soffit cracking of concrete pan joist system

huangyhg

soffit cracking of concrete pan joist system
i recently inspected the soffit of a concrete slab and pan joist floor (slab = 4.5", joists = 10"x16", 20.5" depth overall).  the floor spans some 30' and supports a boiler weighing approx. 8000 lbs.  the center of the boiler's length (6'x10.5') rests at approximately 1/3 span  the original drawings indicate the joists were designed to support the additional load from the boiler.
there are two cracks right below the boiler (about 12' into the span); they occur only at the underside of the slab (perpendicular to the joist span, extending from joist-to-joist). they don't occur at the faces or bottoms of the joists.  i would assume that the slab at this point of the span would be in compression; if there were overstressing of the system (perhaps due to the weight of the boiler), i would expect to see tension-induced cracks at the bottom of the joists (not at the slab).  one of my guesses is that the fairly localized cracking has been caused, not by overstressing, but by damage to the slab perhaps done by the connection for the boiler's steel skid supports (not visible). anyone have additional insight into behavior of pan-joist supported concrete slab?
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despite the slab being in compression due to joist flexure, the compressive stress might have been exceeded by local tension due to concentrated loads on the slab (between the joists) that initiated a crack.  is that possible?  are there concentrated loads from the boiler that perhaps don't align with the joist ribs?

the localized cracking could have been caused by concentrated loading or erection overstress as you and jae have suggested.
when i see a crack in a slab transverse to ribs, i first think of restraint shrinkage cracking.  the slab shrinks more and earlier than the ribs, so the tension in the slab exceeds the strength of the concrete.  have you taken a look elsewhere in the floor to see if this cracking pattern is repeated in other locations?  you said the cracks are only on the underside of the slab.  if my theory is correct, they would probably be on the top also, but they may be inaccessible, under the boiler.
if the joists are not cracked, i don't think the slab cracks as you describe are detrimental.