Shallow drilled Pier

[RFreund]

Looking for some direction on how detailed to get on evaluating shallow drilled piers (about 4' deep) subject to Gravity, uplift, shear and moment forces.

All loads are relatively light P=2.75k+- V=1k+- M=6K+- U=5k+_

I'm wondering if the following procedure seems applicable. I would use the concrete weight to resist uplift. Check bearing as P/A possibly add M/S. However I'm not sure if I would need to add M/S if I can argue that M and V can be taken out by passive pressure (similar to pole embedment). If so checks would be:
Wconc>U
P/A less than allowable bearing capacity
V+M/d < less than allowable passive (pole embedment calc)
This may be a problem because I'm not sure how to accurately handle the moment, not sure if M/d is correct or if I should use some equivalent lateral force above the pier.

See attached sketch as well.

Thanks

EIT

 

[BAretired]

It depends on where the building is. In my neck of the woods, Alberta, Canada, that foundation would be totally within the active frost zone and could be expected to move up and down with seasonal changes.

I would not count on it to resist moment and would expect it to move laterally if the horizontal load was significant. I would not rely on skin friction, but the end bearing would carry some gravity load if you clean the bottom of the excavation before pouring concrete.

BA

 

[azcats]

4' is pretty shallow to get much capacity out of an embedded pole foundation. I would be worried about deflections and I would be hesitant use the increase of 1804.3.1 unless I was sure it wouldn't impact the integrity of the structure above.

I would handle the moment by applying V at a height to get M at grade.

 

[ATSE]

Unless you are in 60+ blow count well-cemented soil, 4' deep drilled pier is actually just an isolated footing.
Very small rotations create a "pin" effect, so moment stiffness and effective moment resistance is near zero (unless embedded in rock).

 

[RFreund]

Thanks,

ATSE-that is a good point.

So I take it the flagpole embedment equations (IBC 1805.7.? - I think) may not be applicable to this situation. I admit I don't know much behind the equations.


EIT

 

[beton1]

How many piers? Would it not be more effective to excavate and use conventional spread footings. Sketch shows this as a building.Is it open?

 

[RFreund]

Thanks for the input. There are 20 or so piers, about 12 O.C.

The building enclosed, it is a greenhouse.

EIT

 

[msquared48]

From what you describe, and being a greenhouse, an agriculural building, I would design this as a pole type structure. embedding the poles of the greenhouse into the augered holes. The 1/2" tolerance at the ground line should be able to be tolerated with proper detailing for the allowable passive pressures listed in the IBC tables.

Based on the allowable soil values, there are equations to increase the allowable as the pier is drilled deeper, plus the uplift will drive the depth, and size too.

If you drill the piers, do not use spread footings for bearing, only the diameter of the pier.

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

 

[BAretired]

Where is the structure? Is it in Indiana? Do you have frost? What type of soil exists at your site...sand, silt, clay, peat moss or a mixture of all these? Is it frost susceptible? Do you care if there are seasonal movements as a result of frost?

The best advice I can offer is to get a soil report from a competent geotechnical engineer, then review the many options for a foundation for this building.

BA

 

[RFreund]

Thanks again for the replies.

Frost line is at 42" below grade. Soils are gravely clay or Clayey gravel (not sure which is correct). However we have not received the official soils report.

I believe they want to use drilled piers because they are thinking it is most economical/cheaper but I will present the option of an isolated square footing to the boss as well.

EIT