Modal analysis question -- "free-free" condition
Modal analysis question -- "free-free" condition
(OP)
Hello all. I have a question that I'm sure sounds very easy to many of you. Advice will be greatly appreciated.
Our team would like to modal test a 3,600 lb (6 ft x 8 ft, approximately) piece of masonry wall (orthotropic) and hopefully determine the flexural stiffness in both directions. We have already sawcut the wall and have it laying on the floor in the lab.
Our idea is to suspend the wall from its 4 corners using our overhead crane, with the idea of creating a condition that's as unrestrained as possible. The crane cables are probably about 3/8 in. diameter and are about 20 ft long. The testing program would be a simple quarter-point modal sweep with impulse hammer strikes at the middle of the wall. From that, we can plot the mode shapes for the different frequencies and hopefully solve for the stiffnesses.
I remember in experimental modal analysis class, we modal tested a small aluminum beam in the free condition by sitting it on soft foam. I also remember the teacher talking about suspending specimens using wire (small EA and large L) to create an unrestrained condition.
Would it be better to suspend the wall vertically from only two corners? Is there a better way to approach this problem than what we're planning so far? We do not have a small shaker, BTW.
Thanks in advance.
100,000e
Our team would like to modal test a 3,600 lb (6 ft x 8 ft, approximately) piece of masonry wall (orthotropic) and hopefully determine the flexural stiffness in both directions. We have already sawcut the wall and have it laying on the floor in the lab.
Our idea is to suspend the wall from its 4 corners using our overhead crane, with the idea of creating a condition that's as unrestrained as possible. The crane cables are probably about 3/8 in. diameter and are about 20 ft long. The testing program would be a simple quarter-point modal sweep with impulse hammer strikes at the middle of the wall. From that, we can plot the mode shapes for the different frequencies and hopefully solve for the stiffnesses.
I remember in experimental modal analysis class, we modal tested a small aluminum beam in the free condition by sitting it on soft foam. I also remember the teacher talking about suspending specimens using wire (small EA and large L) to create an unrestrained condition.
Would it be better to suspend the wall vertically from only two corners? Is there a better way to approach this problem than what we're planning so far? We do not have a small shaker, BTW.
Thanks in advance.
100,000e





RE: Modal analysis question -- "free-free" condition
Usually we'd use 3/8 or half inch bungee cord for this sort of job if it was going to be a long one.
Next time you might try a modal on the wall in-situ, it should be possible to figure out the modes that are due to the foundaton, and still get your stiffness.
One of the more exciting discoveries from doing that is exactly how flexible a foundation is.
Cheers
Greg Locock
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RE: Modal analysis question -- "free-free" condition
We tested the wall in-situ, but couldn't isolate the wall's flexural stiffnesses. It doesn't just connect to a foundation. It starts on top of an elevated slab and then is supported at the top by a steel brace. We have frequencies and mode shape for the entire specimen, but there are too many variables (the wall-to-slab connection stiffness for example).
RE: Modal analysis question -- "free-free" condition
Like I said, you'll learn far more from one hit than from any number of my notes!
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Modal analysis question -- "free-free" condition
RE: Modal analysis question -- "free-free" condition
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Modal analysis question -- "free-free" condition
RE: Modal analysis question -- "free-free" condition
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Modal analysis question -- "free-free" condition
M
--
Dr Michael F Platten
RE: Modal analysis question -- "free-free" condition
"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970
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RE: Modal analysis question -- "free-free" condition
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Modal analysis question -- "free-free" condition
The tests, hanging from two points, went very well today. The first two theoretical mode shapes were about the same shape as the measured ones, but the order was flipped around. No big deal, though, because they were 68 Hz and 69 Hz--identical for our purposes. Rigid body modes were all down around 1 Hz or lower.
We were able to change the model in about 5 min. to get the theoretical to match the measurements, allowing us to back-calc. the elastic modulus. It was fully grouted, so EI was about the same in either direction.
We struck the wall away from the center in 3-4 places to excite some higher freq. modes, but these were of no interest to us. With the first two, we could calc. E, which was the goal.
Now we have another specimen to do the same with, but this one's much larger, like 20,000 lbf and is very orthotropic, like 3x stiffer in one direction than the other. We'll probably chop out a part and test it instead.
RE: Modal analysis question -- "free-free" condition
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Modal analysis question -- "free-free" condition
RE: Modal analysis question -- "free-free" condition
Then you'll see the pitch and bounce modes.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Modal analysis question -- "free-free" condition
RE: Modal analysis question -- "free-free" condition
Just a thought: Sounds like the model you updated from the measurements was a numerical one (FE?). Have you checked if an analytical formula for natural frequencies could be fitted to back-calculate E? I think Blevins has a set of formulaes for orthotropic plates.