Fan Base Mass 3

[Pulpboy]

Does anyone have a recommended ratio of equipment mass to base mass for a small fan. 7.5hp. It's mounted on a cladded metal roof and I'd like to keep vibrations from affecting the roof system. I have ratio's for pumps but I don't think you would need the same mass.

 

[dvd]

A rule of thumb for concrete fan bases is 3 to 5 times the total mass of the fan, but since you are most likely mounting this rooftop fan on structural steel you should concern yourself with the natural frequency of the mounting structure.

 

[boo1]

dvd is refering to a rule of thunb for concrete foundations. This weight acts as an inertia block to stabilize the foundation.

When a structural steel foundation is necessary, it must be sufficiently rigid to assure permanent alignment. It must be designed to carry, with minimum deflection, the weight of the equipment plus the loads imposed by centrifugal forces set up by the rotating element (generally 50% of rotating weight).

Fans installed above ground level should be located near to or above a rigid wall or heavy columns. An overhead platform or support must be rigidly constructed level and securely braced independently from the fan in all directions. In any above ground installation, design of the structure should permit field revisions (e.g: knee braces) if initial operation indicates a need for increased stiffness. Spring mounted vibration isolation bases are recommended for many fans mounted on structural steel to avoid vibration transmission problems.

Very large fans and/or variable speed drive fans require special foundation considerations. The purchaser may elect to perform a system forced response analysis to determine the natural frequencies and expected vibration amplitudes with reasonable rotor unbalance forces. (See AMCA Publication 801).

Vibration isolation bases reduce the transmission of vibrational energy from a rotating fan to the structure on which it is mounted. Robinson recommends that all fans which must rest on steel structures with stiffness < 1.0 x 10^7 lbs/in. be mounted on spring isolation bases.

Transmissibility (the degree of isolation, related to the proportion of the disturbing force) is expressed as:

T = 1 / (f/fn)² - 1

(where f is the fan operating speed, and fn is the spring base natural frequency.)

Transmissibilities of 5% should be targeted, while transmissibilities which remain under 10% are normally acceptable.

 

[Pulpboy]

Thanks for the help. The fan will be sitting on a concrete / steel base located directly above a main wall and perpendicular main girder as long as designs show the roof can handle the additional weight.