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NIOSH Publication No. 2005-132:Preventing Injuries and Deaths of Fire Fighters Due to Truss System Failures |
May 2005 |
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Contents
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Preventing Injuries and Deaths of Fire Fighters due to Truss System Failures
Fire fighters may be injured and killed when fire-damaged roof and floor truss systems collapse, sometimes without warning.
The National Institute for Occupational Safety and Health (NIOSH) requests assistance in preventing injuries and deaths of fire fighters due to roof and floor truss collapse during fire-fighting operations. Roof and floor truss system collapses in buildings that are on fire cannot be predicted and may occur without warning. NIOSH recommends that fire departments review their occupational safety programs and standard operating procedures to ensure they include safe work practices in and around structures that contain trusses. Building owners should follow proper building codes and consider posting building construction information outside a building to advise fire fighters of the conditions they may encounter.
NIOSH requests that the information in this Alert be brought to the attention of all U.S. fire departments and fire fighters. To bring the recommendations in this Alert to the attention of the fire service community, NIOSH requests help from the following individuals and organizations: fire commissioners, fire chiefs, State and lo cal fire district administrators, State fire marshals, safety and health officials, trainers, fire inves tigators, unions, labor organizations, insurance companies, and editors of trade journals and other publications.
BACKGROUND
According to the Wood Truss Council of America (WTCA), wooden trusses are used in roof systems in more than 60% of all buildings in the United States [SBCMAG 2004]. Truss and related engineered wooden floor systems are also becoming more common. Today, more engineered structures use lighter weight materials, producing larger spans and clear openings. Trusses can be designed to carry expected loads, be produced economically, be safely handled, and reduce construction costs (see Figure 1).
Engineered building components may provide adequate strength under normal loading; but under fire conditions, these truss systems can become weakened and fail, leading to the collapse of roofs, floors, and possibly the entire structure. Truss systems are usually hidden, and fires within truss systems may go unnoticed for long periods of time, resulting in loss of integrity. Structural design codes often do not factor in this decreased system integrity, as fire degrades the structural members. Fire fighters typically rely on warning signs to indicate imminent truss failure such as roofs and floors that feel spongy or are visibly sagging. Quite often, these warning signs are not good predictors of truss system failures.
The United States Fire Administration (USFA) reports that during 1990-2000, structural fires and explosions accounted for 46.1% of all reported fire fighter fatalities (500 of 1,085) [USFA 2002]. Statistics compiled by the WTCA suggest that 4.7% of the total fatalities (108 of 2,286) during 1980-2001 were due to structural collapse [Grundahl 2003b]. Fifteen separate incidents investigated by NIOSH identified at least 20 fatalities and 12 injuries that have occurred from 1998-2003 during fire-fighting operations in buildings containing truss systems (see Appendix A).
What is a Truss?
Figure 1. Typical lightweight truss construction. (photo courtesy of Vincent Dunn) A truss can be defined as structural members (such as boards, timbers, beams, or steel bars) joined together in a rigid framework. They are most often in the shape of a triangle or series of triangles. Some trusses are rectangular. Trusses can be built of wood, steel, wood and steel, or aluminum. Concrete trusses are not common but do exist, usually in very large structures (see Appendices B and C for descriptions of different truss types). The truss framework is usually arranged in a single plane so that loads applied at points of intersecting members will cause only direct stress (compression or tension). Three-dimensional trusses (space frames) are very light in weight. The design of a truss, which separates compressive and tensile stresses, allows for a minimum of materials to be used, resulting in economic benefit.
The top and bottom members of a truss are called chords. The top chord of a truss is in compression, and the bottom chord is in tension. The inner members are called webs and give stability to the truss system. The unique characteristic of a truss is the inherent stability of the triangle. Web and chord members arranged in a triangle are much more stable than the same members arranged in a square. The square configuration requires diagonal bracing, which then produces multiple triangles.
