Compression Members Mechanical Engineering Notes | EduRev

Mechanical Engineering : Compression Members Mechanical Engineering Notes | EduRev

 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: 
 
 
• define effective length, pedestal, column and wall, 
 
• classify the columns based on types of reinforcement, loadings and 
slenderness ratios, 
 
• identify and explain the functions of bracing in a braced column,  
 
• determine the minimum and maximum percentage of longitudinal 
reinforcement, 
 
• determine the minimum numbers and diameter of bars in rectangular and 
circular columns, 
 
• determine the longitudinal reinforcement in a pedestal, 
 
• determine the type, pitch and diameter of lateral ties of columns after 
determining the longitudinal steel, 
 
• state the assumptions in the design of compression member by limit state 
of collapse, 
 
• determine the strain distribution lines of a compression member subjected 
to axial load with or without the moments about one or both the axes, 
 
• 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: 
 
 
• define effective length, pedestal, column and wall, 
 
• classify the columns based on types of reinforcement, loadings and 
slenderness ratios, 
 
• identify and explain the functions of bracing in a braced column,  
 
• determine the minimum and maximum percentage of longitudinal 
reinforcement, 
 
• determine the minimum numbers and diameter of bars in rectangular and 
circular columns, 
 
• determine the longitudinal reinforcement in a pedestal, 
 
• determine the type, pitch and diameter of lateral ties of columns after 
determining the longitudinal steel, 
 
• state the assumptions in the design of compression member by limit state 
of collapse, 
 
• determine the strain distribution lines of a compression member subjected 
to axial load with or without the moments about one or both the axes, 
 
• 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: 
 
 
• define effective length, pedestal, column and wall, 
 
• classify the columns based on types of reinforcement, loadings and 
slenderness ratios, 
 
• identify and explain the functions of bracing in a braced column,  
 
• determine the minimum and maximum percentage of longitudinal 
reinforcement, 
 
• determine the minimum numbers and diameter of bars in rectangular and 
circular columns, 
 
• determine the longitudinal reinforcement in a pedestal, 
 
• determine the type, pitch and diameter of lateral ties of columns after 
determining the longitudinal steel, 
 
• state the assumptions in the design of compression member by limit state 
of collapse, 
 
• determine the strain distribution lines of a compression member subjected 
to axial load with or without the moments about one or both the axes, 
 
• 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, Kharagpur 
 
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