BAL system remains valuable in identifying radiant heat and flame proximity for given locations. It allows homeowners, when considering retrofitting, to better prepare land around buildings to reduce heat and flame levels. But with regards to embers, distance from the flame front is not a safeguard.

### *3.3. Australian Standards and Wind Ratings*

Wind ratings for Australian houses are defined by AS 4055 [42], or AS1170.2 [56]. For the purposes of this paper, outlining AS4055 s rating system will suffice for exampling wind speeds in a later discussion. Ratings within AS4055 are given as either N1 through to N6, or C1 through to C4. N or C ratings depend upon regional location; 'C' referencing areas likely to experience cyclonic conditions. The individual ratings suggest limit state (serviceability and ultimate) design wind gust speeds. Stated in m/s, classes N3 through to N6 have the same design gust speeds as C1 through to C4, being: 32 (50 ultimate), 39 (61), 47 (74) and 55 (86) respectively. The difference between the N and C becomes evident when calculating positive and negative pressures applicable to given surfaces or building elements (roof, walls or the like). For example, a C2 rated roof surface has a general ultimate limit state pressure of −3.21 kPa (a net uplift), whereas N4, with the same gust speed of 61 m/s, is deemed to have a net uplift pressure of only −2.21 kPa. These derive from a table of pressure coefficients recognizing the dynamic low-hi-low pressures sustained in cyclonic events. Ghaderi et al.'s [33] study suggests that such dynamic pressures evolve from bushfires as well.

### **4. Retrofitting: Applied Implications**

Conceding that embers cause most bushfire home losses, the ambition of retrofitting is to prevent their ingress and eliminating or reducing ember deposition zones of high ignition risk. The previous section's guidance support's most government or fire authority recommendations. However, the material allows review of existing retrofitting publications more informatively, several of which are outlined briefly below. The following then discusses key areas of bushfire attack—pressure differentials, uplift, angles of attack, counter flowing winds and the like—relative to those structural elements most influenced, seeking to mitigate such attack and inhibit ember and smoke ingress. In making recommendations it is conceded that retrofit activity is framed by what is 'reasonably practicable'. Available time, financial resources, structural element access and emotive connection to home and contents, all influence perceptions of achievability.

A recently published bushfire resource for the construction and retrofitting of homes in Queensland [57], has relevance beyond that state. Particularly useful is the Level of Protection system that informs builders/owners of the protection achievable with a particular range of actions. An extensive document, it also directs builders to particular sections of the relevant standards [27,41,58].

Another developing Queensland publication [59] lays the ground work for retrofitting against extreme winds. Yet despite being a Geoscience Australia and James Cook University's Cyclone Testing Station (JCU CTS) collaboration, under the banner of the Bushfire and Natural Hazards CRC, there is no linkage between bushfires and high wind loads. Despite this lack, it promises to be a valuable resource in identifying levels of protection gained against 0.2 s peak wind gusts gained from different actions undertaken [60].

The Victorian Building Authority (VBA) and Country Fire Authority (CFA) have also produced valuable guides specific to retrofits [61,62]. One deals with domestic homes whilst the other focuses upon commercial buildings defined as Class 9 under the NCC those that might be used as last resort shelters for vulnerable communities—it holds advice relevant to domestic structures, particularly subfloor venting, piping and roof penetrations. As with the recommendations being promoted here, these documents frequently promote methods exceeding AS3959.

Reflecting on these documents in light of research implications presented earlier, the following frequently references Australian Standards and building codes to higher than 'normal' standards.

### *4.1. Pressure Differentials, Dynamic Loadings and Cladding*

These are issues least addressed by current retrofitting advice and AS 3959 generally. Of particular import is the dynamic loading bushfires impose: loads suggesting cyclonic standards (such as cladding fixing AS 1562.1 [63]) whereby materials must satisfy a Low-Hi-Low testing regime that 'rattles' elements over a seven stage sequence of varying pressures. The only guide to this procedure, and any claims of compliance, is a draft document published in April 2009 by JCU CTS [64].

The following 'grounds' this discussion in practice: i.e., how does the preceding data inform the typical Australian home retrofit? Though only common building elements are addressed, the principle of increased attention to tie down may be applied more generally. At the section's end, the discussion turns to idealized means by which internal-external pressure differentials may be alleviated.

