moment transfer at base of retaining wall
moment transfer at base of retaining wall
(OP)
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
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






RE: moment transfer at base of retaining wall
Never, but never question engineer's judgement
RE: moment transfer at base of retaining 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.
RE: moment transfer at base of retaining wall
RE: moment transfer at base of retaining wall
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.
RE: moment transfer at base of retaining wall
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
RE: moment transfer at base of retaining wall
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.
RE: moment transfer at base of retaining wall
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.
RE: moment transfer at base of retaining wall
RE: moment transfer at base of retaining wall
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.
RE: moment transfer at base of retaining wall
RE: moment transfer at base of retaining wall
I disagree. The moment in the toe will be greater than the moment in the heel.
RE: moment transfer at base of retaining wall
Never, but never question engineer's judgement
RE: moment transfer at base of retaining wall
RE: moment transfer at base of retaining wall
RE: moment transfer at base of retaining wall
RE: moment transfer at base of retaining wall
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
RE: moment transfer at base of retaining wall
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.
RE: moment transfer at base of retaining wall