STRENGTH OF MATERIALS for beginners

AE 211
STRENGTH OF MATERIALS
IWRE
FOR BIGINNERS
1st
Edition,2018

1
MOHR’S CIRCLE
Introduction
This is a graphical form of representing Transformation equations for plane stress.Named
after Christian Otto Mohr, (8 October 1835 – 2 October 1918) was a German civil engineer.
Stress Transformation Equations
If we vary θ from 0° to 360°, we will get all possible values of σx1 and τx1y1 for a given stress
state. It would be useful to represent σx1 and τx1y1 as functions of θ in graphical form. To do this,
we must re-write the transformation equations:
Eliminate θ by squaring both sides of each equation and adding the two equations together.

2
Define σavg and R
Substitute for σavg and R to get
which is the equation for a circle with centre (σavg,0) and radius R.
This circle is usually referred to as Mohr’s circle, after the German civil engineer Otto Mohr
(1835-1918). He developed the graphical technique for drawing the circle in 1882. The
construction of Mohr‟s circle is one of the few graphical techniques still used in engineering. It
provides a simple and clear picture of an otherwise complicated analysis.
Sign Convention for Mohr’s Circle
Notice that shear stress is plotted as positive downward.

3
The reason for doing this is that 2θ is then positive counterclockwise, which agrees with the
direction of 2θ used in the derivation of the tranformation equations and the direction of θ on the
stress element.
Notice that although 2θ appears in Mohr‟s circle, θ appears on the stress element.
PROCEDURE FOR CONSTRUCTING MOHR’S CIRCLE
1. Draw a set of coordinate axes with σx1 as abscissa (positive to the right) and τx1y1 as ordinate
(positive downward).
2. Locate the centre of the circle c at the point having coordinates σx1 = σavg and τx1y1 = 0.
3. Locate point A, representing the stress conditions on the x face of the element by plotting its
coordinates σx1 = σx and τx1y1 = τxy. Note that point A on the circle corresponds to θ = 0°.
4. Locate point B, representing the stress conditions on the y face of the element by plotting its
coordinates σx1 = σy and τx1y1 = −τxy. Note that point B on the circle corresponds to θ = 90°.
5. Draw a line from point A to point B, a diameter of the circle passing through point c. Points A
and B (representing stresses on planes at 90° to each other) are at opposite ends of the diameter
(and therefore 180° apart on the circle).
6. Using point c as the centre, draw Mohr‟s circle through points A and B. This circle has radius
R.

4
Stresses on an Inclined Element
1. On Mohr‟s circle, measure an angle 2θ counterclockwise from radius cA, because point A
corresponds to θ = 0 and hence is the reference point from which angles are measured.
2. The angle 2θ locates the point D on the circle, which has coordinates σx1 and τx1y1. Point D
represents the stresses on the x1 face of the inclined element.
3. Point E, which is diametrically opposite point D on the circle, is located at an angle 2θ + 180°
from cA (and 180° from cD). Thus point E gives the stress on the y1 face of the inclined element.
4. So, as we rotate the x1y1 axes counterclockwise by an angle θ, the point on Mohr‟s circle
corresponding to the x1 face moves counterclockwise through an angle 2θ.

6
Example:01
The state of plane stress at a point is represented by the stress element below. Draw the Mohr‟s
circle, determine the principal stresses and the maximum shear stresses, and draw the
corresponding stress elements.

7
Example: 02
The state of plane stress at a point is represented by the stress element below. Find the stresses
on an element inclined at 30° clockwise and draw the corresponding stress elements.

9
INTRODUCTION TO THE STRESS TENSOR
The normal and shear stresses on a stress element in 3D can be assembled into a 3x3 matrix
known as the STRESS TENSOR.

10
From our analyses so far, we know that for a given stress system, it is possible to find a set of
three principal stresses. We also know that if the principal stresses are acting, the shear stresses
must be zero.
In terms of the stress tensor,
In mathematical terms:
This is the process of matrix diagonalization in which the eigenvalues of the original matrix
are just the principal stresses.
Example:03
The state of plane stress at a point is represented by the stress element below. Find the principal
stresses.
We must find the eigenvalues of this matrix. Remember the general idea of eigenvalues. We are
looking for values of λ such that: Ar = λr where r is a vector, and A is a matrix.
Ar – λr = 0 or (A – λI) r = 0 where I is the identity matrix.
For this equation to be true, either r = 0 or det (A – λI) = 0.
Solving the latter equation (the “characteristic equation”) gives us the eigenvalues λ1 and λ2.

