Why Circular beams are not commonly used in construction or in structures?

So you have already observed that in RCC structures, the circular cross-section of the RCC Beam is not adapted.

These are some reasons for not using circular columns for construction purposes:

1.Formwork :

Very difficult. It has to be made out of either MS sheets or Aluminum sections.

Fixing it horizontally is more challenging since the base of the beam formwork shall be semi-circular. The conventional methods of supporting with wooden CHABI is not suitable.

The Formwork shall need to have a provision on the top for pouring concrete.

2.The casting :

Imagine that a conventional letter box is laid Horizontality and you have to pour concrete through the opening at the top.

The concrete has to be high slump concrete so that it moves to all crevices of the section.

Finishing the top portion of the beam shall be a highly skilled job.

3. The de -shuttering :

It shall not be possible to first strike off the sides and then the beam bottom .

The form work shall have to be removed in two halves after the concrete attains the strength.

Comparing rectangular & square section.

1) Beam width could match with the wall – fixing this geometric parameter the depth of
the beam could be decided as per the design requirement.

2) Beam strength varies in cubic magnitude of depth- and rectangular
section is efficient in comparison to square section. moment of inertia is high for rectangular sections.

Any other geometric section
1) Beam sections are subjected to failure in the flexural moment . 
 i) Tension occurs in bottom fibers and compression occurs in top fibers
 reinforcing steel effective in tensile strength and concrete is 
 effective in compression which necessitates wider width at bottom 
 and top.

2) Reduction in web width
 possible in the web,provided the section is safe against shear.and in 
 case of steel structure I section is most preferred for its efficiency.

2) Increase in web width 
 Concrete failure at crushing, if we look at it, we could understand that 
 the specimen exposed sides are subjected to tensile forces and bulges 
 & cracks.
 we can visualize that providing out ward convex curved web for beam, 
 facilitates for web crack due to beam upper part is subjected to 
 compression.

3) Form work handling is difficult.

4) Beam side protruding out of wall in convex shape is not accepted for 
 visual appeal and corner created at beam slab junction along beam 
 length is prone for dust accumulation space for insects lizards etc.

This is why circular columns are not generally used for construction purpose.