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Re: Framing Decisions

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Michael:

Don't worry about the out-of-plumb joists.  1/2" out of plumb will not affect the structural capability of the joists, and in fact is assuming they are fabricated to a much tighter tolerance than is observed in real life.  If the joists are an architectural feature, and your architect has a level or plumb bob, you can specify tilted bearings, but your customer will pay a bunch for that, the erector is likely to get some of them backwards, and the strength, rigidity and serviceability will remain unaffected.  In other words, don't sweat it.

Can't help you with the seismic stuff.

Phil

Michael Zaitz wrote:

 Hello, I have a warehouse building approximately 500' by 320' in dimension.  It is steel framed with joists, girders and tube columns, metal deck,  and precast wall panels.  It is a double sloped roof 1/4:12 with the ridge running parallel to the 500' dimension.  The standard bay size is 40' x 40' with the end bay being 40' x 20'.  Due to the length of the building I am putting in an expansion joint midway down the 500' dimension across the 320' direction.  At present I am running the joist girders parallel to the 320' dimension to give me frame action for the longitudinal direction.  For the transverse direction I am planning on using the panels as shear walls.  Due to the expansion joint I decided to run the girders as I did and that solves any stability problems with the frames at the expansion joint. Here is the problem.  By running the joist girders the way I am, the joists will not be vertical.  With the 1/4 in 12 slope and a 24" joist the bottom chord will be 1/2" out of plumb.  Will the joist manufacturer design for this?  Or can the joist manufacturer slope the top chord to allow the joists to hang vertically and still allow the metal deck to get its required bearing?  Will there be gnashing of teeth trying to do it this way?  On a similar project that the architect was involved in (smaller and not needing an expansion joint)  the engineer ran the joist girders parallel to the long direction which allowed the joists to slope normally.  I can not use x-bracing to get any stability (owner's request) therefore if I run the girders in the opposite direction I will need to do a portal frame. On a separate issue is there any problem using a precast panel in a seismic zone 2 (Av and Aa of 0.18)?