Post Frame Beam Design
Post Frame Beam Design
(OP)
See attached picture of the plans.
The guy who drew the plans shows a 2x8 (12' long) on each side of the 6x6 post. The trusses will sit on these two 2x8. The snow load in my area is 55 psf. Based on this the load on the beam will be 55 psf x 19.5 ft (bldg is 39 feet wide so 19.5 is half) = 1072.5 lb/ft. If I use the M=wl^2/8 I get 1072.5 x(12ft)^2 / 8 = 19,305 ft.lb = 231,660 in.lb. This load is way to much for two 2x8 to carry.
Even if you change the length from 12' to 8' you still get 102,960 in.lb. I just saw a similar building and they used the same construction with the 6x6 posts 8' apart and two 2x12 so I know this type of construction works.
Am I doing my calculations wrong or what am I missing?
Thanks.
The guy who drew the plans shows a 2x8 (12' long) on each side of the 6x6 post. The trusses will sit on these two 2x8. The snow load in my area is 55 psf. Based on this the load on the beam will be 55 psf x 19.5 ft (bldg is 39 feet wide so 19.5 is half) = 1072.5 lb/ft. If I use the M=wl^2/8 I get 1072.5 x(12ft)^2 / 8 = 19,305 ft.lb = 231,660 in.lb. This load is way to much for two 2x8 to carry.
Even if you change the length from 12' to 8' you still get 102,960 in.lb. I just saw a similar building and they used the same construction with the 6x6 posts 8' apart and two 2x12 so I know this type of construction works.
Am I doing my calculations wrong or what am I missing?
Thanks.






RE: Post Frame Beam Design
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Post Frame Beam Design
In addition to the above, it looks like they may be trying to use the struts to reduce the bending moment. This would make the 2x8 act as cont. beams.
Without knowing all of the design assumptions, I cannot say that the design is wrong. I think that you are justified in asking the designer the questions.
RE: Post Frame Beam Design
The trusses have a sloped top chord. Did you account for a reduction for sliding snow?
I see a knee brace, too, which is cutting down the span of the 2x8's.
RE: Post Frame Beam Design
RE: Post Frame Beam Design
RE: Post Frame Beam Design
How do I account for a reduction in snow loading?
4thorns, what load can a 2x6 laid flat handle? Where would I find these values?
Thanks for all the help.
RE: Post Frame Beam Design
As for the 2x6 laid flat........ that's only making a real contribution if it's far from the neutral axis and the fasteners are designed for shear flow. This probably isn't true and I would ignore any contribution from the 2x6 laid flat.
RE: Post Frame Beam Design
The building is going over water treatment equipment so I would say it is a category I building with an un-obstructed slippery surface with a 6:12 roof pitch. Where do you see that the snow load can be reduced to 24.4 psf? Thanks.
RE: Post Frame Beam Design
By NBC, your design snow load would be 55(0.8)0.743 = 32.7 for a ground snow load of 55 psf.
I would seriously question the design provided, not only for the capacity of the two 2x8's but for the attachment to the post.
BA
RE: Post Frame Beam Design
If you use I=0.8, you get a flat roof snow load of 30.8psf. The sliding is only going to make it lower. Without the sliding benefit you're almost cutting the snow load in half.
Factor in the knee braces and it's likely that they're ok.
RE: Post Frame Beam Design
We came up with similar numbers for the snow load, but different conclusions for the adequacy of the members. That doesn't make me feel good about my gut!!
Running some rough numbers, I agree with you.
RE: Post Frame Beam Design
If I am thinking correctly I would need to find the load on the end of each 2x12 and determine what size and how many nails would hold that load. Am I correct?
Anyone know where to find this type of information regarding nails?
Thanks for all the help.
RE: Post Frame Beam Design
I know what you mean...I have frequently been surprised by a calculated result exposing my gut feeling as premature and erroneous. Whenever that happens, I first review my calculation, then try to see where my gut feeling went wrong.
Hopefully the gut feelings improve with age but not always.
BA
RE: Post Frame Beam Design
RE: Post Frame Beam Design
RE: Post Frame Beam Design