This is the second part of a 3-Part series to help insurance adjusters during a claim inspection to make a post-fire assessment of a building’s structural framing system. Part 2 investigates and assesses the future use of common post fire structural framing elements such as steel, concrete, masonry and wood. These more common structural elements take on different and specific characteristics when they are exposed to a fire. It’s important for the adjuster to make reasonable, cost saving assessments on what remains, what is to be repaired, what gets demolished and what gets replaced.
Fire attacks each of these materials in different ways with varying results depending on how much heat and exposure the members have endured. It is essential that a fire analysis be undertaken to determine the characteristics, nature and extent of the fire damage prior to doing an assessment. Below is a review of the structural framing member materials to investigate and assess.
More times than not, just by looking at individual steel framing members and their connections for distortion and deformation, specific measurements to determine dimensional changes can be obtained to verify configuration and size of the framing member. Distortion and deformation include items such as twisting (an out of plane bending) and sagging (over deflection) of beams and joists, plumbness and verticality checks of columns, straightness and bowing of horizontal beams, fractured and displaced connectors and lateral buckling of restrained bearing plates. For most residential structures, 12-inch-deep and smaller steel beams with a ¼ inch or less twisting of the beam is allowed per the American Institute of Steel Construction (AISC) as an installation tolerance. If, however, the amount of observed twist is larger than this, further testing of the steel member is warranted to determine if potential replacement is needed. Light-gage steel members, such as studs, joists and metal truss plate connectors, are highly susceptible to deformation and, if exposed to high temperatures and will more than likely require replacement.
Of the steel’s mechanical properties, hardness testing is recommended to determine the strength and stiffness of the affected structural member. Because the stress-strain behavior of steel is rather predictable as long as the material stays within the elastic range, elastic deformation is reversible. However, once the deformation continues into the plastic range, it cannot be recovered back to its original configuration and must be replaced. Steel framing members can generally tolerate heat up to 550°F without loss of strength but will decline rapidly after 750°F. For example, steel exposed to 1000°F will only retain approximately 60% of its original yield strength and 45% of its stiffness during the fire. Cooling of heated framing members from firefighting efforts also cause distortion and/or fractures (embrittlement or loss of ductility) of members and connections. When carrying out an inspection of a fire damaged building, it is recommended that as part of your focus, inspecting the framing connections for cracking of welds, fracturing of bolts, distortion of end bearing plates and failure of connectors, etc. is required.
As a general rule for fire affected structural steels, if the steel is straight and there are no obvious distortions, then it was probably not heated beyond 1000°F, would not have undergone any metallurgical changes and is probably still fit for use. However, in practice and as mentioned above, it is recommended that, in most instances, some hardness testing should be carried out. Where deflections are visible, the building code has standards on maximum permissible levels of deflection to ensure satisfactory performance. The amount of deflection or distortion must be checked so that its effect under load can be calculated to ensure that the functioning of the structure is not impaired. Therefore, every building should be considered as a separate case and at some point a structural engineer should be involved in the reinstatement exercise to decide what level is acceptable to satisfy the relevant Codes.
The main factor for determining fire damage to concrete framing members is heat exposure including the nature and extent of the fire. Visually, an assessment starts with depicting locations of fire patterns, cracking, spalling and discoloration of the concrete surfaces. In typical residential and commercial fires with limited burn times, structural concrete components typically do not sustain significant structural damage. Although in concrete members, fire and heat can have a damaging effect on the compressive strength and modulus of elasticity of the concrete materials. Typically, the strength of structural concrete members is not affected by temperatures below 550°F. It is important to know where the hot spots from a fire situation are in order to properly assess the structural concrete framing members including their bearing levels and other points of interest. Similar care should be taken when inspecting concrete foundations for anchor bolt failure, cracking, surface spalling, etc. These conditions can also be affected by the cooling efforts from firefighting equipment and use of water.
A color assessment of concrete elements provides a general guide of temperatures to help determine areas where a fire exceeded 550°F. A pink to red surface color indicates temperatures between 550°F and 1100°F, black through whitish gray indicates temperatures from 1100°F to 1700°F and buff or beige over1700°F. The main item for concern is the concrete steel reinforcement within the slabs, beams or walls. Concrete’s compressive strength varies not only with temperature but also with a number of other factors, including the rate and duration of heating, whether the specimen was carrying designed building loads or not, among other concrete material characteristics. In general, concrete heated by a building fire always loses some compressive strength and continues to lose it upon cooling from air temperature or water treatment. Compression tests of core samples can indicate the strength of the concrete, yielding a value for use in calculations if required. In addition, most fine cracks are confined to the surface. Major cracks that could influence structural behavior are generally obvious. A wide crack or cracks near supports may mean there has been a loss of anchorage of the reinforcement.
The affected surface of the concrete should be removed down to this red or pink boundary to determine the depth of material remaining in good condition that can carry structural loads. In addition, any cracks or spalling in the concrete should be analyzed and a determination made on their effect on the structural performance. Typically, any mild steel or hot-rolled, high-yield steel reinforcement will retain its original properties unless high temperatures have occurred to such an extent that the steel has distorted. In some instances, it may be appropriate to take “soundings” on the concrete with a hammer and chisel. A ringing noise typically indicates sound concrete, while a dull thud typically indicates weak material. Once the extent of damage and concrete properties are determined, calculations can be performed to evaluate the load-carrying capacity of the remaining sections for repair versus replacement decisions. In all instances where concrete is exposed to fire, a professional engineer should evaluate the structural members to determine the extent of damage, perform calculations and provide repair or replacement plans for the damage.
