Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations SSS148 on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Pinned base in a concrete wall - is it really achievable? 3

Status
Not open for further replies.

Lion06

Structural
Nov 17, 2006
4,238
I have a somewhat theoretical question regarding concrete walls as they are a reasonably popular topic of discussion on this forum.
I, as many others, often design concrete foundation walls as pinned-pinned when there is a first floor diaphragm to tie into. I understand people have been doing this forever and I am not questioning the practice. My question is this: Is a pinned base really able to be achieved in any typical detailing that anyone does?
I am thinking that any rebar tying the wall to the footing is providing some moment capacity. I understand the pinned-pinned case is the worst case for the wall, so that is being conservative is you get some end moment capacity out of the foundation to footing connection. Assuming a pinned base is unconservative for the footing since there WILL be some moment at the base. Even if the steel yields and a "plastic hinge" forms at the base it doesn't lose its moment capacity.
Also, there must be some steel at the interface of the footing/wall since they are usually seperate pours and the steel is need for shear.
Any opinions on this would be greatly appreciated.
 
Replies continue below

Recommended for you

StructuralEIT,

The point everyone else here is trying to make is that load is attracted to the stiffest path. Forget about moment capacity in the wall-footing interface for a minute. If the footing can rotate enough to "pin" the base of the wall, then the fixed end moment at the base of the wall will go somewhere else (up into the span). We're saying that the wall-footing assembly pins itself at the bottom of the footing. There won't be 17kf under the footing, because the rotation of the bottom of the footing has already released the moment to go elsewhere (up into the span).

Oh, and for the record, the rotation formula I listed above is for uniform load on a simple span. If this is a basement wall, with triangular load, the formula would be a little different.

DaveAtkins
 
StructuralEIT, I sometimes design basement wall as pinned at the bottom (fixed fixed pinned at bottom). The reason you can say it is pinned is because the footing is so narrow therefore it cannot perform like a cantilevered retaining wall. The toe pressure from the overturning moment is so large so the sub-grade should fail and the footing should rotate (pinned) against the slab. That's the way I look at it. Correct me if I am wrong.
 
AlmostPE-
Thank you!! You are the first person to say that. So the soil at the toe would have exceeded the allowable bearing pressure and it will allow the footing to rotate, relasing the moment back into the wall?!
That is what I was getting at.
The question I was asking is if this is acceptable to have the allowable soil bearing exceeded and does this cause any serviceability issues?
 
StructuralEIT:
You are correct in your last post. Technically the soil would have to yield or "fail" just a small amount for the footing to rotate. However, the length that would have failed is very, very small and the remaining footing still has full bearing capacity.

At this point the footing has rotated a very small amount releasing the moment back into the wall, and you have essentially full axial bearing capacity beneath the footing. No serviceability issue here.
 
Structural EIT

You are correct that there is additional soil pressure at the bottom of the wall due to the moment at the bottom of the wall. You are also correct that the maximum moment at the bottom of the wall is limited by the strength of the vertical reinforcing at the top of the footing. However, the maximum moment at the bottom of the wall is also limited by the strength and stiffness of the soil at the interface with the bottom of the footing. If your soil were very strong and stiff, then the reinforcing limit would govern. In most cases, though, the soil stiffness governs the upper limit of the moment.

If the soil governs, the moment at the bottom of the footing is proportional to the soil stiffness and the rotation at the bottom of the wall. The rotation at the bottom of the wall is indeterminate, but itself has an upper limit. The maximum possible wall rotation is the rotation that the wall would have if it were pinned at the bottom.

So, if you calculate the maximum possible rotation at the bottom of the wall and multiply that by the width of the footing, you have the soil deflection due to the maximum rotation. If you multiply that deflection by the soil stiffness (k/in) you have the maximum additional soil pressure due to the wall rotation. If you compare the moment due to the additional soil pressure to the upper limit due to the wall reinforcing, the soil limit should (hopefully) govern. And, the additional soil pressure should (again, hopefully) be small enough to be ignored.
 
The soil doesn't have to exceed the bearing capacity to allow rotation, it moves as a function of the loading and subgrade reaction modulus.
 
