Article and photos by Greg Havel
A truss is “A type of beam that is a framed structure consisting of a triangle or group of triangles arranged in a single plane in such a manner that loads applied at the points of intersections of the members will cause only direct stresses (tension or compression) in the members.” (from the Glossary, Brannigan’s Building Construction for the Fire Service, 5th edition, by Corbett and Brannigan.)
Trusses are usually fabricated of wood or steel, for use as structural supports for roofs and floors, including both long and short spans. Truss bridges are a topic for another time.
One of the common long-span trusses for roof support is the bowstring truss (photo 1, above). This type of truss has a straight, flat-bottom chord that makes attachment of a ceiling simple. The characteristic hump-shaped roof is often partially hidden by a parapet wall (photo 2, below), as in this 100-year-old mercantile building that is being remodeled into a church.
In a bowstring truss, the top chord is curved and in compression; the bottom chord is straight (the bowstring) and in tension; and the shorter web members that separate the two form triangular shapes and are usually in compression. If the truss is of wood, the bowstring is often of the same dimensions as the curved top chord, and sometimes serves as the floor supports for an attic or upper storage space (like at the Ford dealership in Hackensack, New Jersey). If the truss is of steel, the bottom chord is often of smaller dimension than the top chord, since steel has greater strength in tension than in compression, per unit of cross-sectional area.
While a bowstring truss may have a tie-rod as the bottom (tension) member, it usually does not; since the lack of mass of a tie rod limits the way in which a ceiling can be attached to the bottom of the truss; and prevents the use of the bottom chord as supports for an attic (truss loft) floor. Since wood or structural steel elements are usually limited to about 20 feet in length, the bottom chord of the truss with a longer span will be spliced at panel points; and these splice connections are the weak point of any truss of any shape (photo 3).
A parallel-chord truss (photo 4) is similar to a bowstring truss, except that the top chord is flat instead of curved. These trusses are commonly used to support flat roofs, as well as floors and ceilings.
Although it is sometimes assumed that trusses with some members of wood and others of metal are a recent development, the age of a truss cannot reliably be determined from its design. The “Construction Concerns” article dated February 13, 2013, discusses the roof trusses of an 1840s grain elevator with a pitched roof (shaped like a capital “A”). The upper members of the truss, and the bottom chord, are 12×12-inch (30.5 x 30.5 cm) hardwood timbers. Although the bottom chord is in tension, it also supported the elevator machinery in the five-story building, and the walkways for servicing and inspecting the elevators and grain bins. The compression members in the webs of these trusses are also of hardwood timber. The tension members in the web of the truss are 1.25-inch (3.2 cm) diameter wrought-iron rods.
The common news media comment about a truss roof or floor having collapsed “suddenly without warning” is inaccurate. On arrival at a fire incident, observation (or suspicion) of a truss roof or floor by firefighters; or more detailed knowledge of their presence from preincident plans; should be all the warning we need of the impending collapse if the fire continues to grow.
It cannot be emphasized enough that this behavior under fire conditions, and during overloads with no fire present, applies not only to bowstring trusses, but to trusses of any shape, design, or material.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.
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