### 4.1.1. Metal Sheet Roof Cladding

In older homes roof sheeting is commonly nailed, not screwed; whilst capping and flashings to many relatively new homes are pop-riveted rather than screwed—Australian standards [63] only requiring screws from 2018. Both are problematic regarding tie-down the identified potential for high uplift pressures and dynamic wind loadings is likely to exceed their withdrawal capacity. Even if sheeting or capping is not torn away, sufficient gaps may be created at sheet ends and overlaps allowing ember ingress. Sheet roofs should thus be check and fixings upgraded to screw fastenings using cyclonic fixing patterns following AS 1684.3. Gaps between flashings and corrugated profiles should be filled with non-combustible material such as rock wool.

### 4.1.2. Tile Roofs (Terracotta or Cement)

In low wind areas (AS 4055 ratings N1, N2, N3), tiles tend to be tied down minimally or not at all. Given higher and dynamic wind loads associated with bushfires, tiles should be mechanically fastened to a minimum of N4 or C2 wherever there is the possibility to do so. Appropriate sarking should be provided under the tiles, though this may be considered impractical in many instances due to the costs associated. When retrofitting incorporates new cladding, appropriate sarking must be installed, and sheeting rather than tiles considered.

### 4.1.3. Further Roofing Considerations

In the above cases, the tie-down has only been achieved for the cladding. This remains inadequate if tie-down of battens to rafters, rafters to wall plates and studs, is inadequate. Hence the concept of 'reasonably practicable'; i.e., it depends upon owner willingness or financial capacity. Yet, where possible, tie-down fixings should be upgraded at these points. AS 1684.3 provides appropriate guidance in this area. It is important to work to these higher wind loads, not standard contextual loads which may be as low as N1—which the previous information has shown inadequate in extreme bushfire scenarios.

### 4.1.4. Wall Cladding

Fixings to sheet cladding (steel, ply, cement sheet), should be checked for security and upgraded anywhere movement is detected. Gaps and cracks should be filled. Likewise for weatherboards; replacing damaged boards. Older homes may have no sarking, with the upward trajectory of embers, gaps in board overlaps become susceptible, allowing embers to settle within the wall cavity. Eave/wall junctions, re-entrant and external wall corners, should be carefully inspected; loose or damaged stops or cover straps replaced, gaps filled. Intumescent paints may be considered to protect against fire front heat flux, or low areas were embers may settle, however, the service life of such coatings remains doubtful [65]. Closing gaps to prevent ember penetration is the main aim.

### *4.2. Subfloor Protection*

As per existing retrofitting guides, these areas should be shielded by steel mesh and close fitting battens. In the case of brick dwarf walls, gaps or cracks should be filled, vents should be upgraded if vent openings are greater than 2 mm. The junction between dwarf walls and main wall cladding should be inspected and gaps filled.

### *4.3. Fascia, Eave, and Guttering*

This area is particularly open to ember attack due to wind approach angles—upwards at 45–60 degrees from horizontal. Generally, soffits are non-combustible cement sheet, and gutters of galvanized or painted steel. However, fascias are frequently either light steel or timber. Under heat loads, steel may deflect, allowing soffit sheets to drop from groves; joiners between sheets are generally light PVC which may melt, gaps for embers now becoming present. The soffit/wall cladding junction should also be inspected. Fixing should be checked as per previous elements.

### *4.4. Gutter Guards*

This is an area of contention amongst homeowners, builders and suppliers/ manufacturers. This author's experiences and anecdotal reflections of homeowners and architects, suggests that no available system works effectively. Metal guards are considered the most effective in being less susceptible to damage from birds and small marsupials, however, the mesh tends to trap leaf ends and grass still grows under the guard. Many such guards cannot be easily removed for cleaning. When installed, systems for flushing gutters without removal should be integrated, and regularly used.

### *4.5. Sarking*

Sarking has been listed in several of the above recommendations. Leonard et al. [57] recommend against conventional sarking, even flammability index 5 rated (NCC requires 5 or less). That document recommends the use of flame resistant sarking, but fails to identify what that actually means. Kempster [66], however, describes bushfire roof and wall systems incorporating fireproof blankets, sarking and insulation elements tested to AS 1530 [67] offering a flammability index of 1. The materials within these systems can be individually applied as part of a retrofit program but require cladding removal to do so.

