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Re: Structure Magazine Questions

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Without changing the issue, I thought I would bring up another angle on it.
For tall and slender structures all points are well taken but for short,
long span (retaining wall) or shear wall (concrete) in short , long span
structure the chances of a seismic force hitting it orthogonally are
proprortional to 2 in 360 or 1 in 180 and anyone who bowls knows that
oblique is weak and to get to the point, the time history of said shear wall
should be modeled using a finite element program with fine mesh especially
since the oblique force induces torsional , transverse and longitudinal
shear at the wall - footing which combined with the tensile stress i.e.
shear and tension which is not good (cage explodes through concrete cover).
Then there is the subject of lateral stiffness of joints (un-braced) and
torsional shear and bending in columns which gets to eccentricity
etc.,---------having been away from computers since the 70's, I'm
re-learning them and Staad.  It may sound like it.

                                                            Greg










                                                        Greg
-----Original Message-----
From: Lynn <lhoward(--nospam--at)silcom.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Monday, January 11, 1999 7:07 PM
Subject: Re: Structure Magazine Questions


>Mark:
>I understood your remarks in your earlier post, and really do not take
>any exception to them.  I was kind of trying to open a new thread on
>this issue.  Since Bill Allen no longer posts on this list, we don't
>always get many controversial issues to discuss anymore.
>
>Why is it that we do not use the actual accelerations of a building when
>we do overturning design?
>
>Why is it that a steel frame building using ordinary moment frames will
>have a different overturning moment than a special moment frame
>building?  It does not make any sense to me.
>
>>From a quick first look, you would think that for overturning forces, Rw
>should always be 1.
>
>I think we do not really understand how this works, or we have not
>really gone into it in depth and  modified our design based on the
>actual performance of the building.  I think our current design approach
>is conservative.  I just think people should recognize the conservative
>Code design requirements for what they are.
>
>I was also hoping for some people to call me to task on this issue and
>point out where I might be wrong.  I really thought more people would
>jump in and challenge me on this issue. I thought it would generate some
>interesting discussions.
>
>Lynn
>
>
>
>
>Mark K Gilligan wrote:
>
>> Lynn:
>>
>> I was trying to make the point that the earthquake forces on retaining
>> walls from the soil are calculated differently than those for buildings.
>> In the former case the forces are an approximation of the actual forces
the
>> structure will see, while in the later case the calculated seismic
forces,
>> as specified by the code, are often less than the forces the structure
has
>> to resist. My point was also that in the first case a factor of safety
less
>> that 1.5 for seismic loads may be appropriate while in the second case
>> designing the retaining wall for the code forces would overestimate the
>> factor of safety of the wall.
>>
>> I was not suggesting that we design the shear wall or the retaining wall
>> for an Rw=1.0, but rather that we design the retaining wall for the
forces
>> that can be transmitted to it from the rest of the structure.
>>
>> The use of an Rw>1.0 for the super structure is justified because of the
>> ductility that we have designed into specific elements.  Unless the
>> retaining wall is able to exhibit some ductility and still remain stable,
>> then it needs to be designed for the forces that can be transmitted to
it.
>> This statement is consistent with the rest of the code but because it is
>> not explicitly stated in the code, many people do not follow it, and
>> instead just design for the code forces.
>>
>> We have to be careful with statements such as; "Retaining walls which are
>> properly design for static loading conditions do not fail in overturning
>> during earthquakes".  This sort of thinking has lead us to be complacent
>> about some aspects of seismic design only to be rudely reminded of the
laws
>> of physics when an earthquake occurs.   Some of the problems with making
>> decisions solely based on previous earthquake experience arise from
changes
>> in design and construction practices, and because the past earthquakes
may
>> hot have been all that large.
>>
>> The reason why most retaining walls do not fail in earthquakes was
>> addressed in the second part of my posting.  If the only earthquake
forces
>> on the retaining wall are from the soil mass behind the wall, there is
>> rarely a failure.   The problem is that the original question did not
make
>> it clear how the earthquake forces were computed.
>>
>> On the other hand, I have seen designs that rely on a cantilever
retaining
>> wall to resist building seismic forces acting normal to the face of the
>> wall.  Admittedly, this is not a normal situation so it is not surprising
>> that few such failures have been reported, but that is not to say that
this
>> failure mode cannot occur.
>>
>> Mark Gilligan
>
>