Rooftop Generator - Dynamic Loading

[pcronin]

I need to review existing roof framing for a new roof mounted generator. I have all the manufactuer's information for the weight of the generator including the enclosure. The genenerator is for emergency use only and will have isolators to help with noise and vibration and will be mounted on a girder line in a building with bar joists and structural steel girders. The generator is natural gas so there is not a fuel storage tank.

When designing generator pads at grade, manufacturers usually have recommendations for a pad mass of 2-3 times the weight of the generator to address dynamic starting and stopping forces. I can't find any of documentation to quantify the inertia forces and the manufacturer is less than helpful.

Have any technical journals or studies been performed on roof mounted generators? What order of magnitude inertia forces should I expect?

 

[fancypants]

I do not know of any tech articles.

However, I have designed about 10 of what you are describing in the last year. You will find that starting, these generators are very smooth. However, i do recommend isolation springs etc...

See ASCE 7-05 section 4.7.2 for impact loading. I use 20% with these types of generators (Generac is what i am use to).

Do not forget wind, and check shear, bending and the joist seats.

 

[ToadJones]

I apologize in advance for the vagueness I am about to bestow upon this forum:

At one point I had to design structural steel and a concrete slab for a large floor area in a coal fired power plant that supported coal pulverizers that turned coal into dust that fired the boilers.

I don't remember where I got the literature, but I remember that making sure the natural frequency of the combined steel and slab didn't match that of the machine was important otherwise it could cause the structure to resonate when the machine was in operation.
I also remember the mechanical guys suggesting using a massive amount of concrete beyond what was required to dampen the vibrations and inertia forces.

I believe AISC has a design guide on floor vibrations. Some of the references in that guide may be of use.

 

[baw1]

Two additional considerations:
1. Sometimes the generators have short circuit or locked rotor torque that needs to be accounted for to assure the generator does not come off the roof when this upset condition is experienced.
2. For the isolator option the frequencies and limit states of the entire Generator-Isolator-support under the isolator needs to be examined. Most isolator design parameters are based on having a rigid foundation under it. It may not perform as expected if the roof structure itself is flexible, so this needs to be considered.

Limit states for vibrations are typically defined as deflection limits for low frequencies, and particle velocities for high frequencies. Generators typically high frequencies. A lot seem to run at 1800 rpm in the US.