huangyhg

pedestrian comfort?
i am designing a platform and i want to use 3/16" floor plate for the decking. the platform will be an i beam structure with the decking welded down. what is a good span for the floor plate for "pedestrian comfort"? i checked a bar grating span chart and it states that there should be less than 1/4" deflection for a uniform load of 100 lbs/sq. ft., for pedestrian comfort. when i check aisc manual for allowable stress design it gives a maximum span between 24 and 30 inches to achieve that deflection. that seems like way too tight of spacing. i am considering using 1/4" plate but the spans are not much better.
i checked some of the calculations and it seems that the chart in the asd manual assumes that the plate is simply supported on the ends and is only supported in two places, which is not realistic (the plate is supported on four sides).
does anyone know of a more realistic span for this situation? possibly a chart.
thanks,
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one way that we have increased the allowable span and minimized deflection is to have the floor plate supplied shop welded to a suitably sized bar grating. most of the grating manufacturers are familiar with this, so it is not unusual.
have to buy the grating, but may save a lot on reduced quantity of support framing.
that will probably add about $10,000 to the project, probably not a possiblity. my problem is that i do not believe what the chart is telling me. i know that i can span more than 2 ft with 3/16" floor plate. i have spanned that far with 1/8" floor plate.
the problem is that the "comfort" level is usually taken at l/360 deflection. for 1/4" at 2', this equates to l/96 which is well beyond the comfort level.
for the continuous case (fixed ends), the allowable deflection would be .067" (24/360) for a 24" span and .083" for a 30" span. the actual deflection under 100psf would be .038" at a 24" span and .092" at a 30" span. from this you can see that they're in the ballpark.
swearingen,
the asd manual assumes a simply supported beam. it gives a deflection of 0.195" at 24" and 0.462" at 30". i get the same results as you when i model the plate as a beam fixed on the ends. i suspect however that the deflection will be even lower than the values you give. the plate is not really a beam in the sense that it is supported on four sides not just two.
have you tried roark's formulas for stress and strain? look for plate bending equations in there to match your boundary conditions.
for comfort i would look at limiting the natural frequency of the floor to something, say, greater than 3hz or greater than 5hz, or whatever you prefer or can find out. this is equivalent to limiting deflection to a given maximum depending on the assumptions made in the process. limiting deflections is probably easier than calculating frequencies on top of everything else you're calculating.
another idea, is to use 3/16" plate with angle stiffeners welded to the underside intermittently. the angle could have its flange turned downward so that it would work in combination with an effective width of plate as its compression flange. size the composite section to span the spacing of the support beams.
ucfse's suggestion of going to roark's is a good idea if you truly have it supported on 4 sides. of course, there is a difference if those 4 sides are simply supported or fixed, as well.
for typical industrial applications we have used 3/16" flpl spanning 30". this is a realistic span for comfort. from a strength standpoint it is fine, but deflection is an issue. some customers say it is not stiff enough and they want it stiffer. unless your aspect ratio is low you will not gain much by looking at the plate formulas. i would not push it much beyond the 30" for comfort. you will regret it. using 1/4" will typically get you to 2'-9". using stiffners made from angle with the leg welded to the flpl will maintain an economical and easy design to fabricate. (the copes on the angles can be produced on an iron worker instead of having to be torched.)
aggman,
if you are using 3/16" plate for 30" spans, then do you use 1/4" plate for just a 33" (2'-9") span? is that a typo or are you suggesting that for the deflection/comfort issue. i agree that angles used for stiffeners is probably the most economocal way of constructing a platform for industrial purposes. by using differing sizes for longer and shorter spans they will frame into each other nicely too.
my experience with plates and allowable deflections has is that it is hard to predict what people will find acceptable. what's good for one is unacceptable for another.
regards,
-mike
what are you doing checking deflections for pedestrian walkway? overkill.
the idea is to get acceptable levels for everybody.
use 1/4" pl for <= 2'-6" span and get back to designing those wfs! leave 3/16 stuff for the step up to the pickup truck.
going the big inch!

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