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RE: RISA, Lb-comp top and bottom

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If i have simply supported beam w/ steel roof joists bearing on top of it at 4'-0" o.c., is Lb-comp-top = Lb-comp-bottom = 4'-0"?
 
No. You only have Lb-comp-top @ 4'-0" o.c. unless you kick your roof joists to the bottom flange. If the beam is simply supported, and you are only considering gravity loads, your allowable stress will be the same as if the beam is supported 4'-0" o.c. However, if you have a net uplift from wind the beam allowable stress will be reduced because the bottom flange is in compression. (Your unbraced length will probably be the length of your beam for that load case.) RISA uses the two Lb-comp so that you can get different allowable stresses if your beam has load reversals or isn't simply supported.
 
 

Wesley C. Werner


-----Original Message-----
From: john smith [mailto:steelstudent(--nospam--at)gmail.com]
Sent: Thursday, February 16, 2006 11:55 AM
To: seaint(--nospam--at)seaint.org
Subject: RISA, Lb-comp top and bottom

Hi all, i'm new to the listserve and am currently and engineering student.

I started playing around w/ RISA and have a few question regarding Lb-comp.

I know Lb-comp is the unbraced length of the compression flange, but i'm cofused about the top and bottom part.

If i have simply supported beam w/ steel roof joists bearing on top of it at 4'-0" o.c., is Lb-comp-top = Lb-comp-bottom = 4'-0"?

Thanks.

John