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Re: pier design

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At 14:31 3/5/98 EST, you wrote:
>I am currently designing a roof platform. The lateral resisting system is
>composed of moment frames running in both directions. This platform is
>and due to space limitations, I have no other choice than using drilled piers
>for foundations.  For the embendement length, I divided the moment by the
>shear at the base and found an equivalent height for the shear load (ie.
>M/V=h'). My questions are:
>1. What is the minimum reinf. required.
>2. Do I have to design the pier as if it were a column with and axial and a
>moment (eventhough it is lateraly braced by the soil) or should I just
>consider the moment.
>3. Is there a source I can use as a guideline for my design.
>I will greatly appreciate any information regarding this matter. Thank you

[Bill Cain]
1. Make sure you have enough reinforcement to handle the cracking moment to
ensure ductile behavior.
2. Usually axial is small and doesn't effect the design significantly
unless the pier is very long and in soft soil.
3. An excellent resource for design of pier foundations (with a limit of
Length/Diameter = 10 or so because the method assumes rigid behavior of the
pier) is a paper by E. Czerniak that was published in the Journal of the
Strucutral Division, ASCE, Proceeding Paper 1188), Vol. 83, ST2, March
1957.  It gives a method for handling any combination of Moment and Shear
loading (as contrasted with the '97 UBC Section 1806.8.2 approach) and
gives values of maximum moment and shear where they occur (below the
surface).  As for the concrete design, I usually treat the columns as
continuosly braced by the soil unless the soil is soft (in which case this
type of foundation won't work well anyway).

You do have the choice of pinning the column bases and just taking
overturning and sliding forces on the footings in lieu of drilling piers or
trying to make the connection between the columns and a grade beam ductile
to gain fixity.  If a nearby property line is the space problem, tie the
footings together with grade beams to take any eccentric loads.  However,
under this approach your column sizes will increase if you pin the bases.