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RE: RISA, Lb-comp top and bottom

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From what you are saying it sounds like Lb-comp-top = 4'-0".  The top flange is supported only. When you put the beam in with the global Y upwards, it is having the positive local y upwards and this is considered the top of the beam.  If you had the beam rotated around it's x-axis 180 degrees then you would use Lb-comp-bottom = 4'-0".  If you drew the frame upside down in RISA and had your loads going upwards then you would have use Lb-comp-bottom = 4'-0".


From: john smith [mailto:steelstudent(--nospam--at)gmail.com]
Sent: Thursday, February 16, 2006 7:55 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