Tie/Grade Beam for Piled Foundation 1

[AlexSTR89]

Hello,

I am designing a piled building foundation for a metal building. Currently we have two piles under each column (aligned perpendicular to building wall), with pile caps and concrete piers. We are using a perimeter grade beam to tie the pile caps and piers together, and distribute column lateral reactions along the foundation line, as well as support the exterior building wall. See sketch of plan: Plan Layout

The question that I have is this: The grade beam is used to take the lateral reaction from the columns and provide stability for the foundation. Since the grade beam is smaller in width than the piers, the centreline of the grade beam does not coincide with the centreline of the piers/columns. Does this create a concern for the twisting of the piers when considering the load transfer from column to grade beam? Is this a good design layout? Is there something that is better for achieving the design intent?

Any comments would be appreciated
Thanks in advance.

 

[KootK]

I've typically set things out as you have shown and know of no better, practical way to handle the situatio. And, like you, I've worried about the eccentricity between the piers and grade beams. Here's what I've been doing:

1) assume that the grade beams constitute lateral restraint at the top, exterior corners of the piers. Design the concentrated ties at the top of pier to be able to transfer the lateral load from the column back to the line of the grade beams.

2) work out the torque resulting from the pier / grade beam eccentricity and design the pier to transmit that torsion down to the pile cap.

Depending on how the slab on grade is detailed, you can sometimes make use of that too, at least in a belt and suspenders sense.

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.

 

[hokie66]

I would have interpreted "lateral reactions" to mean the thrust parallel to the frames, rather than the way KootK has interpreted. When parallel to the frame, I don't use the perimeter beams, but rather depend on the pair of piles.

 

[kc9508]

I assume since the question was concern about torsion in the pier, aamend is talking about lateral reactions from the columns due to loads applied along the building length, not along each frame. KootK's approach seems reasonable to account for this. If it is lateral load along the frame, the piles/pile cap should be designed to take this force. In this case you might want to batter one or both of the piles.

 

[AlexSTR89]

I am referring to the reactions along the building length, due to braced bays. Thanks for your input KootK, I will try to follow some similar design methodology.

Does anyone know of a good reference that addresses this issue or provides some design examples?

 

[Archie264]

I would have thought the pile group would have taken the lateral reaction in either axis even if it is just a two pile group. If not then perhaps a 3-pile group might be in order? I wonder if one of our geotech guys might care to comment?

Kootk, nice stationary.

 

[KootK]

Was reviewing my retirement account statement for the last quarter. Decided to take solace in some concrete detailing instead.

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.

 

[Archie264]

Love it. I suspect that over the years many important problems have been solved on the back of cocktail napkins...