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# Fw: Structure Magazine Questions

• To: "seaoc list" <seaint(--nospam--at)seaint.org>
• Subject: Fw: Structure Magazine Questions
• From: "T. Eric Gillham PE" <gk2(--nospam--at)kuentos.guam.net>
• Date: Tue, 12 Jan 1999 09:17:51 +1000

```Lynn:  I'l take a poke!

If you want a picture of a shear wall structure which overturned during an
earthquake, take a look at Fig 3-12, page 26 of EERI's "The Hyogo-Ken Nanbu
Earthquake" (Kobe EQ).  Don't know if it was soil related, but it doesn't
look like it, and the building IS very much horizontal.

Regarding the issue of overturning during earthquakes:

I find that looking at a structure during an earthquake in terms of
DISPLACEMENT assists me greatly in understanding maximum demands.  I'm note
sure how many people know this, but if an engineer uses does an RSA to
determine dynamic loads on a structure, he/she is actually using the results
of time history-displacement studies, since the acceleration spectra are
actually derived from displacement spectra (at least according to text
books), which are then smoothed, massaged and tweaked.

This being the case, why not look at a shear wall and its propensity for OT
in terms of displacement?  Calculate an effective period for the structure's
translational modes, find a corresponding maximum displacement, check the
structure for stability at that displacement.

Finding the displaced SHAPE of the shearwall at the max disp could be
tricky, I admit, since if the foundation does not have the capacity to yield
the wall (I am assuming the wall will experience inelastic strains), then
the wall may rock on its foundation.  This rocking will further reduce the
effective period of the structure, and this would have to be accounted for
(and would likely be a real pain in the butt).

This brings me to another of your points,

"I mean how many of us design shear wall overturning forces for an Rw of
1 and then apply a 1.5 safety factor to it?  The principal is the same
is it not?"

It would be very cost-iineffective to design a shearwall for OT, and the
shearwall foundation, based on an Rw of 1.  However, it has always seemed
strange to me that many engineers design foundation systems for factored UBC
load cases, using an Rw>1, without looking at the capacity of the
superstructure supported ON the foundation.  It looks even worse, to me,
when the foundation is such that brittle failure modes (e.g. punching shear,
diagonal tension) are likely to control since there was no effort to
"ductify" (very technical term there) the foundation to allow yielding in
its components.  The superstructure will have an overstrengths associate
with it, by virture higher material strengths, redistribution of forces, and
phi factors, such that it is likely in many cases that the superstructure
will not fully develop a mechanism before the foundation is in danger of
failing.  What is the sense in spending all that time designing and
detailing the superstructure if the foundation system is going to fail?

IMO, the foundation SHOULD be designed for the maximum input from the
superstructure--> just because the foundation is not considered part of the
superstructure DOESN'T mean that engineering design principles pertaining to
ductile/non ductile failure modes shouldn't apply to it.

So, I would say that the foundation system should be designed to yield the
structure it supports, or that the foundation system should be ductilely
detailed to permit yielding.  This would greatly reduce the probability of
the foundation failing before the SS gets to do its job.

Same principle applies (IMO) to OT --> why design the SS for reduced (Rw>1)
seismic loads, then assume it will displace a lot more than the analysis
shows and hence introduce ductile detailing, but check the structure for OT
for the reduced seismic loads (with a FOS, of course)?  Why not check the
structure for OT at some reasonable approximation of its maximum expected
displaced position.  I admit that perhaps the FOS (1.5 or something along
those lines) MIGHT do the trick, but I have a bit of trouble depending an a
might when a 20+ storey structure is on the line.

Anyway, my 2 cents.  Anyone else?

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

-----Original Message-----
From: Lynn <lhoward(--nospam--at)silcom.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Tuesday, January 12, 1999 6:46 AM
Subject: Re: Structure Magazine Questions

>Roger-
>I think the Olive view towers was more an issue of dissimilar structures
>pounding rather than a gross stability issue.  I am sure that someone
>will correct me if I am wrong.
>
>Once the pounding damage had done its damage, the towers did fail.  But
>that was really a secondary failure as a result of the primary failure.
>
>Thanks for your input though, I was sure I would get a whole stream of
>earthquake.
>
>Look, I left myself wide open, aren't there more engineers out there
>that want to take a shot? :)
>
>Lynn
>
>Roger Turk wrote:
>>
>> Lynn Howard wrote:
>>
>> . > Shear walls do not fail in overturning during earthquakes.  Unless
>> . > liquefaction is an issue, global stability of a wall has never really
>> . > been a problem.  I have never seen or read about overturning failures
of
>> . > shear walls.  There may be some I don't know about, but I do study
>> . > actual reports of earthquake failures quite a bit, and I have never
seen
>> . > this issue mentioned.  For those of you who are getting all excited
that
>> . > could lead to overturning issues.
>>
>> What about the stair towers at the Olive View Hospital in the 1971 San
>> Fernando EQ?
>>
>> A. Roger Turk, P.E.(Structural)
>> Tucson, Arizona
>
>
>

```