Bowstring Truss Roof Construction Hazards

Issue 3 and Volume 8.

About 80 years ago, the bowstring truss roof became a popular construction element in the United States. Prior to 1960, it was reportedly one of the most common design types for commercial and industrial structures. The design worked particularly well for structures that had large, open spaces with no supporting columns in the middle, such as car dealerships and other vehicle storage buildings, supermarkets, bowling alleys and skating rinks.

Over time, some of these buildings have been converted to other types of businesses or structures, making the presence of a bowstring truss roof less obvious. Although not used in construction as often today, bowstring trusses are still around, often to create an arched or rounded roof, such as those found at high-end car dealerships. It’s important that every firefighter be able to identify a building with a bowstring truss roof (the arch generally gives it away), and understand the tactics and precautions necessary to handle a fire in a building with this construction feature.

A Tragic History
There have been a number of collapses of bowstring truss roofs that have killed and injured many firefighters:

  • Six FDNY firefighters were killed and 34 injured in August 1978, when the bowstring truss roof of Waldbaum’s Supermarket collapsed during a fire. Many of the firefighters were operating on the roof when it collapsed, which occurred less than an hour into the incident.
  • In July 1988, five Hackensack (N.J.) firefighters were killed when a bowstring truss roof collapsed a little more than 30 minutes after the fire was reported in the Hackensack Ford Dealership. (For more on this fire, visit http://my.firefighternation.com/profiles/blogs/hackensack-tradegy-a-failure.)
  • In May 2009 a San Francisco firefighter was seriously injured while operating a 2½” hoseline when a commercial warehouse’s awning, which was part of a bowstring truss roof system, collapsed and struck him, causing him to be hospitalized (http://my.firefighternation.com/forum/topics/san-francisco-firefighter-hurt). There was no preplan of the building.
  • Two Chicago firefighters were killed and 19 were injured in December 2010, when the bowstring truss roof of a vacant commercial structure collapsed 16 minutes after firefighters arrived and several minutes after the fire had been reported under control (http://my.firefighternation.com/forum/topics/chicago-firefighters-trapped).
  • In September 2011, a collapse occurred shortly after units responded to a fire at a vacant car dealership in Illinois. In this case, firefighters were operating in defensive mode, so there were no injuries/fatalities.
  • One volunteer firefighter was killed and two more were injured in March 2012, when the bowstring roof of the Abby Theatre in Abbotsford, Wis., collapsed (www.firefighternation.com/article/news-2/wisconsin-theater-collapse-kills-one-firefighter-injures-three-others). There was no preplan information about the roof

These are just some of the more glaring examples; there are numerous others.  

Preplans & Communication
Fires in buildings with bowstring truss roofs can confound even the most experienced chief officer; often, they may not know that the building has a bowstring truss roof, which is usually due to either a lack of preplanning or a lack of communicating an informal (or formal) preplan. They also may not have studied the performance of these types of buildings under fire conditions. An effective size-up could identify this hazard.

Local companies initially arriving on the scene of a fire in a building with a bowstring truss roof should clearly communicate any critical preplan information by radio to all incoming units. In larger departments where the battalion or deputy chief is coming from a distance and may not know the building, that information will be particularly critical. (For more on the different types of radio reports and why they’re important, visit www.firefighternation.com/article/communications/radio-reports-review.)

If the information is not yet clear, and firefighters going to the roof discover bowstring truss construction, they must communicate this information quickly and clearly to the incident commander (IC). Note: At the 2009 collapse in San Francisco, the initial report given stated that there was a working fire in a type II (non-combustible) occupancy when it was actually a type IV (heavy timber) occupancy. At the 2012 collapse, none of the initial-arriving companies identified the presence of a bowstring truss roof.

Down & Out Dangers
The Wisconsin collapse was investigated by NIOSH, which clearly documented some of the hazards inherent in bowstring truss roofs in its Firefighter Fatality Investigation Report (www.cdc.gov/niosh/fire/pdfs/face201208.pdf): “The principles of bowstring truss construction are similar to other types of truss construction in that web members are used to form multiple series of triangles that transfer tension from the bottom chord and compression from the top chord of the truss onto the load bearing walls. One big difference with the bowstring truss is that the compressional forces within the top chord act to force the load bearing walls outward as well as downward.

