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RE: "dynamic analysis" for earthquake ?

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-----Original Message-----
From: chuck ritter [mailto:ritter(--nospam--at)jar.com] 
Sent: Tuesday, June 07, 2005 1:56 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: "dynamic analysis" for earthquake ?

>From the ANSYS FEA documentation

"A spectrum analysis ... is mainly used in place of a time-history
analysis to determine
the response of structures to random or time-dependent loading
conditions
such as ... wind loads ..."

-----/Original Message-----

Is this really true, that spectrum analysis is used for wind loads?

That's the first I'd ever heard of it.

Typically, of course, Rigid (i.e. non-flexible), regular-shaped
structures, as defined in Section 6.2 of ASCE 7, are subjected to the
analytical procedures of Section 6.4 (Method 1) or Section 6.5 (Method
2) of ASCE 7. And there are factors given to aid in amplification of
wind load effects for Flexible, regular-shaped structures as well in
Section 6.5.

The only other method listed is the Wind-Tunnel Procedure (Section 6.6)
which I presume must be for "non-regular" structures. Now, I have
absolutely NO experience with wind-tunnel testing at all, so I cannot be
sure, but it seems like "spectrum analysis" would not be appropriate at
all.

Perhaps "spectrum analysis" is somehow involved with the "data
reduction" after the wind tunnel procedure is completed. ASCE 7 Section
6.6.3 says "the structural model _and associated analysis_ shall account
for mass distribution, stiffness, and damping." Perhaps the response of
the structure to the test under proper conditions yields the "spectrum"
that is then input into e.g. an ANSYS model?

Someone with more knowledge of these matters, please inform.


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