Post-Fire Structural Evaluations
Buildings involved in a fire may have damage to the building’s structural integrity. The intensity and duration of heat or direct flame impingement can cause damage to concrete, steel and wood components that reduces the remaining strength of the structural component below the threshold of safety. Post-fire structural evaluation by
a forensic engineer is necessary to determine the extent of damage and determine the repair or replacement options to be considered.
A post-fire structural evaluation consists of determining whether the structural components of a building involved in a fire remain adequate to meet their original design intent. This evaluation can involve both destructive and non-destructive testing of concrete, steel and wood components. Depending on the extent of damage and temperature of the fire, components may need to be cleaned to remove soot and other contaminates in order to allow visual analysis. The manner in which areas are cleaned is dictated by which cleaning method will cause the least collateral damage. The components may be cleaned using abrasive blasting (sand, baking soda or glass), pressure washing, or chemical washing.
A visual examination of collateral items, such as lamps, door handles, plumbing items, windows, and plastics can help shed light on the characteristics of the fire. Having an idea of the fire’s temperature can help in evaluating the duration and intensity of the fire, which in turn, can help determine potential damage to surrounding structural items.
Concrete exposed to fire can be expected to remain unaffected by temperatures up to 500˚F. When exposed to temperatures over 500˚F, concrete is subject to cracking or spalling. Spalling is the fragmentation or the delamination of the concrete surface. At these temperatures, heat from a fire can vaporize water within the concrete, resulting in forces that expand the concrete.
An easy test method involves tapping on the concrete with a hammer. If the concrete emits a ring, that indicates the concrete is still in good condition. If the concrete emits a deep thud, further testing is required. Another test is the use of a rebound hammer to compare the strength of one area of concrete to another. A rebound hammer is a device that exerts a force onto a concrete surface and measures the rebound off the concrete. The rebound is a method by which to measure the relative concrete strength.
Although destructive in nature, the most accurate method of testing concrete is by harvesting a core sample. Core samples are sent to a laboratory for testing to determine the concrete’s actual strength. The color of the concrete after exposure to fire is an indication of the level of damage. The harvested core can also be examined for color differences. For example, a pinkish-reddish color indicates that the concrete did reach temperature levels sufficient to affect its strength. Concrete that is a whitish-grey has lost most of its strength and also the bond with any encased reinforcing steel, if present. When examined after a fire, concrete with a pink tint or concrete that is white and chalky should be considered for replacement.
As the temperatures rise during a fire, structural steel can also become affected. Intense heat can weaken steel members such that they deform in response to the applied loads. After a fire, steel members should be examined for warping, buckling or deformation. Members should be checked for plumb and straightness. The extent of distortion plays a part in the decision as to which members are to be replaced.
Another issue that can arise with metal structural members, post fire, is contamination resulting from exposure to the products of combustion of materials that burned in the vicinity. For example, when some plastics burn, they emit chlorides as products of combustion, which can result in corrosion of the steel on contact. Structural steel suspected as having been exposed to corrosives should be tested. The usual approach is to take wipe samples of the suspected contaminant and then subject the samples to laboratory examination. Should aggressive contaminants be identified, a decision can then be made as to a responsive action. For instance, steel can be media blasted or pressure washed, and then perhaps given a surface treatment such as protective coating.
In certain building types, metal wall panels and metal roofing panels are secured to the building with fasteners that have rubber or neoprene washers. The washers act as waterproofing barriers for the fasteners. Due to the materials from which the washers are composed, damage can take place to the washers from minimal heat. The damage could consist of deformation, or melting. Once the washers are deformed or melted, a pathway of water intrusion has been created that will result in further damage to the building, over time. A visual inspection should take place to examine the washers. In certain circumstances, it may be beneficial to perform water testing on the washers by discharging water on the washers while examining the interior of the building, at each location.
Determining the extent of fire damage to wooden members involves visual inspection. As wood chars, the outer layers of the wood are consumed. The wood member is now reduced in size, and consideration has to be given to whether there remains sufficient integrity to meet original design intent.
When it comes to damage assessment of structural wood, concrete and steel components after a fire, there may be more to the job than meets the eye. Testing may be appropriate to identify and quantify the extent of damage. With this information in hand, repair and replacement alternatives can then be considered.