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Re: RISA, Lb-comp top and bottom

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I think i understand now.

Lcomp-top and Lcomp-bot refer to the location of the compression flange, it's either the top flange or the bottom flange.

I thought it meant the top and bottom of the compression portion of the beam.  So in a simple supported W10 beam, the Lcomp-top is at the top of the flange and Lcomp-bottom is 5" down, or half depth. 

I understand now.  In my simple supported beam, there is no Lcomp-bottom, b/c only the top flange is in compression.

If i cantilever the beam over a support, then the bottom flange is in compression and would need to be braced.

I think i understand now.

Thanks!

On 2/16/06, Eli Grassley <elig(--nospam--at)psm-engineers.com> wrote:

John – you are on the right track with the way a bracing kicker is used.  However…

That does not make sense that the design of your beam would be changing if you brace the bottom flange of the beam.  If you have a simply supported beam, there is NO compression in the bottom flange, and thus no reason to brace it.  A simply supported beam with absolutely no bottom flange bracing will perform exactly the same as a simply supported beam with bracing at 3" oc.  Whether or not RISA realizes this fact is another question.  Check your model and make sure that the local axis of the beam are oriented correctly and the end supports are pinned.

 

~~ Eli G

 

-----Original Message-----
From: john smith [mailto:steelstudent(--nospam--at)gmail.com]
Sent: Thursday, February 16, 2006 10:35 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: RISA, Lb-comp top and bottom

 

So, Lcomp-bottom doesn't mean the bottom of the compression area of the beam (ie. upper half of depth of the beam, the bottom half being in tension)?  But the bracing length for the bottom of the beam.


I was playing around w/ the value for Lbcomp-bot in RISA, and it seems to pass my beam if i put a brace at midspan as a minimum.  So they would build it w/ a 'kicker' from the roof joist?  Is this a steel angle from the bottom of the joist to the bottom of the beams i've seen?


Thanks again!

On 2/16/06, Polhemus, Bill < BPolhemus(--nospam--at)wje.com> wrote:

John:

 

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)gmail.com]
Sent: Thursday, February 16, 2006 10:55 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