Truss Types
Although many types of trusses exist, three typical truss construction methods are most commonly used:
Each of these construction methods is described in detail in Appendices C and D, along with causes of failure for each under fire conditions. CURRENT STANDARDSNational Fire Protection
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Figure 2. Truss loft with space for ventilation ductwork wiring ect. (photo courtesy of National Fire protection Association) |
Figure 3. Truss Placard-State of New Jersey; NJAC 5:70-2.20(a) 1 and 2 [NJAC 1992] |
The authors of this ALERT were Timothy R. Merinar, Richard W. Braddee, Frank Washenitz II, and Tom Mezzanotte, Division of Safety Research, NIOSH; Vincent Dunn, Deputy Chief (retired) New York City Fire Department; and Frank Brannigan, fire and building construction expert. The authors thank the following for their reviews of draft versions: Robert Solomon, PE, National Fire Protection Association; David Stroup, PE, National Institute of Standards and Technology; Rob Neal and William Troup, U.S. Fire Administration; Robert Bland, American Forest and Paper Association; Kirk Grundahl, Wood Truss Council of America; Chief Al Rosamond, Dallas Bay Volunteer Fire Department (representing the National Volunteer Fire Council); Chief Mark Young, Casper Fire Department (representing the International Association of Fire Chiefs); Rob Matuga, National Association of Home Builders; Pat Morrison and Elizabeth Harman, International Association of Fire Fighters.
Please direct any comments, questions, or requests for additional information to the following:
Dr. Nancy A. Stout, Director
Division of Safety Research
National Institute for Occupational Safety
and Health
1095 Willowdale Road
Morgantown, WV 26505-2888
Telephone: 304-285-5894; or call 1-800-35-NIOSH (1-800-356-4674)
We greatly appreciate your assistance in protecting the safety and health of fire fighters.
John Howard, M.D.
Director, National Institute for
Occupational Safety and Health
Centers for Disease Control
and Prevention
Brannigan FL [1988]. Are wood trusses good for your health? The safety issue of lightweight wood truss floor assemblies provokes controversy. Fire Eng 141 (6):73-79.
Brannigan FL [1999]. Building construction for the fire service. 3rd ed. Quincy, MA: National Fire Protection Association, pp. 517-563.
Brunacini A [1985]. Fire command. Quincy, MA: National Fire Protection Association, pp. 37-52.
Cotes A [1997]. Fire protection handbook, 18th ed. Quincy, MA: National Fire Protection Association, pp. 7-61, 7-63.
Cutter B [1990]. The fire and wood truss debate: solution . . . working together! Woodwords May:1-5.
Dunn V [1992]. Safety and survival on the fireground. Saddle Brook, NJ: Fire Engineering Books and Videos, pp. 113, 361.
Dunn V [1996]. Systems analysis size-up: Part 1. Firehouse Oct:18-21.
Dunn V [1998]. Risk management and lightweight truss construction. New York: WNYF, Official training publication of the New York City Fire Department, Vol. 1.
Dunn V [2001]. The deadly lightweight truss. Firehouse Jan:16-20.
Grundahl K [1991]. Fire performance of trusses reference guide. [ www.woodtruss.com]
Grundahl K [1992]. National engineered lightweight construction fire research project. Quincy, MA: National Fire Protection Research Foundation.
Grundahl K [2003a]. Publisher's message: facts on the fire performance of wood trusses. Structural Building Components Magazine June/July:10-15. [http://www.sbcmag.info/Archive/2003/jun/0306%20Publishers%20Message.pdf]
Grundahl K [2003b]. Cause of firefighter fatalities, 1980-2001. E-mail message to Tim Merinar (tmerinar@cdc.gov), December 22, 2003.
Klaene BJ, Sanders RE [2000]. Structural fire fighting. Quincy, MA: National Fire Protection Association, pp. 95-135.
Meeks JE [2001]. Truss plate performance in fire: one person's observations. Structural Building Components Magazine June/July. [ www.sbcmag.info/past/2001/01jun_jul/plateperformance.html ]
NFPA [2001]. NFPA 921: Guide for fire and explosion investigations. Quincy, MA: National Fire Protection Association.
NFPA [2002a]. NFPA 1521: Standard for fire department safety officer. Quincy, MA: National Fire Protection Association.
NFPA [2002b]. NFPA 1001: Standard for fire fighter professional qualifications. Quincy, MA: National Fire Protection Association.
