PEMB Foundation with Expansive Soils

[bbartlet]

I am designing foundations for a PEMB. The issue that we're running into, is that the geotechnical report indicates there are potentially expansive clays under the slab and that the slab be separated from the foundations. This is typical for most traditional buildings that we have done in the area. However, because of the PEMB, we need to resist the kick force, which is traditionally handled with hairpin ties into the slab. I have read numerous posts in this forum about using a grade beam between opposite columnbs, but our reaction is relatively small (on the order of 5 kip or so), so that seems like overkill.

I think that the force could be resisted by the soil on the side of the trench footing. We are in discussion at our office as to whether we should have the PEMB use the top of footing as the bottom of his column (-0'-8") or should we or should have them pour 8" pier/curbs up to (0'-0") to allow them to keep the the bottom of column at the floor level? And if that's the case, do the bolts need to be designed for bending?

As I'm typing this, it seems like it should be relatively easy to ask them to extend their columns down 8", but we were just curious if anyone had come across a situation where they couldn't use hair pins due to the need for slab separation.

 

[msquared48]

Yes, and the only two options are the tie beam or lateral bearing, and I have used both.

If you choose to use the soil, just make sure that the frame can tolerate the 1" or so of lateral spreading to develop the lateral thrust against the soil. It will have to do this to work.

Mike McCann
MMC Engineering

 

[MiketheEngineer]

Or you could remove and replace the expansive clays. Done around here all the time.

 

[dcarr82775]

In this area most buildings on expansive soils are placed on drilled piers. For a PEMB the piers are sometimes sized to resist the thrust.

 

[oldestguy]

Remember that clay shrinks in all directions. Given enough shrinkage, you may have more than 1" lateral movement as those deep cracks develop. Those pressures due to expansion may have you cycling with the seasons, back and forth.