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    Jim Timber's Avatar
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    Truss design help appreciated

    I need to clear span 40' for a second story floor and want to do this with metal trusses.

    Wooden floor trusses max out at 36', and require 12" OC spacing at that length. Not only is that too short, but it's a bunch of material to ship/handle.

    My intent is to make (4) trusses that each support (18) 2x10's, with the outside wall holding the loose ends via a ledger of some kind.

    My current design idea uses a 2x4x.125" top cord, (2) 2x2x.125" angle bottom cords, welded to 1.75x1.75x.125" square tubing diagonal webs, with 3x3x.125" vertical webs @ 24" spacing.

    I'd then laminate the top cord with a 2x4 to provide a nailing flange for the subfloor.

    Two questions: Would this be sufficient @ 18" web depth? If not, would it be @ 24"?

    If not, what would I need to change?
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    Oh, my aching head

    Quote Originally Posted by Jim Timber View Post
    I need to clear span 40' for a second story floor and want to do this with metal trusses.

    Wooden floor trusses max out at 36', and require 12" OC spacing at that length. Not only is that too short, but it's a bunch of material to ship/handle.

    My intent is to make (4) trusses that each support (18) 2x10's, with the outside wall holding the loose ends via a ledger of some kind.

    My current design idea uses a 2x4x.125" top cord, (2) 2x2x.125" angle bottom cords, welded to 1.75x1.75x.125" square tubing diagonal webs, with 3x3x.125" vertical webs @ 24" spacing.

    I'd then laminate the top cord with a 2x4 to provide a nailing flange for the subfloor.

    Two questions: Would this be sufficient @ 18" web depth? If not, would it be @ 24"?

    If not, what would I need to change?
    That calculation is why I decided that I didn't want to be an engineer. . . At one point, I could do it but just didn't want to. . .

    Based off nothing except seeing similar beams in commercial buildings, I'd guess the 18" depth would be enough but I'll wait for the experts.

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    Jim Timber's Avatar
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    I've seen online calculators for it, and am not sure what numbers to plug in to make it spit out something meaningful.

    It's raining pretty good here, so I'm holding out for a break to go grab my notebook with prints.
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    your issue will be deflection no matter your construction.....you need to seriously consider your deflection criteria as it will control your design

    at 40' i think your seriously undersized unless you want the floor to act like a trampoline

    i think you really need to have a direct consultation with a PE to discuss your complete construction picture and the materials you would like to use as this is far beyond internet engineering there are a LOT of pieces to the proverbial puzzle
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    Jim Timber's Avatar
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    Not to scale, but hopefully you can make sense of it.Click image for larger version. 

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    Quote Originally Posted by ttazzman View Post
    your issue will be deflection no matter your construction.....you need to seriously consider your deflection criteria as it will control your design

    at 40' i think your seriously undersized unless you want the floor to act like a trampoline

    i think you really need to have a direct consultation with a PE to discuss your complete construction picture and the materials you would like to use as this is far beyond internet engineering there are a LOT of pieces to the proverbial puzzle
    I'm new here. I was fascinated to see if any of the membership was actually going to give major league engineering advice on this rather esoteric construction problem.
    Last edited by MacCool; 07-28-2019 at 03:47 PM.
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    Jim Timber's Avatar
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    Quote Originally Posted by ttazzman View Post
    your issue will be deflection no matter your construction.....you need to seriously consider your deflection criteria as it will control your design

    at 40' i think your seriously undersized unless you want the floor to act like a trampoline

    i think you really need to have a direct consultation with a PE to discuss your complete construction picture and the materials you would like to use as this is far beyond internet engineering there are a LOT of pieces to the proverbial puzzle
    I've been looking at flat roof trusses in all kinds of buildings around here (MN), and they have very similar construction short of the tubular top chord.

    42psf live load is our snow load here.

    Top and bottom chords will be attached at either end to a pair of cargo containers. No issues with buckling the attachment points.
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    no advisement being given just discussion...


    jim your first post said "floor" ........floor loads are significantly higher than roof loads and are applied differently...again deflection is a issue

    pulled out one of my old Steel Bar Joist books ...thought i would post a design chart from it ...since these are highly optimized designs they provide some implied information can be gleaned from it

    #1 the joist type designation the first two numbers are the height of the joists in inches
    #2 the allowable live load # in blue is based on L/360 defection
    #3 the joist weight in # per/ft....if your assembly is not similar of heavier your not in the ball park

    hope this helps slightly

    ie i cant imagine your "blue" number to be under 200# and probably significantly higher....so looking at the chart ...that implys a 28" member that weighs 13# a running foot...how does that compare to your proposed construction is a question you need to ask yourself....this is a "optimal" chart and i picked the 41' length because it had the header at the top.....obviously i have a full BOOK of every size couldnt post it all just a rule of thumb starting point
    Attached Thumbnails Attached Thumbnails 20190728_154608.jpg  
    Last edited by ttazzman; 07-28-2019 at 04:10 PM.
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    Jim Timber's Avatar
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    Ttazz, that is very useful info!

    I also found a Steel Joist Institute PDF with lots of design considerations. 20" web depth is minimum for 40' span.
    Last edited by Jim Timber; 07-28-2019 at 05:09 PM.
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    Are you going to add plywood to strengthen the... just kidding 🙂

    How long are the 2x10s?

    I think you need to reduce the span. What’s the width the other way? Maybe use a big glulam to reduce the span to 20’ for the trusses (probably won’t need trusses at that length). How much headroom do you have in the floor below?

    I agree with what others have said. If you are not a PE, then get one. I would be surprised if they charged more than $500. This will also go a huge way to squash your building officials!

    With a 40’ span you will be fighting deflections (as was stated earlier). You will feel vibrations. Cracking of drywall will be a big problem if deflections are not managed. If you have any rooms with tile then you have more dead weight and tighter deflection limits.
    I am assuming this a floor as you originally stated, if it is a flat roof you also need to limit deflections to limit the amount of rain that can pond.
    Last edited by swiver; 07-28-2019 at 05:14 PM. Reason: Typos
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