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# 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,

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,

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,

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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