This document discusses reinforced concrete columns. Columns act as vertical supports that transmit loads to foundations. Columns may fail due to compression failure, buckling, or a combination. Short columns are more prone to compression failure, while slender columns are more likely to buckle. Column sections can be square, circular, or rectangular. The dimensions and bracing affect whether a column is classified as short or slender. Longitudinal reinforcement and links are designed to resist axial loads and moments based on the column's effective height and end conditions. Design charts are used to determine reinforcement for columns with axial and uniaxial bending loads. Examples show how to design column reinforcement.

1. Reinforced concrete column
Prepared by: M.N.M Azeem Iqrah
B.Sc.Eng (Hons), C&G (Gdip)
Skills College of Technology

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:
1. Compression failure of the concrete or steel
reinforcement;
2. Buckling
3. 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.

5. Failure modes of columns
Compression
failure Buckling

6. Short and slender columns
(Clause
3.8.1.3, BS 8110)
• A braced column is classified as being short if :

7. Braced and unbraced columns
(clause
3.8.1.5, 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.

8. Braced and unbraced columns (clause 3.8.1.5,
BS 8110)

9. Braced and unbraced columns (clause 3.8.1.5,
BS 8110)

10. 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.

11. 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.

12. 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.

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

14. • 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.
End conditions (clause 3.8.1.6.2, BS 8110)

15. Example 3.17 classification of column (Arya,
2009)
• Determine if the column shown below is short.

16. Example 3.17 classification of column (Arya,
2009)

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

18. • B2 will resist an axial load only, as it supports beams
equal in length and symmetrically arranged.

19. • C2 supports a symmetrical arrangement of beams
but which are unequal in length. If (a) the loadings
on the beam are uniformly distributed, (2)the beam
spans do not differ by more than 15 percent, the
column C2 belongs to category 2.
• If the column does not meet criteria (a) and (b), then
the column belongs to category 3.

20. Theoretical strength of reinforced concrete
column
The equation is derived on the assumption that the axial load is
applied perfectly at the centre of the column.

21. Clause 3.8.4.3 Nominal eccentricity of short columns
resisting moments and axial force
• Toallow for nominal eccentricity, BS 8110
reduce the theoretical axial load capacity by
about 10%.
• Design maximum axial load capacity of short
column is:

22. Clause 3.8.4.4 Short braced columns supporting an
approximately symmetrical arrangement of beam
• The column is subjected to axial and small
moment when it supports approximately
symmetrical arrangement of beams:
•
• The design axial load capacity:

23. Column resisting an axial load and
uniaxial bending
• For column resisting axial load and bending moment
at one direction, the area of longitudinal
reinforcement is calculated using design charts in
Part 3 BS 8110.
• The design charts are available for columns having a
rectangular cross section and symmetrical
arrangement of reinforcement.

24. Column resisting an axial load and
uniaxial bending
• Design charts are derived based on yield stress of
460 N/mm2 for reinforcement steel. They are
applicable for reinforcement with yield stress of
500 N/mm2, but the area of reinforcement
obtained will be approximately 10% greater than
required.
• Design charts are available for concrete grades –
25, 30, 35, 40, 45 and 50.
• The d/h ratios are in the range of 0.75 to 0.95 in
0.05 increment.

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

26. Column resisting an axial load and
biaxial bending
• The columns are subjected to an
axial and bending moment in both x
and y directions.
• The columns with biaxial moments
are simplified into the columns with
uniaxial momentby increasing the
moment about one of the axes then
design the reinforcement according
the increased moment.

27. Column resisting an axial load and biaxial
bending (clause 3.8.4.5, BS 8110)

28. Reinforcement details: longitudinal
reinforcement (clause 3.12.5, BS 8110)
1. 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.
2. The area of longitudinal reinforcement should lie in the
limits:
3. Spacing of reinforcement – the minimum distance between
adjacent bars should not be less than the diameter of the
bar or hagg + 5 mm.

29. Reinforcement details – links (clause 3.12.7, 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.

30. Reinforcement details – links (clause 3.12.7, BS 8110)
1. 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.
2. Arrangement of links

31. Example 3.20 axially loaded column (Arya, 2009)
• Design the longitudinal and links for a 350mm square, short
braced column based on following information.

32. Example 3.20 axially loaded column (Arya, 2009)

33. Example 3.21 Column supporting an approximately
symmetrical arrangement of beam ( Arya, 2009)
• An internal column in a braced two-storey building supporting
an approximately symmetrical arrangement of beams
(350mm wide x 600 mm deep) results in characteristic dead
and imposed loads each of 1100 kN being applied to the
column. The column is 350 mm square and has a clear height
of 4.5 m. Design the longitudinal reinforcement and links.

34. Example 3.21 Column supporting an approximately
symmetrical arrangement of beam ( Arya, 2009)

35. Example 3.21 Column supporting an approximately
symmetrical arrangement of beam ( Arya, 2009)

36. Example 3.21 Column supporting an approximately
symmetrical arrangement of beam ( Arya, 2009)
• Links
• link = diameter of largest longitudinal bar/4
• = 32/4 = 8 mm (equal to minimum bar size
of 8 mm)
• The spacing of the links
• = the lesser of (12 smallest longitudinal bar or
the smallest cross sectional dimension of
column)
• = the lesser of (12x25 = 300 mm or 350 mm)
• = 300 mm

37. Example 3.22 Columns resisting an
axial load and bending moment
• Design the longitudinal and shear reinforcement for
a 275 mm square, short braced column which
supports either
(a) An ultimate axial load of 1280 kN and a moment of
62.5 kNm about the x-x axis
(b) An ultimate axial load of 1280 kN and bending
moment of 35 kNm about the x-x axis and 25 kNm
about the y-y axis

38. Example 3.22 Columns resisting an axial load and bending moment

39. Example 3.22 Columns resisting an axial load and bending moment

40. Example 3.22 Columns resisting an axial load and bending moment

41. Example 3.22 Columns resisting an axial load and bending moment

42. Example 3.22 Columns resisting an axial load and bending moment