Any structural engineer here?

Rustyiron · Sep 16, 2013

I just got some 37' bar joist, enough for a 48' wide shed. I'd like to have 3 - 16' openings (posts) at the front wall, and the back wall could have 8' post spacing. I'm thinking a low pitch roof, around 3/12 so it will be holding the snow. The snow load here is 100 psf, and the roof will be the "pole parn" type metal on wood purlins. The spacing on the bar joists is 4' and I think that I'll need 2x6 on edge @ 24" centers for purlins. What I am wondering is how to support the 16' span...... as cheap as possible! Thinking of a wood girder truss that I could build. The height of this girder is not important to my design, but only to required stregnth. I do have enough LVL's 18" tall (1.75 thick) but they are about $600 dollars in fuel to go get!
I realize that engineering advice over the internet is, risky business, but this is a very simple load and building. I just can not find anything on the 'net about girder truss design. I thought about calling a truss Co. and either getting a quote for these 3 spans, or mabe just paying them for a design. Any takers out there?

rtimgray · Sep 17, 2013

I think it is a pretty simple calculation, but need to understand the concept just a little better. How big is the building overall? The bar joists are 37' long, and the shed is going to be 48' wide, is what I get from your post. Is the roof area then 37' x 48', or something different? Once we get that, all you should need to do is calculate how much roof area is going to be bearing on the 16' clear span, use the 100 psf live load + 10 psf dead load for material and figure out your total load on the member. Once you have that, you can probably figure out pretty easy if you can use a built-up girder, a plywood box beam, steel member, etc.

I'd be happy to give my opinion on this, just need a little more clarification.

BTW - not a structural engineer - I'm an engineer at a cracker factory.

john40981 · Sep 17, 2013

rtimgray said:
I think it is a pretty simple calculation, but need to understand the concept just a little better. How big is the building overall? The bar joists are 37' long, and the shed is going to be 48' wide, is what I get from your post. Is the roof area then 37' x 48', or something different? Once we get that, all you should need to do is calculate how much roof area is going to be bearing on the 16' clear span, use the 100 psf live load + 10 psf dead load for material and figure out your total load on the member. Once you have that, you can probably figure out pretty easy if you can use a built-up girder, a plywood box beam, steel member, etc.

I'd be happy to give my opinion on this, just need a little more clarification.

BTW - not a structural engineer - I'm an engineer at a cracker factory.

So you would be good for him if he needed to blow up his shed then or may be for a party treat to celebrate his shed erection

radioman · Sep 17, 2013

some of the info you posted here doesnt add up. 37' bar joist but you want a 48' shed? how tall is the bar joist? what is your overall dimension of shed you want? I am guessing 48 by 16? what do you plan to do to make up the 11 ft difference ? If you are wondering how to do the 16' span , how about splitting the difference? 8 ' each way? this way you would have a 37 by 14ish with bar joist in the peak with slope on both sides? This would handle the snow load better in maine. If you need bigger, can you tack on to side of barn or something to go 37 by 16 with bar joist in middle so span load is cut in half - 8 feet. Just put roof high enough so you have head clearance from bar joist.

You say you have enough lvl's but what overall dimension are they? why do they cost 600 bucks for fuel? It sounds pretty far just to go pick up. might be cheaper to go local.

Egon · Sep 17, 2013

Go buy some designed trusses.

They are probably cheaper than you can build them yourself.

Truss design can get a little more complicated than appears on first look.

Rustyiron · Sep 17, 2013

Yes rtim. overall 48 (wide) by 37' (deep) Bar joist will be running from the front, (the high side) on posts @ 16' centers, to the back, the lower side with no need for any great beams. The back side will be a bearing wall of some sort, prolly posts on 8' centers with a minor beam between the posts. Basically a 3 sided machine shed with the 48' dimension open at the front. The depth or height dimension on the bar joist is about 28" and meant to span the 37' legnth on 4' centers. I have 11 of these joists and will span an additional 4' at the side walls to make the 48' dimension. No sence building a wall under tha last joist. If I could I would do a little sketch but my pencil will not write on this computer

I appreciate your offer and may just end up seeing what a truss co would charge for the 3 required wood girders, but right now time is easier to come by than the $$$ and I've always done what I could myself, even if it cost more! I'm thinking that this would be a chicken chit order for the truss plant with only 3 pc. needed and the price will be high.

rtimgray · Sep 18, 2013

Alright, I'm with you now. Basically, we will have to hold up a roof area 16' x 18.5' (width of the opening X half the span of the bar joists). This equates to 296 square feet. Our loading will be 110 lbs/ft as mentioned above, so we will need a member to be able hold up 32,560 lbs (wow, that's alot over a 16' span).

A 2x12 will only support about 1,500 lbs over 16', so we're probably not going to be able to just build up a girder. I've got a manual showing the construction of a 24" deep plywood skinned box beam, but even at it's heaviest, it is only rated to support about 10,000 lbs over 16', well short of our goal.

My steel span chart shows a couple of choices - a 12" tall standard I-beam (35 lbs/ft) is rated at 32 kips over 16', so that would probably do it, maybe with a little deflection. A 10x10 wide flange at 49 lbs/ft will support over 40 kips at 16', so that would do it for sure.

I don't have anything on a truss design for that type of loading over that span. Maybe given the nature of the load, someone else could figure it out.

Let me know if there is anything else I can do to be of service, or if I have misunderstood something above.

Good luck and take care.

woodlandfarms · Sep 18, 2013

A truss company is the way to go. For whatever reason they can do it cheaper than you can (unless you are a contractor or have a ton of wood lying around).

Rustyiron · Sep 18, 2013

Thanks for the suggestions fellas, It looks like I need to explore the truss Co. for ideas of what kind of (wooden) truss could be made and compare it to steel prices.

F3506 · Sep 18, 2013

I think you're going to have to use steel to span the 16' opening. That would mean steel posts and an I beam spanning between them to hang your joists on.