Q. Bank For Civil Engineer LDCE Exam - Design a column,square in section, to carry an axial load of 4500000N.

Q - Design a column,square in section, to carry an axial load of 4500000N. 

Ans - 

Step 1:-

Let us adopt M25 grade concrete and mild steel reinforcements.

Allowable direct compressive stress=6N/mm2 in concrete

Allowable compressive stress in steel=130N/mm2

Min. percentage of longitudinal reinforcement=0.8%

Step 2:-

Let “a” be the size of square column

Then area of column= a2

Area of steel =0.008 a2

Area of concrete=a2_0.008a2=0.992 a2

As per condition,

4500000 =6X0.992a2+130X0.008a2

4500000=5.952a2 +1.04 a2=6.992 a2

We get a = 802.24 mm

Size of square column is 802.24 mm

Step 2 :-

Provide square column of size 810 X810 mm with 30 mm chamfering at the corner.

Area of column= 810X810-2X30X30 = 654300 sqmm

Area of steel = 0.008X654300 = 5234.4Sqmm

Let us adopt 28 mm dia bars

Area = 3.142X28X28 4 =615.75 mm2 

No. of bars = 5234.4/615.75 

 = 8.5 say 9 nos

Provide 9 bars of 28 mm dia.

Tie Reinforcement:- 

Using 8 mm dia tie reinforcement, spacing will be

a)16 times dia of main bars = 16X 28 = 448 mmor 

b)48 times dia of tie reinforcement = 48X 8 =384 mmor 

c)Last size of column = 810 mm

Provide 8 mm bars tie reinforcement at 380 mm centre to centre I e. least spacing among the three criterias.