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RE: Steel floor plate
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Steel floor plate
- From: <William.Sherman(--nospam--at)CH2M.com>
- Date: Wed, 14 Mar 2007 10:02:43 -0600
Based on the responses received, I've developed the followng information:
Say span = 4 ft = 48 inches. Per the AISC deflection controlled tables, a 3/8-in plate can carry 127 psf. The calculated deflection is 0.48-in. Recommendations have been made to keep the natural frequency above 5 Hz, or better yet above 11 Hz. So check the natural frequency for a 0.48-in deflection:
Use f = (g/d)^0.5/(2*pi) to estimate natural frequency, where: g = 32.2 ft/s^2 = 386 in/s^2; d = deflection of beam = 0.48-in; so natural frequency f = 4.51 Hz, less than 5 Hz.
Try grating criteria of 1/4" deflection under 100 psf live load: for d = 0.25-in, f = 6.25 Hz, greater than 5 hz.
If we try for f = 11 Hz, then d(max) = 0.08-in; thus, a much thicker / stiffer plate would be requried - does not appear to be practical.
It has been suggested that a 1/4-in plate is suitable for a 4-ft span. Check deflection and frequency under a 300-lb concentrated load:
d = 1.53-in, f = 2.53 Hz; deflection is excessive and frequency is too low.
Check the 1/4-in plate for a two-way simply supported span under a uniform load of 100 psf:
d = 0.36-in, f = 5.20 Hz; meets 5 Hz frequency.
If plate is continuous vs simply supported, the 1/4-in plate is reasonable for 4-ft span each way, but a 1/4-in plate does not appear to be good for a simple span of 4-ft.
In my opinion, the 1/4-in maximum deflection under a 100 psf live load appears to provide an easy criterion with reasonable results.
Actually my application "is" an industrial application - the plate will be used at a platform to be used regularly to access equipment. I don't want the floor plate to feel excessively "bouncy", so I am trying to establish the criteria to use for deflection. And I will need to justify this criteria to other reviewers. It is not clear to me whether AISC's "deflection-controlled" table will accomplish this goal.
I currently plan to use criteria consistent with grating design based on historical practice. However, this platform is movable, so we want to minimize its weight. If I can justify less stringent criteria, then I could reduce the weight.
CH2M HILL / DEN
The deflection limit applies to industrial applications where deflection does not affect serviceability.
Use a lower limit when deemed necessary.
Table 3-18a in the AISC 13th Edition provides tabulated allowable uniform loads for "deflection-controlled applications". The deflection limitation used is L/100. For a 4-ft span, the maximum deflection would be about 1/2-inch - this seems rather "bouncy" to me. Grating tables typically recommend a limit of 1/4-inch deflection under 100 psf live load for comfort - but a 3/8-inch plate at a 4-ft span designed per AISC would have about twice that deflection.
What is the basis for the L/100 allowable deflection? Does this limit provide adequate stiffness for "comfort"?