# Column size for 20m span and Column design Thumb rule

Column size for 20m span and Column design Thumb rule, size of column depend on total load applied on the column, alignment of column and span between two column, in this topic we try to find out column size for 20 metre span.

The size of the columns depends on the total load on the columns. There are axial loads and lateral loads.

Column size depends on various factors like number of floors, span between two column, is it short or slender, For 20m span 1 storey/single floor/ground floor/G+0 residential building, it could be 450mmx600mm (18″×24″), for G+1 it could be 600mmx 900mm (24″×36″) and for G+2 it could be 900mmx 900mm (36″×36″) using M40 grade of concrete of desine mix with Fe550 grade of steel when a tributary area 60m×60m and span between each column is 20m consists of 16 column.

IS 456:2000 recommendation for designing of column
1) minimum clear cover for column 40mm is provided, if size of column is less than 200mm then clear cover is 25mm is provided.

2) minimum diameter of bar 4 nos of 12mm should be taken

3) minimum area of Steel =0.8%of Ag
Maximum area of Steel=6%of Ag ( Steel should not overlap) 4%of Ag if steel is overlap

4) maximum spacing between two longitudes renforcement shall not greater than 300 mm

5) minimum dia of Stirrup 1/4×16 greater

6) maximum spacing of stirrup 16D, B and 300mm which ever is minimum.

Column size for 20m span for 1 storey/single floor/Ground floor/G+0 residential building:-

1) Calculating column size for 20m span for 1 storey/single floor/ground floor/G+0 residential building, we have given number of floor=1, slab span= 60m ×60m, total number of column = 16, fy=550N/mm2, fck=40N/mm2, thumb rule load=20kN/m2, gross area Ag =100%, steel area Asc =1%ofAg =0.01Ag and concrete area Ac =99%of Ag =0.99Ag.

Load = 1×20kN/m2 ×60m×60m = 72000kN,
Load acting on each column=72000kN/16=4500kN, consider 10% increasing load on each column for safety factor, 10% of 4500 =450kN, so total axial load acting on each column=4500+450= 4950kN, putting this value in in formula,

Pu=0.4fck.Ac + 0.67fy.Asc,

4950kN=0.4 ×40×0.99Ag + 0.67×550×0.01Ag

4950×10^3 = 15.84Ag +3.685Ag

4950×10^3 = 19.525Ag

Ag (Gross area=4950×10^3/19.525=253521mm2,
Suppose we square column, so area of square =a2, hence a2 = 253521mm2, a=√253521mm2=504mm, width and depth of column =504mm×504mm, taking in round figure=450mm×600mm(18″×24″).

According to column design and Thumb Rule, minimum column size for 20m span is 450mm×600mm (18″×24″) for 1 storey/single floor/ground floor/(G+0) residential building using M40 grade of concrete of desine mix with Fe550 grade of steel when axial load 4950kN is applied on each column.

Column size for 20m span for 2 storey/two floor/G+1 residential building:-

2) Calculating column size for 20m span for 2 storey/two floor/G+1 residential building, we have given number of floor=2, slab span= 60m ×60m, total number of column = 16, fy=550N/mm2, fck=40N/mm2, thumb rule load=20kN/m2, gross area Ag =100%, steel area Asc =1%ofAg =0.01Ag and concrete area Ac =99%of Ag =0.99Ag.

Load = 2×20kN/m2 ×60m×60m = 144000kN,
Load acting on each column=144000kN/16=9000kN, consider 10% increasing load on each column for safety factor, 10% of 9000 =900kN, so total axial load acting on each column=9000+900= 9900kN, putting this value in in formula,

Pu=0.4fck.Ac + 0.67fy.Asc,

9900kN=0.4 ×40×0.99Ag + 0.67×550×0.01Ag

9900×10^3 = 15.84Ag +3.685Ag

9900×10^3 = 19.525Ag

Ag (Gross area=9000×10^3/19.525=507042mm2,
Suppose we square column, so area of square =a2, hence a2 = 507042mm2, a=√507042mm2=712mm, width and depth of column =712mm×712mm, taking in round figure=600mm×900mm(24″×36″).

According to column design and Thumb Rule, minimum column size for 20m span is 600mm×900mm (24″×36″) for 2 storey/two floor/(G+1) residential building using M40 grade of concrete of desine mix with Fe550 grade of steel when axial load 9900kN is applied on each column.

Column size for 20m span for 3 storey/three floor/G+2 residential building:-

3) Calculating column size for 20m span for 3 storey/three floor/G+2 residential building, we have given number of floor=3, slab span= 60m ×60m, total number of column = 16, fy=550N/mm2, fck=30N/mm2, thumb rule load=20kN/m2, gross area Ag =100%, steel area Asc =1%ofAg =0.01Ag and concrete area Ac =99%of Ag =0.99Ag.

Load = 3×20kN/m2 ×60m×60m = 216000kN,
Load acting on each column=216000kN/16=13500kN, consider 10% increasing load on each column for safety factor, 10% of 13500 =1350kN, so total axial load acting on each column=13500+1350= 14850kN, putting this value in in formula,

Pu=0.4fck.Ac + 0.67fy.Asc,

14850kN=0.4 ×40×0.99Ag + 0.67×550×0.01Ag

14850×10^3 = 15.84Ag +3.685Ag

14850×10^3 = 19.525Ag

Ag (Gross area=14850×10^3/19.525=760563mm2,
Suppose we square column, so area of square =a2, hence a2 = 760563mm2, a=√760563mm2=872mm, width and depth of column =872mm×872mm, taking in round figure=900mm×900mm(36″×36″).

According to column design and Thumb Rule, minimum column size for 20m span is 900mm×900mm (36″×36″) for 3 storey/three floor/(G+2) residential building using M40 grade of concrete of desine mix with Fe550 grade of steel when axial load 14850kN is applied on each column.

We can reduce the size of column by increasing reinforcement quantity, grade of Steel and design mix of grade of concrete for multi storey building.