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Re: Fixed Column Base

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Jim,

	Absolute fixity is absolutely impossible but you can assume "virtual"
fixity at the bottom ends of the piles.  You can probably develop some
tension in the piles; your geotech can tell you how much; but you really
don't need it to develop the analysis.

	I think you should not consider the base as fixed but rather compute a
spring constant for the base.  Given the following pile foundation to
support loads P and M, shown here in profile,


		  P,M		|
	==========================
        |	|	|
	|	|
	|

with four rows of piles you can determine the required pile loadings for
equilibrium using or ignoring pile tensions as appropriate.  From this
you can determine first the pile deflections, then the foundation
rotation, and hence, the rotational (angular) spring constant for the
FOUNDATION.  You should also determine the rotational (angular) spring
constant for the base plate detail connecting the column to the
foundation then you can combine these to determine the TOTAL rotational
(angular) spring constant (in each direction separately) for your
superstructure analysis.

	This can get quite messy (and tedious too), particularly if you want to
take pile cap bending into account, that's why most people assume pinned
or fixed column bases.

				Regards,

				H. Daryl Richardson
			
Jim Kestner wrote:
>
> There is a crane building 90 feet high by 90 feet wide. To control sway, it
> is felt that we have to fix the column bases. The base moments are
> substantial.....about 1000 ft-kips. The columns are supported on a pile
> foundation driven to rock at about 30 feet below grade.
>
> Is it possible to create fixity with a pile foundation? I would think we
> could design the pile cap to resist the moment and make an attachment of
> the piles to resist the up and down forces. I would think the issue comes
> down to whether the pile can resist the upward force.
>
> Is this how others see it? Does anyone else have experience with this type
> of structure? Any guidance is certainly appreciated.
>
> Jim K.
>
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