Timber piles in high seismic region

[k66]

Got a project in Washington State in a high seismic region with timber piles. Have read some areas that do not allow Timber Piles in California due to concerns with ductility. Also, have read articles recommending embedding piles a pile diameter into cap for ductility. Anyone doing timber pile projects in that region? A 2 to 3 inch pile embed may be too shallow for a maximum seismic event. Would you design the timber piles for the design seismic load using the R of the system or consider changing the R value for checking the lateral load on the piles? The passive pressure given by the geotechnical engineer is not helpful only 75 pdf/ft.

 

[msquared48]

At a pile diameter embedment into the pilecap, are you thinking of using only 2 to 3" diameter timber pile? Sounds more like pipe pile to me.

The minimum embedment here that I have used for 40 years is 6" regardless of the diameter. It locks the pile into place from walking due to seismic action, and protects the end of the pile. I uesd the R value for a cantilever pole system.

I did a house on pile of varying lengths above ground several yuears ago in LaConner, Washington, that is still doing well. Just proportioned the lateral load to the pile based on the flexibility so the system was tuned to the response. Worked fine in the last event in 2001 here.

If you have to, consider using batter pile to take the lateral.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[k66]

Thank you for the reply, Mike! This is a commercial masonry wall structure. Seismic R value for the structure was 5. I am not sure that the piles should be checked for that value or a value more like 1.5. As noted tried to use the passive pressure on the grade beams instead, but that is not working so well.

 

[msquared48]

I used that R value because the pile were elevated above the ground and there was no pilecap. Really not your situation...

For your application, with lateral values that low, I would definitely use batter pile.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[abusementpark]

I've noticed the IBC doesn't give requirements for timber piles in higher seismic zones like it does for other types of piles.

 

[k66]

Yes, the building code is silent on additional requirements in high seismic regions for timber piles. Unlike all other types of piles that typically require some additional detailing at the pile cap connection. The Timber Piling Council Manual simply states that the scope of the manual does not include seismic design considerations. It appears that some jurisdictions in California and CalTrans do not allow timber piles to be used for support of permanent structures based on concerns with ductility during a seismic event. There is a couple research papers that suggest embedding the piles approximately 1 pile diameter for ductility. This also provides some fixity of the pile cap.