Earthquake Forces on Pile Foundations

[techiestruc]

Good day!

I am currently designing an equipment foundation on piles. I'm wondering if do I have to include the inertial force of the large mass concrete in addition to the earthquake force due to the equipment and consider these effects on the piles OR I just have to consider the earthquake force due to the equipment only.

Do I have to consider the earthquake effects of foundations OR just the superstructure only?

Please cite references if available.

Help! Your response is very much appreciated.

Thanks!

 

[VoyageofDiscovery]

The answer is in your question, "large mass" piled foundation with seismic waves through the soil is definitely going to be an issue. No reference required, basic understanding of engineering physics is all it needs.

 

[techiestruc]

Does this seem to apply to all of the piled foundations? Do we have to include the inertial force of pile caps as additional shear to the piles?

 

[BUGGAR]

If your soils are capable of some lateral restraint on the caps, you can get some passive pressures to use from your geotech.

 

[msquared48]

That could be problematical if the soil is susceptible to liquefaction. Check with the Geotech.

Mike McCann, PE, SE (WA)

 

[Okiryu]

I see that this discussion is between structural engineers, but I find the question interesting and also would like to have a better understand of the approach of structural engineers when calculating forces that will apply to the foundation. I have seen that when there is a good communication and understanding between the structural-geotechnical disciplines, the project goes very smooth, otherwise discussions are tedious and it results in project delays.
My work is related to geotechnical engineering and when I have to do lateral analysis and if I do not get the enough information from the structural, I try to estimate the static base shear coefficient (Cs) and apply it to the vertical capacity of my element (i.e. pile or soil-cement column for example) to get a preliminary lateral force to check deflections and stresses in the foundation.
So, going back to the OP question, when you do static seismic analysis, I was thinking that pile caps are included in the calculation of the mass of the structure, as this weight may be large and therefore can have an important effect in the base shear value. Anyways, again, I work in geotechnical and would like to see the structural engineers opinions.

 

[Enrique2015]

This question could have two parts, since it depends what are you designing, I am going to assume that you already have all your geotechnical data, and you are designing the number of piles, capacities, and pile cap , in other words foundation design. It's common in the industry and specified by code to assume grade level to be the minimum at which seismic masses are considered; hence in your case you should not account for the pile cap as part of the seismic mass, load combinations as per ASCE 7 also consider part of the vertical mass (i.e Ev)_when designing individual elements such as piles or the pile cap itself, so any part of vertical mass will be taken care during design. I use software from

http://www.structralapps.com

for pile foundation design.

 

[VoyageofDiscovery]

Beyond this, raked (battered) piles have been problematic in California as pile caps were just sheared off. It appears that the stiffness of the pile group has a large influence in such cases. Thus the development of p-y curves.

 

[techiestruc]

Thanks for your responses. I'm currently designing an air compressor which weighs about 46 tonnes and it's foundation (pile cap + pier) is around 360 tonnes. I'm concerned regarding the seismic effects on piles if the inertial force of the foundation MUST be considered since the foundation is massive but I don't want to overdesign the foundation either. As per geotech report, liquefaction is not an issue.

Just to mention, the equipment has also been designed using dynamic analysis and the number of piles has been optimized to prevent the structure from falling within the resonance range. The number of piles provided were checked with static analysis but without the inertial forces due to the foundation. Considering this additional inertial force will affect my initial pile design and providing additional piles will gravely affect the stiffness of my structure, i.e. the system will resonate. I just want to assure that I will be able to justify my design if I have to rework this headache.

Happy Holidays!