**Column size for 7m 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 7m span.

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

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**Column size depends on various** factors like number of floors, span between two column, is it short or slender, For 7m span 1 storey/single floor/ground floor/G+0 residential building, it could be 230mmx300mm (9″×12″), for G+1 it could be 300mmx 380mm (12″×15″), for G+2 it could be 380mmx 450mm (15″×18″),for G+3 it could be 450mmx 500mm (15″×20″) and for G+4 it could be 450mmx 600mm (15″×24″) using M20 grade of concrete ratio 1:1.5:3 with Fe500 grade of steel when a tributary area 21m×21m and span between each column is 7m 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 7m span for 1 storey/single floor/Ground floor/G+0 residential building:-**

1) Calculating column size for 7m span for 1 storey/single floor/ground floor/G+0 residential building, we have given number of floor=1, slab span= 21m ×21m, total number of column = 16, fy=500N/mm2, fck=20N/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 ×21m×21m = 8820kN,

Load acting on each column=8820kN/16=551kN, consider 10% increasing load on each column for safety factor, 10% of 551 =55kN, so total axial load acting on each column=551+55= 606kN, putting this value in in formula,

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

606kN=0.4 ×20×0.99Ag + 0.67×500×0.01Ag

606×10^3 = 7.92Ag +3.35Ag

606×10^3 = 11.27Ag

Ag (Gross area=606×10^3/11.27=53771mm2,

Suppose we square column, so area of square =a2, hence a2 = 53771mm2, a=√53771mm2=232mm, width and depth of column =232mm×232mm, taking in round figure=230mm×300mm(9″×12″).

According to column design and Thumb Rule, minimum column size for 7m span is 230mm×300mm (9″×12″) for 1 storey/single floor/ground floor/(G+0) residential building using M20 grade of concrete ratio (1:1.5:3) with Fe500 grade of steel when axial load 606kN is applied on each column.

**Column size for 7m span for 2 storey/two floor/G+1 residential building:-**

2) Calculating column size for 7m span for 2 storey/two floor/G+1 residential building, we have given number of floor=2, total number of column = 16, span area=21m×21m, fy=500N/mm2, fck=20N/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 ×21m×21m = 17640kN,

Load acting on each column=17640kN/16=1103kN, consider 10% increasing load on each column for safety factor, 10% of 1103 =110kN, so total axial load acting on each column=1103+110= 1213kN, putting this value in in formula,

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

1213kN=0.4 ×20×0.99Ag + 0.67×500×0.01Ag

1213×10^3 = 7.92Ag +3.35Ag

1213×10^3 = 11.27Ag

Ag (Gross area=1213×10^3/11.27=107630mm2,

Suppose we square column, so area of square =a2, hence a2 = 107630mm2, a=√107630mm2=328mm, width and depth of column =328mm×328mm, taking in round figure=300mm×380mm(12″×15″) for rectangular column.

According to column design and Thumb Rule, minimum column size for 7m span is 300mm×380mm (12″×15″) for 2 storey/two floor/(G+1) residential building using M20 grade of concrete ratio (1:1.5:3) with Fe500 grade of steel when axial load 1213kN is applied on each column.

**Column size for 7m span for 3 storey/three floor/G+2 residential building:-**

3) Calculating column size for 7m span for 3 storey/three floor/G+2 residential building, we have given number of floor=3, total number of column = 16, span area=21m×21m, fy=500N/mm2, fck=20N/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 ×21m×21m = 26460kN,

Load acting on each column=26460kN/16=1654kN, consider 10% increasing load on each column for safety factor, 10% of 1654 =165kN, so total axial load acting on each column=1654+165= 1819kN, putting this value in in formula,

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

1819kN=0.4 ×20×0.99Ag + 0.67×500×0.01Ag

1819×10^3 = 7.92Ag +3.35Ag

1819×10^3 = 11.27Ag

Ag (Gross area=1819×10^3/11.27=161401mm2,

Suppose we square column, so area of square =a2, hence a2 = 161401mm2, a=√161401mm2=402mm, width and depth of column =402mm×402mm, taking in round figure=380mm×450mm(15″×18″) rectangular column.

According to column design and Thumb Rule, minimum column size for 7m span is 380mm×450mm (15″×18″) for 3 storey/three floor/(G+2) residential building using M20 grade of concrete ratio (1:1.5:3) with Fe500 grade of steel when axial load 1819kN is applied on each column.

**Column size for 7m span for 4 storey/four floor/G+3 residential building:-**

4) Calculating column size for 7m span for 4 storey/four floor/G+3 residential building, we have given number of floor=4, total number of column = 16, span area=21m×21m, fy=500N/mm2, fck=20N/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 = 4×20kN/m2 ×21m×21m = 35280kN,

Load acting on each column=35280kN/16=2205kN, consider 10% increasing load on each column for safety factor, 10% of 2205 =220kN, so total axial load acting on each column=2205+220= 2425kN, putting this value in in formula,

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

2425kN=0.4 ×20×0.99Ag + 0.67×500×0.01Ag

2425×10^3 = 7.92Ag +3.35Ag

2425×10^3 = 11.27Ag

Ag (Gross area=2425×10^3/11.27=215173mm2,

Suppose we square column, so area of square =a2, hence a2 = 215173mm2, a=√215173mm2=464mm, width and depth of column =464mm×464mm, taking in round figure=450mm×500mm(18″×20″) for rectangular column.

According to column design and Thumb Rule, minimum column size for 7m span is 450mm×500mm (18″×20″) for 4 storey/four floor/(G+3) residential building using M20 grade of concrete ratio (1:1.5:3) with Fe500 grade of steel when axial load 2425kN is applied on each column.

**Column size for 7m span for 5 storey/five floor/G+4 residential building:-**

5) Calculating column size for 7m span for 5 storey/five floor/G+4 residential building, we have given number of floor=5, total number of column = 16, span area=21m×21m, fy=500N/mm2, fck=20N/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 = 5×20kN/m2 ×21m×21m = 44100kN,

Load acting on each column=44100kN/16=2756kN, consider 10% increasing load on each column for safety factor, 10% of 2756 =276kN, so total axial load acting on each column=2756+276= 3032kN, putting this value in in formula,

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

3032kN=0.4 ×20×0.99Ag + 0.67×500×0.01Ag

3032×10^3 = 7.92Ag +3.35Ag

3032×10^3 = 11.27Ag

Ag (Gross area=3032×10^3/11.27=269032mm2,

Suppose we square column, so area of square =a2, hence a2 = 269032mm2, a=√269032mm2=519mm, width and depth of column =519mm×519mm, taking in round figure=450mm×600mm(18″×24″) for rectangular column.

According to column design and Thumb Rule, minimum column size for 7m span is 450mm×600mm (18″×24″) for 5 storey/five floor/(G+4) residential building using M20 grade of concrete ratio (1:1.5:3) with Fe500 grade of steel when axial load 3032kN is applied on each column.