### *4.6. Structural Steel*

Structural steel is common to Australian homes as beams, columns and light gauge wall and roof framing. In normal conditions, these elements are stable, serviceable and non-combustible. However, suffering extreme bushfire temperatures components may buckle under load having lost potentially 60–70% structural capacity. Alternatively, tiedown capacity of joints may become compromised or gaps in cladding generated: the latter through steel's linear expansion coefficient of 1.2 × <sup>10</sup><sup>−</sup>6, meaning with a temperature rise of as little as 60 ◦C (from 40 ◦C ambient to heat flux induced 100 ◦C material temperature), a 2.4 m or 2.7 m steel wall will gain height by approximately 2 mm.

Exposed posts and beams undergo greater elongation. An 8 m long beam, may expand linearly when raised by 100 ◦C, as much as 10 mm. Though non-failure temperatures, issues arise when components are restrained, either by other steel components, or elements such as brick or concrete columns. Baetu et al. [48] demonstrate that constrained structural steel is more likely to fail early—before yield temperatures and loads are reached—due to constrained liner expansion causing premature buckling. This was particularly notable amongst lightweight steel components. The import here is not that structural steel will inevitably fail in bushfire conditions, rather that it may, due to even minor deflection, be a factor in exposing the internal elements of walls and roofs to ember ingress through the generation of gaps as the fire front passes.

Retrofit actions to counter the issue of steel's expansion may be in the form of:

### *4.7. Of Pipes and Penetrations*

Contemporary plumbing invariably involves PVC components, often with long exposed lengths under suspended subfloors. Rigid PVC's flash and sustained ignition temperatures of approximately 400 ◦C and 450 ◦C respectively [68] are achievable in a fire front. Though unlikely to carry fire directly through a wall, roof or floor element, PVC components will melt, droop or compact as the melting range is only 115–245 ◦C (manufacturing process dependent) [69]. It is by this mechanism that embers and flame may gain entry into or through structural and protective elements either during the initial assault or after the fire front has passed.

Fireproof collars are available for retrofitting that hinge around PVC piping. Likewise, there are metal fittings that couple around a pipe and backfilled with fireproof mastics. Where piping is more exposed, consideration should be given to applying intumescent coatings or flameproof insulating wrap, shielding the PVC from heat to prevent unnecessary replacement.

### *4.8. Windows and Doors*

Glazing represents a weak point in the defense against ember attack and the bushfire front itself. Many early homes may have large panes of ordinary annealed gas only 3 mm thick: totally inadequate against a fire front. Likewise, they are easily shattered by flying debris from trees or neighboring properties. More modern homes may have thicker glass, though still generally ordinary annealed.

Upgrading of windows against bushfire needs careful consideration outside the scope of this article as solutions must be highly contextualized; reflecting requirements of AS 3595, the NCC, and through this code, multiple further standards. However, Bowditch et al.'s [32] research offers some guidance. Though reporting on a range of glass and framing types exposed to bushfire temperatures, the simulation was of steady heat flux as against the rapid fluctuations observed by Ghaderi et al. [33]. Likewise, no high velocity winds were brought to bear and the glazing units stood in an open frame. The latter factors meaning that internal-external pressure differentials were not considered. These issues aside, the findings suggest that improved glazing should include 5 mm toughened glass as a minimum (particularly the exterior pane is part of a double glazed unit).

A cheaper, or more easily applied, means of upgrading window security against heat, flame and impact, is shielding. Multiple systems are available, from proprietary roller shutters to cost effective hinged panels, shutters or screens [70]. In either case, 'noncorrosive steel' (generally taken to mean stainless steel) or bronze should be used and installed such that no gaps greater than 2 mm remain.

Doors and windows, designed to open, mean small gaps are common to ensure free movement. It is imperative that these gaps have high temperature resistant seals that will prevent smoke ingress, not just embers. Standard rubber seals are generally inadequate for this purpose, likewise brush seals. High quality neoprene seals should be installed wherever possible.

### **5. Pressure Equalization Strategies—Requisite or Not?**

Notably lacking attention, this area needs applied research and theoretical modeling. The modeling previously discussed [33] is suggestive of pressure differentials affecting bushfire attacked homes but does not confirm it. After the 1983 Ash Wednesday fires this author noted examples of untouched weatherboard homes beside destroyed brick dwellings.  Nearby, stood an old weatherboard house unscathed. This difference between the two outcomes could be 'chance': Alternatively, old cottages leak air with little resistance; whilst the new home, appropriately closed and sealed against ember incursion, failed to quickly equalize the internal-external pressure. Barrow's [1] documenting of timber clad homes with mesh covered windows and vents surviving bushfires led to our understanding of ember attack. Potentially there is another message here too.

