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New tests being conducted are showing that innovative FRP composites have the potential to strengthen structures. What will this mean for the buildings of the future?
By Zachery I. Smith and Edward R. Fyfe, Fyfe Company IN THE PAST FIFTEEN YEARS, FIBERreinforced polymers (FRP) have been used successfully to add considerable blast-resisting capacity to a number of structural elements (for example, columns, walls and slabs). Until recently, the testing of single degree of freedom validation and use of FRPs for force protection has been focused on local component enhancement. Whether this included adding flexural capacity to slabs for uplift pressures, increasing shear capacity in bearing columns or adding flexural capacity to CMU walls for out-of-plane induced blast pressures, it was not focused on global structural enhancement. Progressive collapse retrofit studies take a wider view of the structure. As defined by the U.S. General Services Administration (GSA), progressive collapse is &quot;a situation where local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse. Hence, the total damage is disproportionate to the original cause.&quot; This article looks at new testing and recently completed projects that use FRP composites in combination with innovative FRP composite anchors to establish continuity at beam-column connections, improving catenary action in the event a vertical component is lost. This retrofit scheme reduces the potential for progressive collapse and could be the best design solution for retrofitting existing structures vulnerable to failure because of the as-built reinforcement discontinuities. More importantly, this work broadens the scope of influence of FRP composites from local component strengthening in years past to more global structural strengthening through alternate load paths and/or structural redundancy.
article will focus on the alternate load path procedure. With new construction, there are several conventional steps that an engineer can take to add structural integrity to a new building. The real challenge comes from engineering structural retrofits for existing reinforced concrete and masonry structures. Trying to add redundancy with conventional materials via alternate load paths to cross over failed vertical components in existing buildings, without adding significant structural members and impeding usable space or aesthetics, is challenging to say the least. Reinforced concrete buildings and masonry structures have similar vulnerabilities, in that their discontinuities make them susceptible to progressive collapse. The lack of continuity of the bottom reinforcement at the beam-column connection (reinforcement cutoff ) makes them especially vulnerable.
line, and at mid-span in the negative moment regions. Test specimens were 30 feet long with a cross section of 6 inches wide and 12 inches deep (FIGURE 2). It should be noted that these structural tests neglected any support a typical building would provide via the surrounding slab, column line above, or phenomena such as the Vierendeel truss action. Various configurations of CFRP were applied to retrofit the beams for continuity, so they could develop catenary forces. Generally, combinations included CFRP strengthening to the underside of the beams, with fiber anchors going through the stub column section for positive moment reinforcement or CFRP strengthening on the top side for negative moment strengthening. The CFRP system used was a unidirectional primary carbon fiber reinforced polymer with a published tensile modulus of 11,900 ksi and measured per ASTM D3039 to have a tensile modulus of 12,500 ksi. The specified concrete and steel were fairly standard strengths, at 4,000 psi and 60,000 psi respectively.
Restraints for the test specimens were provided to simulate vertical, rotational and axial factors. The end restraints provided compression and tension supports. The loading was accomplished with three loading points at mid-span and six feet on either side of mid-span. Axial resistance was also provided by a braced frame on both ends of the specimen. Note that for logistical ease the beams were inverted for testing. All specimens had instrumentation for three load cells, five displacement inducers and numerous strain gauges on the CFRP and steel reinforcing bars. developed at the ends and on either side of the column. As the applied load was continually increased, high deflections put the beam in catenary action, when the maximum vertical load reached 52 percent of the prescribed progressive collapse resistance. The load carried by catenary action of the existing steel reinforcement was nearly twice the load carried before the plastic hinges developed, but still half the required resistance to prevent progressive collapse. It should be noted that the test specimens were designed to prevent a brittle shear failure, which could cause a premature failure in an actual structure (FIGURE 3). However, FRP could also provide a substantial amount of shear capacity to an existing beam section to prevent such a brittle shear failure. This was not the intent of the test program, since FRP's ability to add beam shear capacity has been demonstrated numerous times in the past.
The retrofitted specimens can be divided into two categories, those strengthened in the positive moment region and those strengthened in the negative moment region. Both retrofit schemes have the same objective--to develop catenary forces.
the enhanced progressive collapse with CFRP in the positive moment regions may only be accomplished if the designer ensures that the rotational ductility in the concrete section is sufficient to reach catenary action.
The negative moment retrofits are simpler logistically because they do not require developing the carbon fiber anchor through the column section as with the positive moment retrofits (FIGURE 5). The testing also showed that since the CFRP laminates are developing the negative steel reinforcement, they allowed hinges to form away from the sections with limited rotational ductility and avoided fracturing the steel rebar before catenary action was realized. Hence, this proved to be the most successful retrofit strategy for reaching the prescribed progressive collapse resistance with one specimen reaching 108 percent of the progressive collapse resistance target. We'd like to clarify that for this test program, the summary percentages refer to the strength provided relative to total resistance needed to prevent progressive collapse per GSA guidelines.
limited to strengthening walls locally for out-of-plane pressures and possibly spall control. Here the scope of the CFRP composites was broadened to strengthen the entire structure to withstand a threat through added structural redundancy versus trying to strengthen one component against local failure.
In the winter of 2009, a U.S. Army barracks required a complete renovation, which included structural issues related to anti-terrorism/force protection (AT/FP) requirements. To satisfy the progressive collapse resistance requirements, in case the building lost a masonry pier, the engineer-of-record selected the alternate load path design method, which would strengthen the exterior masonry beams to take up the added loads. Thus, the existing masonry beams had to be designed and strengthened to span a length that is twice the span length of the original design (FIGURE 6). This was accomplished by adding CFRP composites