How Hot Is Too Hot?
Structural assessment of fire-damaged buildings reveals what may rise from the ashes.
So far this year, the topic of wildfires has gained national attention across the U.S. In May, the Springs Fire in California threatened 4,000 homes and destroyed 24,250 acres. In June, the record-breaking Black Forest Fire in Colorado destroyed 14,280 acres and about 500 homes and caused an estimated $85 million in damages. The devastation from the unprecedented Rim Fire that burned in and around Yosemite National Park in August and September is still being calculated, but it already ranks as one of the worst wildfires in history.
These disasters easily capture the attention of many, given the resulting devastation to a particular community. In contrast, structure fires actually occur on a much larger scale but seem to escape national attention, perhaps due to their frequency. According to the U.S. Fire Administration, an estimated 449,900 commercial and residential structural fires occurred in the U.S. in 2011, causing about $9 billion in damages. Most of those fires, more than 80 percent, were residential structures.
After a structural fire, engineers often are called upon to determine which portions of the structure can be reused and what needs to be replaced. The ability to reuse components or systems, like the foundation, greatly impacts the time required to rebuild. The answer to the question of what can be reused requires an assessment of the mechanics of the structure and a review of the prevailing building codes.
To assess the mechanics of the structure, one must consider the effects that heat has on common structural elements: concrete, wood, steel, and masonry. Though the effects of heat on different building materials are well understood, the actual heat that occurs inside a particular structure fire depends on a number of factors. Investigators turn to specific visual indicators to provide clues to the conditions to which the building materials were subjected.
Wood. Wood members begin to degrade with prolonged exposure to temperatures in excess of 150 F and it experiences significant degradation at temperatures ranging from 392 F to 580 F. The amount of degradation is dependent on the time of exposure. Because structural fires are generally short-lived (measured in hours rather than weeks), the effects of exposure to heat on wood members are generally limited to the charring of the wood. Depending on the species, charring typically occurs between about 500 F and 600 F. Because the charred portion of the wood has no residual strength, the effective section properties of the member are changed. Surface charring usually can be removed by scraping or sandblasting, and the member can be evaluated for its residual strength. If necessary, the damaged members can be “sistered” with other wood or steel members.
Concrete. Concrete is an excellent fire-resistant building material due to its low thermal conductivity and high thermal capacity. It provides its own fire resistance, depending on the type of concrete and coverage of the rebar. The strength of the concrete member can be affected in two ways: the loss of compressive strength of the concrete and the loss of yield strength of the reinforcing steel (or relaxation of the pre/post-tension tendons). Because of its relatively low thermal conductivity, the concrete member often has a fighting chance for the fire to be extinguished before critical temperatures are reached.
Concrete can spall but otherwise remains generally unaffected at temperatures between 200 F and 300 F. Above 550 F, the concrete surface may begin to exhibit surface crazing and aggregate pop-outs just below 1,100 F. Crazing is the result of shrinkage of the surface of the concrete due to rapid dehydration that forms fine, shallow, and structurally insignificant cracks.
Temperatures of about 550 F can cause a distinctive pink discoloration of concrete with certain mineral content, making the affected regions easy to identify. This is not true with all concrete, so care should be taken when relying on color indicators. The effects of temperature on strength depend greatly on the mineral makeup of the concrete. Concrete with siliceous aggregate can lose about half of its strength at 1,200 F, but carbonate (limestone) and lightweight concrete will retain most of its strength at this temperature.
Steel. Structural steel members, like beams and columns, typically retain around 80 percent of their full yield strength up to about 930 F. Steel has a high coefficient of thermal expansion, and it begins to distort significantly at about 600 F. This creates a condition in which the member may be structurally sound but so distorted that it is no longer serviceable.
Another concern with structural steel is whether it was rapidly cooled during fire suppression, making the member more brittle. Steel embedded in concrete (reinforcing steel) also will retain its yield strength up to about 1,000 F, but can begin to spall the concrete at lower temperatures due to thermal expansion.
