Drilled Pier Design with Moment

[simon17]

I have a small 18"dia x 42"deep drilled sonotube pier being installed for a swing gate. Top of pier will be flush with grade. The gate will impart vertical axial load on the pier and a bending moment. No shear loads. IBC 2012 1807.3, equation 18-1, provides a depth calculation using lateral soil bearing pressure, diameter and applied lateral load. However, in this case, there is no lateral load, but bending moment instead. Is there a way to design the pier for a moment instead?

 

[phamENG]

M=Ph

Find an equivalent lateral load and height combination that gives you the same moment. You'll have to pretend there is a lateral load - you'll have to feel your way through the possible combinations to find the best one. Draw out the soil/structure interaction that corresponds to each iteration's results and pick the one that seems closest to reality.

 

[azcats]

I derived and equation for this about 15 years ago that I haven't looked at in 10. It's based upon the same theory as the IBC formulae.

Unfortunately I did a poor job documenting it. I could have sworn it was from something in SRE's website, but I can't locate it right now.

The formula in my notes is: d = [(M) / (.381 * b * S1)]^.5
Where the variables are the same as in the code. Still iterative to find S1.

If you want to approximate as suggested above by phamENG, I would recommend using a relatively small lateral load (P) with a relatively large (h). Otherwise you throw off the statics in the derivation of the code formula. Something like a 10# lateral load at whatever 'h' it takes to get your moment.

 

[BridgeSmith]

As a starting point / comparison, I input your pier dimensions in Allpile with loose sand for soil, applied a 500 ft-lb moment, and got about 1/2" of deflection. I got the same deflection with a 200 lb lateral load applied at the top of the pier.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[azcats]

I'll also say that I wouldn't be liberal with this design. I've seen so many gates drooping and tilting over the years. An extra foot or two in depth isn't that much cost. And with only 42" embed, if there's a foot of disturbed/loose soil at the surface your design no longer works.

 

[RPMG]

P.equiv = Weight x Mid Gate Width / Mid Height