11
So, the principal stresses are –84.6 MPa and 54.6 MPa, as before. Knowing the eigenvalues, we
can find the eigenvectors. These can be used to find the angles at which the principal stresses act.
To find the eigenvectors, we substitute the eigenvalues into the equation (A – λI ) r = 0 one at a
time and solve for r.
Before finding the angles at which the principal stresses act, we can check to see if the
eigenvectors are correct.

12
To find the angles, we must calculate the unit eigenvectors
And then assemble them into a rotation matrix R so that det R = +1.
The rotation matrix has the form
So θ = 10.5°, as we found earlier for one of the principal angles. Using the rotation angle of
10.5°, the matrix M (representing the original stress state of the element) can be transformed to
matrix D’ (representing the principal stress state).
So, the transformation equations, Mohr‟s circle, and eigenvectors all give the same result for the
principal stress element.

13
Finally, we can use the rotation matrix approach to find the stresses on an inclined element with
θ = -30°.
Example:04
A bar of cross section 850 mm2
is acted upon by axial tensile forces of 60 kN applied at each end
of the bar. Determine the normal and shearing stresses on a plane inclined at 30° to the direction
of loading.
SOLUTION:
the normal stress on a cross section perpendicular to the axis of the bar is
The normal stress on a plane at an angle ϴ with the direction of loading
The shearing stress on a plane at an angle q with the direction of loading
These stresses together with the axial load of 60 kN are represented in Figure below

14
Example:05
A plane element in a body is subjected to a normal stress in the x-direction of 80 MPa, as well as
a shearing stress of 30 MPa, as shown in Figure below. (a) Determine the normal and shearing
stress intensities on a plane inclined at an angle of 30° to the normal stress. (b) Determine the
maximum and minimum values of the normal stress that may exist on inclined planes and the
directions of these stresses. (c) Determine the magnitude of the maximum shearing stress that
may exist on an inclined plane.
(a) We have σx= 80 MPa and τxy = 30 MPa.The normal stress on a plane inclined at 30° to the x-
axis is
The shearing stress on a plane inclined at 30° to the x-axis is
(b) The values of the principal stresses, that is, the maximum and minimum values of the normal
stresses existing in this element,

15
The directions of the planes on which these principal stresses occur
(c) The values of the maximum and minimum shearing stresses
Example:06
Draw the Mohr‟s stress circle for direct stresses of 65MPa (tensile) and 35 MPa (compressive)
and estimate the magnitude and direction of the resultant stresses on planes making angles of 200
and 650
with the plane of the first principal stress. Determine also the normal; and tangential
stresses on these planes.
Solution

16
GRAPHICALLY
Example :07
Show that in astrained material subjected to two dimensional stress, the sum of the normal
components of stresses on any two mutually perpendicular planes is constant.

17
shown
Example:08
For the given state of stress, determine the normal and shearing stresses exerted on the oblique
face of the shaded triangular element shown. Use a method of analysis based on the equilibrium
of that element.

18
Example:09
For the given state of stress, determine (a) the principal planes, (b) the principal stresses.

19
Example :10
Two steel plates of uniform cross section 10 × 80 mm are welded together as shown. Knowing
that centric 100-kN forces are applied to the welded plates and that the in-plane shearing stress
parallel to the weld is 30 MPa, determine (a) the angle β, (b) the corresponding normal stress
perpendicular to the weld.
Using mohr‟s cycle

20
Example:11
For the given state of stress, determine (a) the orientation of the planes of maximum in-plane
shearing stress, (b) the maximum in-plane shearing stress, (c) the corresponding normal stress

21
Example:12
The grain of a wooden member forms an angle of 15° with the vertical. For the state of stress
shown, determine (a) the in-plane shearing stress parallel to the grain, (b) the normal stress
perpendicular to the grain

22
Example :13
A spherical gas container made of steel has a 5-m outer diameter and a wall thickness of 6 mm.
Knowing that the internal pressure is 350 kPa, determine the maximum normal stress and the
maximum shearing stress in the container.

