Depending upon the nature of environments, forces, and boundary conditions applied to the structure, we have to design the shape of the structure as well as individual RCC components.
Theoretical Best place for T-Beam
Consider a case where the dominant load component along the entire length of any RCC component is bending moment in a fixed direction. Hence, the tension side of the component is fixed. Also, both ends of the components are transferring moments smoothly to the other part of the structure.
So, we can design the component in the following ways:
- Remove the tension side concrete completely :
- But tension side reinforcement requires protection  and support . So, dump all reinforcement in a single place and give bare minimum thickness of concrete web.
- But spacing between reinforcements , buckling and local effects  of web, should determine the web thickness.
- Not only tension side of the concrete, compression side of the concrete near neutral axis, where compression strain is low, can be removed .
- But again thickness of the compression flange should be decided up on buckling and other local effects  as well as ductile design requirement of the component .
- Depth of the web will be increased infinitely, such that distance between centre of tension and centre of compression becomes infinite to get infinite moment resistance .
- But buckling and other local effects of web  will decide the depth of the web.
- Reinforcement will not be used in compression side, which will reduce the cost of the component , unless minimum reinforcement should be provided for other local effects .
So, we will design this component as T-Beam. No other sections will be as effective as T-Beam for this theoretical conditions.
In most of the practical cases, following points are also considered for design of RCC components:
- It will be very rare that entire length of the component will be dominant with only bending moments. Bending may reverse its direction.
- Along with bending moment, other forces like axial forces and shear forces will always be acting.
- Slab of concrete as system requirement (floor of the building), which needs separate design for slab. So, the thickness of the T-beam will be compression flange along with its reinforcement details.
- System requirement will decide the depth of the web.
- For very large cross section, pre-stress or post-stress are also considered to increase material utilisation.