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Foundation Analysis of Drilled Piers and Grade Beams

Foundation Analysis of Drilled Piers and Grade Beams

Foundation Analysis of Drilled Piers and Grade Beams

(OP)
I am analyzing a slab-on-grade foundation for a project that is about to go into construction. It is an office addition (See attachment:http://files.engineering.com/getfile.aspx?folder=3... ) . The spacing for the drilled piers are 31'-10",26'-2"& 30'-11". One side of the building has 4 steel columns and the other side is using a CMU load bearing wall. Based on my calculations, I determine that at it's current spacing that 20/60 shaft & bell pier would work instead of 16/48 (this is mainly for the side of the CMU wall). This is primarily due to the spacing of the piers. My main issue is with the Grade Beams. I have analyzed the grade beams as spanning from pier to pier. I checked for the 31'-10" span. Due to the applied loads (wt. of grade beam, 8" CMU Wall @ 12'-0" ht, roof dead and live loads) the grade beam is not passing for strength. I am trying to think of what other acceptable solutions to this issue besides adding additional piers (due to cost) to reduce the span. Am I view the analysis of the grade beam correctly? I have been told it is good to span underreamed footings at 15'-0" o.c. or a max of 20'-0" to avoid these types of situation. What are the potential problems that may arise if it is left as is? Could I count on the soil the grade beams are resting on to help carry the load in this situation? Any suggestions and/or comments are appreciated.

RE: Foundation Analysis of Drilled Piers and Grade Beams

You may want to reduce the spans by adding an intermediate belled pile, albeit smaller because not picking up column loading. The long span may cause cracking in the masonry wall.

Dik

RE: Foundation Analysis of Drilled Piers and Grade Beams

How large is the grade beam (depth and width) and what is the reinforcing?

For masonry support usually try to keep to L/600 on total load deflection to avoid cracking per dik's statement.

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RE: Foundation Analysis of Drilled Piers and Grade Beams

You are analyzing the grade beams as continuous and not simple spans, correct? "I have analyzed the grade beams as spanning from pier to pier. I checked for the 31'-10" span."

RE: Foundation Analysis of Drilled Piers and Grade Beams

(OP)
@haynewp- I analyzed the grade beams as simply supported from pier to pier. I checked the 31'-10" span.

@JAE- the grade beam dimensions are 16"x24" with (3)- #6 top & Bottom

RE: Foundation Analysis of Drilled Piers and Grade Beams

(OP)
@dik - One side of the building does not support CMU wall, only has 4 columns. Besides adding additional intermediate belled pile, I don't see how to alleviate the forces on the grade beams for the 31'-10" span. What potential issues could arise from leaving the grade beams spans as is for the side that doesn't support CMU wall?

RE: Foundation Analysis of Drilled Piers and Grade Beams

haynewp probably hit on your problem - you need to consider these as continuous beams, not simply supported.

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RE: Foundation Analysis of Drilled Piers and Grade Beams

If not supporting anything, then the 31' span should not be a problem, but unless the beam is deep, can be a problem for the masonry. I would not have considered them as simple span (ever, unless a meatball design (quick dirty design for no fees)). You need the continuity to reduce deflections. With the shorter spans, you can more easily treat the masonry as arching between supports, further reducing the flexural steel and shear stirrups.

Dik

RE: Foundation Analysis of Drilled Piers and Grade Beams

(OP)
Thank you everyone for your input.

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