From: Bill Polhemus <bill(--nospam--at)polhemus.cc>
Date: Wed, 08 Jun 2005 06:41:43 -0500
Christopher Wright wrote:
...but do you use the wind tunnel to derive the spectrum?
I truly don't know but I expect you can use instantaneous anemometer
data transformed into a velocity pressure power spectral density curve
for random response. You'd have to use real world transient wind
pressures, not wind tunnel data. Wind tunnels are usually set up to
provide constant aerodynamic conditions, but if you're looking at
dynamic response, you'd need transient wind pressure measurements.
I think that you're thinking of wind-tunnel testing for e.g. aircraft.
For structures, a BOUNDARY-LAYER WIND TUNNEL is used, such as the one(s)
at Texas Tech. These simulate the near-surface wind effects (pressures
vary with height; turbulence due to surface roughness is present, etc.)
Please remember that the use of wind-tunnel testing isn't in question;
ASCE 7 specifically mandates it and describes the type of testing--which
is derived from actual experience with boundary-layer wind tunnels. In
fact, the various pressure coefficients present in ASCE 7 for both main
wind-force resisting system and "components and cladding" derive not
from real-world observations but from boundary-layer wind tunnel testing
of instrumented models.
So my question is still open: How does this correlate with the use of
response spectrum analysis?
> The way you'd define a response spectrum is to get a loading history
and apply the load history
> to a bunch of single DOF systems (mathematically) to find the maximum
You may have answered my question--except that (as I understand it) a
response spectrum is generalized for pretty much any structure. But if
this were true it seems to me that a response spectrum for wind would be
a matter of record just as we have the pressure coefficients for
"regular" buildings in ASCE 7.
But wind tunnel testing is needed because "irregular" structures aren't
amenable to such generalizations--and that would seem to toss the whole
idea of a "response spectrum" out the window.
I'm probably flawed in my thinking because of the paucity of my
understanding in this area. I seek to learn.
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