1. Reinforced concrete column
Structural Design I
Course Code: CIVL312
Reinforced concrete column
By :Dr. Aeid A. Abdulrazeg

2. Introduction to column
Columns act as vertical supports to beams and slabs, and to transmit the loads to the foundations.
Columns are primarily compression members, although they may also have to resist bending moment transmitted by beams.
Columns may be classified as short or slender, braced or unbraced depending on various dimensional and structural factors.

3. Column sections
Common column cross sections are: (a) square, (b) circular and (c) rectangular section.
The greatest dimension should not exceed four times its smaller dimension. (h≤4b).
For h>4b, the member should be regarded as a wall for design purpose.

4. Failure modes of columns
Columns may fail in one of three mechanisms:
Compression failure of the concrete or steel reinforcement;
Buckling
Combination of buckling and compression failure.
• Compression failure is likely to occur with columns which are short and stocky.
• Buckling is probable with columns which are long and slender.

6. Failure modes of columns

7. Short and slender columns (Clause 3.8.1.3, BS 8110)
A braced column is classified as being short if
&
lex effective height in respect of the major axis
ley effective height in respect of the minor axis
For braced columns
For unbraced columns

8. Short and slender columns (Clause 3.8.1.3, BS 8110)
A column may be considered braced in a given plane if lateral stability to the structure as a whole is provided by wall or bracing or buttressing designed to resist all lateral forces in that plane. It should otherwise be considered as unbraced.

9. Effective height of column (clause 3.8.1.6, BS 8110)
The effective height, le of a column in a given plane may be obtained from the following equation:
Where is a coefficient depending on the fixity at the column ends and lo is the height of the columns.
Effective height for a column in two plane directions may be different

10. Effective height of column (clause 3.8.1.6, BS 8110)
for braced column can be obtained from Table 3.19.
End condition 1 signifies that the column end is fully restrained.
End condition 2 signifies that the column end is partially restrained
End condition 3 signifies that the column is nominally restrained.

11. End conditions (clause 3.8.1.6.2, BS 8110)
End condition 1 – the end of the column is connected monolithically to beams on either side which are at least as deep as the overall dimension of the column in the plane considered. Where the column is connected to foundation, it should be designed to carry moment.

12. End conditions (clause 3.8.1.6.2, BS 8110)
End condition 2 – the end of column is connected monolithically to beams or labs on either side which are shallower than the overall dimension of the column in the plane considered.

13. End conditions (clause 3.8.1.6.2, BS 8110)
End condition 3 – the end of the column is connected to members which, while not specifically designed to provide restraint to rotation of the column will nevertheless, provide some nominal restraint.

14. Example 1: classification of column
Determine if the column shown below is short.

15. Example 1 classification of column

16. Short column design
The short column are divided into three categories:
Columns resisting axial load only,
Columns supporting an approximately symmetrical arrangement of beams,
Columns resisting axial loads and uniaxial or biaxial bending

17. It supports a symmetrical arrangement of beams but which are unequal in length. If (a) the loadings on the beam are uniformly distributed, (b) the beam spans do not differ by more than 15 percent, the column C2 belongs to category 2.
It will resist an axial load only, as it supports beams equal in length and symmetrically arranged.
If the column does not meet criteria (a) and (b), then the column belongs to category 3.

18. Both longitudinal steel and concrete assist in carrying the load:
For pure axial load:
Where: Ac is the net area of concrete.
Design maximum axial load capacity of short column is:

19. To allow for nominal eccentricity, BS 8110 reduce the theoretical axial load capacity by about 10%.

20. Example 2: Short column design
Determine the load capacity of the shown column.
300 mm
4T25
300 mm

21. Example 3: Short column design
The load capacity of the column is 1366 kN and column size as shown in figure. Find the area of steel reinforcement is required.
250 mm
250 mm

