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.
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
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.
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.
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
Kenneth S. Peoples, P. E.
Valley Technical Associates, Inc.
1584 Weaversville Road