Concrete Generator Pad Anchorage for out-of-phase electrical "jump"

[jropert]

I am in the process of designing a concrete slab-on-grade for a diesel generator and have come across a design issue with regards to anchorage of this generator to the slab-on-grade.

This appeared to be a basic design in which typical gravity, wind and seismic loads were accounted for and the appropriate slab size and anchorage was designed for those forces. However, the scope of work for this project lists a requirement that I am not familiar with. Under the structural section of the RFP (Request for Proposal) document (this is a government job in which they have required my company to provide designs in accordance with applicable current building codes as well as their own additional requirements) it states, "As a minimum, the generator pad should be sized to be at least 3x the weight of the operating weight of the generator to mitigate the vibration effects, as recommended by most generator manufacturers."

Originally, I had thought this was the "usual" vibration effects that would occur under normal operation. There is a vibration isolation system that will be installed that is rated for seismic loads that I assumed would help "mitigate vibration effects". However, upon further investigation of the supposed reasoning behind this added pad requirement, our electrical engineer found that the user is planning for paralleling another generator to this new one in question in the future (possibly). By wiring the two generators together, there is a potential for an "out-of-phase" (i.e. the cycles of operation of each generator do not match, where 180 degrees is the worst case) reaction to occur in which the generator(s) can "jump" off the foundation causing severe damage to the generator and subsequently the surrounding areas. This is obviously dangerous and the electrical/mechanical engineers will have to consider this in the event of future installation.

I did a quick internet search for this occurrence and this appears to be a known phenomenon when paralleling two generators out-of-phase with each other. However, as far as structural engineering is concerned, I can't seem to find any guidance or documentation of what forces to design for in this specific event. My electrical engineer contacted our generator supplier and they don't seem to know about the design force that would be required. We did find (through another generator manufacturer) that the "3x the operating weight" is used to "hold down" the generator from "jumping" off the ground, but even if that were the case, wouldn't the anchorage of the generator to the isolation base as well as the base to concrete pad need to be designed for this force, whatever it may be? As a quick estimate, since the requirements of the pad is to be at least 3x the operating weight (in this case approximately 35,000 lbs operating weight) would I have to design the pad anchorage for 105,000 lbs uplift and shear (or some combination thereof)? Has anyone encountered this requirement before and what kind of forces are we talking about here?

I find it amusing that the requirement states that this is recommended by most generator manufacturers when the manufacturers that we have contacted basically have no idea about this force! I am going to repost this in the electrical/mechanical forums to get their input as well, but wanted any structural advise as that is the focus of this design question.

I apologize for the long explanation. Any help or advise would be greatly appreciated!

Thanks,
Josh R

 

[JedClampett]

I think that the 3x requirement is a traditional way of making sure the foundation doesn't get into resonant frequency ranges by increasing its mass. I would recommend it even if you have vibration isolation.
You don't need to anchor the generator for the total weight of the foundation. If there was an epidemic of generators jumping off foundations, you can bet the generator suppliers would figure out the force in a big hurry.

 

[JStephen]

It sounds to me like the worst that could happen would be the generator running at speed and then suddenly locking up completely (or less likely, reversing). If you can get or calculate some idea of the moment of inertia of the moving parts, that might give you a way to calculate maximum force or movement. You could potentially come up with very large dynamic forces without requiring that large of a foundation under it.

 

[dcarr82775]

JECLampett is correct. It is an old rule of thumb that says as long as the foundation mass is at least 3* the weight of the unit you do not have to worry about vibration issues in the foundation. If you are less then 3* you need to look at soil/structure interaction for dynamic loads. The NAVFAC manuals have a decent design procedure for this.

 

[jropert]

Thanks for your input!...I would agree with JedClampett in that if there were an "epidemic" of generators jumping off the foundations that the manufacturers would definitely have an idea of what to design for. Again, from the two manufacturers we have talked to, it seems like they don't know what this force would be which would lead me to believe that there is no big deal. However, one of the manufacturers did mention that when the generator is paralleled with the utility in the future that the generator can jump around significantly if initialized incorrectly. From what I've read, most electrical/mechanical engineers should know about this and there are proper procedures that would allow a safe way to parallel the generator without a dangerous occurrence.

dcarr82775, you had mentioned that the 3x the weight factor is a "rule of thumb" for mitigating vibration effects...Is there any official documents to support this? I don't disagree with that at all, it seems logical, but I wanted to read further into this. You had also mentioned that there are NAVFAC manuals regarding this as well. Do you happen to know which manuals these are?

Thanks,
Josh R

 

[paddingtongreen]

Check with whomever is buying the generator if the government has put the same requirement on the anchor bolt/base assembly sizing. There would be no point in going higher than the anchor design capacity.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dcarr82775]

The 3* is mentioned in text books on the matter, and you here it all the time from people in the oil/gas industry. NAVFAC (Naval Facilities) Design Manual 7.3 (that is what it was in the past anyway). You can download it for free from the Feds. You will need additional soil parameters from the geotech and the generator supplier that are sometimes difficult to get. If you can make the foundation block 3* the mass of the generator do it

 

[CTW]

ACI351.R Foundations for Dynamic Equipment has a section dedicated to the rule-of-thumb method. Foundation block weight at least 3x the weight of a rotating machine and 5x the weight of a reciprocating machine. It further offers other rules and criteria for proportioning the dimensions. It's a good reference to own.