GluLam span question

[Hutchman]

Merry Christmas all!
I'm finishing up my 30x50 pole barn style shop, but am having second thoughts about something. I built with attic trusses 4' on center. I was assured by the truss company that this would be sufficient to hold anything I'd put up in the attic. However, I'm kind of having doubts about it being strong enough. I'm not going to be putting heavy machinery up there, but it could be some fairly heavy items. The bottom chord of the trusses is 2x8. I was wondering if I could install 30' 8" glulam beams in between the trusses to make it 2' on center. This is 30' clear span. I am probably going to sheetrock the ceiling, insulate, and finish the attic floor with OSB or plywood.
Thanks for any input.
Hutch

 

[sctattooer]

4' on center?? That can't meet code! Once the sun starts beating on that roof, you can be sure the sheathing will sag. I'd be more concerned with the roof, you could always double up 3/4 plywood, staggering the joints for the attic floor.

 

[IDASHO]

It is a pole barn.

Perlins run perpendicular to the trusses, and a metal roof is installed perpendicular to the perlins.

There is no roof sheeting.

 

[ddawg16]

I don't think the gluelam will be any better.....

I'm willing to bet that your truss has at least a couple of connection points in the middle of the span......in other words, it looks like several triangles put together.....in reality that shape with 2" x 8"s on the bottom is stronger than a single gluelam.

Additionally, I don't think you are going to find a gluelam that will cover that span that is only 7.5" tall.....

The below pic is of the beam that I put in the middle of my grage....and it's only a 20' span....that PSL beam is 7" x 16" and weighs in at around 640 lbs.

 

[6768rogues]

Check the plan that came with the trusses. If you do not have it, contact the company that made them and get a copy. It will tell all the design parameters including spacing and loading, and will show the bracing on the compression members. If you have a 10 psf dead load on the bottom chord and no live load, the trusses are not engineered for an attic load.

 

[Hutchman]

Sorry to have been so long getting back here. I've been out of town, and just got the drawing. It seems I have no live load on the bottom chord. I guess this is another time when I should have done more research and been smarter than the so-called professionals. I ordered these trusses personally, and told the woman,the owner of the company by the way, that I wanted attic trusses and wanted to put anything up there. She asked what I meant by anything and I said "Anything I need to store, I want to be able to put it up there." So she designs the trusses and I verify, "I'll be able to put anything I want up there, correct?" She says yes and I tell her to build and deliver them. Of course, she doesn't remember any of this conversation. It's not like they screwed me deliberately. I guess she just doesn't know what she's doing. If they would have said the bottom chord had to be 2x12, or whatever, I would have said to go ahead and they would have made more money and I'd be happy.
So, here I sit with this big beautiful attic, and I can put hardly anything up there. I guess my only recourse would be to put a beam and columns lengthwise down the center. Nice, I really want a line of columns right down the center of the shop in the way! NOT!
Anyway, thanks for your replies
Hutch

 

[buening]

Ask them what loading it was designed for (attic loading). They have to give you this information, and it should be stated on any plans or specifications that they provide. If this barn does have an attic, they are required by code, to my knowledge, to at least design for 10psf live load (IBC Table 1607.1 #28 Residential uninhabitable attics without storage). May I ask what company did the truss design? If they did not design them to code, you may be able to hold them liable. I would consult a local structural engineer for advice, since he is more familiar with your local code than I will be.

I don't know exactly what type of truss configuration they used, but 2x8 with attic space is pretty skimpy for 4' centers. I assume your two end trusses are on 2' centers, since 50' is not a multiple of 4.

For heavy storage the trusses should be designed for 40psf live load (20psf for light storage) according to the same IBC table quoted earlier.

Glu-Lams typically come in depths of 5.5", 6.875", 8.25", 9.625", and larger. If a properly designed width is chosen, you could use 8.25" glu-lams and then use 3/4" plywood between the glulams or use a 3/4" filler plate....assuming you want to have a solid floor in the attic. This will make up the differences between two depths. I would suggest contacting a local structural engineer and explain your situation. If they have plans for your trusses, they can design some filler joists to take the additional attic load. The joists will reduce the loading on the trusses. This will depend on the truss type used and how much you are willing to pay. You are likely looking at 360 board foot of glu-lams plus engineering fees. You will likely need a higher rated glu-lam, like a 24F due to the beam height limitation and the span length.

 

[Mike in Ohio]

Is it possible to stick build another truss in between each of your existing ones to make it 2' on center? Check with the truss people or an engineer I bet that would get you at least 10 or 20 lbs. per square ft. Just an idea but I'm no engineer so don't take my word for it. Good luck, Mike

 

[Hutchman]

Not sure on building trusses Mike. At this point, I think I'm going to get a structural engineer involved. I shouldn't have to, but that seems to be what I need to do.
Thanks for the replies.
Hutch