At times as a structural engineer you are required to make quick approximate estimations for member sizes.

[1] The sizes for gravity columns can easily be determined if you have a basic approximation about floor loads, tributary area & number of floors.

[2] The floor loads are generally composed of dead load of floor slabs & occupancy loads as per IS 875 or relevant codes.

[3] The dead loads for slabs can be calculated if you can make tentative estimates about the slab depth based on the spans.

[4] Once you have the floor loads and the tributary area for most critical columns can be found by grid spacing.

[5] Total loads on bottom most story can be found as

Total load = Factored floor loads * tributary area from the floor * number of stories.

[6] Once the total vertical loads are known, the column dimension can be found by assuming certain percentage of reinforcement. (2% reinforcement in columns is fair assumption if you are checking for gravity loads only)

P = 0.4 fck Ac + 0.67 fy Ast

Assumed Ast = 0.02 Ac

P = 0.4 fck Ac + 0.0134 fy Ac

[7] From this the area of the column can easily be determined, but this only factors in vertical loads & no moment.

This is post on approximate estimation of sizes of gravity columns. Please what other approximation method you use for rough member size estimation.

Any comments or suggestions are welcomed.