huangyhg

tying wall reinforcement to slab over
hi guys,
i have a situation where i designed a precast slab to sit on top of four walls and doweled in. the contractor has come back and said they would like to use some false formwork and cast the slab in-situ. my original design only tied the walls to the slab over by dowels and therefore designed as a simply supported slab. now, i will cog the top steel over so it laps with some welded wire fabric that i have used for crack control.
my question is, should i be concerned with the negative moment that may occur in the slab at the wall. i will assume the concrete will crack as soon as it becomes stressed and will then behave as a simply-supported slab (as per the original design). the contractor put to me that i should lap the wall steel with the bottom reinforcement but i don't think that this is required.
can anyone see anything wrong with this approach.
i usually use a top bar as though it is fixed at the corner. hook on the end and top bar lapped over the wwf and extended about 0.15 to 0.2 times the span length out.
you really don't want to allow moment cracking right there where you have the highest shear.

asixth,jae,
let's say that the crack formed directly over the support centreline (point of maximum negative moment).
the joint then acts as a pin and allows rotation - so it would then act as a simple support which has already been allowed for...
if it's a highly loaded slab i would agree that the detail would be worth checking, but if the negative service moment isn't going to cause visible cracking issues i would be happy with the additional restraint provided.
the crack doesn't always occur at the centerline. that is the problem. many times the crack is off the face of support (due to the interaction with the shear that is occurring) and you then have a problem.

agree with jae, due to combined effects of bending and shear, the cracks are likely to occur at somewhere beyond the wall face. even with the cracks, you can't assume the slab has became simply supported, because the reinforcement in the negative moment region might still working that would provides some moment capacity.
since you are going to re-design the slab from pc to cip, why not make it as end fixed? if you prefer the simply supported scheme better, would bond breaker help?

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