Bowstring trusses constructed in the 1940’s to 1960’s were very popular in commercial buildings because of their long unsupported spans. However, by today’s standards most are significantly under-designed for a variety of reasons, one of which is the lumber design values that were used to construct them.
At the time these trusses were constructed the allowable design values for dimension lumber were different than those used in construction today. For example, the allowable tension design values are quite different today than the values used in the 40s, 50s and 60s. Changes in allowable design for lumber came about as a result of a large-scale testing program known as the In-Grade Lumber Testing Program. In that test program thousands of pieces of full size lumber were tested by several lumber grading agencies across the United States and Canada. Additionally there were several other research scientists also performing various tests. One of the results of all the testing performed was the finding that the allowable design value for tension was incorrect. The result was a significant change in the published allowable design values for tension in dimension lumber which had a significant impact on bowstring trusses and metal plate connected trusses.
Over the years there have been several instances where bowstring trusses have failed as a result of overloading. One of the most common issues is failure as a result of excessive snow loads. Other factors have also played a role in the failure of these trusses such as water intrusion resulting in wood decay (especially at the exterior wall bearings and roof leaks), damage to trusses while in service, improper repairs, additional loading from mechanical equipment, and the improper modification of some members for mechanical equipment. Recently the New York City Department of Buildings issued a requirement that all bowstring trusses in New York City be properly inspected. Other municipalities around the United States, as well as insurance companies, have also issued requirements for proper inspection of these trusses.
While it is ultimately the responsibility of the building owner to have the trusses inspected, usually by a licensed professional engineer, one of the most important considerations is to be sure that the dimension lumber used in the trusses are also adequately inspected and graded according to the current National Grading Rule for Softwood Dimension Lumber. Timbers must be graded according to the appropriate grade rules of jurisdiction and glued laminated members (i.e. glulam) must be evaluated according to the American Institute of Timber Construction (AITC). Additionally any other deficiencies arising from wood decay, mechanical damage, or additional loading should also be properly documented.