Dynamic Equipment Loading - Generator Weight ACI 351 Rule of Thumb

[JoelTXCive]

I am designing a generator pad for a 400kw generator that has an attached fuel tank capable of running the generator for 72hrs.

I want to comply with ACI 351.3R and have my underlying concrete weight be at least 3 times the equipment weight.

Generator Motor Unit: 7,898 lbs
Steel Enclosure and Empty Tank: 5,120 lbs
Fuel (2,644 gallons @ 7.1 lb/gal): 2,651 lbs

The total is 15,669 lbs. If I hit is with a dead/fluid load factor of 1.4 then I'm at ~22,000lbs. If I then multiply by 3, then i need 66,000 lbs of concrete underneath. This will definitely get me into an 18inch+ slab territory.

Questions:

1) What weight should I use as the generator weight? The whole generator, steel enclosure skid, and fuel tank are 1 unit. But the motor unit is the only thing generating vibrations (no pun intended lol).

2) Should I use a factored load, or service load for the '3x weight factor rule of thumb' (ACI 351.3R-04 section 4.1.2.1)

Thank you in advance.

 

[MotorCity]

Use the service load for the 3x rule of thumb. That said, 3x is on the low side. I usually use 5x as a minimum in my analysis.

 

[SlideRuleEra]

What weight should I use as the generator weight?

15,700 pounds. Vibrating inertia is the cause of problems. You have 15,700 lb. of vibrating equipment - regardless that vibration is caused only by the motor. A proportional amount of non-vibrating mass (the concrete)is needed to sufficiently minimize vibration magnitude to prevent damage to bearings - both the motor's and the generator's bearings.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[WARose]

Some of the numbers I've heard over the years are 5 times the weight of the vibrating equipment or 10 times the weight of the moving parts in said equipment (all unfactored). None of those rules are guarantees (against poor performance) however. It just depends on the circumstances.

 

[TLHS]

If anything, 18" is shallow. The minimum for any real equipment is normally the greater of two feet or a fraction of the width/length. You need to keep it rigid. I'll look up the ratios I normally use later.