I agree, tubes are not available in A36 (at least not readily if at all).
Typically, stich double angles together so that kl/r of one angle does not
exceed kl/r of composite section. You might also look at using single
angles (depends on loads in diagonals and details of the diagonals to chords
Standing seam roof may not require a deck underneath... check with
supplier... some do require supporting substrates and some don't.
Again, I agree with John... anytime I see less than 3 psf collateral load
for ceilings, lights, sprinklers, exhaust fans, etc. I don't feel
comfortable. Probably use between 3 and 10 psf for the collateral loads...
take into account what actually hangs from the joists that might not be
specifically called out (banners, speakers, and other loads you may not
apply as point loads)
Another person had indicated applying a 5 kip load [individually] at each
panel point. This is not a bad idea. If in North Carolina (NC Bldg Code)
or SBC Code area (not worded as strongly in SBC but think still applies...
similar tables) you will have to apply a 2 kip point load at panel points
with 1/2 Live Load applied for truss members supporting more the 200 ft^2
(if memory serves me right).
Even with no snow loads, Live Load should be considered (20 psf reducible or
non-reducible typical, but varies per location and code requirements).
Without knowing the applied loads it is hard to say, but I would expect a
truss 4 to 6 ft deep (or more) for a 110 ft span. Last one I did had 2 50
ft modules (each truss pinned over the interior column and fixed at the
sidewall columns), was a warren truss (without vertials) and was approx. 4
ft deep. Top and Bottom Chords were T-Sections with Double Angle diagonals.
Don't remember loads applied to it though. You can do a quick Moment/Depth
calc and check against 0.6*Fy*(Gross Area of Chord Member) (ASD Tension
Check)... If tension doesn't work for the chords, you won't get the
compression to work either. In actual checks of members don't forget to
apply appropriate Q factors per Appendix B if the members are "slender".
Don't forget bridging/bracing for the trusses... might refer to SJI for
recommend brace locations.
Greg Effland, P.E.
From: John P. Riley [mailto:jpriley485(--nospam--at)peoplepc.com]
Sent: Tuesday, May 15, 2001 3:55 PM
Subject: TRUSS QUESTION
I don't think you can get tube in ASTM A36 steel.
If you use double angles, make sure to stitch them at least at mid-span.
I'd have to go back through my records, but at one time I determined one
stitch plate was required and no more . . . maybe it had to do with the size
and length, don't remember.
You will need a deck under the standing seam roofing, adds at least 3
pounds. Spacing of the trusses will be predicated on the roof deck's
Personally, I would not feel comfortable excluding a ceiling weight, even if
the plan excludes it; since I'm so conservative, the fictitious ceiling
weighs 10 psf. My attitude is that the cost of handling, cutting, bolting,
welding and erecting the trusses establishes a baseline cost. To beef the
trusses up during the design phase adds little. If I'm mistaken on this
matter, so be it, I'm stuck in my ways!
John P. Riley, PE, SE
20 Oakwood Drive, Blue Grass, Iowa 52726
Tel & Fax: 319-381-3949
> I was asked to design a 110 ft span roof truss. The truss will only carry
> the standing seam roof and some lamps inside. No snow, medium risk seismic
> zone and a 75 mph wind. The supporting columns will be made of concrete.
> owner wants to use equal leg angles (L) and tubes (HSS) in A36 steel for
> price reasons.
> Any suggestions on truss shape (Pratt, Howe, scissor, etc), truss depth,
> truss spacing and if intermediate columns are required?
> Thank you for your prompt answers,
> Juan José
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