# How to design column

## How to design column with example

Columns are compression members although some may be subjected to bending either due to their slenderness or due to asymmetrical loading from beams. Columns carry the loads from the beams and the slabs down to the foundations. Columns resist a combination of axial P and exural load M, (or M = Pe for eccentrically applied load).

Below are the examples of column design:

A column carries an axial load of 450KN including its own weight, design a 225 by 225mm column assuming the column is short. Take Fcu = 25N/mm2 and Fy = 410N/mm2.

Solution:

Since the column is short and axially loaded, the area of steel is obtained from

Asc = (N-0.40Fcubh)/(0.80Fy-0.4Fcu)

= (450×103 – 0.40x25x225x225)/(0.80×410 – 0.40×25)

= -177mm2

Since the answer is negative, we provide minimum reinforcement which can be deducted from 0.4%bh

Asc = 0.4% x 225 x 225 = 203mm2

Provide 4-Y12 bars (452mm2)

A uniaxially loaded column supports a total ultimate load of 650KN and a moment of 10KNm. The 300 x 300mm column is short and using 25-250 concrete. Design it completely.

Solution:

N = 650KN, M = 10KNm, size = 300 x 300mm uniaxial column.

Minimum moment = 0.05Nh

= 0.05 x 650 x 10 x 300Nmm

= 9.75KNm

Therefore use M = 9.75KNm

N/bh = (650×103)/(300×300)

= 7.22

M/bh2 = (9.75×106)/(300×3002)

= 0.36

Hence the minimum reinforcement of 0.4%bh:

= 0.4% x 300 x 300

= 360mm2

Provide 4-Y16mm bars (804mm2)

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