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RE: Inverted tapered steel girders

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The Australian steel structures code (AS4100) has a clause in it that permits assessing flexural buckling capacity of segments of varying cross-section (tapered and stepped beams.), I assume the US code as similar.

 

Purlins to the top flange of the rafters would provide lateral restraint (Ly), whilst fly-braces between purlin and rafter bottom flange would be required for torsional lateral restraint (Lz). If metal decking fastened direct to rafters, then it would only provide lateral restraint to rafter top flange, if ribs of cladding profile spanning between rafters, and without purlins have a problem of providing lateral restraint to bottom flange. If the purlins installed between the rafters then they behave similar to the bridging systems placed between the cold-formed purlins themselves.

 

Does the mid-span splice only have bolts to the bottom flange?

 

 

Regards

 

Steven CONRAD Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust

mailto:sch.tectonic(--nospam--at)bigpond.com

Roy Harrison & Associates

Consulting Engineers (Structural)

PO Box 104

Para Hills

SA 5096

tel: 8395 2177

fax: 8395 8477

 


From: jvhannah [mailto:jvhannah(--nospam--at)insightbb.com]
Sent: Wednesday, 1 November 2006 15:21
To: seaint(--nospam--at)seaint.org
Subject: Re: Inverted tapered steel girders

 

Are you saying that there are no lower flange braces to the purlins?  My experience is that the lower flange of the tapered roof beam would be braced on about 10' centers.  It should be possible to design roof beams without lower flange bracing however the roof beam will be heavier.  I'm not sure that it would be visually heavier.  You should be able to check for unbraced length and compact sections in a normal fashion.

 

HTH,  JH

----- Original Message -----

From: Jim Wilson

Sent: Tuesday, October 31, 2006 9:32 PM

Subject: Re: Inverted tapered steel girders

 

When I mentioned that the steel girders lacked diaphragms, I meant that they lacked any bracing between each other, as in bar joist construction.  The building has a metal deck and x-rod bracing for an overall diaphragm.

 

My query is to determine if there is a risk of a lateral torsional buckling type failure because the beam's lower flange is kinked.  I am guessing that at some point the flange could pull to the side as it tries to straighten itself out under load.   But that's just a hunch.

 

Jim

"sscholl2(--nospam--at)juno.com" <sscholl2(--nospam--at)juno.com> wrote:

Tapered steel girders are quite common in my experience. You didn't say what kind of diaphragm there was. It could be steel with or without lightwt. concrete. It will have to take horizontal shear from wind/seismic.

I assume that when you said the top chord was level you meant it was almost level but with slope for drainage.

 

Stan Scholl, P.E.

Laguna Beach, CA