High shear at pier-bedrock contact

[aeoliantexan]

We are using LPILE to analyze existing 48-inch diameter piers embedded in good limestone and supporting a pump station in a lake. A slide in the overburden soil is loading the piers laterally. The resulting shear diagram shows a high shear just below the top of the bedrock. The shear is much higher in magnitude than the value right above the bedrock and of opposite sign. I know the shear is there to develop a couple with the necessary moment. The structural engineers say they never consider this shear in design. Why?

 

[msquared48]

Is the limestone the "bedrock", or is the bedrock below the limestone layer?

If it is the latter situation, then the situation is similar to a pole footing analysis, and the "shear" seen is raally lateral bearing couples between the limestone and bedrock on the 48" piles. Except in the 48" piles, shear really does not even enter into the picture in this scenario.

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

 

[aeoliantexan]

The limestone is the bedrock,and the shear within the pier is the concern.

The piers were installed with a temporary casing extending at least 5 feet into the bedrock and a permanent steel casing extending 3 feet below the temporary casing. The rock socket is 7 feet long below the permanent casing. I would expect, therefore, that the pier is loose in an oversized hole to the top of the rock socket. The lateral support from the rock develops abruptly, but the rock at the top of the socket is confined by 8 feet of overlying rock and 12 feet of overburden soil, so it can develop high lateral resistance there.

 

[msquared48]

Using working loads, and no shear reinforcing, the shear caopacity of the concrete in each pier is somewhere in the neighborhood of 90 Kips, assuming 2500# concrete, which is probably higher.

What are you recommending as the lateral load to each pile?

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

 

[aeoliantexan]

The platform has some restraint from other piers that are outside the slide mass and from the steel discharge pipe. The calculated lateral soil load to the pier is about 240 kips. LPILE gives a shear of about -400kips just below the top of the rock socket.

The concrete was specified to be 4000 psi. It is 35 years old, and I'm assuming it's at least 4500 psi now.

The pier is heaviliy reinforced with 28#11s and has several hundred kips of compressive load. The structural engineer tells me the ultimate shear capacity of the pier is about 400 kips. But he never considers the negative shear within the rock socket. My original question was: how is this justified, and it it common pratice?

 

[dcarr82775]

I believe the high pier shear you are referring to occurs in the drilled pier once it is in the rock. If this is the case, the high shear doesn't matter because it is not possible to fail the pier in shear unless you fail the rock first. If the rock fails then the shear doesn't develop.

 

[aeoliantexan]

That is kind of what I thought. Thank you all.