moment transfer at base of retaining wall

[Structures33]

Hey all - Is the moment that is resisted by a retaining wall transferred into the footing below? Such that the footing would have to resist the same moment that the wall is resisting? This would mean that whatever thickness of the wall is at the bottom, the footing would (essentially) also need to be (depending on r/f).
I always thought that the wall resisted the moment from the lateral forces and that the footing resisted the moment induced by the vertical loads. This has to do with the connection at the footing but all the moment resisting r/f in the wall needs to hook into the footing -- so what does that mean?
Thanks

 

[COEngineeer]

Any examples of retaining wall design will explain this. You will also need hook development length (l dh). So yes, the moment has to transfer to the footing somehow.



Never, but never question engineer's judgement

 

[jike]

The moment at the base of the wall has to equal the sum of the moments in the footing at each face of the wall. The soil pressure or the weight of the soil above creates a resisting moment in the footing on each side of the wall. This must balance the moment at the base of the wall.

The vertical rebar needs to extend into the footing far enough and/or hook to be able to adequately resist the moment at the base of the wall.

 

[yahoo123]

From my understanding. You do not use the same momemnt to design the rebar in the heel side of the footing. Instead you calculate the moment on the heel side of the footing using the soil load above the footing, the surcharge, and the vertical component of the active soil load. Neglect the upward bearing force below the footing.

 

[miecz]

Agree with jike. The moments at the base of the stem, the toe and the heel must sum to zero.

Assuming some of the moment goes to the toe, it would certainly be conservative to design the heel for the stem moment. I've never understood the widely published approach that designs the heel for the overburden load, neglecting the bearing pressure below. This type of overdesign is one reason cast-in-place walls can't compete with other retaining structures.

 

[msquared48]

Also agree with Jike, with a couple of additions....

The stem moment from the lateral soil pressure will equal the toe moment plus the heel moment, plus the effect of any shear key and local geometric eccentricities in the total system. The toe pressure is generated from the reactive soil pressure, the heel from the soil resistance at the rear of the wall due to overturning, and, if there is a shear key, then this will also figure into the total resisting moment equation.

Also included in the base moment distribution is the geometry of the vertical wall and associated footings and any eccentric vertical loads the whole system puts on the foundation.

Mike McCann
MMC Engineering

 

[Structures33]

Thanks for the responses. It sounds like you are all talking about the big picture of a retaining wall system. What I am asking about is the area of the footing directly below the wall. I have a moment at the base of my wall that is 1729 ft-k and the rebar is hooked and fully developed into the footing. Does the footing at that point need to resist 1729 ft-k as well?
Let's just say the wall is at the center of the footing and I sum moments about the base of the footing at the center. This is the moment that I would typically design the footing for -- not the same as the moment at the base of the wall.

 

[asixth]

Structures33,

Yes, the moment at the base of the retaining wall is transferred into the footing. You can visulise this by checking the rotational equilibrium at the wall-footing joint (refer to jike's post).

As for the reinforcement, the bars will need to be fully developed above the footing, therefore they will need to be cogged into the footing to provide adequate anchorage.

Most reinforced concrete textbook's will have a worked example for retaining walls.

 

[miecz]

I'm talking about a simple retaining wall. A wall at the center of the footing is not typical, but the mechanics are the same. With your lateral and vertical pressures, you can calculate the bending moment at the bottom of the stem and the soil pressure beneath the footing. From there you can calculate the bending moments at the beginning of the toe and the beginning of the heel. (Actually, those should be taken at the plane of the vertical reinforcing in the stem, as opposed to the face of the stem.) Those moments should sum to zero.

 

[asixth]

For the situation you described there, I would sum the moment's at the base of the wall, like you have done.

Of the 1729 ft-k you calculated, I would design both the footing heel for 864.5 ft-k, creating tension at the top face of the footing, and I would design the toe for 864.5 ft-k, creating tension at the bottom face of your footing.

So in reality, you would provide enought top and bottom steel to resist +/-864.5 ft-k only for a retaining wall that is located centrally above the footing.

 

[hokie66]

In your case, the 1729 ft-kips will be divided between the heel and toe, and the distribution depends on the dimensions, mass of backfill, etc. The toe will be in bottom tension, and the heel in top tension. The bending moments in the footing are smaller than in the wall. It is normal practice to bend and extend the main wall bars into the bottom of the toe.

 

[miecz]

asixth-

I disagree. The moment in the toe will be greater than the moment in the heel.

 

[COEngineeer]

The moment of the heel is calculated from all of the load from the soil above the heel acting in the middle of the heel length. Just look for retaining wall design example and it should have all of these things people talk about.

Never, but never question engineer's judgement

 

[hokie66]

miecz is correct. The toe moment is larger than the heel with a central wall. Just do the statics by summing the moments about the toe, and you will see.

 

[cap4000]

I have the Ram Advanse Retaining Wall Design Module and it always has an unbalanced moment at the base. I don't know why that is as I never seen it in any book. I also have the Quick R Wall Program but it does not do it that way. Something does not make sense as the summed moments should be zero. Otherwise the footing would be unstable with an unbalanced moment on it. I have done well over 30 retaining wall jobs and never had a problem.

 

[gwynn]

It may be that they are giving moments at the face of the wall, not the centreline. The moments at the face of the wall are typically those designed for, but using those forces in a moment summation is not accurate for evaluating equilibrium conditions.

 

[msquared48]

Asixth:

I too agree with miecz here. You cannot ascribe 50% to the toe and 50% to the heel - you have to look at the soil pressure diagram. P/A +- M/S.

In the scenario you describe, the toe forces would definitely be resisted by the soil pressure. The heel, however, may or may not see full soil bearing over it's length.

Ideally, the soil bearing diagram is a rectangle for P only, and either a trapezoid or triangle for P + M over the entire width of the footing, or a triangle over a portion of the footing. In any event, there is more soil bearing, or force on the toe than the heel when you diagram the soil bearing pressure prism.

Mike McCann
MMC Engineering

 

[asixth]

hookie, miecz and mike,

Correct, I can see that the moment at the toe and heel will not be equal. I was applying pure elasticity when I was distributing the moment to the toe and heel, but I forgot one key fact...soil cannot take tension. You always need to think a bit harder when dealing with soil mechanics.

 

[cap4000]

I have the 2002 CRSI Design Manual and on page 14-12 there is an alternate way noted as "g" to determine the Heel,Toe or Stem Moments. Essentially it sums up all moments about the base including the footing shear key moment.