Seismic Design Utilizing Piles

[mjohan]

I am interested in any published articles or codes referencing seismic design parameters for a building supported on piles.

A quick scenerio of a particular building I am working on is this:
The soil is classified as S3, per BOCA 1996... the soil beneath this 4 story building is 10' landfill, 20' fine to medium sand w/ silt traces, 20' of over consolidated clay layer. Please note that this building is also required to be designed under IBC 2000. The geo-technical report has recommended 8" timber piles stopping above the clay layer for the entire support of the building.

Are there any concerns others have while designing for a seismic event, when the building is supported on piles? Would it be over conservative to assume the landfill was not there and design the piles as cantilevers with a seismic base shear force?

I am involved with the structural design of this building, and naturally, any suggestions I might make to the geo-tech I would want backed up with publishings and other's experience.

Thanks so much,
Mark

 

[FFIELD]

You can find a formula to calculate latterally loaded piles in The 19967 UBC, section 1806.8.2.1. You will need your soils engineer to give you the allowable soil latteral bearing pressures to do this. Should be no problem.

From the results obtained you must calculate the bending moments in the pile. This is difficult. I have researched this and have come up with my own computer program. All I have to do is find it. As I am retired all my brains are in storage in my garage. As it will take only a short time to do the problem I would be happy to do it for you.

From the bending moments and axial load you can design the pile as a short column.

 

[mjohan]

Thanks FFIELD,

I took a look at the geo-tech report. Though I did not find lateral soil pressures, an allowable lateral force of 2 kips was stated. (Specified piles are 8" dia. timber, allowable design load is 25 kips gravity.)
I read further into the IBC 2000, regarding piles in chapter 18. Section 1807.2.9.2 was the answer to one of my questions, "Piles standing unbraced in air, water or in fluid soils shall be designed as columns in accordance with the provisions of this code." So if the landfill will become fluid during a seismic event, then designing the pile as a short column, as you stated, is exactly what needs to be done.
I will give you some more information regarding the soil type, hopefully it will help.
Top Ten Feet - landfill, organics, glass, tires, asphalt, etc. (blow counts / 6" = 1 to 20)
Next 40' - Sand, trace silt (blow counts / 6" = 20 to 58)
Below that is a marine clay - (blow counts / 6" = 10 to 20)
- Piles will stop short of the clay layer.

The seismic base shear is approximately 150 kips, total piles are approximately 100. Seismic activity is very low for this area. Ss = .18g (mapped spectral response acceleration at short periods "per IBC 2000".

I am mostly interested in the theory behind my assumptions. To satisfy my own curiousity.... do these 8" timber piles work for a seismic event??

 

[mjohan]

25 tons per pile - correction for above post

 

[FFIELD]

Yes they will if the soils engineer knows his stuff. You can design for the two kips allowed with possibly a third increase for seismic events. Check with the soils engineer.

The theory behind this is that the lateral force on the pile is transfered to the soil by side bearing pressure. The soil acts like springs and resists the side pressure.

I guess if I pnly had the info that you put in your description I would assume that the fill gives little or no side support. I would then assume that the pile has a point of fixity at about 5 feet below its surface. The bending moment would be approximetly 2 Kips (or 2.66 ) times 10 feet (fill) plus 5 feet or 30ft-kips (39.9).

The theory can be found in any advanced soil mechanics book.

Hope this helps.