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RE: conduit support truss

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Also, don't be misled by a low weight on the bar joists...set up and jig for one or two pieces will be relatively expensive. Also, consider your wind forces for lateral deflection and instability.  For a trussed tower with rectangular struts, I think the area factor is 3.0 (1 sq in of truss = 3 sq in of drag).  A 1.5' deep beam assy at full wind load may be more than your gravity loads.

At 11:56 AM 12/14/2004 -0700, you wrote:
It doesn't sound like the load to your "structure" will be anything major.  The biggest issue is span.  That being said, the "structure" itself will weigh more than anything.  You want to keep your deflections down too.  So, in your case, the cheaper the better.  But a 60 foot span is pretty big.  You might be looking at a truss, but certainly don't keep stock shapes out of the question.  This all depends on how much your airline weighs and if they would be looking at tagging more stuff onto this "structure."  But just by the sounds of it, the cheaper the better.  You might also consider running a tension cable.  It's flexible, but it will support the weight, if you have proper anchorage.  If you don't, well, then your back to your "structure" option.
 
As for your dual bar joist idea, it's actually not a bad one, but the loading seems extremely low for what you are looking for.  Maybe consider using one truss with a shelf angle perpendicular to the span.  And instead of cantilevering, you could run a tie from the end of the shelf angle to the top chord.  You could also get the same result from a WF shape and use the upper surface of the wide flange as your bench to set your lines across.
 
If you go with the WF shape, assume a 25 plf, an LRFD factor of 1.2 (assuming all dead load with no wind pressure), and assuming a full unbraced length, you are looking at a W16x50 with a deflection of 1.14" at midspan (assuming simply supported).  With this option, you are looking at the upper face of the flange having a total of about 6.5" (3.25" each side), which should be more than enough space to fit the 2" line, plus it gives you some space to weld on some clip angles to keep the line in place so it doesn't fall off of the shelf.  You could put the clip angles at 24" on centers, depending on how flexible the line is.
 
A quick glance at the Vulcraft Bar Joist manual shows that for a 60 foot span, you are looking at a 30"+ deep truss and X-bridging is needed.  You save on weight (the K series bar joist weighs 12.3 plf) but you complicate the design by needing the bridging.
 
That's taking a quick look at the thing.  You might be able to grab a couple sticks of WF before you can get a bar joist.  If you are looking at designing and fabricating a truss, then you are going to have to take a serious look at lateral buckling if there is going to be no bracing of that top chord.  But for a 60 ft span, I don't know if there would be sufficient savings going with a WF (even spliced) over a fabricated truss.  Certainly something to consider though.  Don't get a Cadilac DeVille if you can get away with a Ford Taurus will do.  Best of luck.
 
Dave Maynard, PE
Gillette, WY
-----Original Message-----
From: Ken Peoples [mailto:kspeoples(--nospam--at)lvta.net]
Sent: Tuesday, December 14, 2004 10:49 AM
To: Seaint
Subject: conduit support truss

I have a client who wants to install a 2" airline across a 60' outside space with vehicular traffic below so he needs some kind of structure to span the 60'.  The request came in as "design me a truss" and I thought that maybe I could just use a pair of open web steel joists spaced a couple feet apart with bracing in between to make a pipe bridge that can be used for other runs of conduit or piping in the future.  However, I can't help but wonder if there is an easier, more efficient way - like maybe just one larger diameter pipe with the airline run inside or a rectangular HSS with the airline run on top.  Any thoughts/experiences to share?
 
Thanks in advance
 
 
Best regards,
 
 
Ken
 
Kenneth S. Peoples, P. E.
LVTA
Lehigh Valley Technical Associates, Inc.
1584 Weaversville Road
Northampton, PA 18067
 
Phone: 610-262-6345
Fax:610-262-8188
Email: kpeoples(--nospam--at)lvta.net
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