Dampening mechanical vibrations

[knelli]

Hi,

I have been asked to design a slab on grade to dampen the vibrations of a 150 hp dynamic load. The senior strucutral engineer here says that he usually makes sure the slab below and about 2'-0" around the piece of equipment weighs twice as much as the piece of equipment that is vibrating. With that information, I was coming up with an 8" slab. For good measure and rigidity I was going to use a 10" slab. After talking to our client, he says for a load of that size that they would want a minimum of a 24" deep slab below the equipment. Any thoughts? Any recommendations on the type of

FYI: Equip weighs 5k, and is 8' x 5', dymanic load of 150 HP

 

[csd72]

For a 5k load I would expect that the 10" slab could easily take it. I have taken 15k loads onto 6" ground slabs when they are on the right soil.

As for the vibrating bit, I have heard a rule of thumb of three times the weight for vibrating machinery.

Also make sure that 10" gives you enough embedment for the machines anchor bolts otherwise you might need to thicken locally.

csd

 

[Gumpmaster]

The rule of thumb that I use is 5 times the weight of the reciprocating parts or 3 times the total weight of the equipment, whichever is greater.

For large generators, you can come up with pretty large slabs.

 

[J1D]

Our rule of thumb is that once the HP of the vibrating machine is larger than 100 the foundation block will be sepparate from the floor.

The 2'-0" is the minimum foundation block thickness for 1) room of anchorage, 2)good bury of block into soil (foundation block should typically be 12" above grade), etc.

If the 150HP is centrifugal loading type, you don't have to do a dynamic analysis. Some rules of thumb are therefore good guidances, such as minimum width/length in ration to the height of CG of the machine; minimum mass ratio (3 times for centrifugal machine, 5 times for recprocating machine);center of machine in raltion to center of foundation mass, etc.

For machine foundation, 50% soil capacity is the value we usually used.

 

[SlideRuleEra]

knelli - You are receiving good advice - thick, isolated inertia block.

Making the slab wide and long (per your senior engineer's recommendation) does several things:

1. Lowers bearing pressure on the soil
2. Adds to the mass of the foundation
3. Makes for a stable footprint
4. Perhaps most importantly, ensures that the machine's anchor bolts are well within the rebar cage - very important for the day when "something" gets out of balance and excessive vibration tries to tear everything apart.

Making the slab thick (say 2 feet) also addresses several of the above issues and, as you noted, makes the foundation very rigid.

Soil properties are just as important as the design of the concrete. Perhaps a detailed investigation could optimize the foundation size... but this foundation is small and the cost of the extra concrete to satisfy the senior engineer (on the horizontal dimension) and the Client (for the thickness) is certainly worth it.

http://www.SlideRuleEra.net

 

[knelli]

THANK YOU so much for all your words of advice, I really appreciate being able to learn from those with more experience!! We're going with the 24" slab to accomodate the equipment and are working on the rebar design.

 

[Tmoose]

Are the vibrations to be "damped" really coming through the soil?

Pipe and duct mounting can feed plenty of vibration into a structure. Adding a few flex joints here and there is not a cure. The quiet side must be anchored solidly in 3D to force the flex joint to absorb all the motions. Flow noise and vibration dances right across a flex joint, free to excite piping up and down stream.

The slab width should be more than the cg height in any direction. Much more.

Foundations made solely on mass ratios have failed badly when too narrow or ignoring soil conditions.

http://irc.web-t.cisti.nrc.ca/cbd/figures/177f05e.gif

 

[EddyC]

For generator applications, Cummins has an engineering manual that gives guidance on how to size the generator foundation. It is available online.

 

[rob768]

maybe it is not relevant or practical, but can't you use a resilient mount? Steel or rubber springs?

 

[btrueblood]

Um. Everyone is probably correct in assuming that "dynamic load" means a reciprocating, quasi-sinusoidal vibration load. But, dynamic loads from say, a flywheel punch press or drop-hammer forge can be a bit different. Which kind of load are you dealing with?

 

[knelli]

Hi,
Thanks for all the comments.
The load is from a "gas skid blower" but is shown as a compressor on a stand on the shops. I asked the owner if he would use vibration isolators and he said he has mounted lots of similar equip. directly to the slab.
Thanks again....

 

[jt12]

Don't mean to be technical here, but I don't think `dampening' is the correct terminology for what has been discussed. By adding concrete mass, you are changing the frequency of the support to attempt to be non-resonant with the forcing frequency of the equipment. Three times mass is a rule of thumb that often works, but not always. Adding damping would `dampen' the forcing frequency of the equipment so it's effects are seen less by the foundation.

 

[271828]

"Don't mean to be technical here, but I don't think `dampening' is the correct terminology for what has been discussed."

You are absolutely right unless the intention is to moisten the slab, LOL.