N-set, I've seen and done plenty of all-concrete construction, including floors and entire roof structures, standard, pre- and post-tensioned, and also shot-crete. Ironically, concrete itself actually gets stronger over time, for many, many years (more than 60). Although the iron reinforcement does not get stronger, it usually holds up OK, except in salty air or in salt spray, or where it was not sized, spaced, lapped, routed, and/or covered properly to begin with. So, no surprise, it gets down to exactly how it was implemented. In extreme economic/political conditions, some 3rd world concrete construction is nowhere near as strong as typical US post and beam with truss joists (eg., the World Trade Center...), and some would take the weight of a helicopter landing on it. To absorb a gratuitous 500 kg out in a span, let's hope your building is (somehow...) the latter type. Whether "between the chimneys" would help depends on exactly how they did it. If the floor structure is "headed"/cut out around the chimneys, it does not help, unless the chimneys are solid masonry and the racks could be fastened directly to the chimneys, as well as the wall behind them. It also helps if the rack is right up aginst a full bearing wall, since it would then extend as little as possible into the floor span.
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