IBC09 1810.3.13 2

[LPPE]

I have a deep foundation job, and we're driving HP shapes with concrete pile caps. I have some isolated pile caps with no structural slab. This job is designed using the IBC09, and I have a seismic design category "C".

Seection 1810.3.13 Seismic Ties states individual deep foundations shall be interconnected by ties. And it then explains the forces to design for.

However, it is not explicitly clear on the direction of the ties. Are ties to be placed in each of the principle orthogonal directions? Is one tie to each pile cap sufficient?

I already have designed my HP shapes to the criteria in 1810.3.5.3, and am a bit confused by the additional rather unclear design criteria in 1810.3.13.

Any help or clarification anyone has used on 1810.3.13 would be greatly appreciated.

Thank you.

 

[Teguci]

Calculate the lateral forces that the ties are to resist and apply that force in any direction. Can your tie(s) resist that force in any direction?

As an additional warning, if you do design a tie to take side bending (in order to resist the calculated lateral force), make sure it is stiff enough to exclude plastic deformation on your pile.

 

[LPPE]

I was anticipating concrete grade beams between pile caps in each orthogonal direction to satisfy the requirements of 1810.3.13. It just seems that the requirements in that section aren't exactly clear...

 

[Teguci]

ASTM doesn't provide any additional clarification. You might have to go back to NEHRP to find out the specific reasoning for the ties.

As an aside, the code seems to restrict the use of ties to only tension or compression recognizing the inherent flexibility of a lateral beam.

 

[slickdeals]

This is from the NEHRP Commentary Section 7.4.3

A common practice in some multistory buildings is to have major columns that run the full height of the building adjacent to smaller columns in the basement that support only the first floor slab. The coefficient applies to the heaviest column load.
Alternate methods of tying foundations together are permitted (such as using a properly reinforced floor slab that can take both tension and compression). Lateral soil pressure on pile caps is not a recommended method because the motion is imparted from soil to structure (not inversely as is commonly assumed), and if the soil is soft enough to require piles, little reliance can be placed on soft-soil passive pressure to restrain relative displacement under dynamic conditions.

If piles are to support structures in the air or over water (such as in a wharf or pier), batter piles may be required to provide stability or the piles may be required to provide bending capacity for lateral stability. It is up to the foundation engineer to determine the fluidity or viscosity of the soil and the point where lateral buckling support to the pile can be provided (that is, the point where the flow of the soil around the piles may be negligible).

 

[slickdeals]

7.4.3 Foundation ties. One of the prerequisites of adequate performance of a building during an earthquake is the provision of a foundation that acts as a unit and does not permit one column or wall to move appreciably with respect to another. A common method used to attain this is to provide ties between footings and pile caps. This is especially necessary where the surface soils are soft enough to require the use of piles or caissons. Therefore, the pile caps or caissons are tied together with nominal ties capable of carrying, in tension or compression, a force equal to SDS/10 times the larger pile cap or column load.

 

[ATSE]

Agree with slickdeals.
Based on ground motion data from Taiwan and others, ground motion at a single building is not uniform, so despite what we like to think of as "ground motion" there is actually a set of ground motions (plural) that are not 100% in phase, so interconnectivity of the foundation is necessary.