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RE: designing steel joists

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Bar joists are designed as simple pinned trusses.  Since the chords are continuous, the real behavior is somewhat different and the deflections will calculate greater than the tested performance.  Uniform loads are straight forward.  Point loads, uplift loads (roofs), and triangular loads (snow drift) will change tension members into compression members and complicate the design and construction.  Loading will also have an impact on bridging.  A bottom chord of a joist with wind uplift will require more bracing than a bottom chord that remains in tension. 
 
A custom fabrication of bar joists will generally be much greater in cost per pound than what the joist manufacturers will charge because of set up costs vs. assembly line manufacturing and custom rolled angles and web elements.  The light weight members of a custom fabricated truss could get up to about $5.00 per pound as compared to about $1.50 per pound for a simple wide flange beam with minimal fabrication costs. 
 
A lot depends on how a structural steel fab shop is set up.  If they are set up for trusses, automated welding, CNC fabrication, or just a guy with a cutting torch and a welder.  It makes a huge difference in costs. 
 
The least expensive cost per pound for a normal steel fabrication shop is a simple beam.  If you have a lot of repetition, and/or you need the openings, consider a castellated beam especially if the shop has an automated sub arc welder.  If there are a lot of joists to differ the set up costs, a custom bar joist is normal structural engineering.  I would suggest that you load test one to get a more accurate prediction of deflection.  That is what the big boys do. 
 
If you decide to go on with bar joists and it is a regular structural steel fabrication shop, consider using WT's for chords as opposed to double angles.  It simplifies fabrication. 

Regards,
Harold Sprague




From: akester(--nospam--at)cfl.rr.com
To: seaint(--nospam--at)seaint.org
Subject: designing steel joists
Date: Wed, 10 Sep 2008 14:28:57 -0400


A good steel fab client of ours is asking us to design a 12K1 floor joist for them. They have a client that has to have joists ASAP so a regular joist mfr cannot get it done in time. We have the CDs and loading info on the drawings, and I know how to design a joist or truss but never have done an actual steel joist for obvious reasons. Is there anything other than normal structural engineering and steel design principles that I need to know about? Are there SJI provisions we must follow other than bracing/bridging,  or can we just design a joist that exceeds the LL and TL capacity of a 12K1 for that span out of Vulcraft’s book? We have 2-D software capable of quickly modeling it.

 

Any other client I would probably say sorry, but we don’t necessarily want them looking around for other engineers and we are actually a little slow now so why not….

 

Please CC me directly if you would…

 

Thanks,

Andrew Kester, PE

ADK Structural Engineering

Orlando, FL

 

 

 

 



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