Simply Supported Steel I-Beam Test
Simply Supported Steel I-Beam Test
(OP)
I am trying to test a steel I-Beam on simple supports, I've done a swept sine test using a shaker and accelerometer and calculated (Half-power method)the damping ratio to be 2.1%. I think is too high a number for an steel beam, my guess is it should be less than 1/2%. My question is could my setup distort (add) damping to the beam?? Resolution of swept sine is 0.05 Hz.





RE: Simply Supported Steel I-Beam Test
2% is too high, try dangling the beam from an elastic cord and flick it with your thumbnail or a hammer - you should be able to get a good estimate of the damping ratio from the decay rate of the sinusoid.
There are several ways a bad setup can add damping, using a free-free test with an impact excitation will allow you to determine were the problem is.
Cheers
Greg Locock
RE: Simply Supported Steel I-Beam Test
RE: Simply Supported Steel I-Beam Test
Cheers
Greg Locock
RE: Simply Supported Steel I-Beam Test
RE: Simply Supported Steel I-Beam Test
I have a more general comment about your test. I assume that you are trying to find the best place on the beam to position your TMD. That optimium position will be determined by the mode shapes of the beam. The mode shapes in turn will be determined by the boundary conditions. So you should test the beam in the boundary configuration in which it is to be used. Is this beam going to have simple support end conditions in practice?
M
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Dr Michael F Platten
RE: Simply Supported Steel I-Beam Test
RE: Simply Supported Steel I-Beam Test
RE: Simply Supported Steel I-Beam Test
RE: Simply Supported Steel I-Beam Test
Are you using a current controlled amplifier to drive the shaker (one which provides an output current proportional to the input voltage)? If not, then this could also distort your response curven due to a nasty little phenomenon called "force drop-out".
Usung half power points to estimate damping applies specifically to the FRF, which you are not measuring. Greg's rough and ready impact approach will probably give you a better estimate than your half power approach.
A better method is "sine dwell" which is marginally more sophisticated than the impact approach in that it allows you to estimate the damping of individual modes rather than the structure as a whole. It also doesn't require any more equipment than you already have. It should work well on your structure because the natural frequencies are well separated.
1. Put a sine wave through your shaker as close as you can to the natural frequency.
2. Apply it for long enough so that the structure reaches steady state.
3. Switch it off and measure the decaying reponse.
4. Use log-dec or similar to estimate the slope of the log of the decay.
Not so many years ago, we used to estimate the slope by offering a protractor up to the 'scope screen. Fortunately things are a little more sophisticated now.
M
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Dr Michael F Platten
RE: Simply Supported Steel I-Beam Test
RE: Simply Supported Steel I-Beam Test