Normally, we use a main rod in shorter spans in R.C.C. What would happen if we used it in longer spa

Nothing it will just increase the cost.

To avoid higher bending moment and thereby avoiding heavy reinforcement.

one way slab is the slab in which the l/b ratio is more than two and in this type of slab the support is provided along longer direction as follows;

as you can see in above figure the slab is supported on two walls along the length(longer direction) of slab. thus the slab will bend along width (shorter direction). hence the main reinforcement will be provided along shorter direction i.e. width of slab.

now the question arises why we always provide the support along longer direction ?

So lets assume if we provide the support to the slab along shorter direction , then the slab will be bend along longer direction ( vice-versa to our first scenario). And as everyone knows greater will be distance(length) greater will be the moment and greater will be the moment greater will be the reinforcement and greater will be the r/f greater will be the cost.

thus to provide economical section the one way slab is supported along longer span and thereby the main r/f is provided along shorter span.

now what about two way slab ?

Well in two way slab unlike the one way slab the bending is dominant in both of the directions thus we have to provide the main r/f in both direction to have safe structural design of slab.

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