“Bowstring truss roof systems may suffer from a little-known phenomenon related to inaccuracies in early industry-accepted truss design assumptions. One significant design deficiency involves the tensile strength of the bottom chord. Early truss designs assumed wood tensile strength could be defined by bending tests of small, straight-grained wood samples free of common wood defects. Prior to the 1960s, large-scale test facilities were uncommon, so full-size lumber tests were rarely conducted. During the 1960s, full-size lumber tests revealed that construction-grade lumber with natural imperfections (such as knots, checks and irregular grain) provides in-service tensile strength significantly less than that predicted by the earlier small-scale, clear wood tests. By 1968, lumber industry standards established a reduction factor of 0.55 to relate tensile strength to bending strength. Current building codes have increased this factor to 0.60, meaning the allowable tensile strength design values are only about 40 percent of those listed in the early codes. Thus, all trusses constructed prior to the late 1960s have a common code deficiency; the bottom chord members may have inadequate tensile strength to support code-prescribed roof loads.”

To put this into simpler terms, buildings with bowstring truss roofs will not only collapse downward, but can also force the load-bearing walls, upon which the trusses sit, outward in a collapse situation. Normally, this presents the greatest danger on the “B” and “D” sides of a building, if we assume that you can see the roof curve from the “A” side.  However, once the “B” and “D” sides are compromised, wall failure is very likely on the “A” and “C” sides.  Additionally, buildings built with bowstring trusses prior to advances in large-scale fire testing in the 1960s could have a lower safety factor and thus a higher susceptibility to collapse than buildings built in later years.

Other Collapse Hazards
Buildings built with bowstring truss roof structures have a number of other factors that can make them susceptible to collapse. For example, bowstring trusses can be constructed of wood or steel, but most often, they’re made of wood—often heavy timber.

They also have construction features that make firefighting in these buildings different than other buildings. In the San Francisco building, the trusses had a clear span of 50 feet, and were almost 20 feet apart and 50 feet deep. Although the construction materials of the truss were solid, “heavy timber” and connected with bolt fasteners with metal splice plates on the bottom chord, the truss assembly was exposed with no ceiling membrane or fireproofing. This is typical of a bowstring truss roof structure. The NIOSH report (www.cdc.gov/niosh/fire/reports/face200921.html) indicates that: “The curved top chord members were made either by sawing straight lumber into curved shapes or laminating multiple smaller pieces bent over a jig to the desired shape. Bottom chord members were typically constructed with large, straight lumber members joined with either wood or metal bolted splice plates, located near mid-span, to achieve the required length. The top and bottom chord members were fastened together at the truss ends with U-shaped steel heels, or end shoes, bolted to both chord members.”

The integrity/stability of the truss system is based upon all the components remaining intact and connected to transfer a load properly. The large, open spaces in the overhead in these buildings allow for smoke and heat from a fire to travel to that space, leaving things at floor level cool and relatively smoke-free. This gives firefighters entering the space the false impression that nothing major is going on when in reality, a serious problem is brewing in the roof truss space.

Key Characteristics
Characteristics of fires in bowstring trusses include heavy smoke showing from the exterior, with perhaps only light smoke and heat on the interior. Aggressive firefighters working on the inside will report good progress and little fire, and will request that the fire be placed under control. The IC must have eyes on all sides of the building to 1) look for the signs of a bowstring truss roof and 2) correlate the reports being given from the interior. The large, open truss space provides a lot of room for heat and smoke to hide, and heavy smoke around the roof or even facades built later can obscure the true construction type. Important: Roof loading (firefighters, snow/ice, etc.) or roof-mounted equipment installed during the life of the building can add to the potential for an early collapse of the truss.

Attacking the Fire
When working a fire that involves a building with a bowstring truss, there are many critical elements to consider:

  • Be extremely cautious with your risk analysis; carefully weigh the risk vs. benefit of placing firefighters inside or on these structures.
  • Keeping the trusses and truss space cool to prevent collapse should be a high priority. Knock interior fires quickly to prevent heat/fire spread to the overhead truss area, and consider aiming large-caliber streams (unstaffed if possible) into the overhead truss space, placing them around exit doors at end walls.
  • Establish and enforce collapse zones on all sides of the building, but particularly at the walls on which the trusses are resting.
  • If the decision is made to go defensive, make sure all personnel on the fireground are aware of this and stay clear of the interior.
  • Use apparatus-mounted master streams, including aerial master streams from above. Better yet, bring them down to a lower level and direct them horizontally into windows or other building openings.
  • If personnel are on aerial devices operating over the building, they should expect a roof collapse and sudden exposure to heat and/or flames venting from the collapse.

It’s Up to You
All firefighters must understand bowstring truss construction, what it looks like, and what its hazards are. You must also know the locations of all buildings with bowstring trusses in your coverage area, and you must communicate those locations to your firefighters, as well as your mutual-aid organizations.

Now that you’re finished reading this, get out in your area and start identifying these buildings, collect information on each one, and start developing your action plan. Remember: It’s up to you to ensure that incidents like the ones noted in this article don’t happen again.