control joint in structural slab on voidform

huangyhg

control joint in structural slab on voidform
a pump house is designed sitting on concrete slab supported on piles (kind of common in the north area when the building is not considered heated and no uplift movement is allowed). this kind of slab is little bit special, from structural design viewpoint.
the slab is designed as the typical structural slab (like the flat-plate concrete floor), on the other hand it is a slab on 鈥済rade鈥?- the undegradable void form. the slab concrete is reinforced to support the load on floor (with neglecting the voidform) and the frost heave pressure from the voidform. the pressure (somewhere around 600psf based on the locality and voidform property) is usually more critical.
is control joint necessary for this slab?  the house is about 30ftx20ft footprint with 9-24鈥漝ia piles. the slab is 18" thick with double reinf. the cracks (because of the restraint from the piles)on the slab may be a concern.
thanks very much for shedding some light on this!
btw, the punching shear from the middle pile (when frost heave occurs) seems special as well, since the effective area against the shear is much smaller than those in the typical punching shear check.
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control joints are never required for structural slabs.  the control joints in slabs on grade are used to control the locations of cracks in slabs which usually have low quantities of reinforcing and are subjected to subgrade drag forces as the concrete shrinks upon curing.
for a structural slab..on forms or on void forms, the drag forces are less and the reinforcing is higher.  
for an 18" thickness - a control joint wouldn't do you much good unless it was very deep and that would work counter to the structural behavior.
j1d:
see the thread by cdi12 on 'concrete mat expansion joint' that was started last friday. many of the same ideas apply.
if you use .5% reinforcing (.005 times gross cross sectional area of concrete) cracks will stay closed, and no joints are needed.
thanks, jae, for mentioning the drag force. good point! do you think the piles may serve as the source of drag force?
lkjh345, i'm not sure if i have to go up to 0.5% reinf for better crack control. from stregth point of view, the minimum reinf ratio (0.2%) is good enough. some cracks are okay for the slab. i just don't know whether control joint is necessary to locate the cracks.
j1d - now that i read your first post more closely (i admit i am guilty of reading posts way too quickly) - the 24" diameter piles (piers?) would certainly participate in some level of resistance to overall shrinkage of the slab.  
i suppose you could get some sort of spring coefficient and model the thing, but for a house that's probably overkill.  i guess i'd go about it by making sure the concrete mix is chosen to minimize shrinkage (i.e. larger coarse aggregates, low water/cement ratio, water reducer, etc.) and just be careful how you detail the slab top reinforcing to keep cracks closed in likely crack areas.
even though where you think the cracks will appear, they think differently.
for a slab of 30ftx20ft i wouldn't consider control joints. if using 0.2% reinf top & bottom you will have quite effective crack control. if using 0.2% total the control obviously won't be as good, but probably adequate for a basically dry environment.