Wood members are typically defined as either dimensional lumber or manufactured lumber. For dimensional lumber, the fire-damaged wood members are usually either charred or completely destroyed. The destroyed members require complete removal of the remaining pieces and should be replaced with members of similar properties to the original or with new members that meet the load requirements of the most recently adopted building codes. The charring of wood members is more challenging, because the amount of charring will first need to be verified. If the charring is limited and the engineer deems the member to be sufficient for re-use, then the member can be 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 can be added to carry the design loads.

Manufactured lumber members are typically manufactured using glue or other resins to form the member. Examples of these products include glue-laminated or “glu-lam” members, laminated-veneer products, oriented strand board, plywood, and pre-engineered floor joists. Because of the use of resins, these types of products present different issues than 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 the repair. 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 wood 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. Typically used for pre-engineered trusses, “press-plate” connectors 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, it is best to remove and replace them.

Concrete Evidence

In typical residential and commercial fires with limited burn times, concrete structural components typically do not sustain significant structural damage. Although concrete is inherently resistant to fire and does not “burn,” fire and heat can have an effect on the compressive strength and modulus of elasticity of the concrete materials. Typically, where the temperature from the fire event has not exceeded 572 degrees Fahrenheit (300 degrees Celsius), the concrete can be repaired in place without removal and replacement. Above this temperature, physical and chemical changes can occur in the concrete, warranting complete removal and replacement. Based on testing and burn rates of other materials such as wood, the heat of the fire at the structure can be estimated to determine the extent of likely damage to the concrete materials.

When a fire takes place, the depth of concrete will have a characteristic change in which there is a pink or red zone of influence resulting from the fire. When analyzing fire damage to concrete, inspectors should remove the affected concrete down to this red or pink boundary to determine the amount of material remaining in good condition that can carry structural loads. Core samples should be considered to gage the strength of the remaining concrete for use in structural calculations. In addition, any cracks or spalling in the concrete should be analyzed and a determination made on its effect on 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 been distorted.

In some instances, it may be sufficient to take “soundings” on the concrete with a hammer and a chisel. A ringing noise typically indicates sound concrete, while a dull thud typically indicates weak material. Other non-destructive testing methods such as the Schmidt hammer and ultrasonic testing method should also be considered. Once the extent of the damage and concrete properties are determined, an analyst can perform calculations to establish the load-carrying capacity of the remaining sections for repair-versus-replacement decisions.

Where a single concrete member exists that does not support a significant amount of other members, the removal and replacement of the material may be the most cost- effective and practical solution. However, if the concrete member is placed monolithically with other members, or if the concrete member supports a significant amount of other structural members, an in-place repair approach may be necessary, even though the repair could be extensive. When in-place methods are chosen, concrete or shotcrete can be used to increase the size of the sections as necessary to provide the required strength to the member. Cracks in the structural members can be repaired using epoxy-injection, and spalling can be surface-patched with repair mortar. In all instances where concrete is exposed to fire, a professional engineer should evaluate the structural members to determine the extent of the damage, perform calculations, and provide repair or replacement plans for the damage.

Steel and Masonry Take the Heat

Structural steel can be physically altered at temperatures of approximately 800 degrees Fahrenheit, so it is imperative that steel members be evaluated immediately after fire events. The steel member may have a distorted appearance and some discoloration when damaged by high temperatures. For most residential structures, 12-inch deep and smaller steel beams are used at interior locations, and a quarter inch or less “twisting” of this beam is allowed by the American Institute of Steel Construction as an installation tolerance. However, if the amount of observed twist is larger than this, then further testing of the steel member will be warranted, with the need for potential replacement. Also, bolted connections are susceptible to damage at temperatures of less than 1,000 degrees Fahrenheit, so these connections should be investigated for damage. Light-gauge members, such as studs, joists, and metal connectors, are highly susceptible to fire damage. If exposed to high temperatures, they will require replacement. The light-gauge straps that are typically attached to the outside face of the wall sheathing and used as shear wall holddowns should be observed. Replacement of these damaged items is critical at high-seismic areas.

Structural masonry members, such as concrete-masonry unit (CMU) walls, have similar characteristics and properties to concrete. However, examination of these members after fire events 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 wall may not be easily verified. In addition, verifying the bar reinforcement in these cells is not easily accomplished without some advanced form of non-destructive testing. One should note that it is not uncommon for buildings constructed prior to the early 1970s, outside of areas with high seismic activity, to lack vertical reinforcement. A useful guide to determine the damage at CMU walls and pilasters is to observe if the color of the material differs from portions of the wall not exposed to extreme heat.

Close examination of the mortar between masonry members should also be performed. If such discoloration exists, and the mortar is weakened, then replacement of this portion of the structural masonry may be warranted, unless more accurate non-destructive or destructive testing is recommended. Fire damage to masonry walls can also include a reduction in the fire resistance of the materials, which may be necessary for fire separations walls. For non-structural masonry members, such as brick veneer or interior partition walls, replacement may be justified for members that have excessive smoke damage or severe damage to the mortar. Cosmetic repairs should be fully investigated for these non-structural items.

Careful evaluation of each of these structural elements involved in fire-damaged homes will ensure the safest and most efficient structural evaluation of any structure involved in fire losses.

Derek Pumphrey is a forensic engineer at Professional Investigative Engineers. He can be reached at 866-552-5246, [email protected].