THE TIMBER TRUSS: TWO POINTS OF VIEW

THE TIMBER TRUSS: TWO POINTS OF VIEW

We perceive structures to be solid and stable. Nothing could be further from the truth. Every structure is under tremendous strain at all times. The force of gravity constantly is trying to pull it down. By one method or another, the builder provides a resistance system that defies the law of gravity and makes it possible for the building to stand.

Many forces can cause the building to fail, however. An earthquake shakes foundations; weather attacks masonrymortar; insects and chemicals decay and destroy wood. The damaging force of chief interest to us is fire. Fire destroys wooden structural members, distorts steel members, and causes connections to fail.

Firefighting operations often arcperformed under and above structures subject to catastrophic failure when fire disrupts the gravity-resistance system. Firefighters over the years have identified signs of imminent structural collapse, but bloody experience has shown that these commonly recognized warning signs and even the best personal protective equipment are not enough to avert tragedies. The collapse of trusses is sudden and catastrophic. The “warning” against this hazard is knowing the characteristics of trusses of the buildings within your jurisdiction.

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Knowledge of the hazards of trusses has developed gradually. When I wrote the first article on the then new concept of prefire planning to appear in the fire service press i ⅜ years ago, 1 referred to a Navy drill hall that had been converted into a warehouse. The building had 100-foot timber trusses, but this fact was not mentioned in the preplan even though the timber trusses were clearly visible in photos of the building. I hope that any fire department preplanning such a structure today would give serious attention to such trusses.

Neglecting to inspect all structures in your jurisdiction that you know have or you think might have truss constructions can have serious consequences for your department. During these inspections, note all factors that can affect the behavior of the trusses during a fire and incorporate them in your prefire plans, The following will help you to conduct your inspections and write your preplans.

A recent fatal collapse has led to increased emphasis on bowstring trusses. Remember, however, that timber trusses can be bowstring, parallel chord, or gable (triangular)—-all three of which under fire conditions can present dangers to firefighters. The fire chief of a midwestern city’ told me that his department lost three firefighters when the bowstring truss roof collapsed because the roof officer interpreted the absence of the characteristic hump of the bowstring truss to mean that there was no truss. In fact, the wide-span building had a parallel chord truss that collapsed— according to the fire commissioner, “without warning”—during a serious fire and killed the three firefighters.

The bowstring truss sometimes is referred to as an arched truss, since we are most familiar with the segmental arch. In fact, arches can be pointed or even flat. An arch differs from a truss in that the thrust of a truss is downward, while the thrust of an arch is outward. This outward thrust must be resisted. This can be done with a mass of masonry, such as a buttressed wall, or by tying the arch together at the bottom with a steel rod, which is called tied arch. The original Arby’s fast food restaurants had an arched roof. Some of the arches were tied; others had masonry resistance.

CHARACTERISTICS OF TIMBER TRUSSES

Some of the principal characteristics of timber trusses and the hazards they represent follow (no ranking is intended). Some characteristics are common to all trusses; others are specific to a particular structure.

  • Each member of the truss has an assigned function. There is no redundancy. The failure of one member may cause the failure of the truss.
  • The failure of one truss may be catastrophic. One timber truss, of several, failed in a New York supermarket fire. Six firefighters who were venting the roof died.
  • All trusses are designed to be the lightest they can possibly be and still support the designed load under normal conditions. Student engineers sometimes conduct contests to determine who can produce a truss with the greatest superimposed load/ weight of truss ratio.
  • Long spans are characteristic of timber trusses. The consequences of failure—whether it be of the truss or the wall(s) or beam(s) supporting the ends of the truss—can be catastrophic.
  • Multiple connections (panel points) characterize the truss, and all connections are vital. The failure of any connection may be fatal.
  • The top chord of a truss is in compression. It acts like a long slender column (any member under compression acts as a column, regardless of whether it is a horizontal or vertical member) and is divided into shorter segments by connections. This is done because the shorter the segment, the greater its ability to bear a greater load. The failure of a connection causes a longer span between connections and the longer span has only 25 percent of the previous strength. The top chord will buckle at this point.
  • (Photos by author.)

