floor frequency

[rfd23]

Hi,
I am analyzing a industrial building with many vibratory equipment on elevated floor.
I isolated a portion of a floor from col. to col. and performed a dynamic analysis to calculate fundamental mode of floor.

My results are so scattered in x,y and z direction and mass participation is very low in earlier modes.

I have two questions (see attached excel file)

1- How much mass participation is required to consider a mode as fundamental mode for that direction.
2- what happen if natural frequency of floor matches with natural frequency of machine. As per my analysis at resonance vibrations will be under acceptable limits.
Please help me to understand results. Thanks in advance.

 

[JoshPlumSE]

1) This is an engineering judgment call. There isn't any rule of thumb. If a beam has a natural frequency then the mass participation of the overall model may be very low... even though this one beam's behavior is almost totally controlled by this one mode.

That being said, if adjacent members / beams/ bays do not have the same kind of natural frequency, then they will tend to dissipate the energy from that one beams vibration. I tend to think of this as just an increase in the damping of that one mode. Though that may not be a precise theoretical description.

This also depends on the amount of damping you expect for your structure. Do you have architectural elements (full height floor partitions, floor material, cladding and such) that will tend to increase damping?

2) If you've done an analysis of the vibration response at near resonance, then you already know what will happen. You get dynamic amplification of the response. There are hand calc methods you can use to calculate this amplification. Also, it's not just about whether the structure is going to fall down. It's also about whether the equipment operation can tolerate the vibrations and whether an operator can work on the floor without experience significant discomfort.

 

[rfd23]

Hi,
Thanks for your feedback. I will perform analysis more in depth.