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-----Original Message-----
From: Walter Sheen Paoli <wsp(--nospam--at)dhl.com.pe>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Tuesday, March 30, 1999 12:31 AM
Subject: RE: Shear Walls


>
>>> The SMRF, if properly detailed, provides an excellent energy
>>> absorbing system, and it can accomodate some pretty high drift levels.

>This is true, but don't forget that displacement means damage, maybe
>initially the building is cheaper, but after a seismic event, sometimes a
>small one, we have so much damage that fixing the building is very
>expensive. The best you can do is limit the story drift as much as you can.


Actually, that was pretty much the point of my entire response - keeping the
wall structural would, IMO, probably help more than hurt, especially WRT
deformations.


>
>>> The
>>> shear walls, depending on the configuration and sizes, provides some
>>> measure
>>> of deformation control.  Given that the frames are taking most of the
>>> vertical loading, axial load on the walls is usually pretty low,
allowing
>>> them to exhibit good hysteretic behavior.
>>>
>
>Well I wouldn't say that if I don't see the building plans, maybe
>overturning moment gives the wall a big axial load, which is good, at least
>for concrete.


I would have to disagree, in general, with this statement.  A wall with high
axial load would necessarily have higher compressive strains within the
compression region.  This would at best result in more stringent confinement
of the BE's being required, or at worst result in rapid degradation of the
ends of the walls due to compression failure.



>
>
>>> On the other hand, if the walls are in such a configuration that they
>>> bring
>>> about a lot of torsional response, you might look at using soft walls,
but
>>> for a 7 storey structure I would think it wouldn't be too much of a
>>> problem.
>>>
>
>
>I would rather use another simetric wall to control torsional problems.
>
>Just remember, in the life cycle of the structure, it's cheaper to make it
>initially stronger. Don't forget that the "non structural" parts of the
>building are the most expensive.
>
>Just a comment.


I agree with what your saying, to a certain extent.  Making a building
stronger does reach a point of dimishing returns, IMO.  Sure, by increasing
the amount of longitudinal reinforcement, there will be a bit of a DECREASE
in the effective period of the structure, and this may help to lessen the
expected deformations during an actual earthquake.  But if we take two
structures, A and B, which are identical except that B has double the long.
steel, then:

1)  It is likely that the periods of the structures are pretty much the same
(some difference).

2)  If Ta=Tb, then delta max for A will be equal to delta max for B (or
pretty close).

3) BUT, Mo and Vo for B will be a lot more (double) that of A.  Both
structures get to the same deformation level, but it takes TWICE as much
force to get B there than A.

What does this mean?  It means that, for let's say a cantilevered wall, we
have double the Vo to design for in the hinge region, which means a lot more
Av at the base of the wall.  If a mat foundation were used to support the
wall, we have to design the mat for twice as much for B as for A.

Another point to consider is that in a SMRF, for example, putting in twice
as much long. steel in the beams means we have to reinforce the columns a
lot more as well (to preserve a favorable beam/column strength ratio).  Then
there is the added problem of the resulting congestion in both members, and
then there is the problem of congestion in the beam/column joints.

Now, this is a very extreme example, since I don't think anyone would just
arbitrarily double the long. reinforcement, but it is, I think, and
important point.

That said, I do feel that designing for the minimum seismic loads (which I
have thought the UBC to require in order to limit damage during small
earthquakes) is usually not a good idea, especially for SMRF's, since this
could lead to the problems of damage which you mentioned.

BTW, the reason why I feel strongly about this is that here on Guam we have
some truly huge wind loads to design for (155mph UBC).  For hotels with high
height/width ratios, this results in a lot of long. steel, usually a LOT
more than seismic design requires.  Doing this, then proceeding with seismic
design can be pretty difficult.

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

>
>
>
>
>Walter Sheen Paoli
>Civil Engineer
>Paseo de la Republica 6403. Lima 04
>(511) 446-6237 (511) 446-9407
>Lima, Peru
>----------------------------------
>mailto:wsp(--nospam--at)dhl.com.pe
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