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-----Original Message-----
From: Lynn <lhoward(--nospam--at)>
To: seaint(--nospam--at) <seaint(--nospam--at)>
Date: Wednesday, February 24, 1999 12:49 AM
Subject: Re: Dynamic seismic design


I don't do a whole lot of steel design (mainly R/C), but the principles
underlying seismic design are basically the same for both steel and R/C.

IMO, the UBC to date has used a very simplified "performance based" approach
for seismic design.  As it stands now, there are only 2 performance states -
1)  that under the reduced (Rw>1) seismic demands and 2) that under the
actual expected earthquake demands.

The UBC takes care of state 1) by having the designer analyze and design the
structure ELASTICALLY under the reduced seismic loads.  The way I see it,
they are trying to ensure that the structure will not experience
siginificant yielding and damage during an earhtquake which is quite a bit
smaller than the design earthquake.

State 2) is "taken care of" by requiring ductile details which are intended
to allow the structure to deform to the displacement levels associated with
the design earthquake, probably with significant damage (possibly to the
point of it not being economically feasible to repair the structure) but
WITHOUT COLLAPSE.  I put the quotes in because feel that the UBC detailing
methods, and especially their application to particular members, is not
always appropriate (WRT R/C).

So, right now the UBC, IMO, DOES attempt to control damage, but in a very
simplified manner that does not allow for much quantification of the control
since the process is not very transparent at all.  I imagine that the IBC
will probably be more in line with ATC-40 or FEMA 273, in that performance
states (of which there will be more than 2) are explicitly defined and
addressed during the design process.  Hopefully, this will give the designer
a much better tool to gauge the expected performance of the structure during
earthquakes which are not as severe as the "big one".

Regarding the non collapse of steel SMRF's during Northridge - I would think
that the main reasons for non collapse would be:

$.01  Redundancy - the fracture frames sloughed off their load on to other
parallel lateral load resisting systems which took up the slack, preventing
additional excessive displacement which COULD have caused collapse.

$.02  Even the the connections "failed", they were still viable to a certain
degree, so the frames still partially worked to prevent displacement.  In
other words, the failure of the connections did NOT result in a collapse
mechanism, so the structure was still stable.

Again, I don't do steel design much, but that's my two cents.

T. Eric Gillham PE
GK2 Inc.
PO Box 3207  Agana, Guam  96932
Email - gk2(--nospam--at)
Ph:  (671) 477-9224
Fax: (671) 477-3456

>I know this is changing the topic, but I don't think this is necessarily
>the case.  We are hearing more and more about the Code attempting to
>control damage, and that is the justification for all of these code
>Northridge Earthquake: - Billions and Billions (per Carl Segan) of
>dollars in damage and how many lives were lost?  I know, I know, even
>one is two many some people will tell you.
>If all of these steel moment frames moment frames fractured, why didn't
>they collapse?  Hmmmmmm?
>Inquiring minds want to know.
>I know, I just a trouble maker:)