Length of a concrete grade beam with torsion 1

[brph94]

I have an existing pile supported grade beam with no apparent interior resistance to torsion along its 138 feet of length.
The grade beam has steel pipe pile supports centered with the grade beam and spaced at approximately 29.5 foot centers with a maximum of 6 inches of embedment into the grade beam.
At each end of the 138 foot grade beam length there are large multi pile, pile caps.
The grade beams are 42 inches wide x 60 inches deep including the pile embedment.
The bottom steel is about 7.5 inches above the bottom of the grade beam.
Vehicles intended to be moved over the grade beam can exert as much as 164K of factored vertical force spread over 2 closely spaced axles with 4 wheels each.
The factored torsion is as high as 195 ft-kips, and torsion stirrups as well as longitudinal torsion reinforcement must be considered.

I have difficulty believing that the piles can resist the torsional forces with such little embedment in the grade beam, and the details of the pile installation
do not indicate development bars from the piles into the grade beam. Soils in other areas of the building have settled several inches below the pile supported concrete.

Is there any information that limits the length of a grade beam when torsion is considered.

KP

 

[msquared48]

Is the grade beam connected to a structural concrete slab at the top?

Seems like it might be if vehicles are to drive over it. Seems to me that the presence of a slab would constitute torsional restraint, if it exists.

Mike McCann, PE, SE (WA)

 

[Guest262653]

I really don't fully understand your problem (but probably is my fault...).
Could you please provide a sketch? Where do the torsional loads on the beam come from? Which other loads are acting on the beam? Is the torsional resistance really required for equilibrium or is it just a torsional compatibility problem?

 

[KootK]

If there is indeed torsion to be contended with, I believe that your concerns regarding the pile to grade beam are quite valid. I've explored a similar issue in this thread that may interest you: Link.

I know of no maximum length limitation. However, torsional stiffness in concrete beams drops to nothing (5-15%) once cracking occurs. The associated beam rotation will effectively limit how far you can extend a grade beam without intermediate support. Pile connection capacity may also govern as you've rightly identified.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[canwesteng]

Any reference on the torsional stiffness of cracked concrete? Maybe a bit off topic but relevant to a project I'm working on

 

[KootK]

These are both great. Dealt with it myself a couple weeks ago.

Link

Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

And ACI 445.1R-12

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[brph94]

Thanks for the links KootK