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cast iron plumbing spiders in concrete slabs

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greetings:
i am working on a multi-story 2-way flat slab concrete building and have
found out that the mep engineers have designed their plumbing system with
cast-iron 'spiders.' these are effectively collectors for back to back
bathrooms. mep says this is a 'chicago invention' and they are more and more
common in multi family construction. they tell me that typically these are
made in the shop placed and cast into the slab. in plan they look kinda like
this (ascii graphics):

   \    /
    \  /
 --- O ---
    / \
   /   \

each of the spurs is 4" cast iron and then the middle is open to a 6" ci hub
above and and 6" ci tail below. one of the spurs is connected to the vent
riser as well. the vertical risers are installed as the building goes up,
also cast iron. they tell me the economy is that the rough-in is cheaper
this way. these assemblies are supposed to fit in between the layers of
bars. aside from the clearance issues with my bar layers, i am concerned
about the vertical stacks acting as a columns or props. they tell me that
they have a 1/2" contraction joint allowance in the piping between floors.
next to colums, i am not too concerned. however, some if these are located
in the middle of a 25'x20' bay. my slab is 10" right now. i know that the
sum of initial dead load, live load, long term deflection/creep will be more
than 1/2."

the mep tells me 'as of late we are having trouble with these because the
cast iron vertical stacks are exploding when the contractors pull the
forms.' no kidding! supposedly no rubber furnco couplers allowed by the
local plumbing code.

has anyone else had a project with these? is this truely a chicago practice?
i see that my options are limited: thicken the slab, put stricter guidelines
on initial strength required before stripping, requiring strict rules on
form stripping/reshoring, or simply going against the whole detail
(potentialy unpopular).

advise? warnings?

thanks in advance

dennis