PEMB foundation for moment reaction

[todh]

I have designed a lot of PEMB foundations in fat clay that are founded on under-reamed or straight drilled shafts in fat clay. All but one of these foundations had simple pinned connections for the rigid frame to the foundation. One previous (and now another) have moment reactions transferred to the foundation. My allowable soil bearing value at 20 depth is 4ksf and can be increased 20% for transient loads. The PEMB manufacturer has provided a reaction of 878 k-ft. What are some ideas for transferring this load to the bearing depth of 20'? Can you install two drilled shafts three diameters apart (one inside the building and one outside the building footprint) that are connected by a beam that the PEMB column sits on between the two drilled shafts? This would transfer the 878 k-ft moment to the beam and then in turn cause one drilled shaft to be loaded in tension while the other was loaded equally in compression to form a couple resisting the applied moment. Any thoughts or examples of what you may have encountered in the past would be appreciated. My first inclination is to make the PEMB manufacturer provide a frame with pinned connections, but I wanted to see what others have done. This building has a 90' span with a 100 ton crane and an eave height of 40' in hurricane country.

 

[JedClampett]

I would challenge the PEMB manufacturer about that load. It seems very high. Before that load gets to your soil, it has to be taken by the building column (usually these are only about 15 inches deep), the base plate and the anchor bolts. The anchor bolts will be especially tough to get to work. Now they don't care about the anchor bolts, but if the base plate is a stiffened contraption that's 3'-0" square, that might get their attention.
If normal braced design won't work, they can take the moment out in a portal frame with a pinned base.

 

[todh]

We actually received preliminary drawings from two separate manufacturers and the reaction was the same (essentially). It is extremely high though. The typical baseplate is 1' x 5.5' and has 12 1-1/4" diameter anchor bolts.

 

[hokie66]

Your solution of using two bored piles with a coupling beam is what I have done with high base moments. With a 100 ton crane, and an eave height of 40', and also high wind loads, it is not surprising that a fixed (or at least partially fixed) base is an appropriate solution.

 

[FixedEarth]

You will get more capacity in Clay using skin friction instead of end bearing. Is there a reason why you are only using end bearing?