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RE: Design ground accelerations

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Bill raises some very interesting points that are especially germane to
structures where seismic performance is a major issue.  The 1997 UBC
included many provisions contained in the 1997 NEHRP.  But the most
significant element of the 1997 NEHRP was not included in the 1997 UBC.  The
1997 NEHRP uses seismic spectral ordinates as opposed to seismic zones
contained in the 1997 UBC.  If enhanced seismic performance is an issue (as
it should be for essential buildings), the importance factor used in UBC
seismic zones provides a markedly false sense of security that you will
achieve any sort of enhanced performance especially in regard to

Using the UBC seismic zones for an essential building is like putting
tractor tires on a Ferrari, and expecting it to perform like a Ferrari.  (I
watched the Italian Grand Prix over the weekend.)  Proper prediction of the
ground shaking for a given probability is essential in designing a structure
that will provide a predictable level of seismic performance at a given

Harold O. Sprague

> -----Original Message-----
> From:	Cain, William [SMTP:bcain(--nospam--at)]
> Sent:	Monday, September 16, 2002 11:38 AM
> To:	'seaint(--nospam--at)'
> Subject:	RE: Design ground accelerations
> Michael Bryson wrote:
> "Fire the geotech and find one who will design to the minimum code
> requirements, not beyond them."
> Maybe we need to step back and look at what is being asked for and what
> type
> of building we are applying it to.  
> First, the original poster identified the structure as a fire house which
> would place it in the essential services category. 
> Second, the building Code is a set of MINIMUM, not MAXIMUM requirements.
> Third, while I agree that the geotechnical engineer has gone beyond what I
> have found to be normal practice, he has apparently considered the nature
> of
> the building to be designed and in his own way tried to clarify what
> ground
> motions would apply to what level of safety.  I would hope he did this in
> consultation with the owner and structural engineer (although it sounds
> like
> the structural engineer and geotechnical engineer weren't talking in this
> instance).  If there is a provision in the geotechnical report the
> structural engineer is concerned about, he needs to talk with the
> geotechnical engineer BEFORE going to plan check.  Plan checkers do read
> submittals and the better ones do understand what is presented.
> Fourth, if we as structural engineers never go beyond minimum code
> requirements, we are doing our clients a disservice.  That stated, we
> should
> be discussing the level of performance with our clients, educating them on
> what it means and following their instructions (based on their reasonably
> informed decision) using the building code as the MINIMUM acceptable level
> of service.
> Fifth, the 1997 UBC, cited as the controlling code, states in Section
> 1629.4, "Seismic hazard characteristics for the site shall be established
> based on the seismic zone and proximity of the site to active seismic
> sources, site profile characteristics and the structure's importance
> factor."  This provision allows for greater design forces if, in the
> opinion
> of the project team (including the Owner, structural engineer,
> geotechnical
> engineer and the architect (sigh!)), a greater seismic hazard exists. If
> the
> project criteria, as presented by the project team to the plan checker,
> call
> for a higher level of design, I think the plan checker is within his
> rights
> under this section, to ensure that a consistent level of design is used
> relative to the seismic hazard. Without more available information, we
> shouldn't be second guessing the geotechnical engineer.  Rather, we should
> be talking with him or her to ensure we are all speaking the same language
> and have the same project goals in mind.
> Sixth, if it were my community, I wouldn't want MY fire station being
> designed to the MINIMUM code provisions if it is truly to be functional
> after a major earthquake.  Fred Turner of the Seismic Safety Commission
> has
> done some remarkable studies of the failure of fire doors (presumably
> designed to code) to operate following a variety of earthquakes.  A simple
> door failure can take out critical response crews for a considerable
> period
> of time in the important first hours immediately following an earthquake.
> Failure of the structure can take out these crews for an even longer time.
> So Michael, before you "fire the geotech" for designing to greater than
> MINIMUM CODE REQUIREMENTS, you might want to understand what he is saying.
> He may be saving your butt by making the project design appropriate for
> the
> intended use, one that will perform as intended.  On the other hand, he
> may
> be unduly conservative.  Either way, you won't know if you don't talk with
> him.  It is just common courtesy.
> Regards,
> Bill Cain SE
> Oakland CA

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