contraction joints in slab

huangyhg

contraction joints in slab
i have to design storage building with 4 floors, 100x100´ (lxw), 85´height with using of prefabricated concrete elements.
the posts and girders  are in 20´x20´grid, ceilings could be from prestressed hollow panels (thickness 12´´) with
concrete slab (membrane) (th. 3,5´´) on the top of the panels. the slab is reinforced with mat. loading - 500 psf.
my question is - is it neccesary to make contraction joints in the slab or is it possible to let it without? the slab would be used like horizontal stiffening.
thank you very much  for advise.
jb66

yes, you should still place control joints in the slab.  the reinforcing will serve to keep the cracks that occur tightly closed, but nevertheless they will occur.  suggest no more than 20'x20' pattern and you will still get some cracking between.
are you talking about the topping over the hollowcore planks?  if so, control joints do not make sense, since the topping is bonded to the plank.
if you are talking about the slab-on-grade, space the control joints 36 times the slab thickness.
daveatkins
per aci 223, if you use type k, shrinkage compensating concrete it is not necessary to put any control joints.
my opinion varies somewhat from those above.
if the topping is not bonded, i would suggest spacing of joints in the topping similarin spacing to those recommended for a slab-on-grade of the same thickness.
if the topping will be bonded to the precast slabs beneath, then i would still recommend joints in the topping but much fewer in number and spaced much farther apart. the joints have to be located such as not to disrupt the composite action that you are trying to achieve. for instance, you would not want to allow them in the center of the span. i would suggest locating only directly over supports or parallel with the span.
i hope this helps!
thanks to all, i agree with jike - the best solution is probably compromise between two points - slab-on-grade and topping without control joints. the topping will be bonded to the precast slab but not too thickly.
jb66