PEMB Thrust Block?

[waldo459]

A contractor has left out a hairpin for a PEMB. I designed a grade beam to take the horizontal reaction. Seeing the size of the grade beam, he would like us to design a thrust block. As it will be near the ground surface, is this even feasible? If there is a 2500 psf bearing pressure and 100% could be realized it would need to have 17 sq feet of bearing area between 0' and 3' below grade.

 

[DaveAtkins]

I don't believe you can just use allowable bearing pressure--I think you must use passive resistance.

Can you install a threaded rod across the building, under the slab?

DaveAtkins

 

[waldo459]

Yes, passive pressure, making the area larger. The first recommendation was to install tie rods by cutting the slab, the owner insists that the slab remain intact.
If a wedge shape was used for the block, from 2x2 square to 6'dx8'w over a length of 3', we could get the required area based on passive pressure. The resultant of Pp would then be at 4' depth, the reaction at 1'depth, causing an overturning force of 120'k, seems severe. For a 3' constant depth, we would need to taper from a 3'x3' to a 3'dx30'w, also severe.

 

[jike]

Those are the only two solutions that I see (a thrust block or tie rod).

 

[waldo459]

My solution was a thrust block in the form of a grade beam, this keeps it close to the building, but involves more re-bar than a true thrust block. It's looking like it will use less concrete though. The contractor wants to look at alternatives, I'm guessing to save some steel. Any ideas on how to deal with the overturning force economically?

 

[msquared48]

I agree with Dave, try a tie rod back to a deadman if you can. If the rod is permanent, it should be encased in concrete.

Mike McCann
McCann Engineering

 

[waldo459]

A tie rod cannot be used at this point, a thrust block is required. My problem is that it is relatively shallow so develops a low passive pressure. Making it deeper is more economical, but puts a couple into the block from the column thrust at grade level, to the center of reaction of passive pressure, causing it to want to overturn. Is this the right logic, or am I missing something?

 

[muuddfun]

Can you use a battered pile?

 

[hokie66]

Or a bored pile which can take the thrust in bending?