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Re: Golf Course Bridge Design & AASHTO

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At 11:07 PM 7/22/99 -0500, you wrote:
>We're just starting on a job to use 100' salvaged bowstring trusses to make
>a golf course bridge.  It's going to carry carts, people, and golf course
>equipment (tractors).

Watch out for concentrated live loads on typical bowstring roof trusses.
These items were designed for uniform loading, and the curve of the top
chord approximates a pure compression arch without significant flexure
between panel points or between bearings, and without significant shear
being fed as axial load to any of the web members. As a result, web members
are typically very lightly constructed and connected. The bottom chord
accordingly is a tension member of very nearly constant tension to hold the
"abutments" of the "arch" together, like the string of a bow ties the bow
into a curve.

Curved top chords without roof joist load will have upward bending, and
bottom chords are now subject to spot loads and bending stress. Deflection
in the truss profile from any reason can crank destructive cross-grain
splitting stresses into timber bottom chords at bolted steel side plate
splices, and release those bolts. Crosswise "stitch" bolting can halt that
kind of failure. 

The bowstring roof trusses I have investigated all had circular curves for
the top chord, not parabolic or catenary, and the radius equaled the span in
each case. Highway circular curve formulas were convenient aids to solving
the geometry. 

Charles O. Greenlaw  SE   Sacramento CA