Joist-Rafter versus Lightweight Wood Truss
The strength of these assemblie comparable—until fire strikes.
Photos by John Mittendorf.
A recent article on lightweight, metal-plate-connected wood truss assemblies (and other newer building components, such as wooden I-beams) focused on concerns of fire service personnel about the performance of this construction when it’s exposed to fire.
The article relied on laboratory tests to validate a comparative fire performance equivalency between two-by-four-inch wood truss structural members and conventional joist-rafter assemblies.
This prompted a response from Francis L. Brannigan (Fire Engineering, June 1988) which marked the dangers and inaccuracies in using some widely accepted test methods to develop fireground operational procedures and shape general perceptions of these structural assemblies when Sexposed to fire of significant proportions. Using Frank Brannigan’s article as a springboard, let’s expand on the basic differences between lightweight, metal-plate-connected trusses and conventional joistrafter assemblies as commonly used in modern construction.
Structural members of joistrafter assemblies depend on their size for strength. The greater the span or imposed load, the larger the structural member must be to support it. Minimum size of the structural members of these assemblies is generally two inches by six inches, but they may be as large as mill/timber size.
It follows that the larger the structural member, the greater the inherent strength. As the length of the structural member is increased, so is its overall size. Conventional joist-rafter assemblies never “join” or “connect” several members together to form a single, continuous, unsupported structural member. The inherent strength derivative of size and span is reinforced by the method of attaching joists and rafters to other roof and ceiling members: with nails that penetrate the wood three to four inches, or with metal supports which offer similarly adequate strength.
Lightweight Wood Truss
comparable—until fire strikes.
On the other hand, it’s common for the structural members of lightweight wood truss assemblies to be two-by-fours or even two-bythrees. As the span or imposed load of a truss is increased, the size of the truss members is generally not increased. The number of “web” supports (See diagram, page 49) is increased. Remember that the common two-by-four will actually measure 1 ½ inches by 3 ½ inches.
A simple truss derives its strength from multiple members that are in compression and tension to form an integral unit. Its strength, therefore, is dependent on the sum total of the members in the truss. The bottom and/or top chords in truss assemblies are made up of various lengths of 2x4s, joined by metal-plate connectors, to form a continous structural member. 18to 22-gauge metal connector plates, with prongs that produce only 3/8-inch penetration into the wood, are common.
Joist-rafter assemblies have a distinct advantage over lightweight wood truss assemblies in a collapse situation. In the former, the ridge board and rafters are an integral unit; however, they are separate and distinct from ceiling joists. Therefore, if the attic is exposed to significant fire, the rafters and roof may collapse on the ceiling joists, thereby preventing collapse onto firefighting personnel inside the structure. (See Photo 1.) However, since the strength of wood truss assemblies depends on the sum total of interconnected members, expect the rafters (top chord of the truss), the ceiling joists (bottom chord of the truss), and the roof decking to collapse as a unit into the structure. Obviously, this puts firefighting personnel at great risk. This was graphically demonstrated at an attic fire in California when a lightweight, metalplate-connected truss roof suddenly collapsed without warning into the structure, severely injuring nine firefighters.
The common building practice of “butting” together varying lengths of 2x4s in lightweight wood truss assemblies to form a single structural member is also extremely dangerous in the event of fire. Notice in Photo 2 that the bottom chord of the truss is comprised of two 2x4s butted together and held in place by a single connector plate. Size and thickness of metal connector plates may vary, but their prongs, in most cases, produce the same result: penetration into the wood of only 3/8 of an inch. Tests and fireground incidents have indicated that when truss members are “charred” to a depth of ¼ inch, such connector plates suddenly fail (see Photo 3). Notice also in Photo 3 that the connector plate has conducted heat to the wood under it, which facilitates plate failure.
It’s easy to see that when the connector plate in Photo 2 fails, the bottom chord of the truss will separate and fail, allowing the vertical supports for the roof to fail, which, in turn, can cause the entire roof and the attic assembly to fall into the floor space below it.
There’s no comparison in size of structural elements between joistrafter assemblies and any type of lightweight truss. This fact bears directly in determining collapse time during a fire. A recent test by the Los Angeles City Fire Department indicated that metal-plateconnected trusses, open-web trusses, and wooden-I beams collapsed in less than five minutes when exposed to fire. Photo 4 is a good example of wooden I-beams that were exposed to fire. Notice that fire only traveled across the underside of the roof (the bottom portion of the vertical studs are not charred), burning away some “stems” of wooden I-beams. Yet, several feet away, the I-beams aren’t charred. Obviously, this roof is not safe — but the top of the roof looks normal. Other portions of this roof suddenly collapsed without any warning. While it’s true that protected truss assemblies may achieve one-hour fire ratings under full-design load, few are constructed with any type of fire protection rating, leaving unprotected truss members vulnerable to fire exposure.
The popularity of lightweight, metal-plate-connected wood truss construction and other types of lightweight trusses is increasing due to cost-effectiveness and speed of installation. However, use of relatively small structural members in tension and compression should be a major concern to engineers and fire service personnel. Fast failure rates, unpredictable collapse, and a lack of warning before collapse are common characteristics that fire service personnel should always consider when confronting a fire in a lightweight wood truss construction.