Design Of Columns - Axial Load 493r6n

Design of Compression -Axially by S.PraveenKumar Loaded columns

Assistant Professor Department of Civil Engineering PSG College of Technology Coimbatore [email protected]

Introduction ►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. ► 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. 2

Classification of columns

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,

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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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► 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 e x= Mx /P and ey = My /P, as shown in [Fig.(c)]. 5

► 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 6

Based on Slenderness Ratio Columns (i.e., compression ) 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..  Columns with low slenderness ratios, i.e., relatively short and stocky columns, invariably fail under ultimate loads with the material (concrete, steel)7 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. Braced columns & unbraced column In most of the cases, columns are also subjected to horizontal loads like wind, earthquake etc. If lateral s are provided at the ends of the column, the lateral loads are borne entirely by the lateral s. 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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In such cases, the effective length ratio k varies between 0.5 and 1.0

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

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Reinforcement in column ► Concrete

is strong in compression. ► However, longitudinal steel rods are always provided to assist in carrying the direct loads. ► A minimum area of longitudinal steel is provided in the column, whether it is required from load point of view or not. ► This is done to resist tensile stresses caused by some eccentricity of the vertical loads. ► There is also an upper limit of amount of reinforcement in RC columns, because higher percentage of steel may cause difficulties in placing and compacting the concrete. ► Longitudinal reinforcing bars are “tied” laterally by “ties” or “stirrups” at suitable interval so that the bars do not buckle 11

Codal Provisions(IS-4562000)

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Page No:41 & 42 –

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Page No:94–IS 456-

Page No:42–IS 456-2000

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Clause26.53.1- Page No:48–

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Functions 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. 17

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Clause 26.53.3.2Page

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Clause 26.5.3.2 Page

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Clause 26.5.3.2 Page

Functions 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. 21

Clause 26.5.3.2 Page No:49–IS 456-2000

Cover 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 Clause 26.4.2.1 Page does not exceed 12mm, cover of 25mm may No:49–ISa456-2000

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SP 34- 1987 Page No:88

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Assumptions in Limit State of Collapse -Compression

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Clause 38.1 Page No:69–

Clause 38.1 Page No:69– IS 456-2000

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Clause 38.1 Page No:69– IS 456-2000

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Detailing of columns

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a) Termination of column bars inside slab b) Fixed end t in a column

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c) Typical detail of beam column junction at external column

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