Drilled pier lateral load 2

[jxzohio]

What's effect of axial load on drilled pier's lateral capacity? I used LPILE to design a 0.9m dia. drilled pier and limit the deflection to 0.5 inch. I found the lateral capacity when pier in tension is significantly less than the capacity when pier in compression (about 50% reduction). What's your experience on this?

 

[miecz]

Could be your shaft is limited by the tension in the reinforcing steel. Try using more vertical reinforcing.

 

[jxzohio]

Here is my understanding after playing Lpile program for a while. Any comments are welcome.

Uplift force will accelerate cracking of concrete on a laterally load pier. The decreasing EI will make the pier to deflect more. By using a deflection criteria (0.5"), the pier lateral capacity will decrease when the tension load is increased. On the other hand, if a pier is compressively loaded, the lateral capacity will increase when the compressive load is increased.

 

[Panars]

jkxzohio is correct, but perhaps I can add to the explanation.

Concrete is strong in compression but weak in tension. When you apply a bending moment to a reinforced concrete drilled shaft, one side of the shaft goes into compression and the other side goes into tension. When the concrete starts to crack, the stiffness of the drilled shaft drops (LPile calculates the "cracked" EI), and the lateral deflections increase.

Now if you apply an axial compressive load to the shaft, the side that was in tension may not be, depending on the magnitude of the axial load. Up to a point, increased axial compressive loads will increase the bending capacity of the drilled shaft. Engineers commonly create an interaction diagram to graphically show this relationship. LPile will also generate an interaction diagram.

If you apply an axial tension load to the shaft, the opposite happens. The side that was in tension due to the bending moment is now in even more tension, and more of the concrete cracks.

Peter Narsavage
Columbus, Ohio