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shobroco (Structural) (OP)
7 Jun 10 12:17
I am designing a grade beam on helical pier foundations to carry a 2 storey residential building on a site with 15' +/- of fill. I have designed concrete beams for lintels & suspended floors before and am looking at a similar design for strength to carry my loads but 2 pier contractors I have talked to say my spacing is longer than they usually see. I am 14'-16' in different areas but one guy says average 8' & the other says 10'-12'. I want to keep the grade beam only about 3' deep & I also want to minimize the number of piers. Do you see any problems?
emmgjld (Geotechnical)
7 Jun 10 20:34
If your calcs indicate that you can properly span the distance, what the piering contractor says is secondary. You should do your computations and look the situation over. Modify as required.  Be the Engineer.

I know that sounds a little harsh but...Part of their mindset may be stuck in selling piers and you have proposed a significant reduction in their product.

For example, I have been involved in a number of residential underpinning jobs (expansive soils) which the existing stemwall/gradebeam was adequate to span 14 to 20 feet and I generally placed the piers from 10' to 16'. The contractor tried to talk me into shorter spans. That is what their books say. I looked at their books and the assumed construction was for grade beams which could only span 8' to 12' and sometimes a little less.  I am also afraid the contractor had hoped for a higher product volume job.
JAE (Structural)
8 Jun 10 7:58
Your grade beam probably isn't the issue they are seeing.  

Most helical piling are spaced 6 to 10 feet due to either limitations in the pile capacity or concern over the spanning capability of existing structure.

 
msquared48 (Structural)
8 Jun 10 11:21
I agree with JAE here.  

I did a helical pile foundation for a PEMB about 10 years ago that had a structural slab, and found that I needed the pile grid at 10 feet each way with the slab loads seen.  Otherwise the slab thickness got too great, overloading the pile.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

shobroco (Structural) (OP)
9 Jun 10 5:11
Thanks, you have agreed with what i already thought.
Zambo (Civil/Environmental)
10 Jun 10 10:39
What did you already think?

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