Embedment depth/diam for post frame building. Something does not smell right.
Embedment depth/diam for post frame building. Something does not smell right.
(OP)
We are working out some calculations for depth and diameter for posts in a post frame building. We are coming up with some rather ridiculous values. Not off the wall, but certainly way conservative.
So we decided to examine something that works.....electric poles. By our calculations and soil mechanic values based off the EP486. from the ASAE and IBC, we are looking at electric poles needing embedment depths of 14ft or more. We already know they are being installed at 4-6ft and usually just break off well above the ground line here.
So we have a feeling either our math is right and our soil data is wrong, or we have our zeros mixed around....
Based right out of the EP486.1, we have some design values for our soils somewhere around 400lb/ft2*ft. We also have concrete backfill in ours which further increases the area.
I am curious how we might verify or obtain more accurate soil data in our area to verify this part of our work?
Right now we have columns spaced on 12ft centers, 5ft deep, with 3ft diam of concrete around the side wall columns.
So we decided to examine something that works.....electric poles. By our calculations and soil mechanic values based off the EP486. from the ASAE and IBC, we are looking at electric poles needing embedment depths of 14ft or more. We already know they are being installed at 4-6ft and usually just break off well above the ground line here.
So we have a feeling either our math is right and our soil data is wrong, or we have our zeros mixed around....
Based right out of the EP486.1, we have some design values for our soils somewhere around 400lb/ft2*ft. We also have concrete backfill in ours which further increases the area.
I am curious how we might verify or obtain more accurate soil data in our area to verify this part of our work?
Right now we have columns spaced on 12ft centers, 5ft deep, with 3ft diam of concrete around the side wall columns.





RE: Embedment depth/diam for post frame building. Something does not smell right.
https://www.scribd.com/document/29750872/Post-Fram...
http://nfba.org/uploads/Simplified_Lateral_Design_...
RE: Embedment depth/diam for post frame building. Something does not smell right.
RE: Embedment depth/diam for post frame building. Something does not smell right.
RE: Embedment depth/diam for post frame building. Something does not smell right.
RE: Embedment depth/diam for post frame building. Something does not smell right.
RE: Embedment depth/diam for post frame building. Something does not smell right.
RE: Embedment depth/diam for post frame building. Something does not smell right.
Mike McCann, PE, SE (WA)
RE: Embedment depth/diam for post frame building. Something does not smell right.
We double checked a very standardized design of a post frame building in our county in which, if these general design parameters are followed, you do not need to even get engineered drawings. This would be a 40x60x14 building. By examining their specifications and comparing to our calculations driven by the soil properties from the ASAE/NFBA/IBC,etc, etc, we determined that their design is actually in the red by a large margin.
FYI, the vertical loading all looks good. The design is being driven by the lateral forces in the soil.
If anyone is will to help here, I would like to pitch a load case to check some math?
1ft round pole, 5000lb shear, 20,000 ft/lb moment, class 4 soils. How deep does this need imbedded?
RE: Embedment depth/diam for post frame building. Something does not smell right.
Mike McCann, PE, SE (WA)
RE: Embedment depth/diam for post frame building. Something does not smell right.
RE: Embedment depth/diam for post frame building. Something does not smell right.
Mike McCann, PE, SE (WA)
RE: Embedment depth/diam for post frame building. Something does not smell right.
Mike McCann, PE, SE (WA)
RE: Embedment depth/diam for post frame building. Something does not smell right.
20 x 7 x 10 = 1400#.
Adjust the forces for your circumstance.
Mike McCann, PE, SE (WA)
RE: Embedment depth/diam for post frame building. Something does not smell right.
Allowable deflection on a structure are also significantly less than on a utility pole, which is what you're generally controlling for when you're analyzing lateral capacity of something like this. Ultimate capacity never comes into play. It's pretty normal to see utility poles that have deflected out of plumb to a level that would be unacceptable for a building.
Also, are you sure the framing of the example buildings are what you think they are? If you can restrain moment at the top of the posts using knee braces, or running the posts from floor to ceiling with a shearwall or something similar you could pin the base, which would significantly lower your embedment requirements due to the reduced base moment.
RE: Embedment depth/diam for post frame building. Something does not smell right.
Targeting an exposure C condition
Building specs 60x100x 17ft sidewall
3:12 pitch freespan
Columns are glulam 5ply 2x8 yellow pine on 12ft centers
Soils are sandy clay loam
End walls designed as shear walls and diaphragm roof loading
There are two load conditions. Our min code requires a 75mph cont/90 3sec gust. We need to at least achieve this with no concrete floor or unconstrained conditions.
A floor will be poured a few months later in which we are targeting about a 130mpg rating with windward side constrained by the floor and unconstrained on the leeward side but are examining the possibility of pinning the columns into the floor to get constraint on the leeward wall. The concerns there is settling causing concrete cracks over time.
Obviously the columns under highest load are the ones at mid length on the side walls.
Again, our county minimums are much lower and this is our own building so we will not be documenting or signing off the higher rating, but we know the building will be subjected to 100mph a few times. 130mph is likely only from a tornadic event in which all bets are off.
Our design right is is 6" thick concrete cookies on the bottom of each bore hole for vertical loading. Bore hole to be 2ft diam x 4ft deep, filled with min 3000psi concrete and rebarred.
Our county roof loads are much higher than we would ever really experience so we are not pushing beyond the minimum which is 10D, 15S, 20L. Our design will see a realisic 3-4D which completed, and 15S would never happen.....But we have to design for it.
RE: Embedment depth/diam for post frame building. Something does not smell right.
I do not have time to check right now, but I believe that the IBC has the same provision.
Mike McCann, PE, SE (WA)