VIBRATION ANALYSIS- EFFECTIVE MASS OF (WAREHOUSE) FLOOR SYSTEM ON DETERMINING ITS NATURAL FREQ. 1

[eljineer]

Hi,

I'm going to analyze concrete floor system with different loading. I want to know the effective mass for determining the natural frequency.

As per AISC DESIGN GUIDE SERIES 11, a live loads of 11 psf and 6 psf are recommended for vibration analysis for typical office and residential occupancies and zero live load or at least nearly so at foot bridges, gymnasium and shopping center floors.


How about for a warehouse with high racks with live load 15 kPa, can i assume the load of rack alone for consideration of vibration analysis or it is still suggested to use zero live load too? I'm still studying vibration analysis but I want to know the logic behind it?, it is related to the damping?, because in basic is vibration is a function of deflection.

Thank you very much..

 

[structSU10]

Is vibration a concern in a warehouse for some reason? If its for storage, with no people really hanging around, I would think vibration doesn't really matter.

 

[msquared48]

If you're storing Nitroglycerine, it does!

Mike McCann, PE, SE (WA)

 

[WARose]

I don't have a specific recommendation.......but as someone who has done a lot of vibration work let me tell you: what mass to use is something everyone struggles with (where it is variable). I've found that it is best to work backwards from the limits of acceptability and see if any potential combination of mass, stiffness, damping, etc could be an issue.

The good news is: since you are on a elevated slab.....the stiffness and damping should be pretty well defined. (This is as opposed to foundations.)

 

[oldestguy]

Is this a student "project"? If so read the rules for this medium; not allowed.

 

[JoshPlumSE]

Don't use the maximum or minimum loading of the structure. Use the most likely mass / weight under normal function of the building. If this is highly variable, then run the calcs a couple of different ways. Or, (as WARose suggests) work backwards to see if there is a loading within the normal range for your building that will cause an issue.

 

[gravityandinertia]

As has been said already, mass is a difficult choice to make. However, also consider this, mass generally lowers frequency. So if you're natural frequency fully loaded is high enough above the expected excitation frequency you're likely fine. If not, then it becomes a very difficult guessing game.

 

[eljineer]

hi all,

thanks for comments and opinion,

i want to share my experimental analysis for the effects of live loads/sdl consideration in natural frequency for the vibration analysis.

the said recommended value for the offices,residential etc, even if you use 15 kpa (full live load) have no significant effects in natural frequency of the floor system,

what really affects to this parameters (nat. freq.) is the stiffness of the floor system.

 

[271828]

"the said recommended value for the offices,residential etc, even if you use 15 kpa (full live load) have no significant effects in natural frequency of the floor system,

what really affects to this parameters (nat. freq.) is the stiffness of the floor system."

Are you saying this is what you measured?

What is the percentage increase in uniform mass when you add the live load?

If the stiffness remains constant, then the addition of significant live load should make a noticeable difference in natural frequency.

For analysis, do what JoshPlum typed. Use your best estimate of the actual mass in place. If it's variable, then analyze the system for various masses to determine which is the most severe case.