Embedment depth/diam for post frame building. Something does not smell right. 1

[fastline12]

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.

 

[BigH]

you might want to look at this:

https://www.scribd.com/document/29750872/Post-Frame-Building-Design-Manual

http://nfba.org/uploads/Simplified_Lateral_Design_of_PF_Buildings.pdf

 

[darthsoilsguy2]

a little fyi....i've never designed it since these things usually belong to the utility company and they have their own designers, but it has been my impression from discussions with them that the designer considers the layout of wires and anchors and those tensions in their calculations for stability. utility poles may not be a good example.

 

[MotorCity]

If you want accurate soil data for your particular location, you need to hire a geotechnical engineer and have field testing performed. There is no crystal ball or rule of thumb that can provide accurate soil data for your site. Also, comparing utility poles to building columns is not comparing apples to apples. Utility poles are loaded by several tensioned cable is in various directions near the top of the pole. Building columns can have loads anywhere along their length and may be subject to numerous combinations of forces.

 

[oldestguy]

I have used the charts provided by Griffin (Construction Design Charts), 1967 and have had no problems. The main factor in any of these methods is selecting the proper "horizontal bearing capacity". I'll attach the chart as a jpg file and the text, if it will attach also, or another post.

 

[oldestguy]

Ok here is the chart

 

[oldestguy]

Of course the downward loading need to be addressed, in addition to the overturnig loads. For simplicity one might assume two different designs and take the one requiring the largest size as governing. I'd consider 400 #/sf bearing cap'y as very weak soil.

 

[msquared48]

400 psf/ft2/ft is not low for lateral bearing, but definitely is for vertical bearing.

Mike McCann, PE, SE (WA)

 

[fastline12]

Thanks guys. All we can tell right now is either others are using some other soil data available through a geotechnical eval, or we are simply missing something in the calculations.

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?

 

[msquared48]

So the height of the pole is only 4 feet?

Mike McCann, PE, SE (WA)

 

[fastline12]

NO, sorry. In the example, we would consider a 14ft above ground height. Both constrained and unconstrained load conditions.

 

[msquared48]

So the moment is more like 5 x 14 = 70 kip feet?

Mike McCann, PE, SE (WA)

 

[msquared48]

And would this moment be resisted by one or two poles?

Mike McCann, PE, SE (WA)

 

[msquared48]

Also, 5 kips seems high. Based on 20 psf wind the force per 10 foot wide bay would be s
20 x 7 x 10 = 1400#.

Adjust the forces for your circumstance.

Mike McCann, PE, SE (WA)

 

[TLHS]

Don't use utility poles emedments as an example. They're often unengineered at the distribution level and installed such that embedment is 2ft plus 10% of the length, and when they are designed their factors of safety can be low. It's a known issue in the industry. They can also be exempted from hurricane loading sometimes, if that's an issue in your jurisdiction.

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.

 

[fastline12]

I will try to explain more and share our specs to see how you guys might approach this. The information about utility poles is only being used as a common sense reference. We do not intend to use utility poles or their embedment data. One point of cross check may be in the wind pressure we present. We are using pressures consistent with the ASCE references.

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.

 

[msquared48]

Mind you that the old UBC code allowed you to double the allowable code table Passive pressure if the structure could tolerate lateral deflections beyond the norm, as in the range of L/120 or so. Such would be the case for a pole structure, or a telephone pole.

I do not have time to check right now, but I believe that the IBC has the same provision.

Mike McCann, PE, SE (WA)