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RE: "Cantilevered Column" Structures

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Per UBC, the base of the structure for seismic design purposes is "the level
at which the earthquake motions are considered to be imparted to the
structure or the level at which the structure as a dynamic vibrator is
supported". This still leaves alot up to "engineering judgment". Unless the
soil around the piles is considered to act more like a fluid during an
earthquake (e.g., liquefied soil), I would expect there to be lateral load
effects between the piles and the soil starting immediately below the
concrete foundation level. Thus I would argue that the earthquake force
starts being imparted to the structure at this level (although it likely
wouldn't be fully imparted until the bottom of pile is reached). 

A true cantilever structure would have greater deflections and would fail if
excessive deflection occurs. But piles would deflect less due to soil
support and collapse due to excessive deflection is not likely. 

> -----Original Message-----
> From: Kevin Harris [mailto:Kevinh(--nospam--at)jhiengineering.com]
> Sent: Thursday, July 19, 2001 2:24 PM
> To: Seaint(--nospam--at)seaint.org
> Subject: "Cantilevered Column" Structures
> 
> 
> Greetings
> 
> I have a question that has been bugging me for awhile that I 
> hope someone can give me some guidance on.
> 
> Here are the facts:
> 
> 1) A lightweight pre-engineered building (say 20 psf) bearing 
> on a pile supported mat that supports tanks and equipment 
> (say 500 psf+).
> 
> 2) UBC seismic zone 3.
> 
> 3) Point of fixity of piles is 20 ft below grade, per geotech.
> 
> Would this structure have to be considered a "cantilevered 
> column building system" (R=2.2)?  I was wondering if I would 
> be released from this low R value because the structure is 
> essentially just an equipment foundation.
> 
> Also, if I have to use R=2.2 for the foundation, would the 
> light metal building shell have to be designed with R=2.2 as 
> well (unless it can be shown to have 10 times the metal 
> building stiffness)?
> 
> Thank you for your thoughts.
> 
> Kevin Harris, P.E.
> 
> 
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