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# Composite structure nat'l frequency

• To: "Struct EngAssoc" <seaint(--nospam--at)seaint.org>
• Subject: Composite structure nat'l frequency
• From: "ASC" <ggg(--nospam--at)bigpond.net.au>
• Date: Mon, 12 Jun 2006 21:09:23 +1000

 A comment on the discussion below:   A quick estimate of NF can be obtained as follows:   Calculate static deflection under gravity, ust.   Circular freq. squared = g/ust   where g is accel. of gravity in consistent units.   nat'l freq. in Hertz = circular freq./(2pi)   Sincerely, Gregory from Oz     -----Original Message-----From: Brian S Bossley [mailto:BSBossley(--nospam--at)venturaengineering.com]Sent: Sun 6/11/2006 11:24 AMTo: seaint(--nospam--at)seaint.orgSubject: Natural Frequency of a Noncomposite Floor System=20=20I 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.=20There 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. =20=20I 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?=20