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RE: RISA, Lb-comp top and bottom

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Welcome to the wonderful world of structural engineering.


The “bottom” and “top” distinctions are important for beams where you have ‘reverse curvature’ bending. That is, you have both “positive” and “negative” bending moments in a single beam.


If you have steel roof joists bearing on the beam—and assuming the joists themselves are securely in place, having e.g. a roof deck attached—the Lb-top is going to be the spacing of the joists.


For the bottom chord, if you have no “kickers” from any of the joists, the Lb-bottom is going to be the full span between beam supports. However, if you DO have “kickers” that securely support the bottom flange to the joist framing, then the distance from the farthest support will be Lb-bottom.


Hope that’s clear.


From: john smith [mailto:steelstudent(--nospam--at)]
Sent: Thursday, February 16, 2006 10:55 AM
To: seaint(--nospam--at)
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"?