huangyhg

roof design - sloped/skewed bar joists or light gage steel t
we have a whole series of two story steel structures (5 building total), and the architect is showing a pitched roof with hips and valleys, much like is typically found by framing with gang-nailed wood trusses.
the shape of the roof just screams light gage steel trusses, but with all kinds of mechanical ductwork to design around, we want to be able to design the roof structure ourselves and have more control over the final product. also, we have very strange bearing locations at the end of the structure, with 11 or 12' cantilevered soffited eaves, which will be more difficult with light gage steel.
with the red-iron steel option, we'll have steel beams sloping down the hips of the roof, and then bar joists bear onto those sloping steel beams. this would occur at only a couple of areas (ends of the buildings, then at the corner of the l-shaped buildings), but these areas are going to be complicated to design and build, in my mind.
so my question: both methods we can make work, both have pluses and minuses. our dilemna is that we have to choose one method and run with it, and be able to support that decision when the building goes into value engineering. i can get feasability and cost values on the light gage steel easy enough.
but for the bar joists, do any of you know what kind of a premium you pay in order to get a lot of sloped bearing seats, and quite a few bearing seats that are both sloped and skewed. does using steel bar joists with three dimensional connections get too cost prohibitive, or is it just something they deal with. can you call vulcraft and get basic feasability information on a design of the roof, just enough information in order to choose which method to use?
thanks for any gut reactions you can provide.
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your best bet is to call vulcraft or other joist manufacturer. work with a local steel fabricator of erector to get an in place erected price.
if i recall correctly, you can get only sloped seats and not skewed. you might want to confirm that with a joist manufacturer.
i've never used them, but it is my understanding that light gage metal trusses are meant to be spaced at 16 or 24 inches. with bar joists, you can space them whatever your sheathing will allow (1 1/2 inch "b" deck; say 6'-0"). so you're probably going to need three times as many light gage trusses as joists. you're also going to need a lot more attachments of the sheathing to the light gage joists than to the bar joists. of course, the light gage trusses are much lighter to move around.
i bet after you get all the numbers, it's close enough in price to be decided by your preference.
light gage trusses are either spaced at 24" oc or 48" oc depending upon load requirements.
alpine said they could design them at whatever spacing you needed, provided it didn't overstress their proprietary chords and webs, and that 4' or larger spacings were "common." of course, they may have just been saying that to sell some trusses to me.
i would bet that the light gage trusses will be less expensive overall. you can attach wood sheathing directly to the light gage or use steel decking, depending on need. if you go with red iron, you'll need supplementary blocking if you want to use wood sheathing.
the trusses can be designed for the conditions you noted.
i have contacted vulcraft in the psat and asked about the extra cost for deeper bearing seats. they are more than happy to give that kind of information out. spacing metal trusses farther apart than 24" is when metal trusses start to become competitive. i ususally specify factory built metal trusses (i had a bad experience with site built trusses). if using metal deck, i try to space them as far apart as practicable. bracing for the lateral load resistance system is by the eor like with wood trusses. connection details (truss to structure) can be by the eor or by the supplier with loads provided by the eor. i have done it both ways depending on the truss supplier.
will you be using the roof as a diaphragm?
yes, the roof will be used for a diaphragm.
i ran into some problems finding diaphragm values for metal deck on metal stud trusses not too long ago. the roof was changed due to other reasons to steel joists so i stopped looking at some point for the values. i'm not saying they don't exist, but i couldn't find them at the time. you may search for previous threads about this also.

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