one way slab / two way slab, which one is better?
one way slab / two way slab, which one is better?
(OP)
as per subject, need to know things to consider.
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one way slab / two way slab, which one is better?
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RE: one way slab / two way slab, which one is better?
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.
RE: one way slab / two way slab, which one is better?
Carl Bauer
www.bauerconsultbotswana.com
RE: one way slab / two way slab, which one is better?
RE: one way slab / two way slab, which one is better?
RE: one way slab / two way slab, which one is better?
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.
RE: one way slab / two way slab, which one is better?
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
RE: one way slab / two way slab, which one is better?
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.
RE: one way slab / two way slab, which one is better?
RE: one way slab / two way slab, which one is better?
RE: one way slab / two way slab, which one is better?
Sufficient cover is between 50 to 75mm. the later is preferred.
Cement type and content is also important.