NFPA [2002c]. NFPA 13: Standard for the installation of sprinkler systems. Quincy, MA: National Fire Protection Association.
NFPA [2003a]. NFPA 1620: Recommended practice for pre-incident planning. Quincy, MA: National Fire Protection Association.
NFPA [2003b]. NFPA 5000: Building construction and safety code. Quincy, MA: National Fire Protection Association.
NFPA [2003c]. NFPA 501: Standard on manufactured housing. Quincy, MA: National Fire Protection Association.
NIOSH [1998a]. Commercial structure fire claims the life of one fire fighter-California. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Fatality Assessment and Control Evaluation (FACE) Report 98F-07.
NIOSH [1998b]. Fire fighter dies while fighting warehouse fire when parapet wall collapses-Vermont. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Fatality Assessment and Control Evaluation (FACE) Report 98F-20.
NIOSH [1999]. NIOSH Alert: request for assistance in preventing injuries and deaths of fire fighters due to structural collapse. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 99-146.
NIOSH [2000]. Restaurant fire claims the life of two career fire fighters-Texas. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Fatality Assessment and Control Evaluation (FACE) Report F2000-13.
NIOSH [2001]. Roof collapse injures four career fire fighters at a church fire-Arkansas. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Fatality Assessment and Control Evaluation (FACE) Report F2001-03.
NIOSH [2002]. First-floor collapse during residential basement fire claims the life of two fire fighters (career and volunteer) and injures a career fire fighter captain-New York. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Fatality Assessment and Control Evaluation (FACE) Report F2002-06.
NIOSH [2004]. Partial roof collapse in commercial structure fire claims the life of two fire fighters-Tennessee. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Fatality Assessment and Control Evaluation (FACE) Report F2003-18.
NJAC [1992]. Identifying emblems for structures with truss construction. New Jersey Uniform Fire Code. New Jersey Administration Code 5 :70-2.20.
SBCMAG [2004]. U.S. Structural building components usage and market share statistics. Madison, WI: Structural Building Components Magazine. [ www.sbcmag.info/legislative/itcfinalreport/US_Structural_Building_Components_Usage_and_Market_Share]
Tapley B [1990]. Eshbach's handbook. 4th ed. New York: John Wiley and Sons, Inc.
USFA [2002]. Firefighter fatality retrospect ive study. Arlington, VA: U.S. Department of Homeland Security, Federal Emergency Management Agency, U.S. Fire Administration. FA-220.
USFA [2004a]. Structural collapse prediction technology research. Arlington, VA: U.S. Department of Homeland Security, Federal Emergency Management Agency, U.S. Fire Administration. [ www.usfa.fema.gov/inside-usfa/research/safety/nist1.shtm]
USFA [2004b]. Building performance awareness of lightweight construction during fires. U.S. Department of Homeland Security, Federal Emergency Management Agency, U.S. Fire Administration. [ www.usfa.fema.gov/inside-usfa/research/construction.shtm ]
The NIOSH Fire Fighter Fatality Investigation and Prevention Program
has investigated the following incidents that involved structures
containing truss construction. The complete NIOSH investigation
report for each incident can be obtained at www.cdc.gov/niosh/fire/.
