Lightweight Wood Truss Floor Construction: A Fire Lesson

Lightweight Wood Truss Floor Construction: A Fire Lesson



A recent San Antonio fire provides some hard-hitting lessons about the safety of this common type of construction.

Recently, a multiple-alarm fire destroyed half of a three-story apartment building in San Antonio, Texas. It was one in a series of extra-alarm fires in which lightweight, wood-frame trusses with metal surface fasteners proved to be an important factor in firefighting operations.

Upon arrival at the fire building, firefighters noted smoke issuing from the area of the fireplace chimney at the building’s roof level. Firefighters entered the building and advanced to the second-floor area where smoke had been detected by occupants.

During fire search and occupant removal of a second-floor apartment, firefighters noticed that fire was travelling up the walls at the rear of the apartment’s fireplace. At about the same time, firefighters outside the building saw that fire had broken through the roof around the fireplace chimney. A second alarm was transmitted.

Handlines were quickly stretched to the third floor. The ceiling surrounding the fireplace flue was pulled in an attempt to stop the fire from taking possession of the wood-truss, gabled attic space.

After a few minutes, firefighters operating on the third floor noticed that the floor they were standing on began to feel “spongy.” The company officer decided to evacuate the area, and firefighters moved out of the apartment to an open breezeway/stairway. (The breezeway/stairway ran the entire width of the building and separated the building into two halves.)

Approximately 30 to 45 seconds after moving out of the apartment and into the breezeway, the thirdfloor apartments where the firefighters had been working collapsed onto the second floor. About one minute later, the “combined” second and third floors collapsed in pancake fashion onto the first floor. However, the walls surrounding the collapsed floors, as well as the breezeway/stairway, remained fairly intact.

Firefighting operations were shifted to support a defensive/offensive strategy. Handlines were withdrawn across the breezeway and operated on all floors from exposure 2. A third alarm was transmitted for additional personnel, and a ladder pipe was placed into operation in front of the fire building.

Draft stops in the attic and the presence of the breezeway/stairway in the middle of the building provided fire breaks, and the fire was kept from extending to the other “half” of the building. A vertical and horizontal collapse zone was defined and maintained. The fire was extinguished with this third-alarm assignment.

Post-fire analysis

A subsequent fire investigation reconstructed the events leading to the fire. An apartment dweller on the second floor had attempted to start a fire in his fireplace using alcolrol Some of the alcohol spilled and leaked down through the floor around the fireplace. Kvidentlv, ignition took place inside this metaltastened wood floor truss. The fire subsequently travelled throughout the floor truss area. It also travelled up tho walls of the building into the third store floor truss area and into the gabled truss attic area.

firefighters operating on the third floor noted that, prior to collapses there was no tlamethrough ot the flooring above the truss and no sagging ot the floor. The only indication ot problems with the floor was its leel ot “sponginess.”

The rapid failure of these open floor trusses is important to note. It’s estimated that the third floor collapsed only 10 to 15 minutes after receipt ot the alarm at the fire alarm office. It appears that the fire was reported to the tire department soon after ignition took place.

General analysis

Current model building codes require that draftstopping be installed in wooden floor trusses, attic trusses, and similar concealed space areas in multiple-family dwellings and hotels. Such draftstopping must be installed “in line with walls separating tenants from each other and separating tenants from other areas.” (Uniform building Code 1985 edition, section 25.16 subsection 74). The draftstopping need not be “rated” (plywood is even permitted!).

Hie amount of required draftstopping is a relatively recent provision in the codes. Older editions of the codes permitted less draftstopping, resulting in very large, open areas in the attic and floor spaces. There are literally thousands of buildings across the country that have little or no draftstopping present. What draftstopping is in place is very often full of openings created by installation of electrical systems, cable TV, heating and air conditioning equipment, and plumbing.

Early collapse of this three-story lightweight wood truss residence could have caused serious injury to operating firefighters. Pancake collapse on the top two floors also triggered the curtainfall collapse of brick veneer.

Photos by Glenn P. Corbett.

Lessons Learne Recommendations Made

  • Probably the only thing predictable about lightweight wood floor trusses is their unpredictability.
  • As in any other building with voids, rapid deployment of firefighters to pull ceilings where fire is detected or suspected in the floor or attic trusses is imperative. Quickly determine the extent of fire involvement in the truss area. An advanced fire in the truss area is a condition dictating evacuation of the building. Defensive attack may be called for.
  • Smoke or flame pushing out from behind the siding of the building at the level of the floor truss probably indicates involvement of the truss area.
  • Keep track of time spent inside of the building. As the dock ticks away, collapse is approaching. Expect quick collapse of these trusses in as little as ten minutes of fire involvement.
  • Keep aware of the conditions of the floor. A fee! of “sponginess” as experienced in the San Antonio fire described on pages 41
Fire originated in flooring under this second-floor fireplace. Note the areas of fire travel both vertically and horizontally because of construction.
  • and 42 may be present. Don’t expect sagging or flamethrough as an indicator of impending collapse. Have a quick means of egress to the outside the building.
  • When operating under a floor truss (as when pulling ceilings), be wery of any movement of the floor above. Smoke conditions will :make this difficult. Try to stay in an area where rapid egress from the area is possible. Once you duties have been completed inside buildings get outside.
  • Conduct inspections frequently at construction sites employing litghtweight wood-frame floor trusses. Verify that the trusses have not been damaged prior to or during installation. Make sure you inspect just prior to “cover up” to ensure that the draftstopping is complete and in place correctly (that is, tight to the floor deck above).
  • Explain to your building and housing departments why it’s so important that the draftstopptng is installed correctly-then make sure they do their job!

At recent San Antonio fires, smoke and/or fire has been seen “pushing” from behind the wooden siding of the buildings directly in line with where the floor trusses are located inside the buildings. This possibly indicates that the fire has penetrated the floor truss area. In buildings with masonry veneer, indication of involvement of the floor truss area may not be so evident.

It’s no wonder that fires in floor trusses fail so quickly. Wooden members as small as two-by-fours are fastened together with metal plate fasteners that penetrate only about three-eighths of an inch into the wood. The open nature of these trusses very closely simulates a wood crib with its good ventilation, large surface-to-area ratio, and substantial fuel load. These are perfect conditions for rapid failure.

Perhaps our best defense against wood floor truss collapse is to have suppression capabilities right near the point of ignition at the time of ignition—in other words, install automatic sprinklers in the void. It’s interesting to note that the newly proposed “life safety” sprinkler standard for low-rise apartment and hotel buildings, National Fire Protection Association Standard No. 13R, won’t require sprinklers to be placed in the floor truss areas or in unusable attic spaces.

As more experience concerning the fire characteristics of wood floor trusses is acquired, we may be able to more closely predict their collapse. Hopefully, building codes and standards will be modified to better protect against this dangerous type of construction.

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