Post-tension tendons are installed in forms, and concrete is placed around them. When the concrete has reached a certain compressive strength, the tendons are stressed. In theory, the majority of this stress remains in the tendons through the life of the facility. The tendons “relax” when they lose this tension. Post-tension tendons embedded in concrete are the most sensitive to heat because they begin to relax at about 600 F before their strength is affected at 1,000 F.
Masonry. Clay masonry will typically retain most of its strength with exposure to temperatures of up to 1,800 F. The mortar between the masonry units may lose strength at 500 F to 700 F, but usually this loss of strength is limited to the surface of the mortar. Cracking also is possible with rapid cooling associated with fire suppression. The reinforcement inside grouted cells in structural masonry will be affected by the temperatures mentioned above.
Armed with the knowledge of the physical effects of extreme heat on structural members, the investigation would seem to be pretty straightforward. Unfortunately, unless the structure is equipped with thousands of thermal couples and a data collection system, the actual temperatures experienced are closer to a guess than a known fact.
What can be done to make it an educated guess? Begin with a visual assessment, looking for clues mentioned above and indications of heat based on the known melting point of other materials. This usually requires at least two trips to the loss location.
Preferably, the first site visit would be conducted quickly after the fire is extinguished and when deemed safe to note the condition of glass, plastic, carpet, metal, and other items in the debris. For example, melted glass would be indicative of temperatures in excess of 1,100 F. Additionally, copper pipes melt at 1,980 F, aluminum window frames melt at about 1,200 F, and undamaged polyethylene around a drain pipe might indicate temperatures at the concrete slab-on-grade were less than 250 F.
A second trip occurs after the site has been cleaned up to observe the condition of the structural components. Once debris has been removed, the condition of the slab, beams, columns, joists, and roof/floor deck can be observed. On this trip the investigator is looking for surface crazing, spalled concrete, discoloration, exposed reinforcing steel, distorted members, and excessive cracking, all of which can be indicators of the temperatures the components were exposed to and of structural distress.
In the case of a post-tension slab, the concrete in selected areas may need to be removed down to the tendons to observe the condition of the plastic sheathing on the tendons as an indication of whether they have reached a temperature where they may have relaxed. Concrete can be cored and tested for compressive strength verification and samples sent for petrographic analysis. Petrography is a specialized laboratory macro and microscopic examination of a concrete sample. This examination can shed light on the temperatures that the concrete was likely subjected to, mix information, mineral makeup of the aggregate, crystalline structure, and microscopic damage.
Worth the Investment
A post-tension slab was all that remained after a structure fire destroyed the clubhouse of a manufactured home neighborhood in central Texas. The building owner requested that a structural engineer evaluate the slab for the appropriateness of its reuse. The evaluation occurred after demolition of the remaining structure and cleaning of the property.
The fire reportedly started in the attic where files had been stored, and eventually burned through the ceiling and on top of the slab for a short time before the fire department extinguished the fire. The above-slab structure was a total loss.
The visual assessment of the slab revealed that no spalling or discoloration of the concrete had occurred, with the exception of a small area (less than 20 square feet) of discoloration below the reported point of origin (Photo 1). The lack of spalling and discoloration indicates that the temperature at the surface of the slab was probably less than 300 F to 500 F, which is less than the expected relaxation temperature of the post-tension tendons. Because the aggregate used in the concrete was limestone, the compressive strength of the slab would not be expected to be affected at these temperatures.
To further confirm the likelihood of temperatures at the surface of the slab, observations were made of wood expansion joints, under-slab plastic drain lines using cameras, and the polyethylene sheathing around the plumbing (Photo 2). Lack of heat-related distress to these components support the assertion that the actual temperature experienced at the surface of the slab was much less than would be expected to affect the compressive strength of the concrete or the tension in the tendons. This slab was deemed suitable for reconstruction.
Every year, structural fires result in property losses valued in the billions across the U.S. A thorough assessment of the condition of the structure can identify structural components or systems that can be reused, ultimately returning the building into service in a timely, cost-effective, and safe manner.