By WiD Guest Blogger
Matthew T. Blackmer, M.S.C.E., P.E. & Pie Staff
Pie Consulting & Engineering
Anomalistic weather is becoming the norm, causing an increasing number of billion-dollar-plus disasters annually. In 1980, the National Oceanic and Atmospheric Administration (NOAA) began reporting an average of three to four of these major storms a year, but the numbers have increased exponentially over the past several years. In 2011, there were a record 14 disasters of this magnitude. And on top of this virtually every federal, state and forest service agency across the nation forecasted that 2012 is very likely to become a record year for extreme wildfire activity in the western and southwestern U.S., with the future heralding a year round state of danger. And it has proven to be the case already. Just recently wildfires were burning in nine U.S. states, already inflicting heavy property damage.
It is important for the insurance and restoration industries to understand the exact type of material damage that can occur tot residential and commercial structures during fire and fire suppression events. Most of these structures are composed of wood, concrete, steel (structural and light-gage) and masonry. However, a combination of these materials is typically present in all existing buildings, so different approaches are required for the different materials present at the site.
Wood – Wood members are typically defined as either dimensional lumber or manufactured lumber. For dimensional lumber, the fire-damaged wood members are typically 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 amount of charring will need to first 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 utilizing glue or other resins to form the member. Examples of these products include glue-laminated (“glu-lam”) members, laminated-veneer products (“LVL’s”), 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 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. The typically used “press-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.
Concrete – In typical residential and commercial fires with limited burn times, concrete structural components typically do not sustain significant structural damage. Although concrete is inherently resistance 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 300º Celsius (572º Fahrenheit), the concrete can be repaired “in-place” without removal and replacement. Above this temperature, physical and chemical changes can occur in the concrete that may warrant 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, the affected surface of the concrete should be removed 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 determine 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 their effect on the structural performance. Typically, any mild steel or hot-rolled, high-yield steel reinforcing 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 sufficient 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. Other non-destructive testing methods such as the Schmidt hammer and ultrasonic testing could also be considered. Once the extent of damage and concrete properties are determined, calculations can be performed to determine 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, a repair “in-place” approach may be necessary, even though the repair could be extensive. Where in-place methods are chosen, concrete or shotcrete can be utilized 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 utilizing 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 damage, perform calculations and provide repair or replacement plans for the damage.
Steel – Structural steel can be physically altered at temperatures of approximately 800o Fahrenheit, so it is imperative that steel members be evaluated immediately after fire events. Typically, the steel member will have a distorted appearance and some discoloration when damaged by high temperatures. For most residential structures, 12 inch deep and smaller steel beams are utilized at interior locations, and ¼ inch or less “twisting” of this 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 will be warranted, with the need for potential replacement. Also, bolted connections are susceptible to damage at temperatures of less than 1,000o Fahrenheit, so these connections should be investigated for damage. Light-gage members, such as studs, joists and metal connectors, are highly susceptible to fire damage and, if exposed to high temperatures, will require replacement. Light-gage straps that are typically attached to the outside face of the wall sheathing, utilized as shear wall hold-downs, should be observed; replacement of these damaged items is critical at high seismic areas.
Masonry – 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. Verification of bar reinforcement in these cells is also not easily verified without some advanced form of non-destructive testing (it is not uncommon for buildings that were constructed prior to the early 1970s, outside of high seismic areas, to not have vertical reinforcement). A useful guide in determining the damage at CMU walls and pilasters is to observe whether the material is a different color than for portions of the wall that were 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 resistivity 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 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.
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.
Concrete Society Technical Report No. 33, Assessment and Repair of Fire-Damaged Concrete Structures, The Concrete Society, 1990.
U.S. Department of Housing and Urban Development, Appendix A – The Effects of Fire on Structural Systems, February 2000.
Robert S. Vecchio, PhD, P.E., Condition Assessment of Steel Structures, Structure Magazine, November 2006.
National Fire Protection Association, Beverly Hills Supper Club Fire, Southgate, KY, May 28, 1977, Fire Journal, January 1978.
James E. Amrhein, Reinforced Masonry Engineering Handbook, 5th Edition, Masonry Institute of America.
Gillian Flacus. “1500 homes lost; $1B loss in San Diego area.” http://seattletimes.nwsource.com/html/nationworld/2003971082_wildfires24.html, Seattle Times, October 24, 2007.
Insurance Information Institute, http://www.iii.org/media/facts/statsbyissue/wildfires/