Laterally loaded Pile. Point of zero shear

[tmoe]

This is a cross post. Not sure if thats frowned upon, but I know you structural folks will have good input:



Happy New Years Eave everyone. I have a question that I know is relatively basic, but I cant seem to wrap my head around.

At the most basic level of my design, I have a pile that will be embedded into native soil some distance to be determined. It will also be exposed about ground a distance of about 5.5 ft, and functioning as a retaining element of wall. I need to stress that this is a SMALL project and Im not looking for a rigorous driven pile analysis. for now using the embedded pole formula in Enercalc and the California Building Code is just fine. I am curious though how to approach a simplified bending design for the pile. My mentor gave me the following instructions before he left out of the country on vacation:

" Resolve the the forces on the retaining side of the wall such as active pressure, surcharge, wind load if applicable, into a resultant force. Do the same for the passive pressure side. Now determine the location of the point of zero shear. This is where the moment will be maximum. Design the post for bending based on this moment."

My confusion is that when I draw a simply picture of this scenario, I'm not sure what assumptions Im making in terms of where this pile is supported. Am I assuming pinned supports? If so where. As is, all I see is a vertical beam with two unequal loads, both of which will cause the pile to rotate counterclockwise.

Can you guys help me resolve this confusion?

 

[dcarr82775]

Find the point of zero shear. For your purposes assume a rigid cantilever support exists at the point of zero shear, and sum the driving and resisting moments about that point (use only the forces above the point of zero shear in your sketch, ignore everything below it). That is the moment your mentor tells you to use.

 

[tmoe]

dcarr82775, thank you for your input. The the rigid cantilever support concept was exactly what I was looking for!

Cheers

 

[tmoe]

how exactly do I find the point of zero shear in the first place though? It seems that I would need support conditions in order to do this??

 

[AELLC]

I don't have Enercalc any more, but as I recall, the maximum moment is assumed to occur at 1/3 depth, but for some reason the output shows only the moment at ground surface - so I think it would be conservative to extrapolate that moment to the 1/3 depth by using the ratio Moment@1/3 = Moment@Groundsurface X (L+0.333 X embed depth) divided by L, where L is the moment arm measured form the resultant of the applied load to the ground surface.


Perhaps the program RetainPro would do this design better, give the Enercalc guys a call because they own RetainPro now.

The problem with embedded "poles" on Enercalc is that it only analyzes the required embedment depth but not the "pole" itself

 

[AELLC]

I believe this attached shows the statics of all this

It is easy to say maximum moment occurs at zero shear, but how to determine zero shear is unknown in this situation.

 

[paddingtongreen]

There is a very different result if the pier is constrained at the surface, perhaps by a slab.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[dcarr82775]

Just sum the forces in the horizontal direction. The driving forces are easy. Now calculate what you are using for your passive EFP multiplied by some tributary width of your own choosing to account for the individual piers. Solve for what depth your passive forces will equal your driving and that is your point of zero shear. You can trial and error it in Excel or I think you end up with a quadratic if you want to just solve for the exact answer.

Don't use 1/3 the embedment depth. Not a terrible number for a trial, but it varies with soil properties. Figure it out yourself based on the actual soil properties you are working with.

 

[BAretired]

The pressure diagram and resultant diagram in the original post is not correct. The pile is not in equilibrium under those forces. You need another force near the bottom in the same direction as Ra. If it is called Rb, then Rp = Ra + Rb. Also the sum of moments of the three forces about any point must be zero.

The pressure distribution shown by AELLC is about right, but an exact solution is not possible without considering the stiffness of the pile and the properties of the soil.

BA

 

[JLNJ]

You can come up with a solution to AELLC's method if you grossly simplify the system to uniform side-bearing pressures. It ends up as being a simple quadratic equation. I usually choose my pressure zone lengths based on my best trial guess of the pier embedment. After you solve for the bearing pressure you can look at that pressure and see how reasonable it is. Set up the quadratic using the equilibrium equations. Or course, this involves quite a few assumptions, so be careful.

Short piers often work in real life but it can sometimes be difficult to get the numbers in the analysis to work out, especially if you discount the first few feet of embedment since the soil is "disturbed".

One way to get the additional horizontal force BA mentions is through friction at the bottom of the pile. Of course, you need dependable vertical loads at the bottom of the pile to give you that friction force.

Sometimes I look at the old UBC method as a baseline, but I find that it yields fairly liberal results.