BIG Pole Barn - Long-Span Roof Trusses in RISA 3D 2

[ThorenO2]

Hello,

I am analyzing a new barn with long-span trusses having no records of the design. The field verified chords are 2x8 SP 2400F with very large pressed plates, which makes me believe they were designed for considerable loading. However, according to my analysis, the bottom chords and tension diagonals do well, but the top chords are absolutely "crushed" due to the unbraced length from the 4'-0" o.c. purlins. If the top chords had purlins @ 2'-0", my analysis of 10 psf DL + 17 psf LL (0.85*20) will work. A solution that would achieve this is to specify additional 2x4 purlins @ 4'-0" o.c. at the underside of the top chords to create the desired unbraced length. In addition, I will need to specify lateral bracing for the compression webs, which currently have none. Has anyone done anything of the sort or is able to provide input on analyzing this type of truss. I've heard these trusses are designed for 12psf LL, which would ease things, but still require the 2'-0" unbraced length. Thank you for any insight,

 

[JoshPlumSE]

My guess is that (if there is a roof diaphragm) the original designer considered the 2x8 continuously braced. Though for a chord this deep, that sounds like a dubious assumption to me.

Your solution of adding 2x4 purlins to reduce the unbraced lengths sounds like reasonable solution to me.

Not sure what you're getting at for the compression webs. Normally, I use the webs full length (joint to joint) and K = 1.0. Every once in awhile I might reduce this slightly based on the connection lengths. But, I try not to do this unless I have to and unless the connection provides significant restraint.

 

[ThorenO2]

Great, JoshPlumSE - thanks. I've seen disputes here on the 29 gauge metal panels over 4'-0" o.c. purlins acting as a diaphragm, but it's my opinion it has to be this way or they wouldn't have put plywood and diagonal bracing at the endwalls. I haven't started the lateral analysis in RISA, but I am counting on the diaphragm with (2) new interior metal shearwalls, which ought to really bring down the forces on the frames. I see there is an ancient DOS program DAFI that's used by folks, but I prefer my own ancient DOS software, RISA-3D v17.

 

[southard2]

Not sure about the 29-Ga panels acting as a diaphragm.

My first comment is that when you answer the phone and someone utters the words pole barn, laugh, and then hang up the phone. About once every five years I make the mistake of entertaining some poor soul with a pole barn related project and it always boils down to someone trying to get the structure to do something it can't or shouldn't, and trying to save money while thinking somehow they've outsmarted the rest of the construction world.

I am currently designing a house for a buddy of mine and because of OSB prices he is insisting on a pole barn like structure. What I've designed is more like a metal building structure only I'm using wood trusses bearing on 8x8 timber posts.

Fabral has an ICC report using their PBR panel (Mighti-Rib) as a diaphragm. So I had to use light-gage steel purlins to use the PBR panels as a diaphragm.

Here is a link to the report ESR 2059.

https://icc-es.org/report-listing/esr-2059/

In the report you will find their PBR panel can be used as a diaphragm at 26-GA. You will also see they have a panel called the grand-rib 3 which is like a PBR panel but the ribs are shallower and closer. That panel is listed at 29-Ga and appears to have to be fastened to wood supports. But the diaphragm values aren't very much. But I hope this helps you somewhat. See if the panel on your project matches any of their panels.

I can't say I'm comfortable using an exposed fastener roof panel as a diaphragm and I've tried to talk my buddy out of this. To be honest he is going to pay more money for a building that is going to go up slower, cost more, be less strong, have a shorter life, and an awful resell value compared to conventional construction or even a typical all steel metal building. I just don't get it, but he can't be reasoned with. It is as if he thinks I'm lying to him. I tried three times to talk him out of this design but he wants what he wants. Planning on delivering the plans to him later this week where I will with one final plea tell him not to build this building. I really think in 15 years he is going to regret it big time and have a big hole in his wallet.

Regarding the existing trusses I've seen in plenty of trussed housed using 2x4 running laterally from truss to truss to brace compression webs along their weak axis.

If this is a new pole barn why have you been asked to analyze it? Is it moving or something? Did they get busted for not having a permit to build it? If there are no design plans than it is highly likely that if there ever was a plan for those trusses that someone probably deviated from them anyways. 12 psf is an appropriate live load if the trusses has a large enough effective tributary areas and certain roof slopes. Consult the appropriate code for your area to determine if you can reduce the live load that much. But I would also watch the wind load case for download pressures as well. .75 Roof Live load plus .75 wind download... This is why I rarely reduce live loads.




John Southard, M.S., P.E.

https://www.pdhlibrary.com/

 

[ThorenO2]

Thanks for the great post, southard2. I've gotten involved due to not hanging up the phone as recommended. I did chase away a customer once who wanted to build a Pole Barn as his main residence.