Whilst awaiting further research it is worth considering potential remedies, particularly from a retrofitting perspective. One solution is release valve venting. Associated with every room, they must allow rapid release of pressure but disallow incursion of embers or smoke. Notably, the hypothesis is less about high pressures forming suddenly within the house, and more about sudden low pressures—relative to the interior—becoming dominant outside the structure. It is about reducing that pressure differential sufficiently to limit structural flex and component creep, or the fracturing of already heat shocked glazing, either of which may allow ember incursion and destructive backdraft ignition.

### **6. Defense through Water**

Dousing a structure with water before a bushfire arrives has long been considered a reliable defense strategy. Several studies have been undertaken [71–74] reporting on the effectiveness of bushfire defense sprinkler systems. To this may be added news reports and testimony of those who have saved or lost homes where sprinkler systems have been deployed [40]. However, discussion remains as to its potential for heightening risk through over expenditure of water—leaving inadequate supply for fire front defense.

Water storage and supply are crucial to active fire defense systems; particularly for those who remain and defend a property believing themselves equipped to do so. Wilkinson & Eriksen [75] outline water scarcity, pump failures, power outages and melting componentry—pipes lines, hoses, fittings and even water storage tanks—as the 'weak link' in seemingly well prepared systems. They also note that reticulated supply cannot be guaranteed due to power cuts to pumping stations, or through drought depleted reserves being drawn down prior to fire front emergence. Some cities or towns may impose restrictions on reticulated water supply to conserve storage, small town or farm homes use tank or bore water and are not required to comply, making conservation discretionary.

Current water storage policy, though varying state to state, requires maintaining dedicated firefighting reserves of between 10,000 L to 22,000 L to be maintained on site depending upon block size [76–78]. This raises the question of what a viable storage container looks like. Previous research [4,6,79] suggests that polyethylene (PE) tanks are inadequate in the face of significant heat sources, even when full. However, when appropriately sited, shielded or installed underground, they may offer low cost alternatives. No data is available regarding the effectiveness of intumescent coatings to such tanks. Concrete or metal tanks were shown to have no such limitations. Of importance in all cases, however, is that connections and above ground piping should match those of the local fire authority and be of metal.

### *Spray and Sprinkler Systems*

When appropriately designed and deployed, spray systems have shown high value in protecting homes [40,80]. What is appropriate remains widely discussed in a range of reports in Australia and internationally [40,57,61,71–75,81]. A recent study into the efficacy of airborne droplets for attenuating the effects of bushfire offers useful insights, whilst raising further doubts [73]. The key findings from these studies show that:

Post bushfire blogs and news reports tell of sprinkler efficacy in some instances, and failure in others [40,80]. The main questions arise around spray head location, pressures, flow volumes and spray direction. In the 2020–2021 Australian fires, a rural property was actively defended by the occupants with the aid of preinstalled sprinkler systems [80]. The design of the home and immediate landscape aiding their success (a green roofed structure). However, the most important asset was the water supply: more than 200,000 L held in concrete tanks. Saving the property took over 150,000 L, most expended through the sprinklers. Such volumes of water are seldom available. Arguments against sprinklers have focused on this expenditure of water prior to fire front arrival. Yet another commentary [40] argues that sprinklers did little to prevent the destruction of the home despite plentiful water supply.

Most researchers and manufacturers concur that spray head type and location are the keys to a system's success [71,82,83]. Recommendations include static low volume, high pressure, spray heads that generate a 'screen' of fine water droplets. Others use rotating heads to similar effect, deploying heads around eaves and fascia. Systems located on the roof ridge line are considered ineffective due to the high uplift winds taking the water away from the property: a premise supported by the modeling discussed earlier [33]. The modeling of Green [71], suggests spray systems should be close to a structure's walls

for efficacy, directed in such a manner that water is carried back to the upper walls, wall bases and under eaves by the encroaching winds and vortices.

Most importantly, spray systems and service lines must be fireproof. Whilst CPVC piping is fire resistant and has been successfully tested for unshielded interior fire suppression systems, questions remain about their resilience after extensive exposure to UV [84,85]. The recommendation for metal systems thus remains.