23
Example :14 (English SI UNITS)
The unpressurized cylindrical storage tank shown has a -in. wall thickness and is made of steel
having a 60-ksi ultimate strength in tension. Determine the maximum height h to which it can be
filled with water if a factor of safety of 4.0 is desired. (Specific weight of water = 62.4 lb/ft3
.)
(refer test 1 2017/18 AE 211) for new concept of thicknes t.

24
BEAMS
A beam is a structural element that primarily resists loads applied laterally to the beam's
axis. Its mode of deflection is primarily by bending. The loads applied to the beam result
in reaction forces at the beam's support points. The total effect of all the forces acting on the
beam is to produce shear forces and bending moments within the beam, that in turn induce
internal stresses, strains and deflections of the beam. Beams are characterized by their
manner of support, profile (shape of cross-section), length, and their material.
Beams are traditionally descriptions of building or civil engineering structural elements, but
any structures such as automotive automobile frames, aircraft components, machine frames,
and other mechanical or structural systems contain beam structures that are designed to carry
lateral loads are analyzed in a similar fashion.
Types of beam supports
 Pinned support or Hinged support
 Roller support
 Fixed support

25
Types of beams
 Simply supported beam
 Cantilever beam
 Beam with overhang
Types of Loads
 Concentrated load
 Uniform distributed Load (UDL)
 Linearly Varying load
 Concentrated Moment

26
EVALUATION OF REACTIVE FORCES
A beam structure reaction can be calculated by determining its equilibrium such that:
Lets make beam analysis by categorizing in the following groups
1. Shear Forces and Bemding Moment
2. Stresses in beams
3. Deflection of beams
1. SHEAR FORCES AND BEMDING MOMENT
Types of beam
a) Determinate Beam
The force and moment of reactions at supports can be determined by using the 3
equilibrium equations of statics i.e. Fx= 0, Fy= 0 and M = 0
b) Indeterminate Beam
The force and moment of reactions at supports are more than the
number of equilibrium equations of statics. (The extra reactions are called redundant and
represent the amount of degrees of indeterminacy).
In order to properly design a beam, it is important to know the variation of the shear and
moment along its axis in order to find the points where these values are a maximum

27
PRINCIPLE OF MOMENTS
In the 2-D case, the magnitude of the moment is: Mo = Force x distance
Example:01
The beam shown below is supported by a pin at A and roller at B. Calculate the reactions at both
supports due to the loading.
SOLUTION

28
Example:02
Determine the reactions at support A and B for the overhanging beam subjected to the loading as
shown.
SOLUTION

29
Example :03
Determine the reactions from figure shown below.

30
Example:04
A cantilever beam is loaded as shown. Determine all reactions at support A.
SOLUTION

31
SHEAR FORCE & BENDING MOMENT
DIAGRAM
V = shear force
= the force that tends to separate the member
= balances the reaction RA
M = bending moment
= the reaction moment at a particular point (section)
= balances the moment, RA.x
From the equilibrium equations of statics

32
Shape deformation due to shear force:

33
Shape deformation due to bending moment:
Sign Convention:
 Positive shear force diagram drawn ABOVE the beam
 Positive bending moment diagram drawn BELOW the beam
Example:05
a)Calculate the shear force and bending moment for the beam subjected to a concentrated load as
shown in the figure. Then, draw the shear force diagram (SFD) and bending moment diagram
(BMD).
b)If P = 20 kN and L = 6 m, draw the SFD and BMD for the beam.
SOLUTION

36
Example:06
Calculate the shear force and bending moment for the beam subjected to a concentrated load
as shown in the figure, then draw the shear force diagram (SFD) and bending moment
diagram (BMD).
SOLUTION

38
Example:07
Calculate the shear force and bending moment for the beam subjected to an uniformly
distributed load as shown in the figure, then draw the shear force diagram (SFD) and bending
moment diagram (BMD).
SOLUTION
These results for V and M can be checked by noting that dV/dx = -w. This is correct, since
positive w acts downward. Also, notice that dM/dx = V. The maximum moments occurs when
dM/dx = V = 0.

