Calculating tension in lowest grid of MSE wall 2

[jsnider]

We have a 2-tier MSE wall. When using Keystone Keywall, we get two different values for tension in the lowest grid (from chaning the design preferences option “Assume zero friction at base for tension in lower grid”). What is the reason? How is this calculated? Which should we use? It appears "Assume zero friction" is AASHTO's approach and using friction is NCMA's approach.

 

[RFreund]

I believe AASHTO and NCMA both assume zero friction at the base (see the keywall help - screens - design preferences).

However the only difference is the tributary are to to the lower grid.
If you assume zero friction at base then the tributary width to the lower layer is half the distance to the layer above and the foundation.
Using friction the distance would half the distance to the layer above and half the distance to the foundation.

Where the Tension is proportionate to the trib area (F=g*k*D*A)

If you wanted more info, the engineers at Keystone are pretty helpful

EIT

 

[jsnider]

This makes sense now for the tension on the lowest grid:
(a) if you assume zero friction at the base between the reinforced soil and the foundation soil, the entire reinforced soil zone would be assumed to move under active pressure without friction; thus the tributary area for tension in the grid and against the block would go fully down to the foundation level and 1/2 way up to the overlying layers.

(b) if you assume friction at the base, the very lowest level of reinforced soil would be assumed to have frictional resistance from the foundation soil; thus the tributary area for tension in the grid and against the block would go only 1/2 way down to the foundation level and 1/2 way up to the overlying layers.

option (a) is conservative, especially if you were not certain how the wall would behave, but if there was friction between the reinforced soil and the foundation soil then option (b) would be more likely.

Thanks. I get it now.