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RE: CBC CODE SR VALUE

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I would recommend that engineers not use equation 31A-1 unless they are
required to since this provision is technically flawed.

The SR term was apparently introduced to account for the near field effects
that occur near a fault.  When the near field provisions were introduced
into the code dealing with schools and hospitals the SR adjustment was not
removed.   As a result it has been my experience that the SR term is
significantly less than the 1.25 multiplier.

There are several difficulties with this provision.  First is the
requirement  that V, in equation 31A-1, is the base shear calculated using
formula 30A-4.  If you have a building with a short period you can find
that the required base shear is much larger than is reasonable.  When this
provision was originally written section 1630A.2 was organized quite
differently and thus you did not have the problem.  When they changed the
way we calculated the base shear they did not coordinate the change with
equation 31A-1 thus causing the problem.

In addition I do not believe that the term PMm is clearly defined.  

Finally I believe that equation 31A-2 is not effective in adjusting the
target base shear.  This is because the equation gives equal weight to all
modes even including those that have a small or no base shear associated
with them.

The lesson here is that,  just because something is in the code does not
mean that it is technically valid.

Mark Gilligan


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