39
Example:08
Calculate the shear force and bending moment for the beam subjected to the loads as shown in
the figure, then draw the shear force diagram (SFD) and bending moment diagram (BMD).

41
Example:09

43
RELATIONSHIP BETWEEN LOAD, SHEAR FORCE & BENDING MOMENT
When a beam is subjected to two or more concentrated or distributed load, the way to calculate
and draw the SFD and BMD may not be the same as in the previous situation.

44
REGION OF DISTRIBUTED LOAD
 We can integrate these areas between any two points to get change in shear and moment.

45
HINTS TO KNOW
 Slope of bending moment always determined by the shape of shear force lines. The
changes in slope (sagging or hogging also depends on the changes in shear force values)
 When shear force intersects BMD axis, there is a maximum moment
 When SF maximum, BM minimum and vice versa
 SFD and BMD always start and end with zero values (unless at the point where there is a
moment/couple)
 When a moment/couple acting: –Clockwise (↓) (+), Anticlockwise (↑) (-)
Example:10
SOLUTION

46
Example:11
SOLUTION

48
Example:12
SOLUTION

49
Example:13
SOLUTION

50
Example:14

52
Example:15
SOLUTION

53
Example:16(discussion)
Draw s.f and b.m diagram,given data in a figure below.

54
CONTRA POINT OF SHEAR FORCE & BENDING MOMENT
(Point of Contraflexure)
 Contra point is a place where positive shear force/bending moment shifting to the
negative region or vice-versa.
 Contra point for shear: V = 0
 Contra point for moment: M = 0
 When shear force is zero, the moment is maximum.
 Maximum shear force usually occur at the support / concentrated load.
STATICALLY DETERMINATE FRAMES
For a frame to be statically determinate, the number of unknown (reactions) must be able to
solved using the equations of equilibrium.

55
Example:17
Calculate the shear force and bending moment for the frame subjected to the loads as shown in
SOLUTION

58
SUPPLEMENTARY QUESTIONS
Question 1: A cantilever beam is subject to a uniformly distributed load of w N per meter
length. Write the equation of the shearing force and bending moment at any point x along its
length l. Also sketch the shearing force and bending moment diagrams.
Question 2: A simply supported beam which is 4m long is subjected to a uniformly distributed
vertical load of 1600 N/m. Draw the shearing force [S.F]and bending moment [B.M]diagrams.
Question 3:A simply supported beam carries a vertical load that is increasing from zero at one
end to a maximum of 8000 N/m at the other end. Draw the shearing force and bending moment
diagrams.
Question 4: A beam which is 4 m long is simply supported over a span of 2m and overhangs
both supports by a span of 1 m on either side. The right-hand overhanging portion carries a
uniformly distributed load of 80 kN/m and a concentrated load of 20 kN at the extreme end while
the left-hand overhanging portion carries a uniformly distributed load of 40 kN/m and a
concentrated load of 30 kN at the extreme end in addition to a concentrated load of 160 kN at its
mid-span. Draw to scale the S.F and B.M diagrams and find how much may be added to the load
at mid-span without increasing the maximum B.M on the beam.
Question 5: A horizontal beam AB is 8 m long and carries a total uniformly distributed load of
300 kN. The beam is supported at the end A and at a point C which is at a distant x from the
other end B. Determine the value of x if the mid-point of the beam is to be a point of inflexion
and for this arrangement draw the S.F and B.M diagrams, indicating the principal numerical
values of each.
Question 6: A girder 10 m long, carrying a uniformly distributed of w N/m, is to be supported on
two piers 6 m apart so that the greatest bending moment on the girder shall be as small as
possible. Find the distances of the piers from the ends and the maximum bending moment
Question 7 A beam is loaded as shown in figure. Find the reactions at A and B also draw S.F
and B.M and thrust diagram

