as per IS 456-2000

1. Design of Compression members-
Axially Loaded columns
by
S.Praveenkumar
Assistant Professor
Department of Civil Engineering
PSG College of Technology
Coimbatore

2. IntroductionIntroduction
► A column is an important components of R.C. Structures.
► A column, in general, may be defined as a member carrying direct axial
load which causes compressive stresses of such magnitude that these
stresses largely control its design.
► A column or strut is a compression member, the effective length of
which exceeds three times the least lateral dimension.(Cl. 25.1.1)(Cl. 25.1.1)
► When a member carrying mainly axial load is vertical, it is termed as
column ,while if it is inclined or horizontal, it is termed as a strut.
► Columns may be of various shape such as circular, rectangular,
square, hexagonal etc.
► ‘‘Pedestal’ is a vertical compression member whose ‘effective length’ isPedestal’ is a vertical compression member whose ‘effective length’ is
less than three times its least lateral dimension [Cl. 26.5.3.1(h)].less than three times its least lateral dimension [Cl. 26.5.3.1(h)].
2

3. Classification of columnsClassification of columns
3
Based on Type of Reinforcement
a) Tied Columns-where the main
longitudinal bars are enclosed within
closely spaced lateral ties( all cross
sectional shapes)
b) Spiral columns-where the main
longitudinal bars are enclosed within
closely spaced and continuously wound
spiral reinforcement (Circular, square,
octagonal sections)
c) Composite Columns-where the
reinforcement is in the form of structural
steel sections or pipes, with or without
longitudinal bars

4. Based on Type of Loading
a) Columns with axial loading (applied concentrically)
b) Columns with uniaxial eccentric loading
c) Columns with biaxial eccentric loading
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5. 5

6. 6

7. ► The occurrence of ‘pure’ axial compression in a column (due to
concentric loads) is relatively rare.
► Generally, flexure accompanies axial compression — due to ‘rigid
frame’ action, lateral loading and/or actual(or even,
unintended/accidental) eccentricities in loading.
► The combination of axial compression (P) with bending moment (M) at
any column section is statically equivalent to a system consisting of the
load P applied with an eccentricity e = M/P with respect to the
longitudinal centroidal axis of the column section.
► In a more general loading situation, bending moments (Mx and My) are
applied simultaneously on the axially loaded column in two
perpendicular directions — about the major axis (XX) and minor axis
(YY) of the column section. This results in biaxial eccentricities ex=
Mx/P and ey= My/P, as shown in [Fig.(c)]. 7

8. ► Columns in reinforced concrete framed buildings, in general, fall into
the third category, viz. columns with biaxial eccentricities.
► The biaxial eccentricities are particularly significant in the case of the
columns located in the building corners.
► In the case of columns located in the interior of symmetrical, simple
buildings, these eccentricities under gravity loads are generally of a
low order (in comparison with the lateral dimensions of the column),
and hence are sometimes neglected in design calculations.
► In such cases, the columns are assumed to fall in the first category,
viz. columns with axial loading.
► The Code, however, ensures that the design of such columns is
sufficiently conservative to enable them to be capable of resisting
nominal eccentricities in loading
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9. Based on Slenderness Ratio (Cl. 25.1.2)(Cl. 25.1.2)
Columns (i.e., compression members) may be classified into the following
two types, depending on whether slenderness effects are considered
insignificant or significant:
1. Short columns
2. Slender (or long) columns.
 ‘Slenderness’ is a geometrical property of a compression member
which is related to the ratio of its ‘effective length’ to its lateral
dimension. This ratio, called slenderness ratio, also provides a
measure of the vulnerability to failure of the column by elastic
instability (buckling) — in the plane in which the slenderness ratio is
computed.. 9

10.  Columns with low slenderness ratios, i.e., relatively short and stocky
columns, invariably fail under ultimate loads with the material
(concrete, steel) reaching its ultimate strength, and not by buckling.
 On the other hand, columns with very high slenderness ratios are in
danger of buckling (accompanied with large lateral deflection) under
relatively low compressive loads, and thereby failing suddenly.
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11. Braced columns & unbraced column
In most of the cases, columns are also subjected to horizontal loads like
wind, earthquake etc. If lateral supports are provided at the ends of the
column, the lateral loads are borne entirely by the lateral supports.
Such columns are known as braced columns.(When relative
transverse displacement between the upper and lower ends of a
column is prevented, the frame is said to be braced (against sideway)).
Other columns, where the lateral loads have to be resisted by them, in
addition to axial loads and end moments, are considered as unbraced
columns. (When relative transverse displacement between the upper
and lower ends of a column is not prevented, the frame is said to be
unbraced (against sideway).
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12. Unsupported Length
Code (Cl. 25.1.3) defines the ‘unsupported length’ l of a column
explicitly for various types of constructions.
Effective length of a column
The effective length of a column in a given plane is defined as the
distance between the points of inflection in the buckled configuration of
the column in that plane.
The effective length depends on the unsupported length l and the
boundary conditions at the column ends
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13. Effective lengths for idealised boundary conditions (Euler’s theory)
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14. Code recommendations for idealised boundary conditions
(Cl. E–1)-Use of Code Charts
Charts are given in Fig. 26 and Fig. 27 of the Code for determining theCharts are given in Fig. 26 and Fig. 27 of the Code for determining the
effective length ratios of braced columns and unbraced columnseffective length ratios of braced columns and unbraced columns
respectively in terms of coefficientsrespectively in terms of coefficients 1 and 2 which represent theβ β1 and 2 which represent theβ β
degrees of rotational freedom at the top and bottom ends of thedegrees of rotational freedom at the top and bottom ends of the
column.column.
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15. 15
In such cases,
the effective
length ratio k
varies between
0.5 and 1.0

