Lateral Shaft Capacity 1

[Atooz]

Trying to design a drilled shaft which has high moment (1200 kip-in) and axial load of 10 kips. The steel column on top is concentric to the shaft. But e =M/P = 120 in which is too high. Trying to use column interaction curve ACI c4-60.6 and C4-60.7 to come to the required size but while calculating Ag = Pn/(fc'*Kn) for the shaft I get very less value (about 5 sq inch) due to Pn being very low. Using 1% reinforcement. Almost a pure bending case

Any thoughts on how to reach at shaft size?

 

[WARose]

Using some tables I have (based on 1.5" of cover, 4 ksi concrete, and 60 ksi steel), I came out with a 16" diameter column with 7-#8's. Actually I could get a 14" to work.....but not without too much steel. This also assumes the loads you give are ultimate (i.e. factored). You will probably need more cover, but this gives you an ballpark idea of what you need.

I'd check deflection here as well. From the title of the thread, this appears to be caused by a lateral load. So you need to check that out. (I'm not sure if your load included any P-Delta or not.)

 

[Ingenuity]

As you indicated, it is near pure bending. K_n = 0.013 based upon a 16" dia section.

If you enter the interaction diagram based upon a value of R_n then you will get a trial size and rebar %.

 

[BridgeSmith]

If the 1200 kip-in moment is the applied moment at the top of the shaft, it is not the max moment in the shaft. That will be determined by an analysis of the shaft bending using the lateral resistance of the soil around the shaft. This is most often accomplished using a P-y curve method, such as the that used by Allpile (COM624P) or Lpile. For this, you will need soil properties for the embedment material.

 

[Atooz]

Thanks for all the replies but don't you all think that lowee region of curves are amounting to high strain and lot of tension developing into concrete. I was worried about the tensile stresses. Some books say that if monet too high comapraed to the axial load than it should be designed as a beam and not column. And yes it includes P-Delta effect.

 

[WARose]

A lot of times I just take the guesswork out of it by designing for the max. moment + P-Delta while forgetting about any help from axial compression (if there is any).

It can't get much worse than that.