Seismic Design of Piles

[Hromis1]

Maybe some one here can point me to the correct sections of the code. When designing piles supporting something like a large mat, what is the correct seismic weight? Should it include the weight of the mat? (I would think so) Most the ASCE formula appear to be set up for forces to the foundation. Not forces generated by the foundation itself in response to a EQ. Is there an equiv. response modification factor? I have some odd situations where the mat/foundation weighs significantly more than the supported structure.

ASCE 12.13.3 does cover some information on the subject concerning the stiffness of the system. However, I do not see any equiv. response modification factor.

 

[Hromis1]

Let me add some more to this..

From my point of view..the piles and the cap or mat form a "frame". The frame itself may have it's own response values..

For lower design categories there are some empirical formulas such as 0.1 Sds for the lateral force. But what about the higher design categories?

 

[outatime2002]

Let me add some more to this..
From my point of view..the piles and the cap or mat form a "frame". The frame itself may have it's own response values..

For lower design categories there are some empirical formulas such as 0.1 Sds for the lateral force. But what about the higher design categories?

I have 3 comments:

1. My interpretation of the code is that the effective seismic weight of a structure does not include the weight of it's foundation. Therefore, the lateral force resisted by the piles will only be from the superstructure. Including the weight of the pile cap in the base shear calculation would be too conservative.

2. One can perform a vertical force distribution to determine the lateral force on the piles from the pile cap only. Since the pile cap is essentially at grade (Level 0), then the lateral force at level 0 = ((w*h^k )/sum(wi*hi^k))*V. Since, h = 0 at grade, then the whole term goes to zero. Therefore the pile cap itself does not place additional lateral force on the piles. It distributes the lateral force from the superstructure down to the piles.

3. I agree that the piles do support the vertical dead weight of the pile cap along with any additional soil or surcharge loads.

 

[IL-SE]

You have to remember that an "earthquake force" is an inertial force caused by the building trying to stay where it is while the ground moves below. The foundations are moving with the ground, therefore no siesmic force is acting on them. The rest of the building is trying to maintain it's original position which engineers evaluate by applying an equilvalent lateral force to the structure. It's a way of a applying force to mimic the reaction that a building has during an earthquake.

 

[outatime2002]

You have to remember that an "earthquake force" is an inertial force caused by the building trying to stay where it is while the ground moves below. The foundations are moving with the ground, therefore no siesmic force is acting on them. The rest of the building is trying to maintain it's original position which engineers evaluate by applying an equilvalent lateral force to the structure. It's a way of a applying force to mimic the reaction that a building has during an earthquake.

Well put, IL-SE.

 

[McEngr]

You have to remember that an "earthquake force" is an inertial force caused by the building trying to stay where it is while the ground moves below. The foundations are moving with the ground, therefore no siesmic force is acting on them. The rest of the building is trying to maintain it's original position which engineers evaluate by applying an equilvalent lateral force to the structure. It's a way of a applying force to mimic the reaction that a building has during an earthquake.

IL-SE, I have to respectfully disagree. The RESPONSE of the building is what induces the force, not the motion of the ground. There will still be a seismic force induced as a response to the foundation. Hromis, I'm not sure if you've performed a strut-and-tie model before, but it's pretty intuitive and can be used to solve the foundation whether it's seismic/wind/gravity controlled. I wouldn't worry too much about ACI 318 chapter 21 as it is more geared towards seismic DETAILING than design. When I say detailing, it is intended to have the structure respond in a ductile way so that it performs better under seismic events. The piles will not need to be detailed for seismic as there is no benefit for the seismic forces to be visually inspected for ductility. There will be more soil-structure interaction effects that will have to be resolved with your geotechnical engineer and can sometimes be iterative. Hopefully you have a good geotech that is responsive.

Having done consulting on my own without a colleague to bounce things off of, I understand your predicament. Hopefully, we can respond collectively to help you.

 

[Hromis1]

McEngr,

I solved the problem "some what". Or I at least determined that it was not the governing load case. The problem had large lateral loads that were still higher than the EQ loads.

Regardless of what the various codes say, there is a difference between the response of the ground to an earthquake and the response of the pile and cap system. The geotech engineer was able to provide a lateral spring constant in this case from which I could find shears in the piles at various depths. Really bad soil in this case that provides little lateral support for the first 30' of length.

Again, this case was an "odd" machine foundation, where the wgt of the foundation and cap itself is nearly an order of magnitude greater than the loads above elevations "zero".

Just another real world case that helped me understand where the codes fail.

Hromis1

 

[kevo_55]

Hromis1,

I would recommend getting your hands on the 2009 SEAOC Blue Book. It has a really good chapter in there with respect to seismic & foundations.

I must say that reading the new blue book helps me understand the code much better.

 

[Hromis1]

kevo_55, thanks. I will look into that book. Again for this case I was able to determine that EQ was not the governing load case, but some lateral impacts were. They put a lot of torsion into the mat over the pile cap. I use these odd cases to try and understand the use of the "code" better. Regardless of what the codes says, you still have to be the engineer. This was one of those cases. I am simply using this board to think out loud.

Hromis

 

[McEngr]

Hromis1,
I would recommend getting your hands on the 2009 SEAOC Blue Book. It has a really good chapter in there with respect to seismic & foundations.

I must say that reading the new blue book helps me understand the code much better.

Thanks kevo. Good info to discover. I'll check on the cost of that book. I think it's spendy.

 

[kevo_55]

^^ I believe that the cost was reasonable.

I think you can get it for $80 if you're a member of the ICC.