Shear wall footing 1

[haynewp]

I got into a debate on shear wall footing design for moment. I am talking about moment that is along the length of the footing, not transverse to it.

For a continuous masonry or concrete shear wall, the wall stiffens the footing along its length. To bend the footing you have to bend the wall in-plane with it. But for discontinuous elements like tilt-up panels or more flexible wood shear walls, would you design the footing longitudinal reinforcing for the in-plane moment considering the wall is continuous on top of the footing? I say yes for wood.

 

[BAretired]

Yes, but might use additional tiedowns to reduce moment.

BA

 

[haynewp]

What about tilt up? The panels would close at the top and preclude failing the footing, in my opinion.

 

[JAE]

Hmmm....sounds like a run-in I had a few years ago with a popular structural engineering software (rhymes with Pam).

Their continuous footing design created footings with enormous amounts of longitudinal rebar. When I called them to challenge the idea that footings under walls don't bend, they seemed not to appreciate the subtleties of Hooke's Law. If a wall can't bend much, neither will the footing below it.

For tilt wall footings, the individual wall panels might try to rotate independently of their adjacent panels - I think this might create some bending but not as much as you think as they are also stiff and the uplift tension in any hold-down anchor at the end of one panel is resisted by the downward pressure of its adjacent panel across the joint. Seems like there might be some significant shear there but not bending.

 

[haynewp]

I noticed the same thing using that software years ago.

 

[CELinOttawa]

In New Zealand tilt up walls are designed to rock. The footings are only anchored to the panels at the ends, with rare exceptions, and the footings designed in this way have not been foudn to have failied in the ChCh EQs.

I'd say there is now absolutely no question that minimum longitudinal reinforcing and sensible anchoring at the ends is sufficient to resist the actual forces which will occur in service.

The major caveat here is that I don't know what you guys in the states do with your panel fastening. The panels I'm talking about are *not* fastened rigidly to one another, but have alignment brackets that keep the panels in plane and allow them to slide along one another's edges (thus permitting them to rock in the longitudinal direction.