16. 16
In such cases,
the effective
length ratio k
varies between
1.0 and infinity

17. Recommended effective length ratios for normal usage (Table 28,
Page 94)
1. columns braced against sideway:
a) both ends ‘fixed’ rotationally : 0.65
b) one end ‘fixed’ and the other ‘pinned : 0.80
c) both ends ‘free’ rotationally (‘pinned’) : 1.00
2. columns unbraced against sideway:
a) both ends ‘fixed’ rotationally : 1.20
b) one end ‘fixed’ and the other ‘partially fixed’ : 1.50
c) one end ‘fixed’ and the other free : 2.00
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18. Reinforcement in columnReinforcement in column
► Concrete is strong in compression.Concrete is strong in compression.
► However, longitudinal steel rods are always provided to assist inHowever, longitudinal steel rods are always provided to assist in
carrying the direct loads.carrying the direct loads.
► A minimum area of longitudinal steel is provided in the column, whetherA minimum area of longitudinal steel is provided in the column, whether
it is required from load point of view or not.it is required from load point of view or not.
► This is done to resist tensile stresses caused by some eccentricity ofThis is done to resist tensile stresses caused by some eccentricity of
the vertical loads.the vertical loads.
► There is also an upper limit of amount of reinforcement in RC columns,There is also an upper limit of amount of reinforcement in RC columns,
because higher percentage of steel may cause difficulties in placingbecause higher percentage of steel may cause difficulties in placing
and compacting the concrete.and compacting the concrete.
► Longitudinal reinforcing bars are “tied” laterally by “ties” or “stirrups”Longitudinal reinforcing bars are “tied” laterally by “ties” or “stirrups”
at suitable interval so that the bars do not buckleat suitable interval so that the bars do not buckle
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19. Codal Provisions(IS-456-2000)Codal Provisions(IS-456-2000)
19
Page No:41 & 42 –IS 456-2000

20. 20
Page No:94–IS 456-2000

21. 21
Page No:42–IS 456-2000

22. 22
Clause26.53.1- Page No:48–IS 456-2000

23. 23

24. Functions of longitudinal reinforcementFunctions of longitudinal reinforcement
► To share the vertical compressive load, thereby reducing the overall
size of the column.
► To resist tensile stresses caused in the column due to (i) eccentric
load (ii) Moment (iii) Transverse load.
► To prevent sudden brittle failure of the column.
► To impart certain ductility to the column.
► To reduce the effects of creep and shrinkage due to sustained loading..
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25. 25
Clause 26.53.3.2Page No:49–IS 456-2000

26. 26
Clause 26.5.3.2 Page No:49–IS 456-2000

27. 27
Clause 26.5.3.2 Page No:49–IS 456-2000

28. Functions of Transverse reinforcementFunctions of Transverse reinforcement
► To prevent longitudinal buckling of longitudinal reinforcement.
► To resist diagonal tension caused due to transverse shear due to
moment/transverse load.
► To hold the longitudinal reinforcement in position at the time of
concreting.
► To confine the concrete, thereby preventing its longitudinal splitting.
► To impart ductility to the column.
► To prevent sudden brittle failure of the columns.
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29. 29
Clause 26.5.3.2 Page No:49–IS 456-2000
Cover to reinforcementCover to reinforcement
For a longitudinal reinforcing bar in a column, the nominal cover shall not
be less than 40mm, nor less than the diameter of such bar.
In the case of columns of minimum dimension of 200mm or under, whose
reinforcing bars does not exceed 12mm, a cover of 25mm may be used.
Clause 26.4.2.1 Page No:49–IS 456-2000

30. 30
SP 34- 1987 Page No:88

31. Assumptions in Limit State of Collapse -CompressionAssumptions in Limit State of Collapse -Compression
31
Clause 38.1 Page No:69–IS 456-2000

32. 32
Clause 38.1 Page No:69–IS 456-2000

33. 33
Clause 38.1 Page No:69–IS 456-2000

34. 34

35. 35

36. 36

37. 37
Detailing of columnsDetailing of columns

38. 38
a) Termination of column bars inside slab
b) Fixed end joint in a column

39. 39
c) Typical detail of beam column junction at external column

40. 40

41. 41

42. 42

43. 43

44. 44

45. 45