Bored piers subject to axial tension and shears 2

[ap7775]

Does anyone know of any interaction formulas or methods for calculating bored pier diameters and depths subject to axial tension and shear loads?
A typical example is a pier to support one of the columns in a trussed tower. The column baseplate/pier is subject to both tension and shear loads simultaneously.
I am aware of Brohms methods, but how to combine with tension forces?

 

[asixth]

You can design for the two load cases independently. For pull out you are looking at the hold down provided by the soil, whether this be skin friction alone or you provide some anchoring. The passive resistance of the soil due to shear force can be analyses separately. The two will be interactive for the structural design of the pile, you will need to look below the balanced moment on the interaction diagram and see what is the capacity of combined tension and moment of the pile. This is a structural requirement, as far as I am aware to satisfy geotechnical requirements you need to ensure the soil has passive resistance to resist the shear and skin friction for hold down with seperate factors of safety. As long as this is satisfied the the cast-in-place pile has been designed.

 

[rowingengineer]

I agree with asixth,
From all the references I have reviewed over the years there has never been any mention that lateral load on a short cast in place pier would reduce the axial tension capacity.


When in doubt, just take the next small step.

 

[VoyageofDiscovery]

This is an interesting topic as I believe past design philosophy just designed piles specifically shear capacity and others for tension.

I believe the OP needs to deal with the interaction and thus it should be accounted for with the basic structural interaction formula for tension and shear.

VOD

 

[csd72]

In my (not so expert) opinion:

Cohesive soils - the cohesion is not overly affected by pressure and therefore would resist uplift to basically the same extent regardless of lateral load.

Frictional soils - friction is proportionate to pressure. In a laterally loaded pile the stress is increased on one side and reduced on the other but the overall net pressure remains basically the same along the length.

This assumes that there is not a gross deformation of the soil due to excessive deflections, if this occurs then friction/cohesion may be reduced in the disturbed soil.

It would be interesting to see if there are any papers on this.