Page 1 Module 10 Compression Members Version 2 CE IIT, Kharagpur Page 2 Module 10 Compression Members Version 2 CE IIT, Kharagpur Lesson 21 Definitions, Classifications, Guidelines and Assumptions Version 2 CE IIT, Kharagpur Page 3 Module 10 Compression Members Version 2 CE IIT, Kharagpur Lesson 21 Definitions, Classifications, Guidelines and Assumptions Version 2 CE IIT, Kharagpur Instructional Objectives: At the end of this lesson, the student should be able to: â€¢ define effective length, pedestal, column and wall, â€¢ classify the columns based on types of reinforcement, loadings and slenderness ratios, â€¢ identify and explain the functions of bracing in a braced column, â€¢ determine the minimum and maximum percentage of longitudinal reinforcement, â€¢ determine the minimum numbers and diameter of bars in rectangular and circular columns, â€¢ determine the longitudinal reinforcement in a pedestal, â€¢ determine the type, pitch and diameter of lateral ties of columns after determining the longitudinal steel, â€¢ state the assumptions in the design of compression member by limit state of collapse, â€¢ determine the strain distribution lines of a compression member subjected to axial load with or without the moments about one or both the axes, â€¢ explain the need of the minimum eccentricity to be considered in the design of compression members. 10.21.1 Introduction Compression members are structural elements primarily subjected to axial compressive forces and hence, their design is guided by considerations of strength and buckling. Figures 10.21.1a to c show their examples: pedestal, column, wall and strut. While pedestal, column and wall carry the loads along its length l in vertical direction, the strut in truss carries loads in any direction. The letters l, b and D represent the unsupported vertical length, horizontal lest lateral dimension, width and the horizontal longer lateral dimension, depth. These compression members may be made of bricks or reinforced concrete. Herein, reinforced concrete compression members are only discussed. Version 2 CE IIT, Kharagpur Page 4 Module 10 Compression Members Version 2 CE IIT, Kharagpur Lesson 21 Definitions, Classifications, Guidelines and Assumptions Version 2 CE IIT, Kharagpur Instructional Objectives: At the end of this lesson, the student should be able to: â€¢ define effective length, pedestal, column and wall, â€¢ classify the columns based on types of reinforcement, loadings and slenderness ratios, â€¢ identify and explain the functions of bracing in a braced column, â€¢ determine the minimum and maximum percentage of longitudinal reinforcement, â€¢ determine the minimum numbers and diameter of bars in rectangular and circular columns, â€¢ determine the longitudinal reinforcement in a pedestal, â€¢ determine the type, pitch and diameter of lateral ties of columns after determining the longitudinal steel, â€¢ state the assumptions in the design of compression member by limit state of collapse, â€¢ determine the strain distribution lines of a compression member subjected to axial load with or without the moments about one or both the axes, â€¢ explain the need of the minimum eccentricity to be considered in the design of compression members. 10.21.1 Introduction Compression members are structural elements primarily subjected to axial compressive forces and hence, their design is guided by considerations of strength and buckling. Figures 10.21.1a to c show their examples: pedestal, column, wall and strut. While pedestal, column and wall carry the loads along its length l in vertical direction, the strut in truss carries loads in any direction. The letters l, b and D represent the unsupported vertical length, horizontal lest lateral dimension, width and the horizontal longer lateral dimension, depth. These compression members may be made of bricks or reinforced concrete. Herein, reinforced concrete compression members are only discussed. Version 2 CE IIT, Kharagpur Version 2 CE IIT, Kharagpur Page 5 Module 10 Compression Members Version 2 CE IIT, Kharagpur Lesson 21 Definitions, Classifications, Guidelines and Assumptions Version 2 CE IIT, Kharagpur Instructional Objectives: At the end of this lesson, the student should be able to: â€¢ define effective length, pedestal, column and wall, â€¢ classify the columns based on types of reinforcement, loadings and slenderness ratios, â€¢ identify and explain the functions of bracing in a braced column, â€¢ determine the minimum and maximum percentage of longitudinal reinforcement, â€¢ determine the minimum numbers and diameter of bars in rectangular and circular columns, â€¢ determine the longitudinal reinforcement in a pedestal, â€¢ determine the type, pitch and diameter of lateral ties of columns after determining the longitudinal steel, â€¢ state the assumptions in the design of compression member by limit state of collapse, â€¢ determine the strain distribution lines of a compression member subjected to axial load with or without the moments about one or both the axes, â€¢ explain the need of the minimum eccentricity to be considered in the design of compression members. 10.21.1 Introduction Compression members are structural elements primarily subjected to axial compressive forces and hence, their design is guided by considerations of strength and buckling. Figures 10.21.1a to c show their examples: pedestal, column, wall and strut. While pedestal, column and wall carry the loads along its length l in vertical direction, the strut in truss carries loads in any direction. The letters l, b and D represent the unsupported vertical length, horizontal lest lateral dimension, width and the horizontal longer lateral dimension, depth. These compression members may be made of bricks or reinforced concrete. Herein, reinforced concrete compression members are only discussed. Version 2 CE IIT, Kharagpur Version 2 CE IIT, Kharagpur This module is intended to explain the definition of some common terminologies and to illustrate the design of compression members and other related issues. This lesson, however, explain the definitions and classifications of columns depending on different aspects. Further, the recommendations of IS 456 to be followed in the design are discussed regarding the longitudinal and lateral reinforcing bars. The assumptions made in the design of compression member by limit sate of collapse are illustrated. 10.21.2 Definitions (a) Effective length: The vertical distance between the points of inflection of the compression member in the buckled configuration in a plane is termed as effective length l e of that compression member in that plane. The effective length is different from the unsupported length l of the member, though it depends on the unsupported length and the type of end restraints. The relation between the effective and unsupported lengths of any compression member is l e = k l (10.1) where k is the ratio of effective to the unsupported lengths. Clause 25.2 of IS 456 stipulates the effective lengths of compression members (vide Annex E of IS 456). This parameter is needed in classifying and designing the compression members. (b) Pedestal: Pedestal is a vertical compression member whose effective length l e does not exceed three times of its least horizontal dimension b (cl. 26.5.3.1h, Note). The other horizontal dimension D shall not exceed four times of b (Fig.10.21.1a). (c) Column: Column is a vertical compression member whose unsupported length l shall not exceed sixty times of b (least lateral dimension), if restrained at the two ends. Further, its unsupported length of a cantilever column shall not exceed 100b 2 /D, where D is the larger lateral dimension which is also restricted up to four times of b (vide cl. 25.3 of IS 456 and Fig.10.21.1b). (d) Wall: Wall is a vertical compression member whose effective height H we to thickness t (least lateral dimension) shall not exceed 30 (cl. 32.2.3 of IS 456). The larger horizontal dimension i.e., the length of the wall L is more than 4t (Fig.10.21.1c). Version 2 CE IIT, KharagpurRead More

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