LIGHTWEIGHT WOOD TRUSSES: MORE TO CONSIDER
Photos by John Hayes.
The weaknesses and unacceptability of lightweight wood truss construction and sheet metal surface fasteners, or “gang nailers,” have been given considerable attention over the past two to three years. I agree wholeheartedly that they are a hazard to the health and well-being of firefighters and civilians. In addition to the points already published about trusses, the following are some considerations for those who test these assemblies and those who tout the dependability of truss construction, as well as for firefighters. Keep in mind that although this article addresses wood truss construction, similar consideration, where applicable, should be given to metal truss construction.
TEST CONDITIONS VS. REALITY
Test loads are static and usually are spread out, distributing the load over more than one support member; on the other hand, a firefighter is in motion and represents a concentrated load imposed on a relatively small surface area. Vibrations from firefighter motion and from tools in use are transmitted to support members already weakened by fire conditions. The chance exists that a concentrated load in motion will be imposed on just the right spot at just the wrong time.
It is necessary for firefighters to search above the fire for trapped victims and for fire extension. It is also important to check the roof for ventilation possibilities as well as for victims. More than one firefighter could be assigned either of these tasks. Keep in mind that firefighters come in all sizes, shapes, and weights. Generally, while operating on the surface above the fire, they’re unaware of weakened supports beneath them or of the exact location of such weakened supports.
AGING STRUCTURES VS. CONNECTOR PLATE STABILITY
Over a period of time, nails often tend to work their way out of structural members to some degree and are in need of reseating. Possible reasons for this are the drying and shrinking of wood, settlement, and the movements (and vibrations) of people, machinery, nearby heavy vehicular traffic, trains, and so forth. This occurrence has been observed even when the nails were long and deeply seated into a wooden member.
In many truss assemblies, one-halfinch-long nails are part of a solid plate that holds together multiple members of the assembly. It stands to reason that under the aforementioned conditions, the individual plates would be subjected to multidirectional stresses. Since truss members at different directions are joined to the relatively inflexible connector plate, each will tend to move in a slightly different direction. The stresses eventually could cause many, if not all, of the nails on the connector to loosen from their seating: Nail holes become enlarged, and any movement of the connector plate tends to be toward the direction of least resistance, out and away from the object to which it is attached —in this case, the wooden structural members forming the truss assemblies.
AGING STRUCTURES VS. ROTTING WOOD MEMBERS
Aging wooden structural members frequently become rotted and weakened because of water leaks, alternate wetting and drying, and so on. Fortunately, to date I have encountered this condition only in a structure of solid wood beam construction. When rotted beams are encountered in solid wood beam construction and the damage is confined to a limited number of such members, collapse is not necessarily an imminent probability under fire conditions because the added load may be carried by the unaffected beams.
It does not appear that the same result could be expected of a structure of lightweight wood truss construction. The possibilities for disaster are compounded because only one lightweight truss member (of smaller dimensions than the solid wood beam) needs to fail to precipitate a collapse. If this hypothesis is valid (and from my experience I have reason to believe that it will bear some degree of significance), during fire operations we should expect less stability and earlier failure potential of lightweight wood truss assemblies as the structures age.
Older structures of solid beam construction frequently collapse. Floors sag, beam ends pull out, walls bow, and so on. Consider the added complication that only one member of the truss assembly need fail to cause total failure of the entire assembly and potential subsequent failure of the entire roof or floor. Furthermore, consider the added number of parts of the truss assembly that could fail, as opposed to one solid beam: The more parts there are, the more chances for failure. I have extreme misgivings about aging structures with lightweight w’ood truss construction.
THE CONNECTOR PLATE AS HEAT REFLECTOR?
Some have argued that metal connector plates act as heat reflectors. To my knowledge there have been no extensive studies undertaken to demonstrate the success rate of connector plate reflectivity under actual fire conditions. It is well-known that lighter-colored objects with smoother surfaces reflect radiant heat better than darker-colored objects with rougher surfaces. “Lighter color” certainly would include a shiny-surfaced metal object.
However, I pose the following questions for full consideration and research before I can accept the assertion that connector plate reflectivity is a significant mitigating factor in the collapse of lightweight wood trusses at a fire: Do all connector plates have shiny metal surfaces or are some refinished? How long will a shiny metal surface remain shiny, considering the various conditions under which it may exist—for example, at the seaside, in dampness, in an industrial atmosphere, and so forth? How shiny and reflective is a metal surface that’s completely obscured by heavy smoke and affected by combustion products such as soot? Of what significance is the reflectivity of a connector plate when the entire upper level of the fire area, including all parts of the truss assembly, are being heated to flashover by convection, when the direct flame exposure to the truss plate is nil, and when the transfer of heat through the metal plate to the wood surfaces beneath it is considerable?
These are just a few of the arguments pointing to the extreme danger of lightweight wood truss construction under fire conditions. Although the subject has received attention in the fire service press anti fire departments around the country are aware of the dangers of this construction, the truss remains. Ways must be found, with all affected parties participating, to conduct more realistic firetests so that the true results of lightweight wood truss construction in aging buildings can be known and so that appropriate actions to protect lives can be taken *