huangyhg

pt slab
the pt slab in question is a parking deck and a roof for a parking garage building. the top of the slab has high and low point elevations. low point being at the drain location and high point half way between drains at 5" higher than the low point. the slab thickness is 8" at the drain so that makes the slab 13" thick at the high point.
the location of these crickets and slope lines are arbitrarily layed out in relation to the supporting columns. the column spacing is good for 8" thick slab.
so the question is how to specify the location of the top reinforcing?
a) should it be uniformly lacated at say 7" from the slab soffit?
or
b) should the reinforcing and cables follow the top of slab profile by maintaining the 1" cover?
option b) will make calculations and modeling of the slab very complicated and near imposible. and the placement of the reinforcing in the field will be very tedious and not practical as well.
the concern with the option a) is the 5" of concrete that is basically unreinforced or too much of concrete cover that may lead to excessive tensile stresses in concrete and therefore cracking. or the possibility that the 13" slab areas may be under-reinforced.
any ideas wil be helpful...
thanks,

l775,
i agree with your problems with option a. crack control would be non-existant in the 13" areas as well.
i would follow option b personally and put the extra effort into design. it is not near impossible to design or build. too may people are trying to take too many ill-conceived short cuts these days to "simplify" design for themselves.
i would normally try to negotiate with those in control to get the high points near to columns and low points at mid span/panel and possibly reduce the minimum slab thickness at mid span to 6" with 11" over the columns if possible. simplifies the design and puts the extra concrete where you want it. using small diameter bars in the top will allow the top reinforcement to mould to the shape better.
why don't you use option c and keep the slab thickness uniform throughout. this is often done and the contractor simply has to set the formwork at the profiles you've set. this adds some to the cost of the project, but for post-tensioning slabs, it creates an accurate design. with varying thicknesses, you've got varying prestress forces along and across your spans. very complex and i'd worry about the slab's behavior.
concur with jae, bridge decks have been designed this way for decades (uniform thickness with typically a 2% slope for drainage). much easier to build, too.

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