Report ID |
State |
Truss Type |
Number of injuries and fatalities |
Event leading to death or injury |
98F005 |
Illinois |
Heavy timber |
2F*, 3I |
Backdraft |
98F007 |
California |
Heavy timber |
1F |
Roof collapse |
98F020 |
Vermont |
Heavy timber |
1F |
Roof / wall collapse |
98F021 |
Mississippi |
Lightweight wood |
2F |
Roof collapse |
99F002 |
Indiana |
Lightweight wood |
1F |
Roof collapse |
F2000-13 |
Texas |
Lightweight wood |
2F |
Roof collapse |
F2000-26 |
Alabama |
Lightweight wood |
1F |
Floor collapse |
F2000-43 |
Delaware |
Lightweight wood |
3I |
Fire spread through truss voids |
F2001-03 |
Arkansas |
Lightweight wood |
4I |
Roof collapse |
F2001-09 |
Wisconsin |
Heavy timber |
1F, 1I |
Roof / wall collapse |
F2001-16 |
Ohio |
Lightweight wood |
1F |
Floor collapse |
F2001-27 |
South Carolina |
Lightweight wood |
1F |
Roof collapse |
F2002-06 |
New York |
Lightweight wood |
2F, 1I |
Floor collapse |
F2002-50 |
Oregon |
Heavy timber |
3F |
Roof collapse |
F2003-18 |
Tennesse |
Lightweight wood |
2F |
Roof collapse |
*F=Fatality, I=Injury |
Triangular trusses are the most common trusses used in single-family dwellings. Triangular trusses provide a peaked roof. | Figure B-1 Triangular truss |
Scissor trusses are common in construction with cathedral ceilings. They are often found in churches. | Figure B-2 Scissor truss |
Parallel chord trusses provide a flat roof or floor. The top and bottom chords are parallel. They are commonly used in single-family dwellings, row houses, apartment buildings, and smaller office buildings. | Figure B-3 Parallel chord truss |
Bowstring trusses get their name from the curved shape of the top chord. Parapet walls may hide the curved roofline on large commercial buildings. | Figure B-4 Bowstring truss |
Inverted king/queen post trusses are used in place of support columns to provide open floor space under the truss. | Figure B-5 Inverted king/queen truss |
Heavy timber trusses are often engineered to provide large open areas-such as under a cathedral ceiling. The timbers in a heavy
timber truss are usually joined together by bolts that pass through the center of metal or steel plates. The most common connector is the split-ring metal connector that is embedded in prepared depressions on the face of the timber. The embedded plates are
used to transfer shear stresses and increase the load-carrying capacity of the bolted connection. Until the 1960s, the bowstring timber truss was one of the most common designs used in commercial construction and can be recognized by its curved top chord (see Figure B-4 in Appendix B). A classic example of a fire in a bowstring truss roof is the Hackensack, New Jersey, automobile dealership fire in 1988. Bowstring truss roofs are sometimes incorrectly described as arches or arched roofs [Brannigan 1999].
Engineered lightweight construction trends result in buildings designed and constructed using trusses manufactured from lumber (2×4, 2×6, or 2×8 inches)- where such trusses meet the engineering specifications and applicable building codes. Engineering and construction economics cause the design process to use the minimum-sized structural members necessary to support or carry the anticipated load. Engineered lightweight truss systems are the most commonly used truss systems in residential and single-family structures, and they are also used in many commercial buildings. This type of truss became popular in the early 1950s after the invention of the metal connector plate, also known as the gusset plate, gang-nail, nailer plate, or truss plate. This plate is used to rigidly connect the different wooden web members into various truss shapes.
The fastener is designed to connect the truss members by small teeth punched out of 20- , 18-, or 16-gauge galvanized steel sheets (see Figures C-1 and C-2). The teeth are hydraulically or roller pressed into the lumber so that the metal plate forms a bridge across the joint between the wooden web and chord members. These teeth may vary in size and length, but they typically do not penetrate more than ½ inch into the wood.
Concealed spaces are created when both the top and bottom of lightweight wooden truss systems are covered with wooden sheathing, gypsum wallboard, or other materials enclosing the area from roof to ceiling. These concealed spaces are also known a s truss voids or truss lofts. Joist and rafter roof systems also have these concealed spaces. This truss void space is often used for HVAC ductwork, plumbing, and wiring. Truss voids are also found between floors of buildings constructed with floor trusses. The open area from the top chord of the truss to the bottom chord can create a path for rapid horizontal fire spread. Even if fireblocking is placed in the voids, openings for ductwork, appliance piping, electrical wires, conduit, or additional utility installations can still create a path for fire to spread throughout roof and floor systems (Figure C-3). The truss void provides a reservoir for hot gases that may flash over when the void is opened as a result of ceiling collapse or fire-fighting tac tics (such as pulling ceiling or searching for the seat of the fire). Horizontal flame spread is also possible through open space created between roof rafters.
Steel trusses are available in different types and shapes. The current trend is toward engineered, lightweight steel construction.