  • The bottom chord of a truss is in tension and is being pulled like a rope. Just as a rope fails totally when cut once, the bottom chord fails if one connection fails.
  • Steel bolts conduct heat into the wood, thus destroying the wood by pyrolytic decomposition and possibly causing the steel to fall out.
  • Steel tension rods can slack off as they first elongate and then reach failure temperature. Failure of any member can cause the truss to fail.
  • Wood is combustible. In some designs, parallel wooden members are spaced away from each other; the space makes a flue that can accelerate burning.
  • When trusses collapse, a pressure wave of air or fire blows out of every opening, and glass windows burst into shards. After withdrawing, forces ensure their safety by providing for an adequate collapse zone.
  • Even if the trusses do not fail, the wide span of roof boards between them can fail. Such a failure caused the death of a fire lieutenant in a Yonkers, New York fire.
  • Do not confuse the lattice truss with the timber truss. It is made up of thin members, such as 1 X 6 boards. In a fire all the boards are subject to early and rapid ignition, and failure of the assembly is certain.
  • Wood trusses have failed as a result of decay of the wood, chemical action, or wet rot.
  • Trusses are beams and exert a vertical load. This load is delivered to the two opposite walls. Unless the wall is perfectly vertical —that is, if the wall is eccentrically loaded (out of plumb) —it might overturn. In some cases, this problem is resolved by tying the bearing walls together with cold drawn steel cables, which fail completely at 800°F. In other cases, the walls, particularly those
  • made of wood, arc tied together with steel tie-rods. Often the turnbuckle in the center of the tie-rod is concealed with a lathe-turned wooden block. The failure of such a tie-rod cost the life of a New Jersey officer when the weight of the trusses caused a wooden bearing wall in a church to collapse.
  • I noted a Southern California timber truss building that had steel rods parallel to the bottom chord. My guide informed me that they were required by the community’s building department. The rods may have been required to tie the walls together to increase earthquake resistance, to compensate for walls that were eccentrically loaded by the trusses (out of plumb), or to provide greater tensile strength in the bottom chord of the truss. The latter two reasons could be very significant in a fire; the first one may or may not be, depending on the wall.
  • Other structural elements may be tied to the trusses. In a Washington, D.C. theater incident, the tie-rods holding the suspended-beam canopy were tied to the trusses, which became heavily involved in fire. The loosened tie-rods allowed the canopy to become an undesigned cantilever that pulled the top 20 feet of the masonry front wall out, almost to the point of overturning. Alert junior officers who were studying the building construction caught the hazard and cleared the areas.
  • The trusses may resist the overturning of a masonry wall by a cantilevered business sign. The only fuel in a fire in an abandoned commercial garage was the roof. The wall was eccentrically loaded by a heavy projecting sign, and the trusses were resisting the overturning of the wall. No one was aware of the imminent collapse, and the wall collapsed just as the companies were about to move in to finish up. Seventeen firefighters were injured.
  • The trusses may be supported on unprotected steel, as is the case in an
  • Orlando, Florida country club. Heavy timber parallel chord trusses are supported on an unprotected steel beam set in a wooden wall. When the heated beam attempts to elongate during a fire, it may overturn and dump the trusses. At a slightly higher temperature, the beam will fail and drop the roof.
  • Trusses generally are designed to support only the roof. Substantial undesigned loads such as shelving for material; metal, lath, and plaster ceilings; and possibly even sprinkler piping may contribute to early failure.
  • Ceilings sometimes installed beneath the assembly in an attempt to shield the trusses from fire may serve to conceal the extent of the fire from firefighters working in the building. Fire coming out of the roof and visible only from the exterior indicates that the trusses almost certainly are burning, and the building should be evacuated.
  • Some gusset-plate trusses may appear to be timber trusses. In Nova Scotia I noted that one large truss, a girder supporting other trusses, has a bottom chord that consists of four 2 x 10 planks, side by side. The length of the chord requires splicing of the planks, and they are held by gusset plates. All the splices are located at the same point, instead of being overlapped. Fire at the spliced point could cause the bottom chord to fail. Since the bottom chord is under tension like a rope, one failure would cause the entire truss to fail, thus dropping all the trusses.
  • Do not place uncritical reliance on automatic sprinklers. Dry pipe systems that have “gone wet” a number of times may be clogged with scale. The fire load and rate of heat release of the contents may be capable of overw helming the sprinklers, rendering them ineffective.

Fire units should carefully examine trusses in the area and note any characteristics that might cause early collapse. In my opinion, when trusses are involved in fire, firefighters do not belong on or under them or close enough to be injured by a collapse.

This Nova Scotia heavy truss is no different from the lightweight truss. Note the gusset plates. The bottom chord is four 2 x 10's, all spliced at the same place. The bottom chord is like a rope under tension. A failure here would collapse the truss and the trusses it is supporting.Many Arby's restaurants have tied arch roofs.This San Francisco inverted queen post truss supports a heavy load. The architect highlighted it as an interesting feature. Failure of the steel tension members would precipitate serious collapse.

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