×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Rooftop Generator - Dynamic Loading

Rooftop Generator - Dynamic Loading

Rooftop Generator - Dynamic Loading

(OP)
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?   

RE: Rooftop Generator - Dynamic Loading

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.

 

RE: Rooftop Generator - Dynamic Loading

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.

RE: Rooftop Generator - Dynamic Loading

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources