Reverse engineering trusses to know their capabilities?
I traded a guy some months back for 8 pole barn trusses. He is further north 3-4 hours in Indiana. I am in Nashville area zone 4a.
I am going to build a pole barn shop with them and I would like to make a vented attic area to keep the place a little comfortable for working in the weather extremes.
They are much beefier than what I have seen here. 2×8 top and bottom chords vs the 2×4 I normally see. I assume they have more snow load to deal with uop their.
Because, I do not know the spacing they had or if they will accommodate my plan for some fiberglass batts in ceiling and a lightweigth roof sheathing such as 1/4 or 3/8 ply, or?? I thought I would ask you folks if you can point me in a direction to solve the math.
I am going to make the shop with these. If I cannot figure it out, I will plan on some recycled 2″ foam between the metal and the purlins.
I assume the barn they came off had 8ft truss spacing because it was a 56 ft barn and I have 8 trusses. However, for all I know the end gables were not trusses, but rafters? I do not know.
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I can’t imagine sheathing a roof with 1/4” plywood. Standard is 1/2” and even that has some noticeable flex to it when you walk around. The trusses should generally be able to accomdate the usual roof construction since that’s what they’re designed to do. Insulation isn’t generally a problem either.
To get the specifics, if you don’t have any docs on the ones you have, is to either contact an engineer and ask to have the numbers run, or contact a truss manufacturer and give them the dimensions and have them give design specs. The truss manufacturer will probably want to sell you more trusses, but if you only have 8 now you might need some more.
Can you get the ordering information from the guy you got the trusses from? If you can get a manufacturer and order number you can probably get full specs with a simple phone call.
Bill
Sorry for confusion. I meant the underside/ceiling would be some lightweight sheathing to keep weight down as I doubt pole barn trusses are made for handling the weight of drywall. I have no intention of putting any sheathing under metal roofing; so, it would be metal on purlins for roof side of trusses.
Drywall is a very distributed load, so it’s not usually a problem for a truss to support. Even a 2x4 lower chord is not generally a problem for drywall support.
You should really check with the manufacturer of the truss or an engineer before making any design decisions with your mystery trusses though. That’s the only way to know for sure what they can handle.
Bill
Mike,
Unfortunately I've never been able to get a truss manufacturer to allow their trusses to be re-used, even if they have the original engineering. I doubt an engineer would touch them either.
Malcolm, I’m assuming he traded for leftovers, so new trusses. I’m not so sure I’d trust used ones, either, due to concerns over damage to the panel points.
Bill
Bill,
From his last paragraph I think they are salvage.
Yea, I did not contact a local truss maker as I figured they would be afraid of lawsuits. I am going to use them no matter what. The question is, do I have the strength to space them at 8ft and support drywall. I know they are not at factory new condition, but I want to know what the design strength is. I will inspect and make sure they are all in great condition for my shop. I just do not want a sagging roof because I overloaded them. I had posted here because I thought someone might know of a way for me to calculate it. No worries though, I will keep searching around. Thank you all for chiming in.
-Mike
You might be able to get a local engineer to do the calcs for you. They'll want some sort of deniability since these are salvage. Truss engineers generally use software that makes the math pretty easy, but we all learn how to do it longhand in school. I don't know any engineers that still remember the longhand method, though. Besides the basic math, there are all sorts of derating factors that you have to apply (probably including the fact of salvaged parts), and you've got to get snow and wind loads right, etc. It's pretty easy when you've got the tools and do it all the time, but not a DIY sort of calculation.
I had to fix some trusses in a house. I found a free on-line calculator for trusses (no I don't remember what site). You do need to know the grade of the lumber (are there grade stamps on your trusses?), although you could just guess as #2. I assume you know the species. Most trusses I see here are SPF. You also have to assume that the plates are appropriate to the loads. I would not assume 8 ft. centers. My current pole barn, which is a home-built affair, has trusses at 6' o.c. I believe, with 2x4 purlins standing on end attached to top of the trusses. Purlins on top spanning more than one truss with lapped joints would be much stronger than purlins in between.
Let me know if you can't find a free calculator. I can dig up my original calcs somewhere.
Some years ago when our local used building materials operation was building an open shed for sheltered material storage, we got a local engineer to inspect salvaged roof trusses and give us what we needed to reuse them. If I remember correctly, in any areas that looked "stressed" from the salvaging process, we simply did some gussets to "repair."
I would see if a local (or maybe even not local if you can get him or her to "inspect" remotely or using FaceTime) structural engineer will work with you.
Peter