BOWSTRING TRUSSES

BOWSTRING TRUSSES

The need for large, open floor areas, without the inhibiting presence of columns, is one of the most appealing reasons why building owners select trusses for their buildings. Supermarkets, bowling alleys, and automobile repair shops often use trusses, and. in particular, bowstring trusses.

Bowstring trusses are named for obvious reasons. The shape of their curved top chords and horizontal bottom chords resemble an archer’s bow and string.

Two-by-fours are the minimum size for members of wooden bowstring trusses. To join them together, “split ring” connectors—metal plates embedded in the face of the truss through which bolts are passed to hold the truss together—are often used.

Beyond the normal fire hazards of trusses in general, one of the most dangerous aspects of a wooden bowstring truss is the large area in which smoke and heat from a fire can collect whilestill maintaining a “clear” fire smoke condition at floor level below it. With the attachment of a ceiling to the bottom chord (as was the case in Hackensack), a heavy fire condition can be totally obscured from firefighters operating below.

It is of vital importance to firefighting operations at a building constructed with bowstring trusses that the presence of the truss is identified and that this information is relayed to the commanding officer and other firefighters on the fireground. Preplanning will greatly assist in this endeavor.

Determining the extent of truss involvement in fire is also very important. This determination in many cases will be guided by firefighters operating on the roof, rather than firefighters operating underneath the truss. Even without a ceiling attached to the bottom chord of a truss, smoke will often obscure the fire involvement of exposed trusses to the view of firefighters operating below.

Rapid failure and absence of forewarning typify’ this type of construction under fire conditions. Interior firefighting operations involving bowstring trusses require close coordination of companies, monitoring fire involvement of the trusses, and close observation of progress made of these companies by the fireground commander. A thorough discussion of firefighting operations in bowstring truss buildings is contained in Vincent Dunn’s Collapse of Burning Buildings and in Frank Brannigan’s Building Construction for the Fire Service, Second Edition. These should be consulted for complete information.

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