Dampening mechanical vibrations
Dampening mechanical vibrations
(OP)
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
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






RE: Dampening mechanical vibrations
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
RE: Dampening mechanical vibrations
For large generators, you can come up with pretty large slabs.
RE: Dampening mechanical vibrations
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.
RE: Dampening mechanical vibrations
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.
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RE: Dampening mechanical vibrations
RE: Dampening mechanical vibrations
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.
htt
RE: Dampening mechanical vibrations
RE: Dampening mechanical vibrations
RE: Dampening mechanical vibrations
RE: Dampening mechanical vibrations
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....
RE: Dampening mechanical vibrations
RE: Dampening mechanical vibrations
You are absolutely right unless the intention is to moisten the slab, LOL.