Figuring for Wood Girts

[tobystevens]

Hello all new here and to the industry.

I am trying to find any information on wood girt design. I am in the steel industry and have a customer that is using 2x10 wood for girts. The buiding is a 30LL 16' Bay Spacing, 90MPH Exp B building, 2' OC Purlin/Girts. His permit department needs our drawings to say that 2x6 will work for his girts although he is going to use 2x10's. Any help with this would be greatly appriciated.

 

[MiketheEngineer]

I assume these are wall girts turned flat to the wind. Roof purlins are typically upright. If so - not a problem!!

Even a 2x10 flat to the wind at 16' span maybe problematic.

Say 10 psf wind - minimum - adjust if needed
24'' o.c. = 20 plf

M = wL^2 /8

M = 640 ft-lbs x 12 = 7680 in-lbs

Fb = M / S Syy - 2x10 flat = 3.468 in^3

Fb = 7680 / 3.468
Fb = 2214 psi / 1.33 reduction for wind = 1665 psi

So you need a fairly good grade of 2x10. Some codes allow for a 1.6 wind reduction instead of 1.33. That will help if applicable.

I got you this far - now check for deflection - that could be a real problem.

Good Luck

 

[tobystevens]

No they will be on edge to wind. Does that change things??

 

[MiketheEngineer]

OH YES!!!

The section modulus - Sxx - just went up to 21.4 in^3

SO NO PROBLEM - either in bending or deflection

 

[csd72]

Sag of the girts under dead load could be an issue.

You either need adequate bridging to take this or you need to design them for biaxial loading and for buckling along the full length.

 

[MiketheEngineer]

CSD72 -

Good Point - I am hoping sheathing will take care of that

 

[csd72]

only if the sheathing is on both sides otherwise there be a tendency for them to twist.

 

[NKT]

Can the sheathing hang from a beam at the eave? There should be a member there to handle collector loads anyway... If the collector member and sheathing connections are designed to hang the full weight of the sheathing, there will be minimal weak-axis bending on the wood girts below.

 

[msquared48]

The only dead load the girts will see will be their own self-weight and the weight of the sheeting. Most of the load from the sheeting will be transferred to the lowest gort that will probably be a sill plate. That should support this dead load.

Regarding the twisting action csd refers to, just add vertical blocking at midspan. This coupled with the girt end connections should suffice.

Mike McCann
MMC Engineering