Modeling Drilled Shaft Supports

[abusementpark]

I posted this recently on the foundation engineering board without a single response. Perhaps, I'll do better here:


Have there been any simplified methods developed for modeling drilled shaft supports as translational and rotational springs for the purposes of structural analysis?

I would assume it would be based on shaft diameter, shaft length, and some soil parameters like cohesion. I understand that any simplified procedure will be limited in accuracy since the soil response will not be linear, but it may be more accurate than simply assuming infinitely rigid supports, like most people do.

 

[JAE]

One thing that I've done in the past is to visualize what sort of deflection I would (at a gut level) expect in my drilled pier.

For example, I might perceive a 1/2" downward deflection due to loads as "just about right". Also I might think that a lateral deflection of 3/4" might be appropriate for a given situation.

With those deflections, and with the known service-level loads, I can essentially calculate a spring reaction.

I realize that this is far from scientific or accurate but it is definitely closer than an infinitely rigid condition.

 

[vandede427]

in your structural analysis program, they will be nodal spring supports. use a program like Lpile to model the shaft and apply a unit load at the top to get the stiffness.

 

[BAretired]

Interesting problem. It is a beam on an elastic foundation, the shaft being the beam and the soil being the elastic foundation. The only problem is that the soil is not perfectly elastic and its properties may vary throughout the length of shaft.

The stiffness of the pile is easily determined, so consider a pile of length L with vertical support at the bottom. Attach horizontal springs to the side of the pile based on your knowledge of the modulus of subgrade reaction at various depths in the soil.

Apply a moment, shear or both moment and shear to the top of the pile and use a simple frame program to determine deflections and rotations at various points along the pile.

BA