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Re: "dynamic analysis" for earthquake ?

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On Jun 8, 2005, at 6:41 AM, Bill Polhemus wrote:

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.)
The turbulence is really a micro effect, fairly high frequency, which lies under the natural time variations in gusting and overall wind speeds. The dynamics of the situation you'd try to mimic with a response spectrum or PSD response are lower frequency variations and don't have much to do with surface roughness. We had a helluva storm here last night, and you could hear how the wind speeds changed because of gusting, even though it was a straight-line wind pattern. (Someone else got the tornadoes.) You don't hear turbulence because the scale is so much smaller.

You may have answered my question--except that (as I understand it) a response spectrum is generalized for pretty much any structure.
I didn't do a very good job. A response spectrum is is a general response becasue it covers a wide band of response frequencies. Imagine a certain load time history--you might stick an anemometer out into our storm last night and record the output over time. You take the velocity over time and make a wind pressure history. If you apply that wind pressure as a force to a structure which you represent as a single degree of freedom system and you can solve the equation of motion and find the peak response at some point over time. If you repeat the solution with structures having different frequencies the peak response changes--it'll be a different magnitude and will occur at a different time. Now if you plot the peak responses as a function of the structural frequency, you'll get a response spectrum. The response spectrum tells you what the peak response is for any structure as long as you know its natural frequency, so it's general in that regard.

With FEA you can figure all the natural frequencies of the structure and combine the response from each mode into a probable maximum loading, considering contributions from all modes. You can simply add all the maxima if you want, but it'll be very conservative because all the maxima aren't simultaneous, since they apply to different frequencies. The advantage of using FEA on irregular structures is that you can get all the vibration modes resulting from the irregularities and apply the loading accurately.

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

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