does placing shelf legs on a piece of 6x6 steel change the c

huangyhg

does placing shelf legs on a piece of 6x6 steel change the c
my boss, a mech eng. is convinced that placing shelving feet on small steel plates will reduce the change of overloading our concrete mezzanine. this doesn't make much sense to me but, my elec eng dergee just didn't prepare me to explain why. can anyone give me a hand?
i imagine that you're putting the shelf feet directly on the concrete floor, so that the shelf is bearing on only a very small area of concrete.
by adding a steel spacer, you're allowing the small plate to distribute the load over a much larger area ('cause steel has a much higher bearing stress than concrete).
jclough,
while your post doesn't clearly define your problem, i think you can both be partially correct.
will placing shelving feet on small steel plates reduce the change of overloading our concrete mezzanine?
not necessarily.  the load will still be present on the mezzanine, no matter how you try and distribute it.
however, the steel plate should help distribute the weight over a larger portion of concrete.  if the failure mode is punching shear, then yes, a plate would help, but you would have to design the plate correctly.  if the failure mode is bending, the no, you would need something to distribute the load to a framing   
thanks rgerk.
you've got the idea. the shelves are standard 4 ' high units with probably as much as 500# on them. they are sitting on a 6" thick prestressed concrete mezzanine rated at 200lbs/ft2.
i doubt shear stress generated by the shelving would cause the legs to punch through.
what do load ratings really mean...bending stress?
all possible failure modes should be investigated under the design load (load rating).  of course you have to be clear on the distinction between uniform loading and point loading.  
jclough - the live load rating (200 lb/ft2 in your case) assumes that that the entire floor has an evenly distributed weight on the entire surface - think of sand (at about 100 lb/ft3) spread 2 feet deep. that total weight is the (live load) design structural capacity of the 6" slab, the supports for the slab, the foundation, and the soil that supports the foundation.
200 lb/ft2 is a "heavy duty" floor rating usually found in industrial settings. for reference, single family homes are typically designed for 40 lb/ft2 and many commercial facilities use 100 lb/ft2 (or less). even the "stack rooms" of libraries are often designed for 150 lb/ft2.
i do not disagree that point load should be considered separately. but in the "everyday" world, the slab you have described will not even "know" that 500 lbs on a single shelf (with four legs to distribute the 500 lb. even more, i presume) is there - with or without the plates.