59
Question 8 A simply supported beam of length 'l' is subjected to a symmetrical uniformly
varying load with zero intensity at the ends and intensity w (load per unit length) at the mid span.
What is the maximum bending moment? Ans (wl2
/12)
Question 9 A cantilever beam of 2m length supports a triangularly distributed load over its
entire length, the maximum of which is at the free end. The total load is 37.5 kN.What is the
bending moment at the fixed end? Ans (50x106
Nmm)
Question 10 A simply supported beam of length 10 m carries a uniformly varying load whose
intensity varies from a maximum value of 5 kN/m at both ends to zero at the centre of the beam.
It is desired to replace the beam with another simply supported beam which will be subjected to
the same maximum 'bending moment‟ and „shear force' as in the case of the previous one.
Determine the length and rate of loading for the second beam if it is subjected to a uniformly
distributed load over its whole length. Draw the variation of 'SF' and 'BM' in both the cases
(hints use sampled diagram below)
Question 11 Calculate the reactions at A and D for the beam shown in figure. Draw the
bending moment and shear force diagrams showing all important values.

60
HINTS
Equivalent figure below shows an overhanging beam ABCDF supported by a roller support at A
and a hinged support at D. In the figure, a load of 4 kN is applied through a bracket 0.5 m away
from the point C. Now apply equal and opposite load of 4 kN at C. This will be equivalent to a
anticlockwise couple of the value of (4 x 0.5) = 2 kNm acting at C together with a vertical
downward load of 4 kN at C. Show U.D.L. (1 kN/m) over the port AB, a point load of 2 kN
vertically downward at F, and a horizontal loadof 2 √ kN as shown.
Question 12 Construct the bending moment and shearing force diagrams for the beam
shown in the figure.
Question 13 Two bars AB and BC are connected by a frictionless hinge at B. The assembly is
supported and loaded as shown in figure below. Draw the shear force and bending moment
diagrams for the combined beam AC. clearly labelling the important values. Also indicate your
sign convention.

61
HINTS
There shall be a vertical reaction at hinge B and we can split the problem in two parts. Then the
FBD of each part is shown below
Question 14 Determine the shear force and bending moment equations and plot them for a
cantilever beam loaded with a moment MB = 40kNm and a force F= 10kN.
Question 15:
Draw the S.F and B.M of the loaded beam shown below in diagram (a) , (b),(c) and (d)
And clearly show the maximum bending moment and state its value at diagram (c) .

63
2. BENDING STRESS IN BEAMS
Stresses in Beams
Forces and couples acting on the beam cause bending (flexural stresses) and shearing stresses
on any cross section of the beam and deflection perpendicular to the longitudinal axis of the
beam. If couples are applied to the ends of the beam and no forces act on it, the bending is said
to be pure bending. If forces produce the bending, the bending is called ordinary bending.
Any deformable bar subjected to a bending moment causes the material within the bottom
portion undergo tension and the top portion undergo compression.
The region between the tension and compression lies the neutral surface, which the material do
not undergo changes in length.
Several assumptions:
 The longitudinal axis, x, which lies within the neutral surface does not experience any
change in length.
 All cross sections of the beam remain plane and perpendicular to the longitudinal axis
during the deformation.
 Any deformation of the cross section within its own plane will be neglected.

64
 The axis lying in the plane of the cross section and about which the cross section rotates
is called the neutral axis.
 The material is homogeneous, with same cross sectional area along the length and
constant Elastic Modulus (E).
 Longitudinal strain varies linearly from zero at the neutral axis.
 Hooke‟s law applies when material is homogeneous.
 Neutral axis passes through the centroid of the cross-sectional area for linear-elastic
material.
BENDING DEFORMATION

65
Resultant moment on the cross section is equal to the moment produced by the linear normal
stress distribution about the neutral axis.
REMEMBER !! Also, moment may be positive or negative (sagging or hogging moment)

66
Example:01
Determine the maximum bending stress in beam AB as shown in the figure and draw the
stress distribution over the cross section at this location.

67
Example:02

68
Example:03

70
Example:04

73
Example:05
Determine the maximum compressive and tensile stress in beam AB as shown in the figure
and draw the stress distribution over the cross section at this location.

76
SHEAR IN STRAIGHT MEMBERS
 When a shear V is applied, non-uniform shear-strain distribution over the cross section
will cause the cross section to warp.
 The relationship between moment and shear is given as V= dM/ dx
The shear formula is used to find the transverse shear stress on the beam‟s cross-sectional area.