The soil bearing capacity doesn't need to be exceeded to allow the rotation, but would you agree that a higher bearing stress will occur at the toe as a result of this rotation? It may not necessarily exceed the allowable, but it will be higher than under pure gravity load, correct (and lower at the heel)?
 
StructuralEIT, when I desing a basement wall with pinned at the bottom, I do not reinforce the concrete footing. The only thing it has is the dowel going into the wall. I am just using the footing as a base for the wall. I dont really care if it cracks since you wont see it anyway. So theoratically it should rotate at the stem connection onto the footing.

Now if you do reinforce the footing and also has enough hook embedment length, it will rotate at the base of the footing. Yes, the soil will not be able to resist the toe pressure and it will start to rotate but it will only rotate so much because the horizontal rebar will transfer the moment to the both sides of the wall.

I am just using my engineering judgement on this design(I am sure you do that a lot). Other engineers in my office seem to agree with me. The didnt agree with me at first.
 
StructuralEIT-

I think you are right. There is some additional bearing pressure at the toe due to the rotation. However, I think the additional pressure is small and can be ignored, in most cases.
 
AlmostPE-
You do reinforce it transversely, don't you? If you do not and it cracks, such that your 3'0" wide ftg is now 2'0" then you will likely exceed your allowable bearing stress and have problems, no?
I can almost see not reinforcing it longitudinally, because no load is getting transferred in that direction, it is really on for T&S.
 
Usually we let the narrow footings (2 or 3 feet wide) work as plain unreinforced concrete. We have longitudinal bars for shrinkage and add transverse bars at 48" oc to wire them to so that they don't roll off the bricks into the sand as they are pouring concrete.
 
jmeic-
Thank you. That is all I was looking for.

All,
This is my understanding now (which makes sense) - There will be some moment at the base of the footing due to the reinforcement from wall into footing. Unless the subgrade is infinitely stiff, this will be less than the capacity of the connection at the wall/ftg interface due to the finite stiffness (some give) of the soil. This moment can be ignored as long as the wall is designed as pinned at the base.
 
The unreinforced footing I use is 1'-6. It is only sticking out 4" both sides of the wall. Plus it does have a hook @ 16" o.c.
 
A couple more thoughts.

First, the vast majority of these footings are have design bearing pressures nowhere near the allowable. Thinking back, I can't think of a single one I've ever designed that wasn't like this. I'd make the footings 2'-0" just by default. If I have 3 ksf soil, I need 6 klf to have a problem. It's fairly hard to have a wall with 6 klf, although I'm sure they exist. All the heavily loaded walls I can recall ended up on footings on solid rock.

Second, the allowable bearing pressure in your geotech reqport is for larger scale bearing failure, like the pressure bulb stuff you see in geotech books. I'm not sure whether very localized little areas of high pressure really are of any consequence.

I still invite you to do one or both of the following:

1. Make a little model of a wall with footing on soil springs. See what moment is generated at the base.

2. Calc the rotation at the bottom fo the wall in the upper bound case of being pinned. Calc how much the footing tip would be pressed into the soil to make that rotation happen.
 
Stiffness attracts load, if you reinforce you must then account for the potential to attract load. If sufficiently reinforced you must take the load into the connected element the footing the mat what ever it is. Modulus of sub-grade reaction the final stiffness your dealing with. Where the heck does it come from? Spring constants leave it to the dirt guys. pinned pinned well then ensure you reinforce accordingly and pick up the remaining resultants. In anycase fixed at the base of the wall we do and then transfer this load into the sub-soil at the toe(S) rocking otherwise. We keep all our foundations in the elastic realm period. Not sure about this plastic hinge thing would imply yielding and ability to transfer load ie fixed. Anyways what's the slab in the basement have to do with it, unless you detail it accordingly? Never designed a mat that wasn't rigid which was my comment, long ago, otherwise. You can detail a pinned connection or fixed connection and a footing size will result based on the dirt guys opinion of either the allowable bearing capacity or the modulus of sub-grade reaction,one ULS and the other SLS.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor