Strip Foundation Uplift

[KootK]

Here's my situation:

I have a 12 ft wide by 16 ft hight shear frame that's carrying about 20 kips worth of lateral load at the top.

The columns of this frame tie into strip foundations below (at a corner location on one side of the frame). Therefore, I need to be able to get at least 27 kips worth of uplift resistance out of that strip foundation.

My coworkers tell me that by tying into the strip foundation, I will have ample uplift capacity because to experience uplift, the frame would have to lift up the entire strip foundation assembly with it.

I don't doubt that this is correct but I would like to have an analytical way to justify that assumption (at least once).

I don't think that it's realistic to expect to be able to pick up huge portions of the foundation without damaging the foundation itself. Here's how I'm thinking of modelling the situation:

1. Maybe I could treat the strip foundation as an upsidedown beam with the uplift load acting as a point load at mid span. Then, I would calculate the span of beam that could be picked up without causing it to fail in bending tension at the top face (Assuming the beam ends fixed I think??). Then, I could use that span x the cross sectional area of the foundation * 150 pcf to get the amount of uplift resistance.

I would appreciate any feedback that anyone could offer with respect to my analysis approach. I'm kind of just making this up as I go along.

Also, where the uplift capacity is needed at a corner, could I just treat it as a pointload at the end of two cantilever beams? This is tough without sketches. I hope it makes some sense. Thanks in advance.

Adam

 

[ishvaaag]

Well, a strip foundation with some columns and one shear frame somewhere that can add uplift tension at one column...

Respect the analyisis for local forces and effects from uplift tension imprinted by the frame, you put the stabilizing (other than those of the shear frame) columns at 0.9 or 1 the dead weights, then you enter the factored compressive and uplift forces of your shear frame, all the strip foundation on just exclusively compression-only springs. That way you identifiy worse factored effects of uplift where it occurs.

Of course ensure proper transference of the uplift forces by check of the anchor device.

Of course the ordinary factored weight (say without wind) loadcases need be compounded for the design of the strip foundation.

The presence of uplift may signify a stronger strip foundation surrounding the shear frame zone.