Hollowcore Concrete Plank Vibration Question

[Berniedog]

I'm back. In the 6th Edition of the PCI Design Handbook, there is an equation (eq. 9.7.6.1) to determine the Min. Natural Frequency of a floor system by walking. I want to confirm that (sometimes I am not too bright)the actual frequency (caused by walking) must be greater (Eq. 9.7.4.4) than the minimum frequency.

Is this correct?

 

[Lion06]

No, you want the natural frequency of the member to be greater than the forcing frequency (the step frequency) to get away from resonance.

 

[dougantholz]

First - minimum natural frequency is a terrible label. It should probably read first mode shape natural frequency, often referred to simply as natural frequency.
Excitation frequencies, from walking, seismic or machines can be higher or lower than the natural frequency. Dr Murry provides a great description of frequency and how to calculate it in the AISC Design Guide for floor vibrations. Although this is written for steel, the equations are the same, stiffness is stiffness and mass is mass. After that everything is just a little algebra.

 

[youngstructural]

Spot on, both StructuralEIT and dougantholz have given you the exact right advice...

I will just clarify one point: The AISC manual that dougan has recommened is in fact "Design Guide 11" and is a joint AISC/CISC publication. Messers Ungar, Allen & Murray have produced an excellent resource. It is available as a free download to AISC members. If you do any amount of steel design, joining AISC is definately worth your time.

Also, you might want to look at the ATC floor vibration publication... It is a more recent resource, and also deals with non-steel stituations such as hollowcore floors, concrete bleechers, etc.

And don't feel too embarrassed about the vibration theory; It's really not obvious unless you get deeply into vibration. A lot of engineers have a hard time wrapping their brains around it. I very much like the added clarity of dougan's suggestion regarding calling the min. natural frequency the First Modal Shape. That way you automatically think to check the harmonics.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...