Before we look at the reasons, please have a look at the framed structure where the conventional Rectangular section has been adapted :
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 form work shall be semi circular. The conventional methods of supporting with wooden CHABI is not suitable.
The Formworm 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.
Hope you’re clear with certain difficulties associated with the circular sections.
Thank you so much for the A2A.
Happy to help you.
Edit 01 :
Have a look at this elevated water tank. you shall observe that while the staging columns are all circular, the RCC tie beams are all RECTANGULAR.