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RE: Natural Frequency of a Noncomposite Floor System

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Title: RE: Natural Frequency of a Noncomposite Floor System

Brian:

It would be helpful you to look to Section CB serviceability considerations of  ASCE 7-02. It says that fundamental natural frequency for mitigating walking vibrations should be above 8 Hz which in turn limits static deflection under uniform load to be 5mm independent of span. But the code says many floors not meeting this criteria are perfectly serviceable.

Hope this would help you.

Regards

Ashraf

 


From: Brian S Bossley [mailto:BSBossley(--nospam--at)venturaengineering.com]
Sent: Monday, June 12, 2006 2:24 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Natural Frequency of a Noncomposite Floor System

 

Charlie –

 

Would it matter if this was equipment vibration though?  I know that floor vibrations due to human activity are on a low frequency, but this equipment may be on a higher frequency.  Would I have to consider my natural frequency in two different ways (one for low frequencies and one for higher frequencies)?  If so, what would be the cut-off vibration frequency?

 


From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
Sent: Sunday, June 11, 2006 12:55 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Natural Frequency of a Noncomposite Floor System

 

Floors tend to behave as composite -- whether they have studs or just puddle welds -- within the low range of loading that vibration is a design criterion, hence the recommendation. I'm not sure what software program you're using, but you might ask them why. Perhaps they have followed a different method or have accounted for the composite action in another way.

Charlie


-----Original Message-----
From: Brian S Bossley [mailto:BSBossley(--nospam--at)venturaengineering.com]
Sent: Sun 6/11/2006 11:24 AM
To: seaint(--nospam--at)seaint.org
Subject: Natural Frequency of a Noncomposite Floor System


I am trying to calculate the natural frequency of an existing floor system and though I've read through the appropriate portions of Design Guide 11, I'm still having a little trouble understanding a few things.



There is a line in the Design Guide that states that the transformed section properties are to be used if the deck is attached to the supporting beams, regardless of whether or not shear connectors are used.  The natural frequency of the beam is then estimated based upon the composite beam deflection under dead load and vibration loading.  So am I to assume that the full composite section properties (using 1.35x Ec, .) should be used in calculating deflection, also?  My software package seems to disagree with me on that point, but then it offers no explanation as to why not. 



I think the actual initial deflection of the beam would be based upon the bare beam section for the weight of the beam+slab, and then possibly based upon the composite section for the equipment load.  Is it correct to look at the actual deflection in considering the natural frequency?  If so, wouldn't it be based only on the equipment load, and not on the dead load + equipment load?