Impact Hammer Test Question (tester on the specimen) 5

[271828]

Good evening vibe experts.

I have the opportunity to modal test a really nice stair specimen this weekend. We have no small shaker, so I am stuck with using our impact hammer as the excitation. All along, I had planned to stand below the middle of the stair and strike the specimen from below, and rove the accelerometers. Now I find out that there's a huge fountain under it, so there's no way I can do that.

I can't stand beside it and strike downward because of the guardrail totally blocking access from the side (can't strike the guardrail).

I could strike about 3 ft (at the 2nd or 3rd tread) from the end of the 32 ft long stair. However, I am concerned that there might be some moment restraint at that end, causing there to be a modal node near where I'd be striking. Seems to me that option is no good.

The stair weighs about 10,000 lb and I weigh about 180 lb in case it matters. I expect the stair to have about 1% of critical damping without anyone on it.

I am left with only one apparent option (other ideas are more than welcome) and that's to stand on the stair and strike a tread near where I'm standing. Problem 1 is that I add damping and mass. Problem 2 is that my inertial forces, such as during the hammer strike, will not be measured.

Is it totally hopeless to try a modal sweep while standing on it? It would be acceptable if the modal results were good, but not near-perfect quality. However, if they're total garbage, then that's obviously no good.

Advice would be greatly appreciated.

 

[GregLocock]

Let's see

a 2 lb hammer is enough for a car, if you are desperate, so I hope you are using a 4lb or 8lb sledgehammer.

I've never done what you are proposing, it sounds a bit iffy.

But, I know the PERFECT solution

: Relaxation excitation

Get a cable with a release trigger and a tensioner. Attach your load cell to the stairs. Tighten the cable between the load cell and the building. Release the trigger.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[271828]

Thanks Greg. Believe it or not, we use a pretty small hammer for structures such as these. Tests are pretty consistent with shaker chirp tests, so I think they're ok. We use very small force inputs because we're interested in the response due to very small forces.

No go on the relaxation excitation idea, although I'll certainly keep that one in my toolbox. In this case, there's nothing to connect it to. The excitation needs to be vertical and there's a large fountain below it. Above it is several stories to the top of the atrium.

 

[GregLocock]

Hmm, I don't suppose you could make some force-measuring boots?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IceStationZebra]

I have one idea - Use something like portable scaffolding and some ballast to cantilever yourself up and over the bottom of the stairs.

Also, speaking of sledge hammers I found this paper.

http://www.mecheng.adelaide.edu.au/...lic_papers/2005/preprint_hammer_vs_shaker.pdf

ISZ

 

[271828]

Force-measuring boots? No, but I do have a force transducer that I could heeldrop onto. There are two problems with this, however. First, my mass and damping are still there. Second, the treads are narrower than the force transducer, so it wouldn't fit onto a tread.

Unfortunately, I can't use scaffolding and cantilever myself over the stair. It's extremely fancy and the structure opens Monday. They'd never let me try that.

Unless they drain the fountain (been requested), I think I'm stuck with the following:

1. Stand on the tread and strike downward, trying to minimize my inertial force. Do the modal sweep.

2. Stand below the bottom of the stair and strike the tread that's as far up the stair as I can reach and take a couple of measurements. I can at least compare the natural frequencies obtained to the ones from #1. I can also see if H12 is approximately equal to H21 (where 1=midspan and 2=the tread used in #2). I would probably actually do this step first. That way I'd know standing on the tread causes massive problems before going through the sweep.

Any other ideas?

 

[btrueblood]

Just drop the hammer, don't swing it, and the inertial forces of your body (arms, shoulders) should be minimized. Yes, you've added a mass, and a minor amount of damping, but you should be able to compensate for your mass in your model. Try dropping the hammer to the tread below yours.

Any way to suspend the hammer and drop it (e.g. via a cable thru a ceiling fixture, or a cantilevered boom?

 

[MikeyP]

Surely as long as you are not hitting EXACTLY on a node then you should be OK.

... and don't call me Shirley...


M

--
Dr Michael F Platten

 

[271828]

Heading out first thing in the morning for my adventure. I appreciate all the thoughts and advice. They said yesterday that the fountain will be drained, which gives access from below. If they fail to do that, I think I have a couple of decent backup plans. I'll try to remember to report back and describe how it went.

 

[271828]

Well, the tests went well. They drained the fountain, so I could strike from below. I did learn a lot, though.

The biggest lesson (which is already obvious I guess) is that it's a lot harder to get high quality modal data from hammer tests than shaker tests. I really missed being able to control the magnitude and frequencies that were excited.

 

[GregLocock]

You control the frequency by changing the mass of the hammer or the material on the tip of the load cell, and you control magnitude via practice.

In some circumstances I got better data off a hammer than a shaker, but wouldn't wnat to do more than 30 points with 5 strikes per point!



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[271828]

Yeah, I know. We only have one hammer and it extends to high frequencies (for us -- we deal with 3 Hz - 15 Hz +/-). I don't have the skill to know when I'm hitting it hard enough to cause a peak acceleration of 0.5%g or whatever. Perhaps with enough practice.

It was tiring standing on the ladder waiting for the next frame to start--over and over and over.

 

[GregLocock]

What size hammer? 2 lb?

Yes, getting good hits is tiring - I'd often do a few for practice and then when I got a good one just use that one, ie no averaging at all.

Anoher technique that isn't in the books is just to bash it in lots of likely places and average the lot together to get a sort of averaged mobility.

If I was trying to maximise the thump I'd tune the frequency response of the tip by supergluing discs of rubber sheet to it - then check the power spectrum to see if I was getting any rolloff in the frequency range of interest.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.