Ran across this in my archives and thought I’d post it for others…
An Update on Bowstring Truss Issues .pdf (2.4 MB)
Interesting read. Here is the introduction:
“In 2009, engineers from Western Wood Structures (WWS) published a white paper entitled,
“Bowstring Trusses “Fail” to Meet Current Code Requirements.” [1] That paper described two
common bowstring truss types and discussed three significant issues related to the design of
bowstring trusses. Bowstring trusses are characterized by a circular curved top chord and a
straight bottom chord. The top chord is usually made with a radius of curvature equal to the span
of the truss. This radius provides for the most economical design and results in a 30-degree
spring angle at the truss heel.
This paper adds to the knowledge base of the previous one. The history of these trusses,
including changes to snow loading requirements, changes to the allowable timber stresses, and
changes to timber grading rules are examined to provide a more thorough understanding of truss
performance.
The truss types described in the previous article were ARCH-TECO Trusses (TECO) and Tim
Trusses. Both TECO Trusses and Tim Trusses were designed and manufactured by Timber
Structures, Inc. The TECO Trusses were prevalent in construction from the mid-1940s to the
mid-1950s. The basic configuration of the TECO truss is included in reference [1]. Several
common failure modes observed in TECO are included in this paper. Tim Trusses had spans up
to 160 feet and were used in construction beginning in the mid-1950s. Since the publishing of
the white paper in 2009, WWS has encountered two other common types of bowstring trusses,
the Summerbell Truss and the Bellfast/Mckeown Lattice Truss…”