22. Column Resisting an Axial Load and Uniaxial Bending

23. Construction of Design Chart
A design curve can be drawn for a selected grade of concrete and reinforcing steel for a section with a given percentage of reinforcement, 100Asc/bh, symmetrically placed at a given location d/h.
The curve is formed by plotting values of N/bh against M/bh2 for various positions of the neutral axis x. Other curves can be constructed for percentages of steel ranging from 0.4% to a maximum of 6% for vertically cast columns.
The family of curves forms the design chart for that combination of materials and steel location. Separate charts are required for the same materials for different values of d/h which determines the location of the reinforcement in the section.
Groups of charts are required for the various combinations of concrete and steel grades. Design charts are given in Part 3 of the code. The process for construction of a design chart is demonstrated below.

24. Example 4: Construction of Design Chart
The process for construction of a design chart is demonstrated below.
Calculate the point on the design curve, when the moment is zero (pure axial load).
Point No. 1 (1944, 0.0)
300 mm
d=255 mm
45 mm
6T20

25. Calculate the value of the axial load and moment when the tension steel is at yield.
d=255 mm
300 mm
Steel in comp. yielded

26. Point No. 2 (591.4, 125.7)

27. Calculate the value of the axial load and moment when the neutral axis lies at the edge of the column
d=255 mm
300 mm

28. Point No. 3 ( , 45.62)

29. Calculate the value of the moment when the axial load is zero
Calculate the value of the moment when the axial load is zero. The value of x can be determined by successive trials to give the case when the sum of the internal forces is zero.
Point No. 4 (0.00, 90)

30. Construction of Design Chart
(1944, 0.00)
( , 45.62)
(591.4, 125.7)
(0.00, 90)
(0.00, )

31. Further points can be calculated for the % of steel up to max
Further points can be calculated for the % of steel up to max. of 8 %, curve by taking other values for the depth x of the neutral axis.
If b, h, d, d’ on fcu, fy are changed, the interaction diagram must be redrawn.
To simplify the use of design chart N may be replaced by and

32. Design chart for column resisting an axial load and uniaxial bending moment, (Part 3, BS 8110)

33. The area of longitudinal reinforcement should lie in the limits:
Reinforcement Details: Longitudinal Reinforcement (clause , BS 8110)
Size and minimum number of bars – bar size should not be less than 12 mm in diameter. Rectangular column should reinforced with minimum 4 bars; circular column should reinforced with minimum 6 bars.
The area of longitudinal reinforcement should lie in the limits:
Vertically cast
Horizontally cast

34. Reinforcement details: longitudinal reinforcement (clause 3. 12
Reinforcement details: longitudinal reinforcement (clause , BS 8110)
Spacing of reinforcement – the minimum distance between adjacent bars should not be less than the diameter of the bar or hagg + 5 mm.

35. Links are passing round the bars to prevent buckling.
Reinforcement details – links (clause , BS 8110)
The axial loading on the column may cause buckling of the longitudinal reinforcement and subsequent cracking and spalling of concrete cover.
Links are passing round the bars to prevent buckling.

36. Reinforcement details – links (clause 3.12.7, BS 8110)
Size and spacing of links – the diameter of the link should be at least one quarter of the largest longitudinal bar size or minimum 8 mm. The maximum spacing is 12 times of the smallest longitudinal bar.
Arrangement of links

37. Example 5: Short column design
A short braced column is subjected to an ultimate load of 1480 kN and ultimate moment of 45 kN.m. the column section is 300 mm x 300 mm.
Determine the area of steel required.
Assume 25 mm diameter bars for main reinforcement and 8 mm diameter links.
The cover on the links is 25 mm.

38. Example 5: Short column design
BS8110- Chart No. 28

39. Provide 4T25mm to give an area of 1963 mm2
Links:
The diameter of the links is one – quarter times the diameter of the largest longitudinal bar, that is
The spacing of the links is the lesser of (a) 12 times the diameter of the smallest longitudinal bar, that is
Or (b) the smallest cross- sectional dimension of the column
That is 300 mm
Provide 300 mm