[b]CONNECTING PILE CAPS TOGETHER WITH GRADE BEAMS[/b]

[tbone73]

For the foundation design experts. I have a cast in place post tensioned building with frames spaced at 27 feet typical and 62 feet 2 span beams supporting 7 inch slab. The foundation type is auger cast pile groups with individual pile caps. Under frame action alone the dead and live load produce large column shears and consequently large reactions on the pile caps. Large enough in fact to dictate oversizing the pile groups to limit diplacements. However we are considering using grade beams as ties from the exterior pile caps to the interiors to limit the displacements. The ties will be in tension. Does anyone have some similar experiences with using ties in foundation? Thanks for your input.

 

[rowingengineer]

tbone,
I am confused large horizontal shears? Are these generated from the PT or Live and dead, if live and dead how are these generating horizontal thrust?

As for the question in using grade beams for tension ties, yep have done before and will do again. What exactly do you want to know?


A clear conscience is usually the sign of a bad memory

 

[slickdeals]

Under frame action alone the dead and live load produce large column shears and consequently large reactions on the pile caps.

I don't understand this part. Is this some sort of crazy eccentric structure with cantilevers? Otherwise, how would your gravity loading produce huge lateral forces?

 

[slickdeals]

@RE:
Love your signature

 

[InDepth]

You need to define what your intent is. Building codes usually have different requirements of wether you are designing a tie beam or a grade beam. Tie beams are usually required to connect all pile caps and have a minimum tension capacity based on the axial load. Grade beams are usually used for moment and axial load to stiffen the rotational rigidity at the base of the frames.

I believe you want to activate additional pile caps (usually gravity pike caps) to resist the lateral shear developed at a single column. You can do this using a tie beam and sizing the rebar for the axial load (see ACI or Building Code). The rebar of the tie beam should be developed into the pile cap, or even extended on to a second set of pile caps. Note that you may be able to use the lateral resistance of the pikes and the passive pressure developed in front of the pile cap, and possibly an effective width of perpendicular tie beams.

 

[tbone73]

If you run a frame analysis with two 62 ft spans say three supported levels with a tributary width of 27 feet. The self weight is that of a 7 inch slab and beams and say 40 psf live load gives you fairly significant shears, when you assume the column fixed to the pile cap.

 

[tbone73]

By fairly significant shears I'm talking about 50 kips at service load.

 

[rscassar]

The 50kips does sound very excessive for gravity loads alone. What column stiffness have you assumed in your design and can you release some of the fixity at the base of your structure without causing excessive deflections elsewhere. I would be trying to use a two or three pile group to resist these reactions before trying to link adjacent pile caps.