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Re: Trussed arch effective length

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Review AISC's current specification for what constitutes a bracing point.  It is fairly specific about stiffness & strength.  

If I understand you correctly, you should be very careful with the proposed model.  If the panel points have a girt on top of each one, and the girts are adequately tied to a horizontal braced bay (think PEMB), you  are fine.  If there aren't girts at the panel or the girts aren't tied to a horizontal brace, you have dominos.  Once one reaches it's limit, they will all fail together.

The easiest way to understand the problem is to follow the load.  Assume one panel point tries "kick out" and fail.  Will it push/pull on a girt or cable?  What then stops that girt or cable from moving?  If you can understand what stops the panel point from moving, you have a load path.

Jake Watson, P.E.
Salt Lake City, UT

P.S. What braces the bottom chord of the truss?  Can the bottom chord ever go into compression due to wind uplift?

---------- Original Message ----------------------------------
From: "Haan, Scott M." <HaanSM(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
Date:  Wed, 19 Jan 2005 12:52:48 -0900

>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?
>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.

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