rebar - floor pad

huangyhg

rebar - floor pad
we have a combustion chamber/boiler - probably weighs about 60,000 lbs - that is to be placed on a concrete pad.  
we were given back a plan view drawing showing the rebar placement.  for the entire boiler room the spacing is 12" apart, but right at the feet of the chamber the spacing is 20".  the feet on the chamber are somewhere between ~16"-18" square.  they seem to stress the importance of this increased spacing too.  
why is rebar spaced further apart at this point than everywhere else?  i would think that at the loading points the rebar would be closer together.  i am just curious, we do not do concrete and are rarely even shown the drawings so i am not saying this is wrong but i do find it interesting.
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maybe there is suppose to be a pad underneath each boiler's support that goes from the boiler to underneath the slab.  maybe they dont want it to sit on the slab.  this is slab on grade right?
aci 318 7.12 doesn't really apply to slabs on grade, but if i'm specifying steel, i use this as a minimum.  note 18" min. spacing.
the only thing i can think of for the increased spacing is if holes for anchors are going to be drilled after the concrete is poured, so less likely to hit rebar when drilling?  are the feet going to be attached to the slab?
coe,
yes, the slab is on grade.  
aggie,
we do used anchors in the floor so that could be why.  perhaps if they were to plan the rebar to be a safe distance inside the anchors the rebar becomes too close together - if that's possible.
forgive me if this is a silly suggestion, but could it be that those rebar below the chamber feet at 20"c/c are in addition to the ones 12"c/c?
they do not show any rebar at 12".  they show in detail how the spacing changes for that one portion of the floor and made a note of it.
this is not critical as our job is the boiler, i was really just curious and wanted to hear your opinions.  if i get the chance i will ask the guy who did the design, but that may not happen.
look forward to hearing any other thoughts though.
is the slab thickened at the of the chamber?  are the bars that are 12" oc the same size as those under the feet?
ucfse,
the floor is thicker under the chamber - thickness increases from 6" thick to 12" thick.  the rebar is 12" oc under the chamber and 20" apart around the support legs.  
having the extra bars at the thickened area of the slab isn't unusual.  when you have a slab on grade, it is common to thicken the edges or the areas that receive concentrated loads and add some bars.  20 inches is a bit farther than is really good, not necessarily for strength but there are other issues, such as cracking and limiting the crack width, with which to be concerned.