Figure C-1 Gusset plates used to connect wooden truss members (photo courtesy of Kirk Grundahl) |
Figure C-3 Utility lines routed through truss voids. (source NIOSH [2000]) |
Figure C-2 Gusset plates. Note length of teeth/penetration depth (photo courtesy of Vincent Dunn) |
Figure C-4 Lightweight steel trusses (Photo courtesy of Francis Brannigan) |
Engineered structural steel building materials have been developed that are more cost effective than solid wood or masonry construction. For example, modern lightweight building practices have led to long-span steel trusses designed with a steel cable as the bottom chord [Brannigan 1999]. The steel bar joist (a parallel-chord truss) is commonly used for both roofs and floors in commercial buildings (see Figure C-4). To give steel trusses increased fire rating, they must be encased or covered with a spray-applied material to provide for the proper hourly fire resistance rating. This material insulates the steel from heat exposure and increases the time for the metal to reach the critical temperature (in the range of 800° to 1,200° F) at which the steel has lost too much strength and can no longer support its load. The insulation increases the time to failure but may not totally prevent it.
Steel trusses may fail in less time than a wooden truss under the same conditions. Some of the worst incidents involving fire fighter fatalities have involved metal trusses. These include the Brockton, Massachusetts, Strand Theater disaster in 1941 (in which 13 fire fighters were killed) and the Wichita, Kansas, automobile dealership fire in 1968 (in which 4 fire fighters were killed) [Brannigan 1999]. The twin towers of the World Trade Center contained 60-foot steel bar joists in their floor construction.
For more information about truss construction and educational or training materials being developed for the fire service, visit www.usfa.fema.gov/inside-usfa/research/construction.shtm [USFA 2004b].
All parts and connections of a truss are vital to the stability of the truss system. The bottom chord of a truss is under tension. A tension member acts like a rope. If the bottom chord of the truss breaks, the truss system may fail by pulling apart. Conversely, the top chord of a truss is under compression. The top chord acts like a column. Failure of a compression member reduces the overall load-bearing capacity of the truss. The failure of any one element can lead to failure of the entire truss. The failure of a single truss transfers additional load to the surrounding trusses, which results in multiple truss failures. The failure of one truss can cause serious problems to other parts of the structure, even parts separate from the initial failure point. However, when a truss member is cut or fails, the load may be redistributed to adjacent structural elements to mitigate the domino effect. The overall collapse potential depends on the supported load and how many adjacent trusses are also weakened. Fire fighters need to be aware of this phenomenon and use extreme caution when working around cut or damaged trusses.
An often overlooked hazard is found where interior trusses or wooden beams extend beyond the exterior wall to provide a balcony or a stairway landing. Fire burning inside the building can degrade the truss or beam, resulting in collapse of the cantilevered bal cony or stairway landing. Fire fighters standing on or under the collapsing exterior landing may be injured or killed. Different types of trusses can fail in different ways, as described in the following subsections.
Heavy timber truss systems may be constructed of wood or wood and steel. Heavy timber members are defined in building codes and are at least 6 inches wide and deep. The wooden web members connecting the top and bottom chords may be smaller dimensionally; however, they are critical to the overall strength of the truss section during a fire. In general, heavy timber trusses are long span and are placed at wide on-center spacings because they have such high load-carrying capacity. When impinged by flames and weakened to the point of collapse, large areas generally collapse. However, heavy timber trusses have the longest time to failure of any truss type because as the outer wood burns and turns to char, the char acts as an insulator and slows the rate of degradation to the inner wood [Grundahl 1992]. Hazards include the following:
Findings reported from the National Engineered Lightweight Construction Fire Research Project indicate that unprotected wooden assemblies fail within 6 to 13 minutes of exposure to fire [Grundahl 1992]. This 1992 report provides time to failure under laboratory conditions for a number of structural members and may not be truly representative of fireground conditions. Fire fighters should never rely solely on time-to-failure data to initiate fireground procedures. Continual evaluation of the fireground conditions, with emphasis on size-up and structural integrity, is necessary to ensure that fire suppression is carried out safely.
