Cracked EI Analysis for Drilled Shafts

[ktwannabe]

We are running COM624 for a drilled shaft design. I am researching the behavior of the shaft when using a cracked EI analysis as I am not sure of the implications of this analysis. The deflection are less if I assume cracked EI. Can anyone point me to a good reference for this topic?

 

[Polecat]

ktwannabe:

I am puzzled by your statement: "The deflections are less if I assume cracked EI." My experience has been just the opposite.

If we are talking about a laterally loaded drilled shaft with a rebar cage in it, then you essentially have a concrete column subjected to a high moment. Under bending, concrete remains uncracked only until the modulus of rupture is reached (at approximately 475 psi for a normal 4000 psi foundation). From that point on, cracking occurs and will propagate in depth and frequency as the bending moments increase.

ACI 318-02 discusses the treatment of cracked moment of inertias at some length in Chapter 10.11 and R10.11, indicating that the Ig should be reduced down to as much as 0.35Ig under large bending stresses.

Since deflections are in inverse proportion to the EI product, any reduction in Ig will cause a direct increase in deflection. Hence, my confusion as to why you said you got less deflection with cracked sections. You might want to recheck your program inputs to be sure that you are telling it to use a cracked EI stiffness.

For more info, you might also want to visit this site:

http://ensoftinc.com

, the developers of L-Pile4, which, I believe is actually the next generation of COM624.

http://www.spiraleng.com

 

[Focht3]

Polecat is right - deflection is inversely proportional to the EI of the shaft.

COM624 is not set up to model variations in the section modulus with applied moment. You will have to get a copy of LPILE4 to do that.