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Re: EQ resistant Design

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In the nuke industry, they specify to use either SRSS or the 100%-40% rule,
while in building codes they specify either SRSS or 100%-30%.  Whether you use
SRSS or "components" depends on what you are designing - in some instances, such
as when checking steel members using code equations, the 100-30 or 100-40 method
works better than the SRSS approach.  The SRSS approach is actually a bit
conservative, since it assumes that the structure is being subjected to the
"maximum" acceleration in both directions simultaneously.  Since earthquakes
have a lot of randomness, it is not so likely that the peak acceleration in each
direction will occur in the same instant.  For inelastic (i.e., R-factor)
designs, the SRSS method also tends to significantly overestimate vertical
forces - the 100-30 or 100-40 methods work better.  Why did they choose 30% or
40%??  It is an approximation of the statistical likelyhood of the forces.

Of course, building codes only require you to consider this if you either have a
non-orthogonal structure (such as having skewed wall alignments), or if the LFRS
elements are subjected for forces from either direction (such as a column and
foundation supporting bracing from two directions).  For simple buildings having
separate LFRS systems, there should not be significant orthogonal effects.

regards,
Martin Johnson



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