Three thumb rules to be followed are as follows:
Size of the Columns
Distance between Columns
Alignment of columns
Minimum Size of RCC columns
The size of the columns depends on the total load on the columns. There are axial loads and lateral loads. Large beam spans induce bending moment not only in the beams but also in columns which are pulled by the stresses in the beams. It is important to use advanced structural design software like ETabs or Staad pro. I highly recommend every structural designer learn this software. The thumb rules are for general designing in very small projects.
For this general thumb rule, we will assume a structure of G+1 floors high, using standard 6″ walls.
The minimum size of an RCC column should not be less than 9” x 12” (225mm x 300mm) with 4 bars of 12 MM Fe415 Steel.
This setup of 9″ x 12″ RCC columns is safe for G+1 Floors. There are a lot of other considerations, but this is just a thumb rule.
Span (distance) between two columns
For the above column setup, a span of up to 5 meters is quite safe. One can use beams of size 9″ X 12″ (225 MM x 300MM) with a slab thickness of 5″ (125 MM) cast in M20 concrete for spans up to 5m. There are other considerations like secondary and tertiary spans, point loads and wall loads which have to be considered. It is complicated, but thumb rules can work if the structure is simple. It is always recommended to use structural design software like ETabs or Staad pro for design.
In a beam of up to 5 meters length, secondary spans of up to 4 meters, wall loads of up to 8 kN per running meter, I can use steel as below.
Top Steel – 2 bars of 12 MM
Crank bars – 2 bars of 12 MM, cranked at an angle of 45° at a distance of L/4 at both ends of a simply supported beam
Bottom Steel – 3 bars of 12 MM.
This configuration can change depending on a lot of factors.
Alignment of Columns
Placing of columns depend completely on the plan. A planner has a very important job. A grid column placement is always preferred in order to reduce point loads and unnecessary complications while construction. This reduces the cost of construction as well as time required for construction. Beams which have continuity with other simply supported beams have reduced bending moments, and thus require less steel and concrete depth to be safe.
Columns have to be connected with each other for a smooth transfer of loads. An experienced planner will keep such things in mind when planning the structure.
Ekeeda - Civil Engineering