generator foundation

[gte447f]

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.

 

[JedClampett]

If they're so confident, why don't they design their own foundations?
My experience is that these recommendations go out the window once there's a problem.

 

[Mccoy]

Do they provide working frequencies of the machinery? Together to all the other basic stuff (loads, foundation geometry, soil properties) that's all you need to check the dynamic stability. Often the dynamic soil reaction is the same as the static reaction, but you should prove it.

 

[JoshPlumSE]

I can't say that I'm an expert on the subject, but I have been reading a lot about it recently My thoughts:

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.

 

[dcarr82775]

Everybody claims their machines are perfectly balanced, and I believe them. Then some darn fool loads it onto a truck and moves it somewhere else for some reason and turns it on. Now it is no longer perfectly balanced.

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)

 

[gte447f]

I have the static loads and the operating frequency, but I do not have the dynamic forces (or the masses in order to calculate the dynamic forces myself). I have basically communicated to the client that, since I have requested, but have not been provided the necassary info to complete a thorough dyanamic analysis, the contractor/supplier of the spring isolators will be responsible for ALL vibration control.

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.

 

[dcarr82775]

Since you have 3* mass in the foundation I would call it a day and move on.

 

[gte447f]

dcarr, I agree whole heartedly. I am really just curious now about how to properly incorporate the spring isolators into the dynamic foundation analysis. Any good references?

 

[DR901]

I would approach your client by saying the cost of the foundation will be small when compared to the cost of the generator. 3*mass is a GREAT rule-of-thumb.

 

[JedClampett]

My experience on designing these foundations is that you're pretty much stuck with rule of thumb designs. You never get the right soil parameters from the Geotechnical Report (not their fault, we don't know enough about the equipment to ask for them initially), it's hard to get the vibratory information from the machine supplier and why violate the rule of thumb anyhow? You might get a different piece of equipment from a different vendor. The machine might not be operated at its maximum frequency. There's ramp up frequencies. Plus how much time do you want to spend on saving very cheap concrete?
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.