One-way or Two-way Concrete Slab?

[amecENG]

I have a multi-storey concrete building with an essentially square column grid that has band slabs along the column lines in one direction only (i.e. spanning North-South).

I am wondering how stiff do the band slabs have to be in order to consider this slab as a "one-way" slab?

I know one of the criteria for two-way slabs with beams is a ratio of the stiffness of beams in N-S direction vs the beam stiffness in the E-W direction. In this case I do not have any beams in the E-W direction. Can I assume an equivalent beam width for the slab in the E-W direction in order to compare it to the stiffness of the band slab? If so, what "width" should I use?

For those of you with the Canadian CSA A23.3 Concrete Design Handbook, there is a solved example of this type of slab on page 5-21 in the handbook section but they do not prove that the slab meets the two-way criteria.

Here is some specific design information on the slab:
Grid spacing = 8.0m
Band slab thickness = 450mm
Slab thickness = 200mm
Column size = 500mmx500mm

 

[rscassar]

Is the slab post-tensioned? If the slab was post-tensioned then I would say that it is definitely one-way spanning.

You could compare the stiffness of the 'column' strips in each direction and find that the stiffness of the banded direction would be 5-10 times stiffer than the unbanded direction. On this theory alone I would say the structure would want to behave in a one-way spanning manner. I read a good saying in a strut-tie article which was:

It is useful to remember that the structure tries to carry loads as effectively as possible with the least amount of deformation.

Authors were Nori and Tharval.

I think this is very applicable to your situation. There will be some two-way behaviour of the system and concentration of stresses in the column strip in the unbanded direction but I would say that the one-way action is prominant.

 

[hokie66]

Banded slabs are designed as one-way systems. There is always some two way action, but that is just a bonus.