77
SHEAR STRESSES IN BEAMS
For rectangular cross section, shear stress varies parabolically with depth and maximum shear
stress is along the neutral axis.

78
Example:06
Determine the maximum shear stress in beam AB at 1 m from A and draw the stress
distribution over the cross section at every 25 mm interval.

81
Example:07
Determine the shear stress distribution over the cross section at A, B, C and D as shown in the
figure. Given V = 25 kN and I = 4008 cm4
.

83
Example:08
Determine the shear stress distribution over the cross section at A, B, C, D, E and F as shown
in the figure. Given V = 25 kN.

85
3. DEFLECTION OF BEAMS
INTRODUCTION
Deflection is a result from the load action to the beam (self weight, service load etc.)
If the deflection value is too large, the beam will bend and then fail. Therefore it is vital that
deflection must be limited within the allowable values as stipulated in the Standards
•The theory and background of deflection comes from ‘curvature’

86
CURVATURE
The deflection diagram of the longitudinal axis that passes through the centroid of each cross
sectional area of the beam is called curvature or elastic curve, which is characterized by the
deflection and slope along the curve
ELASTIC CURVATURE
• Moment-curvature relationship:
 Sign convention:

88
Several methods to compute deflections in beam
 Double integration method (without the use of singularity functions)
 Macaulay‟s Method (with the use of singularity functions)
 Moment area method
 Method of superposition
 Conjugate beam method
 Castigliano‟s theorem
 Work/Energy methods
Assumptions in Simple Bending Theory
 Beams are initially straight
 The material is homogenous and isotropic i.e. it has a uniform composition and its
mechanical properties are the same in all directions
 The stress-strain relationship is linear and elastic
 Young‟s Modulus is the same in tension as in compression
 Sections are symmetrical about the plane of bending
 Sections which are plane before bending remain plane after bending
Non-Uniform Bending
 In the case of non-uniform bending of a beam, where bending moment varies from
section to section, there will be shear force at each cross section which will induce
shearing stresses
 Also these shearing stresses cause warping (or out-of plane distortion) of the cross
section so that plane cross sections do not remain plane even after bending.

89
SLOPE & DEFLECTION BY DOUBLE INTEGRATION
BOUNDARY CONDITIONS
 The integration constants can be determined by imposing the boundary conditions, or
continuity condition at specific locations
 3 beam cases are considered:

91
SIGN CONVENTIONS
Example: 01 (Double Integration)
Determine the mid-span deflection of beam shown below. Given E = 20 kN/mm2 and
I = 1600 x 106 mm4
.
SOLUTION

94
MAC CAULAY METHOD
This is a simplified method based on the double integration concept. In this method, only ONE
section will be considered which is at the last loading type.

95
USE OF CONTINUOUS FUNCTIONS

96
Example 02: mac caulay method
The cantilever beam shown in the figure below is subjected to a vertical load P at its end.
Determine the equation of the elastic curve. EI is constant.

97
Example: 03 (Mac Caulay Method)
Determine the mid-span deflection of beam shown below. Given E = 20 kN/mm2 and
I = 1600 x 106 mm4
.

100
Example:04
Determine the maximum deflection of the beam shown in the figure below. EI is constant

102
Example:5
Determine the slope and deflection at x = 3 m. Also, determine the location and magnitude of the
maximum deflection. Given EI = 4000 kNm2
.

105
Example:06
Find the deflection equation for the given beam. Then, determine the maximum deflection at
mid-span along span AB. Given E = 200 kN/mm2
and I = 10 x 106
mm4
.

108
MOMENT AREA METHOD
 Based on the properties of elastic curve and bending moment diagram.
 Suitable to use for determining deflection and slope at a particular point.
 Also suitable for beam with different cross-section.
 There are two important theorems used in this method
Theorem 1
The angle between the tangents at any two points on the elastic curve equals the area under the
diagram between these two points.
This equation forms the basis for the first moment-area theorem:

109
Theorem 2
The vertical deviation of the tangent at point A on the elastic curve with respect to the tangent
extended from another point B equals the moment of the area under the diagram between
these two points (A and B). This moment is computed about point, A where the vertical deviation
tA/B is to be determined.

111
Example:07
The beam is subjected to the concentrated force shown in the figure. Determine the reactions at
the supports. EI is constant.

