# Earthquake Forces and Design Deformation of a Pile Group

• To: <seaint(--nospam--at)seaint.org>
• Subject: Earthquake Forces and Design Deformation of a Pile Group
• From: "Brian S Bossley" <BSBossley(--nospam--at)venturaengineering.com>
• Date: Tue, 5 Apr 2005 16:28:23 -0400
```Ok - once again - another question for the structural giants of SEAINT:

Correct me if I'm wrong, but under ASCE 7-98 or ASCE 7-02, the
earthquake forces are ULTIMATE (yet elastic) forces and then these are
factored down if you are using WORKING STRESS.  But I seem to remember
someone telling me (at an ASCE seminar, I believe) that the design
deformation is the deformation that you get from your ULTIMATE
combinations, regardless of which set of combinations you chose for
design.  I know that in ASCE 7, they have an equation delta(x) = Cd*
delta(xe)/I for the calculation of the story drift determination, where
Cd is the deflection amplification factor to get you from a ELASTIC
ELASTIC deformation based on the shears you've calculated for the
ULTIMATE earthquake forces.  So I assume that the INELASTIC deformation
(delta(x)) is what would actually occur when the big one hits - or at
least for design considerations.

Now I have pretty big lateral forces transferred to a pile group (HP's)
and I am told by the geotech guy that the forces in each pile are not
based on relative stiffness of the piles (weak axis vs strong axis), but
rather on some fifth order interaction equation that I don't know
anything about because I'm not a geotech.  But, for a set of given
deformations, he gave me the shears at the top of the pile for both weak
axis and strong axis and also, the corresponding bending moments.

Now, would the force transferred to these piles be based upon the base
shear or upon the design deformation?  If deformation, is that the
inelastic ultimate deformation or the elastic ultimate deformation?

Brian S. Bossley, PE
Ventura Engineering
7610 Olentangy River Rd.
Columbus, OH 43235
(614) 847-1110 x121

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