Seismic "D" and Battered Piles

[papa2ten]

Does anyone know of any IBC code limitations regarding Battering of piles in Seismic Site class or Design Category "D." A pile Contractor told me that IBC prohibits the bettering of piles in certain seismic site classes or categaories.

Is this True? If yes, How would one resist lateral shear loads at the base?

Thanks!

 

[JAE]

IBC 2000 section 1807.2.23.2.3 "Flexural Strength"
(this is a section under 1807 - Pier and Pile Foundations" and is under the SDC D, E, or F section.)

It states: "Batter piles and their connection shall be capable of resisting forces from the load combinations of section 1605.4."

...so it refers to battererd piles applicable to SDC D.

I looked over the rest of Chapter 18 in the "high seismic" paragraphs and there is no mention of battered piling NOT being allowed...

You might ask him for the exact reference...bet he doesn't know it.

 

[cap4000]

While IBC dictates the "pile" flexural strength requirements it makes no mention of the surrounding soil capabilties. Battering piles for horizontal loads is the only easy way to go. Typ. batters are 1 to 5 on 12V. After 3 on 12 special pile driving equipment is required. LPile Software and py curves get complicated assuming no batter.
Good Luck.

 

[BigH]

bnannis - not a codal point, but I bring to your attention the results of 1964 Nagata earthquake reviews. They found (I believe this is one of the ASCE Geo Journals) that pile caps of battered pile groups behaved rather badly in the seismic event. The idea in Vancouver (at least in the mid-80s) was to use vertical piles so that the group translated together rather than setting up stresses of the event hitting different parts of the piles/pile cap at different times.

 

[JAE]

BigH - that makes some sense in that a battered pile group would be somewhat "triangulated" and rigid while piles in a vertical configuration would seem to be more ductile and allow for more lateral movement via bending in the piling.

 

[haynewp]

I think there were some other threads about battering piles you can search for.

From my research, this is what I know:

The piles at the edges of a group at the highest angle will take a lot more axial load than the other piles that are more vertically oriented like stated above. You need to be very careful about punching through the cap at these locations. Be careful of the connection of the pile to the cap at these locations as well.

Some experts in foundations I have talked to do not seem to like using battered piles for foundations in seismic regions at all (like BigH stated above).

A lot more research needs to be done for Codes to give adequate direction on the subject of pile performance under seismic forces. For instance, as the horizontal earthquake waves move upward through the soil, they are first felt by the piles. Before the building begins to react at it's own period, the piles themeselves must react in their own way to the waves as they translate upward to the surface of the soil through the length of the piles.

Then the builing forces have to be resisted by the piles at the same time new waves are coming up through the piles themselves. And changes in soil profiles at you go up toward the ground surface makes things even more complicated. But I couldn't really find much design guidance on any of that.


HTH

 

[haynewp]

When I said the battered piles will take more axial load, I want to clarify I mean they are more rigid in the direction of the majority of the seismic force being applied (horizontally) so they will be taking more of the seismic lateral in the form of axial load thus damage to the cap at these locations may be a problem. I hope that makes sense.