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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,
I've never encountered the 100-40 rule. It's usually SRSS or worst
orientation for the kind of equipment I do. I get separate EW and NS
spectra about half the time and rarely get a 'worst orientation'
specification. The rest is a single set of horizontal spectra to be
applied simultaneously in both directions. OTOH for some of the
non-nuclear equipment, usually machine support, some people are still
using seismic zones and very surprised when I say that that approach has
been dead for at least 5 years.

>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.
This is strictly true only if the seismic loading is equal in both
directions. It tends to emphasize the larger the two loadings if they're
unequal. It's conservative to the extent that it subject the structure to
a net loading greater than the required design loading, but sometimes
it's easier to do that than to figure out which is the 'worst
orientatation.' In fact usually when I get a worst orientation request, I
apply the loading simultaneously along both horizontal directions unless
it's obvius which is the worst case. On the occasions when I've gotten
curious, the 'worst orientation' is invariably worst for some members in
one direction and worst for other members in the other direction, but
never worse than both applied simultaneously. But that's a 'for
instance,' not  proof.

Most of my stuff is mechanical equipment which is rarely orthogonal and
usually has inclined inertia axes.  FEA and dynamic analysis is the
calculation method of choice. That means combining seismic and dead
loading is a little trickier, but usually easier than figuring out 'worst
orientation' with equivalent static loading. Is dynamic analysis catching
on in building design?

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)

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