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Trussed arch effective length

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Title: Trussed arch effective length

Anyone:

Anchorage has been seeing fabric covered steel frame structures being erected frequently.  The model being used has arched trusses in the transverse direction and tension only cable bracing in the longitudinal direction of the buildings.   There are girts / compression struts in the longitudinal direction at 10 feet on center along the exterior/top of the trusses.  The interior of the trusses are unbraced.  The exterior is covered with fabric.

The question has been raised over the braced length of the compression members in the trusses.  The manufacturer is using an effective length factor of K=.5 and using the length between panel points to calculate the compressive capacity. The point of an arch is to carry the loads in compression and resist the moment with thrust at the supports.

Is it standard practice to assume a braced length between panel points when panel points aren't braced out of plane? It seems like the length used to calculate the compression strength should be the distance between girts. If the interior member is in compression for it's entire length and is only braced out of plane by web members,  would the effective length be from the center of the arch to the foundation? How should the compressive stress be calculated?

Thanks.

Scott M. Haan P.E.
Deputy Building Official

Municipality of Anchorage
Development Services  Department
Mission Statement:  Guide safe construction and responsible development for the community.