wo way pt slab - concentration of bottom steel at columns

huangyhg

two way pt slab - concentration of bottom steel at columns
in plan review on a few projects, i have noticed that some two way p/t slabs specify a group of bottom bars centered over the columns that is on par with the total amount of top steel added above the column.  i am a little bit embarassed, but i am not sure what this bottom steel is for.  on three separate projects looked at over the last couple years, i have seen anywhere from (3) #5 by 9'-0" long to (8) #6 x 12'-0" long placed directly over the column in each direction.
any idea on what code provision requires this rebar?  is it for transferring the unbalanced moments in the slab into the column via flexure?  a seismic requirement for continuity?  the amount of rebar placed here seems somewhat random when looking from project to project, many times they just pick an amount of rebar and use it for every column on the job.  
thanks for any input on this . . . i guess you learn something new every day.      
the bottom steel is to resist tension at the bottom of the slab caused by the stressing of the pt tendons (if the tensile stresses exceed the allowable).  see aci 318 section 18.4.1.
i didn't think that was the purpose.  a typically sized slab loaded with a single floor's amount of load wouldn't have much more than its self weight "balanced," so it should be relatively rare that you would have tension at the bottom of the slab (maybe at short bay conditions where somebody forgot to raise the tendons back up, or something similar).  what i am seeing is every column at every joint on a 20 story building receiving (5) #6 bottom bars x 12'-0" long, typically for a 9" slab.  
i am guessing it is a seismic requirement, but i can't seem to find it anywhere.  
it could be for structural integrity. if you look in chapter 7 of aci, you will find requirements for bottom steel, which typically never gets placed by most eor's. there is also a chapter on aci manual for designing slab-column connections, that suggests using continuous bottom bars for a catenary action.
three thoughts:  1) probably for continuity/structural integrity for seismic loading; 2) could be to improve punching shear capacity, as allowed in some codes; 3) compression steel?
fredpe, for service conditions you don't have tension at the bottom of the slab.  but immediately after the tendons are stressed you often do have tension in the bottom of the slab.  at this time, the pt force is at its highest value (before losses) and the concrete strength is at its lowest value (usually about half of the specified 28-day strength).
the use of the bottom steel at the columns is to ensure continuity of the slab, should there be a failure. similar to the requirements of providing a minimum of (2) tendons in each direction directly over the columns. there is not a code provision for this requirement, however i have seen it used for a number of years by various engineers.
regards,
auce98
if the lateral system is shearwall/frame interaction you will often get bottom reinforcement due to moment reversal.