generator foundation
generator foundation
(OP)
Does a foundation for a 2 MW Caterpillar 3516C generator (80 kips operating weight) need to be analyzed for dynamic effects if it is going to mounted on spring isolators, or do the isolators effectively eliminate concerns about vibration/dynamic loads to the foundation (and the adjacent building)?
What about without spring isolators? Do these generators produce significant vibration/dynamic loads or are the loads negligible? Some say modern engines with an even number of cylinders are "balanced" and so the dynamic forces are negligible.
I have requested dynamic load information from the vendor, but they say the information is not readily available, and that the foundations are routinely designed when they provide only the static loads. They did provide a Caterpillar Installation Guide that basically discounts vibration as a concern. It says,
"Massive concrete foundations are unnecessary for modern multicylinder medium speed generator sets. Avoid excessively thick, heavy bases to minimize subfloor or soil loading. Bases should be only thick enough to prevent deflection and torque reaction, while retaining sufficient surface area for support."
The Installation Guide goes on to suggest sizing the depth of the foundation so that the weight of the foundation equals the weight of the generator (not 3-5 times the weight that used to be frequently quoted as a rule of thumb).
So is vibration not an issue with these machines?
Input from anyone with experience with these types of installations is much appreciated.
What about without spring isolators? Do these generators produce significant vibration/dynamic loads or are the loads negligible? Some say modern engines with an even number of cylinders are "balanced" and so the dynamic forces are negligible.
I have requested dynamic load information from the vendor, but they say the information is not readily available, and that the foundations are routinely designed when they provide only the static loads. They did provide a Caterpillar Installation Guide that basically discounts vibration as a concern. It says,
"Massive concrete foundations are unnecessary for modern multicylinder medium speed generator sets. Avoid excessively thick, heavy bases to minimize subfloor or soil loading. Bases should be only thick enough to prevent deflection and torque reaction, while retaining sufficient surface area for support."
The Installation Guide goes on to suggest sizing the depth of the foundation so that the weight of the foundation equals the weight of the generator (not 3-5 times the weight that used to be frequently quoted as a rule of thumb).
So is vibration not an issue with these machines?
Input from anyone with experience with these types of installations is much appreciated.





RE: generator foundation
My experience is that these recommendations go out the window once there's a problem.
RE: generator foundation
RE: generator foundation
ACI 351.3R-10 is a good resource for this topic. It gives a general equation for calculating the amplitude of the dynamic force as:
Fo = Mr*e*w^2*SF
Fo = amplitude of dynamic force (lbs)
Mr = rotating mass = pound force / (386 in/sec^2)
e = mass eccentricity (in.)
w = circular operating freq. (rad/sec)
SF = safety factor.
To be considered "well balanced" ANSI and ISO standards would generally require a force of less than:
Fo = Mr *0.25*wo*SF
The references I have seen, use 6 kips (for the weight of equipment) as the cutoff below which you don't really need to be concerned about a dynamic analysis.
If your have a block concrete support, then the size of your FDN should limit the response if it is greater than 3 or 4 times the weight of the equipment.... At least that's the "rule of thumb" that I've seen.
RE: generator foundation
If I can't make a footing with 3* the mass of the machine, I generally obtain the operating frequencies and mass of the rotating objects (CAT has all this easily available, don't let them fool you the salesman is just being lazy) and then estimate some minimum eccentricity to come up with the dynamic load. Then I go through the ACI and NAVFAC design methods, If I need the spring isolators to make the foundation work I figure the spring stiffness I need and spec them myself (just a spring stiffness kips/inch)
RE: generator foundation
For cya, I have sized the foundation at 3 times the static weight, and I have sized the footprint to keep the static soil bearing pressure at 25% allowable (I know this is on the conservative side... some sugeest 25%, others suggest 50%, and still others suggest static plus dynamic less than 75%).
I have also checked the natural frequency and the resonance frequency (you can do this part without the magnitudes of the dynamic forces) of the foundation in vertical, horizontal and rocking modes, and have verified that the natural frequency of the foundation is less than 0.5 times the operating frequncy of the generator.
I think this is as much as I can do without the dynamic forces from the manufacturer.
The one thing I don't know how to account for is the spring isolators. I know the genset will be mounted on isolators. How do I account for the isolators in my check for resonance? I suppose I would need to know at minimum the spring constant and maybe damping, etc. for the isolators. I don't think I will get this info, so, again, this will be the responsibilty of the contractor/supplier, but I would like to know for my own sake.
RE: generator foundation
RE: generator foundation
RE: generator foundation
RE: generator foundation
Don't believe the equipment suppliers. A lot of time their isolators are nothing more than rubber pads that dry out over time. Provide 3 times or 5 times their machine mass and be done with it.