Structural masonry members, such as concrete-masonry-units (CMU), modular bricks and structural clay tiles, have similar characteristics and properties to that of concrete. However, examination of these members is typically more difficult than for concrete members because of the variability in the construction of these members. For example, the location of grouted cells in a masonry wall may not be easily verified. Likewise, verification of reinforcement in these cells is also not easily verified without some advanced form of non-destructive testing. For masonry structural framing elements, the physical properties and mechanisms of failure when exposed to fire have never been analyzed in detail. Behavior is generally influenced by edge conditions and if there is a loss of compressive strength as well as unequal thermal expansion of the block, brick or tile’s two exposed faces. For solid brick, resistance to the effects of fire is directly proportional to thickness. Modular or perforated bricks and hollow clay units are more sensitive to thermal shock. There can be cracking of the connecting webs and a tendency for the wythes to separate. Exterior walls can be subjected to more severe forces than internal walls by heated and expanding floor slabs. Brick or block walls with plaster applied give much better performance that improves the insulation value of the material and reduces thermal shock.
As with concrete, it is possible to determine the degree of heating of the masonry wall from the color change of the mortar and masonry units exposed to extreme heat. Close examination of the mortar between masonry members should be performed. If such discoloration exists, and the mortar is weakened, then replacement of this portion of the masonry units may be warranted, unless more accurate non-destructive or destructive testing is provided. For solid brick walls without undue distortion, the portion beyond the pink or red boundary may be considered serviceable and calculations should be made accordingly. Modular brick and hollow block or clay tile walls should be inspected for the effects of cracks indicating thermal shock.
Fire damage to masonry walls can also include a reduction in the fire resistivity of the materials, which may be necessary for fire rated separation walls. For non-structural masonry members, such as brick veneer or interior non-load bearing masonry partition walls, replacement may be warranted for members that have excessive smoke damage or severe damage to the mortar. Cosmetic repairs should be fully investigated for these non-structural items.
Wood members are typically defined as either dimensional lumber or manufactured lumber. For dimensional lumber, the fire-damaged wood members are either charred or completely destroyed. The destroyed members require complete removal of the remaining pieces and replacement with members of similar properties to the original members or with new members that meet the load requirements of the most recently adopted building code in effect. The charring of wood members is more challenging, because the depth of charring will need to first be verified. If the charring is limited the member may be deemed sufficient for re-use and left in place. If the charring is sufficient to require repair, then either the member will need to be removed and replaced or a new wood member sistered alongside the damaged member and installed to carry the design loads.
Manufactured lumber members are typically manufactured utilizing glue or other resins to form the member. Examples of these products include glue-laminated and laminated-veneer products, oriented strand board (OSB), plywood and pre-engineered floor joists. Because of the use of resins, these products present different issues than does the dimensional lumber used in framing. Typically, removal of repetitive members, such as joists and wood decking, is warranted when these members are exposed to high temperatures from fire events. However, the larger beams, girders and columns will generally warrant further investigation to determine a repair or replacement. If a wood deck is removed and replaced, the deck must be replaced with materials of the same thickness and diaphragm capacity, and the nailing must be able to resist the design lateral loads as specified in the most recently adopted building code.
During fire-suppression efforts, portions of floor and roof decks are typically removed to release the heat and smoke from the fire event. These portions must be replaced with the same thickness deck, and usually requires replacement of the deck to the supporting members with sufficient nailing to resist design lateral loads. Where possible, replacement with full sheets is more easily installed than for partial sheets. The use of pre-engineered trusses is common in residential and commercial buildings. When such products are used, an engineer must evaluate whether the connections were damaged from the heat or if only the wood members were damaged. The typically used metal plate connectors for pre-engineered trusses are very susceptible to damage in fires and the connections can be weakened in a fire event. When the damage is localized to a single connection or only a few members of the truss, the trusses can be repaired in place. When significant damage exists to the trusses, it is best to remove and replace the trusses.
Knowing the pre-fire and post-fire characteristics of structural building materials can be a difficult and challenging task when assessing how a fire affects these building materials. Let the experts at Warren investigate and assess the extent of structural damage from fire insurance claims to any home, business or building. Part 3 – Observations for Reuse, Salvage and/or Repair of post-fire damaged buildings continues with providing the adjuster some basic knowledge and insight to properly assess the post-fire structural integrity of a building and to determine any cost saving repairs.
Allan Abbata is a senior consulting engineer at Warren and a licensed professional engineer in South Carolina, North Carolina, New York, New Jersey, Pennsylvania, Massachusetts, Missouri, Texas, Alabama, Maryland, Minnesota and Virginia. Allan holds a Bachelor of Science in Civil Engineering. He has more than 45 years of applied engineering expertise to include in-depth knowledge of building codes, rules and regulations that guide design. Allan has also prepared construction drawings and specifications, provided on-site supervision and inspection of construction projects, and. has overseen project management and responsibility for overall performance of building contracts while also serving as the client’s liaison with local, state and federal agencies and municipalities.