Post Frame Beam Design

[jmme]

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.

 

[msquared48]

You have conside3red the snow load, but not the dead load of the structure which is probably an additional 10 to 15 psf, depending on the roof framing and roofing used. You need to revise your figures to include that.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[OHIOMatt]

Couple of things to consider, if in the US and using the IBC, a 55psf ground snow can be reduced to about 24.4 psf roof snow. This is based upon a Category I building, an un-obstructed slippery surface with a 4:12 roof pitch. It is unlikey that you have all of these.

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.

 

[Lion06]

Is the 55psf ground snow or roof snow?

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.

 

[4thorns]

You've noted the 2x8's. There is a 2x6 between them as well. It's laid flat but should have a structural effect if it is in pure tension. Do you have any other views of this design?

 

[JAE]

jmme - are you doing the design of this or are you checking someone's design? What is your role in this?

 

[jmme]

Someone asked me to take a look at the plans just to see what I thought but I don't have much experience with these types of buildings. I am really just trying to learn for my own sake.

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.

 

[Lion06]

Look in the Snow Load chapter of ASCE 7 for sliding snow (and roof snow for that matter if you were using the ground snow in your calcs).

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.

 

[jmme]

OHIOMatt ,
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.

 

[BAretired]

In the National Building Code of Canada, the roof snow load is 0.8 times ground snow. The slope factor is (60 - [α])/45 for an unobstructed slippery roofs with [α] between 15 and 60 degrees and 0 for [α] > 60.

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

 

[Lion06]

Look at ASCE 7-05 Table 2 for the sliding snow factor.

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.

 

[Lion06]

BA-
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.

 

[jmme]

I was going to ask about the connection to the posts. The building I saw the other day used two 2x12 and the end of each 2x12 was in the center of the 6x6 post. There were about 5 nails attaching the 2x12 to the post. I would think it would be much easier to nail the 2x12 to the post.

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.

 

[BAretired]

Lion,

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

 

[boo1]

The struts and beams are not well secured, lags are too few. The bare steel gusset and common galv nail may also have corrosion issues in your enviroment. Consider double 2x12s PT w no struts, through bolts w two 5/8" hot dip galv. bolts and change the nails for the H2.5SS clips to SST.

 

[jmme]

boo1, what is the strength capacity of the 5/8" bolts? Where can I find this information? Thank you.

 

[Lion06]

You can get the capacity of the bolts, nails, screws, and members from the NDS.