### **7. Retrofitting and the Context: Landscaping**

In 2011 Honey & Rollo [34] analyzed the influence of structural form, proximity, and the arrangement and alignment of multiple structures upon the transport, passage and deposition of embers. They also investigated the significance of air pressure and velocity, with findings not dissimilar to Ghaderi et al.'s modeling. In dealing with the form they experimented with alignment and, through raising structures off ground, introduced interrupting airflows limiting ember build up on the lee side. Potentially the most important aspect of this study is their observationally derived hypothesis concerning land sculpting in front of buildings in order to confuse wind flows and dictate preferred ember deposition [34].

A useful CFA [86] document speaks to appropriate landscape design for bushfire, however, the focus is upon materials, layouts and plant/tree selection. Sculpting of the landmass itself is not raised. In practice, such sculpting may be problematic for many homes due to block size restrictions, suggesting collaborative sculpting across multiple blocks, or council-assisted sculpting of government or state forest lands. Further studies need to be undertaken, however, before such works could be assured of success; particularly in light of Sharples et al.'s modeling and experimentation demonstrating powerful reverse airflows driving fires laterally upon ridges and rapidly up the lee sides of hilly terrain in extreme bushfire contexts [46].

Controlled burning, another landscape strategy, also needs further research having received varied commentary. A recent publication, whilst questioning clearing burns deep within the forests, strongly promotes burns proximal to infrastructure: Arguing that lower fuel loads near housing reduces heat flux levels and fire spread, making fringe control more achievable [87]. Indigenous burning and land care practices have recently been acknowledged as effective through changing the density and species of forestation: opening the landscape, slowing down encroaching fires and reducing their intensity [88–90]. Only this latter approach would appear to have a broader influence upon ember attack reduction from extreme bushfire events—through potentially limiting the likelihood of such events. It is suggested here that these practices should be promoted as part of resilient and regenerative communities: as retrofitting beyond the boundaries of structure and personal land holdings.

### **8. Conclusions and Retrofitting Limitations**

As most previous writing on retrofitting suggests [57,59,61,62], such actions have their limitations: many alluded to, or boldly stated, in the sections above. Not least is the cost. In 2017 Penman et al. [91] found that preparing occupants and homes for bushfires ranged from AUD 8500 to as high as AUD 47,000. With 2020 construction industry annual price growth being 3.6% [92], and CPI averaging 2% over the past 4 years [93], a rise of approximately 10% on these prices is reasonable: i.e., the contemporary cost for retrofitting approximates AUD 9350 to AUD 51,700. To which end of that spectrum any retrofit comes from being totally dependent upon the depth of actions taken.

Cost, however, is not the only limitation. Retrofitting is about working with what is, towards a frequently unobtainable ideal. In many cases, gaps will remain, likewise timber window frames and walls, and the tiles will remain tiles. Installing sarking behind existing cladding is a major exercise at which many will balk. Fitting out a house with appropriate spray heads, fire pumps and adequate water storage is full of complexities—and for some, it can be as simple as a question of aesthetics on a heritage home, foolish as that may sound.

This paper has sought to consolidate existing knowledge surrounding retrofitting actions for Australian homes facing bushfire threats: augmenting that knowledge through inferences drawn from otherwise uncorrelated fields. Additionally, the concept of retrofitting has been expanded beyond singular structures, conceptualizing its role in resilient communities and collective defense strategies. In offering directions for further research, the influence of the immediate landscape and other structures has been included. The purpose of retrofitting has also been broadened to consider sheltering occupant's air quality in light of recent evidence showing significant mortalities, hospitalizations and in utero risks from smoke inhalation. More significantly, the work expands our knowledge of why homes are open to ember attacks in the first instance. In so doing, a pressure differential hypothesis has been explored using the existing engineering and modeling data. This data suggests that such a hypothesis has a level of merit that should be pursued through detailed engineering modeling and practical testing. At a minimum, the existing data strongly suggests attention to retrofitting actions that may counter the influences, however minor, of these potential pressure differentials upon glazing, claddings and openings. This, because such influences, coupled with component movements brought about by extreme temperatures and wind forces, may create small openings through which embers and smoke may encroach upon a structure's interior, leading to its demise.

**Funding:** This research received no external funding.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The author declares no conflict of interest.

### **References**

### *Article* **Camera-Driven Probabilistic Algorithm for Multi-Elevator Systems**

### **Yerzhigit Bapin 1,\*, Kanat Alimanov <sup>1</sup> and Vasilios Zarikas 1,2**

Received: 31 August 2020; Accepted: 10 October 2020; Published: 24 November 2020