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Re: Completly shearing a building (also

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Dennis,

In structural design there are always *some* things that we ignore.  In all 
structures we ignore the contribution of non-structural items.  In steel 
construction, we ignore the fact that shear connections *do* have the ability 
to transmit some moment.  In concrete construction we ignore the fact that 
tie bars *do* contribute something to moment resistance.  We ignore the 
fact that lateral loads from earthquakes may be much greater than the forces 
that we design for.  In structural analysis, we ignore the fact that the 
building is not completely constructed before any load is placed on it.

There are some things that we have ignored that we have learned that we 
should not ignore.  We have learned not to ignore partial height infill walls 
that reduce the effective height of columns.  We have learned not to ignore 
reentrant corners.

We have ignored the affect of completely sheathed wood walls on lateral 
resistance in frame construction as compared to just considering the walls 
that we designate as "shear walls" as being the only part of the structure 
capable of providing lateral force resistance.  If completely sheathing walls
contributes something to the lfr, fine, that is a little bit more safety that 
we have.  If it reduces the lfr, then, we have to consider how much the lfr 
is reduced and whether it is significant.  If the lfr resistance is 
significantly reduced, then we should not ignore the effect of completely 
sheathing the wall.  But at this point in our knowledge, we can't say that 
the lfr is significantly reduced.  At this point in our knowledge, we don't 
even know the force that the walls will be subjected to.

"Liability," how I hate that word!  We have no "liability" until a court 
says we are liable, or we, as individuals, voluntarily accept being liable.  


A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Dennis Wish wrote:

>>Dave,
Thank you and thanks to the others who responded with the same eloquently 
stated opinions. My comments were, at most, cynical to the current design 
methodology. How the heck are we going to change the course that Seismology 
Committee is taking which affects our practices yet has little if any 
calculable effect on the performance of residential structures.
In my example, the contractor may be doing a service by increasing the 
stiffness of the overall structure, but he is also ruining the calculated 
distribution of force by stiffness which may, theoretically, alter the 
performance of the building. Do I believe it to be a negative effect - no 
way. However, the liability that is transfered due to over-specification of 
the methods is unwarranted in residential design which experiences field 
changes as a normal part of bulding a home. I don't believe that the same is 
true of commercial and industrial stuctures which are built to much more 
ecconomical standards and do not generally experience major changes in the 
field.

As I mentioned to others - how do we change the course that Seismology is 
taking by trying to tie up all methodologies into one neat design so as to 
comply with what is believed to be the answer to Vision 2000. They really did 
not listen at the August 14th meeting which was meant to apease the community 
to a small degree but planned to proceed into publication of the blue book 
provisions already drafted by Kelly Cobeen prior to the opinions expressed by 
those in attendance?<<