Energy absorption of seismic isolator mounts 1

[dpa]

I am looking for information on how to calculate the energy abosorbed by a rubber or other elasitic seisimic isolator based on durometer hardness, mass of the elastic material applied loads etc.

Thanks in advance
DPA

 

[NickE]

I do believe (I'm not sure however) that energy absorption of any material is related to the area inside the hysteresis curve of the commpression/relaxation graph. I dont believe that mass of the elastomer and applied load are all that significant. You might be able to take some pieces and send them to a test lab. If a random compression/relaxation signal is sent into the actuator of a sevo-hydralic test machine and the resulting Force vs Deflection curve is plotted and you know the exact size of the test sample you might be able to read the amount of hysteresis and convert it to an energy term.

As far as how to theroetically calculate this I have no Idea.

nick

 

[dpa]

Thanks Nick E that's a start and it sounds sort of familiar from days gone by.

DPA

 

[ElastomerXpert]

Energy absorption is a function of the actual dynamic property profile of the elastomer, and that changes with the strain and frequency of the actual deformation. Most seismic bearings are made of fairly resilient compounds (Natural Rubber or Neoprene) and their function is to isolate (absorb little energy) rather than to damp (absorb lots of energy). So you may not find the absorbed energy adds up to very much for any seismic event. And by the way.. why do you want to know about the absorbed energy?

 

[dpa]

Hi ElastomerXpert,

Thanks for the response. No pun etc. I am working on the design of a dock float which is attached to one end of a 2500 pound steel ramp. The float is exposed to some fairly heavy wave action. A storm this winter tore the original float loose and put some dramatic bends into the connecting bolts. I would like to absorb as much energy as possible in order to protect the connections as well as any boat that might happen to be left there during a storm.

DPA

 

[ElastomerXpert]

Aha, you don't really care about how much energy is absorbed directly, you just want the dock float to work and prevent damage. And wave action is not "seismic" in the sense that adjective is normally applied in the industry; earthquakes have short duration and put building foundations into shear, waves go on and on and pound on things in shear, compression, and tension at various stages.

So you have a different design challenge than I'd thought, and the approach to it would require a lot more input than can be provided here. Maybe you should look at Gent's book on Engineering with Rubber (available from the Rubber Division of the American Chemical Society). Your other avenue is to find a competent consultant, preferably an engineer who really knows rubber or a rubber technologist who really knows engineering. (Neither group is very large!)

RJD

 

[dpa]

ElastomerXpert,

Thanks for the tips. I'll look around. I guess I could always go to steel springs as a last resort. Lots more people understand those than rubber including me. Weight is a problem though not to mention arrangement.

DPA

 

[ElastomerXpert]

Actually, I think the properly designed rubber part will do this job better than steel springs, weigh less, and of course not be subject to corrosion. That's why they build rubber dock bumpers for freighters, etc, that keep the big ships from pounding the docks. Check Gent's book, if that doesn't give you ideas I can recommend (I hope I'm allowed to do that) a guy up in Massachusetts who's an excellent engineer with long experience in designing rubber parts. He might give you some input gratis, or you&he may decide to explore a consulting relationship.

R J Del Vecchio
Technical Consulting Services
(consultant in Applied Rubber Technology)

 

[Focht3]

Is this for a RO/RO facility?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[dpa]

Thanks ElastomerXpert,

That is a very helpful tip.

Focht3,

This is just a private dock on a lake in Washington State but the guy's property just happens to be situated such that the wind and waves come crashing directly at him. There is a solid sandstone bottom that goes out 72 feet from his retaining wall where the water depth varies from about 6 feet at the deepest part down to almost zero (at the deepest part) depending on the time of year. The sandstone ends abruptly in an almost sheer cliff that appears to drop off to several hundred feet. I went down to 85 feet looking for a lost piece of lead ballast and it was still dropping off at about an almost vertical down angle. The float is out over the deep part. In addition the cliff does not stick out as far on the neighboring property so his part of the cliff area is fully exposed to wind and wave action. If we had had experience with the weather a that location we would not have taken on this job for any price although the guy is willing to pay whatever it costs.

DPA

 

[Focht3]

Wouldn't a "better" answer to make the floating dock hinged so that it can be stored at the bulkhead when not in use? Otherwise, you're going to have a tough time with this design -

Of course, you could install anchors at depth - but that isn't too practical because of the steep bottom slope.

You've got a tiger by the tail!



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[dpa]

The floating dock is hinged but the ramp weighs 2500 lbs so remving the float is tough. Plus when the water level is down you cannot move the float in close to the seawall. Would have been a good idea if the weights were not so large.

DPA

 

[Focht3]

The water level problem is somewhat self-correcting, right? (It sounds as if the water level is down, the dock rests on the bottom.) As to moving the dock horizontally, you could use a 12V winch - 2,500 pounds shouldn't be a problem. And remember that the dock should be floating, so the forces are wind and wave - not the weight of the dock.

Is this the owner's permanent residence - or a weekend retreat? How much warning does he have about drops in the water level (lake) or is this a tidal issue (Pacific Ocean)? What does he do with his boat(s)?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[dpa]

The float is out beyond the edge of the cliff so that part is always afloat. The problem with dragging it in to the seawall is damage to the flotation. The water level is low much of the year except early summer. Next time I am over I will try to remember to take some photos. I can email you some if you like.

DPA

 

[Focht3]

Sure! Post to this thread when you have them -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[dpa]

Can you post photos to a thread like this? How do you do that?

DPA

 

[Focht3]

The [blue]Process TGML[/blue] link is just above the "Submit Post" button of the 'Your Reply' box below. Instructions for linking images are provided there.

Basically, you will have to put the photos on a publically accessible web site and create links to them in your message. It isn't hard, but it can be tough getting the images the right size. Don't make the images any wider than about 600 pixels - for 1024 x 768 screens. You will have to play with it to get it right.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Tmoose]

http://wemyssbay.net/images/jetty.jpg

??

Are you asking the ramp to locate and restrain the float?