Lightweight chords are often continuous, and connecting web members often transfer substantial loads to other parts of the truss. This means that cutting a member may not automatically result in truss failure. There is much debate over whether fire immediately weakens or loosens the connecting gusset plates. Some researchers [Dunn 2001; Brannigan 1999] contend that these metal gusset plates can contribute to the degradation of wooden truss members through pyrolysis. Heat transferred through the metal fastener's teeth may destroy the wooden fibers held in tension by the gripping action of the metal teeth. This process loosens the plate and leads to a weakened truss and possible catastrophic failure if the gusset plate falls away and allows the weakened truss to pull apart. Other researchers [Grundahl 2003a; Meeks 2001; Cutter 1990] suggest that the metal plates protect the underlying wood during the initial stages of a fire. They suggest that the wooden members between truss joints may burn before the areas underneath the metal plates. The unprotected areas become charred to a depth that reduces the strength of the wooden member. Eventually, as the fire progresses, wood charring takes place underneath the metal connector plate. This causes the load-carrying capacity of the metal-plate-connected joint to be reduced. This reduction in the joint capacity eventually causes the metal connector plate to pu ll out and the joint to fail.
Lightweight wooden trusses are prefabricated at a factory and shipped to the construction site. If these trusses are improperly transported or stored at the site (exposed to the elements), or if they are dropped or handled improperly, the gusset plate or the entire truss can be significantly damaged. This can cause the plate to pull away from the wood surface or become weakened or loosened. In such cases, where the truss has not been properly repaired, the truss is weakened before installation and could fail under fire conditions much sooner than normally expected. Unexpected failure caused by mishandling is not unique to trusses and is difficult or impossible to predict during initial size-up.
The following are common causes of lightweight wooden truss failure that may be encountered in a fire:
All-steel trusses present their own hazards when exposed to fire. The mass and surface area of steel truss components are factors
that determine time to failure. A heavy, thick section
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of steel has greater resistance to fire than a lightweight section of the same length because of the increased mass. A large, solid steel truss can absorb heat and take longer to reach its failure temperature, whereas a lightweight steel truss such as an open-web bar joist will be heated to its failure temperature much faster. Once the
failure temperature is reached, heavy steel trusses and lightweight metal trusses will react to the fire and fail in a similar manner. A steel member fails at the internal temperature of the steel and not at the ambient air temperature. This temperature is often referred to as the critical temperature of the steel member.
Findings reported by the National Engineered Lightweight Construction Fire Research Project indicate that unprotected lightweight steel C-joists fail within 4 to 6 minutes of exposure to fire [Grundahl 1992]. Testing conducted by the U.S. Bureau of Standards (now known as the National Institute of Standards and Technology, or NIST) showed that unprotected steel open-web bar joists reached 1,200º F in 6 to 8 minutes [Brannigan 1999]. Table D-1 illustrates that steel retains only 25% of its original strength at 1,200º F and retains only half its original strength at approximately 900 ºF. Building design calculations are based on original strength at normal temperatures. At elevated temperatures, steel may retain no excess strength.
Steel is noncombustible and does not contribute fuel to a fire. This property may cause a false sense of security and overshadow the fact that steel loses strength when exposed to temperatures commonly found in structural fires. Steel has a high thermal conductivity, which means it can transfer heat away from a localized source and act as a heat sink. As long as the flame impingement is localized, the steel can transfer heat to other regions of the member-and thus the time to reach the critical temperature is delayed. If an intense fire is evenly distributed along the steel member, the critical temperature may be reached very quickly. Steel also has a high coefficient of expansion that results in the expansion of steel members as they are heated. As an example, a 50-foot-long steel beam heated uniformly over its length from 72° to 972° F will expand in length by 3.9 inches. The same beam uniformly heated to 800° F would expand by 3.2 inches; if heated to 1,200° F, the beam would expand by 4.9 inches [Grundahl 1991; Cotes 1997].
This unique truss contains web members made of steel cables.
Figure E-1. Heavy timber truss with steel cable web members. ( Photo courtesy of Francis Brannigan. )
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DHHS (NIOSH) Publication Number 2005–132
May 2005