huangyhg

load distribution in mezzanine floor
hello-
i have a mezzanine consisting of steel bar joists spanning 40', spaced at 24" o.c. with a 3 1/2" non composite concrete slab.
my client wants to install a new rack system in which the the legs of the rack fall on every 5th joist or so. my question is, if the legs fall directly over a bar joist, do you think the conc. slab is stiff enought to spread the load such that the adjacent bar joists will share the load? or should i analyze the one joist directly below for the entire load?
any comments will be appreciated.
thanks, ice
31/2" is not much of a slab for point loads, i doubt you will be able to get it to span 80" under 2/3 of the load. you may be able to provide cross bridging between the joists at the point of load to ensure this.check to see if they have the capacity to take the load in truss action.
this may share the bending but you will still need to design the joist under the post to take local effects all by itself.
if i can get the load spread out to three joists, the floor system will be ok.
if i provide adequate cross bridging between the three joists under each rack leg, they will deflect the same amount and therefore evenly share the load. does this reasoning seem correct?
make a simple computer model with a continuous slab with one concentrated load and spring constants for the joists and see how it spreads out.
the live loading that is not from the rack system will remain for any open areas in addition to the rack loading. (aisles and exit lanes.) if forklift trucks are contemplated a impact factor and concentrated load at the wheel location will apply.
icenine,
rather than try to use bridging to make the joists work together, can you use a frame on top of the floor to support the legs and distribute the load to the joists? would eliminate working from below, and also eliminate punching shear as concern if leg occurs other than on top of joist. would still have to analyze these frame reactions to assure that load distribution to joists is acceptable.
thanks for your valuable tips!
ice

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