Influence of Counterforts in Ret Walls 1

[TonyES]

In a cantilevered concrete retaining wall what are some standard practices that any of you take for including the influence of either a 90 degree bend in the wall or a counterfort. I have typically used a 1 to 1 ratio but think that may be unconservative. I was wondering what other engineers do. Thanks

 

[teleBob]

The best way I have found to look at realistic behaviour of a retaining wall with frequent jogs (like a walk-out basement wall for a custom residence) is to model it in 3-D using Finite Element Analysis, like in StaadPro, Risa 3-D, etc. If you have width-to-height aspect ratios less than 3, you will be surprised how much of the wall works in horizontal bending. We often place a single mat (for 8" and 10" walls) of reinforcing steel in the center of the wall thickness to work for all directions of bending, with added dowels at wall corners and into footings at strategic locations and spacings.

 

[canstruct]

This all depends on the height and allowable bearing capacity. A rule of thumb is to have a base equal to 0.6 times the height of wall, but when you have low bearing capacity this rule does not work and this ratio might approach to unity.

 

[TonyES]

I've done a brief finite analysis and was a bit confused on the results, what I'm looking for is a way to quickly and conservatively treat a bend/counterfort in a retaining wall. The situation I have now is a long 12' tall ret wall that has 10' long counterforts every 15', the wall has already been built and only has 3' footings, so clearly a simple analysis without including the counterforts says the wall is overturning and the eccentricity is beyond the footing width. If I assume the counterforts to help the overturning for a 1 to 1 ratio than the wall is OK, what do you guys think??

 

[teleBob]

As long as the counterfort wall is properly connected to the retaining wall, you should be able to depend on it, its footing and the soil weight above its footing to resist overturning. I would just check overturning on a 15 foot section of wall, or put 15 feet of soil pressure on the counterfort section. It's hard to imagine that the wall would tear away from the counterfort in an overturning failure, although there may be substantial horizontal bending along the top sections of the wall between the counterforts. That's where I have seen problems in similar situations, especially when the reinforcing mat is on the earth side of the concrete.

An FEA analysis is most helpful to see bending forces in wall elements and, if the footings are also modeled with springs (compression-only springs, spring constant = Modulus of Subgrade Reaction), to see soil pressure distributions. Stability for overturning is probably easier by free-body hand calculations.

Does anyone else have experience with retaining wall FEA modeling?

 

[connect2]

Try:
Moments and Reactions for Rectangular Plates, U.S., Bureau of Reclamation, Monogram 27, W.T. Moody. Should be able to find it on the web, formed the basis of,
PCA, Rectangular Concrete Tanks. Try also;
Bowles, Foundation Analysis and Design,, 12-14 Design of Counterfort Retaining Walls.
Don't find much Finite Difference work, but thats the analysis method utilised in the first two references.

 

[connect2]

Oh yes, not mentioned in any of the above, if the Counterforts and wall are poured as one, dry shrinkage between the two orthogonal elements can be a problem.
I have Monogram 27, let me know how you make out. I'm only mildly in the computer age though.

 

[connect2]

Yes, was just reviewing the post, soil foundation interaction is very nebulus and requires a great deal of conservatism. Stability analysis critical to all the above. This is a pretty complex structural form. Try to keep the heel big enough to maintain a fairly, and I say fairly again, unform contact pressure across the base of the footing. The connection condition of the wall to the footing, pinned or fixed, neither, re soil interaction requires a parametric study, and the involvement of the dirt guy to determine rotation characteristics of the wall and footing for the various contact pressures.