SOLUTION

Beams with large depths in relation to spans are called deep beams. A simply-supported beam is classified as deep when the ratio of its effective span L to overall depth D is less than 2. Continuous beams are considered as deep when the ratio L/D is less than 2.5. 

A beam, whether deep or conventional, is always designed for bending moments. The moment of resistance corresponding to the bending moment is directly proportional to the moment of inertia. So for a greater moment of resistance on which the bending moment shall act, we have to provide a greater moment of inertia which can be achieved only by providing depth more than that of width. That’s why deep beams are designed.