Example 3.9 - Design of Short Column Subjected to Uniaxial Bending

COLUMN SUPPORTING AN AXIAL LOAD AND UNIAXIAL BENDING

EXAMPLE 3.9

An internal column sized 300 x 300 mm in a multi-storey building is subjected to an ultimate axial load (NEd) of 1600 kN and bending moment, M of 60 kNm including the effect of  imperfections. 2

2

Design the column assuming f ck ck = 30 N/mm , f yk yk = 500 N/mm and nominal cover, c = 30 mm. Solution:

1. Check the design moment, M Ed . Moment due to geometric imperfections, Mimp = ei. NEd Eccentricity due

Minimum eccentricity

to imperfections

ei = max { lo/400, h/30, 20} mm 

No need to calculate M imp since it has already been considered.

2. Design of main reinforcement. 

Use the design chart

a) Calculate d value. Assume bar = 25 mm and link = 8 mm d

d = h – c - link - bar/2 = 300 – 30 – 8 – 25/2 = 249.5 mm

b) Calculate d/h (to determine the chart to be used) d/h = 249.5 / 300 = 0.83  0.85 2 c) Calculate N/bhf ck  ck  and M/bh  f ck  ck 

     





               

  

  

h

COLUMN SUPPORTING AN AXIAL LOAD AND UNIAXIAL BENDING

EXAMPLE 3.9

Intersection point

0.59

0.074

d) Determine the value of A s f yk  / bhf ck  from the chart (point where the lines intersect). From the chart,

      

 

    

  

2

Therefore, provide: 4H25 (1960 mm ) e) Check the reinforcement provided with the minimum and maximum requirement.

  

  

       

 



       

         

     2

As,max = 0.04 Ac = 0.04 (300 ) = 3600 mm

2

As,min (368) < As,prov (1960) < As,max (3600) OK!

COLUMN SUPPORTING AN AXIAL LOAD AND UNIAXIAL BENDING

EXAMPLE 3.9

3. Design links a) Minimum link diameter = max { 6 mm, ¼ of the maximum longitudinal bar diameter) i)

6 mm

ii)

¼ bar = ¼ (25) = 6.25 mm Use:

8 mm

b) Maximum link spacing, s cl,tmax The least of: i)

20 bar = 20 x 25 = 500 mm

ii)

Lesser dimension of the column = 300 mm

iii)

400 mm

Provide spacing, scl,tmax = 300 mm

Therefore, for the links, provide H8  – 300 c/c spacing. 4. Sketch the cross-sectional detailing of this column.

H8-300 300 4H25

300