Well there are two conditions for two way slab corners
When there is upper floor or wall or column or any other type of load is acting on the slab it’s called as slab corners are held down i.e. slab corners are not allowed to lift up because of those load.
While when there is no such type of load is acting on slab corners then the slab corners are free to lift up when load is applied at slab’s center, This condition is known as slab corner are not held down. This generally occur with one storeyed building.
For most of the students it is quite difficult to visualize the torsion in a slab system. After all when we read about torsion most of us see/imagine pictures something like this,
i.e. most of us see/imagine a bar to which a moment is applied about its longitudinal axis. Due to which the given bar get twisted as shown in the picture above. Now the question arises, What is the longitudinal axis of the slab ? & which moment are we talking about ?
so in In slabs we don’t talk in terms of longitudinal or lateral axis but we talk in terms of the local X,Y & Z axis. X & Y axis are always in the plane and Z axis is normal to the plane as demonstrated in figure below ;
As we all knows Torsional moment is the moment that leads to the rotation of the plane about the axis. Mxx torsion would twist the slab about x-axis whereas Myy torsion would twist the slab about y-axis. These torsional moments are quite high nearer to the corner of the slab. In two-slabs which are restrained along the edges & are not free to lift, cracks get developed at the corners because of the high torsional moment as shown in fig. below ;
Thus we need to prevent such types of the cracks formations in the two-way slab. Therefore we reinforce these sections by providing reinforcement to resist the crack formation at the corners. whereas in case of the slab is continuous along the edge then as per the design we have to provide some reinforcement to take care of the negative bending moments (Bar Mark A and D in figure below) at the edges and thus corner reinforcement is not called for.
But in case of the edges are discontinuous then we specifically need to provide corner reinforcement. By experimentation it has been seen that these cracks develop up-to a distance 0.2 times the length of short span. as shown in fig. below. Thus corner reinforcement is provided in this zone.