Both have their place and I usually treat a slab as 2 way if possible or if the slab is going to function as one unless it is small and there is no benefit.
Briefly:
There are a couple of types, those supported by columns and those supported by a fairly rigid beam/wall system.
The advantage lies with the latter; it is possible to reduce the slab thickness, reduce the amount of reinforcing within the slab (even with the increase in top reinforcing, because it's often required anyway), and reduce the cost. It is also possible to increase the overall stiffness of the slab. This is applicable only if you have supporting walls/beams already; to construct them specifically for support is more costly and the benefits are lost.
The important considerations are the edge support conditions, the cost of the additional support structure if required, and the quality of the labour or documents. This type of construction requires a better skilled workforce.
As Dik said two way is often more economical. Even if you don't design it that way it's going to do a certain amount of spanning in the longer direction using the distribution steel you need to put in anyway. However, when your slab has a length/width ratio of more than 2:1 you can design it as one way because you won't get much adavantage from the long span any more. Carl Bauer
Sometimes u can make a slab 1 way even if the ratio is < or = 2. This is specific for cases where edge beams along the shorter span are to be treated as dummies.
One problem with one way slabs is that shrinkage can cause large cracks right through the slab. This is not so much of a problem in the load carying direction because flexural cracking forms a close crack spacing and hence tapered cracks of narrow width. Sometimes in long one way slabs you can need more reinforcement in the secondary direction to control cracking than is needed in the primary direction.
StructuralDzine:
Can you provide some additional info/detail on the cracking through phenomenon?
The only time I've encountered that type of cracking, it was running from centre of column to centre of column in the middle band with deep drop panels. They were providing the influence so that the entire slab between panels was in tension.
This sounds like someone has had a thermal cracking problem. If the slab is long and is designed as one-way spanning then there will be a large amount of rebar in the direction of the span. There will obviously be less rebar in the orthogonal direction as this is the distribution rebar.
If the slab is cast in one hit and the pour is restrained on all sides (say it is cast onto and tied into walls on all four sides), then the restraint against thermal shrinkage is very large and you will need a substantial amount of rebar to deal with the induced thermal stresses and strains. The large amount of rebar in the span direction will cope with this but I suspect that the distribution rebar has been designed to the minimum structural steel clause rather than to thermal requirements. Hence the large crack. Andy Machon
Can you provide some additional info/detail on the cracking through phenomenon?
The first time I encountered this was soon after graduation when I was asked to do a report on a cracked slab.
Briefly this was a long narrow slab supported on walls and exposed to the weather. The slab had cracked right through with the cracks at right angles to the long axis. Minimum code reinforcement for the time had been used in the long span direction. It acted as a one way slab spanning in the short direction but with significant restraint in the long direction.
Concrete shrinks with time which creats tensile stress in the long direction of a restrained slab. Eventually the concrete will crack. If the tensile strength of the concrete exceeds the yield strength of the reinforcement crossing the crack further shrinkage will cause the crack to widen, giving a wide crack right through the slab.
If the steel is stronger than the concrete further shrinkage will cause another crack to form in the concrete. This results in more but narrow cracks; they are held tight by the reinforcement.
This is a summary and I haven't gone into all effects, just the major ones. From memory I think Martin Murray published some papers on this about 20 years ago.
Thermal expansion is not the major problem since it tends to counter the drying shrinkage, but in cold weather thermal contraction will add to it.
Thanks Ginger and 'dzine... I was thinkin' a much wider crack, one that daylight, or in the case of the drop panel, a matchbook cover could pass through.