113
Example:08
The beam shown in the figure is pin supported at A and roller at B. A point load of 60 kN is
applied at 6 m from A. Determine:
i)Deflection at B
ii)Slope at B
iii)Maximum deflection

116
Example:09
A cantilever beam of length „l‟ is subjected to a concentrated load P at a distance of l/3 from the
free end. What is the deflection of the free end of the beam? (EI is the flexural rigidity)
SOLUTION

117
Example:10(choice question)
At a certain section at a distance 'x' from one of the supports of a simply supported beam, the
intensity of loading, bending moment and shear force arc Wx, Mx and Vx respectively. If the
intensity of loading is varying continuously along the length of the beam, then the invalid
relation is:--------------------------------------------------------------------------------------(a)

118
COLUMN AND STRUTS
CONCRETE COLUMN
STEEL COLUMN

119
WHAT IS A COLUMN?
 Columns normally take compression force
 In vertical direction; if not, is called strut
 Divided into 2 categories:
CRITICAL LOAD
 A slender column pinned at both end subjected to load, P.
 The maximum axial load that a column can support when it is on the verge of buckling is
called the critical load, Pcr.
(hints, figure should be read vertically)

121
EULER THEORY (IDEAL COLUMN)
Analysis Assumptions
 The column is perfectly straight before loading
 The column is made of homogeneous material
 The load is applied through the centroid of the cross section
 The material behaves in a linear-elastic manner
 The column buckles and bends in a single plane
PROBLEMS

122
BUCKLING OF COLUMNS
IMPORTANT POINTS
 Columns are long slender members that are subjected to axial loads
 The critical load is the maximum axial load that a column can support when it is on the
verge of buckling. This loading represents a case of neutral equilibrium
 An ideal column is initially perfectly straight, made of homogenous materials, and the
load is applied through the centroid of the cross section
 A pin-connected column will buckle about the principal axis of the cross section having
the least moment of inertia
 The slenderness ratio is L/r, where r is the smallest radius of gyration of the cross
section. Buckling will occur about the axis where this ratio gives the greatest value
COLUMNS HAVING VARIOUS TYPES OF SUPPORT
•Euler is used to determine the critical load provided “L” represents the distance between the
zero-moment points.
•It is called the column‟s effective length, Le.
•A dimensionless coefficient, K (effective-length factor), is used to calculate Le. Therefore,
𝐿𝑒=𝐾𝐿
•Thus, we have:

123
WHAT IS THE VALUE OF K?
Example:01
A 6 m long concrete column having an Elastic Modulus of 200 kN/mm2 and the cross section
shown in the figure is to be used in a building. Determine the maximum allowable axial load
(Pallow) the column can support so that it does not buckle. The safety factor is taken as 2.0. Given
that:
a)Both end of the column are pinned ends
b)Both end of the column are fixed ends
c)One end is fixed and the other end is free to move

125
Example:02
A 6 m long steel column having an Elastic Modulus of 200 kN/mm2
and the cross section shown
in the figure is to be used in a building. The safety factor is taken as 2.0.
a)Determine the slenderness ratio of the column
b)Determine the critical load (Pcr)
c)Determine the maximum allowable axial load (Pallow)
Assumed both ends of the column are pinned.

127
Rankine’s Formula
We have learnt that Euler's formula gives correct results only for very long columns. But what
happens when the column is a short or the column is not very long. On the basis of results of
experiments performed by Rankine, he established an empirical formula which is applicable to
all columns whether they are short or long. The empirical formula given by Rankine is known
as Rankine‟s formula, which is given as

128
Rankine’s constant for various materials
QUIZ 1
A column of 9 m long has a cross section shown in figure. The column is pinned at both ends. If
the column is subjected to an axial load equal in value ¼ of the Euler‟s critical load for the
column. Determine the factor of safety on the Rankine‟s ultimate stress value. Take 𝑓𝑐 = 326
N/mm2
, Rankine‟s constant 𝑎 = 1 7500 , 𝐸 =200 Gpa. Properties of one RSJ area = 5205 mm2
.
𝐼 𝑥𝑥=5943.1× 104 𝑐𝑚4 , 𝐼 𝑦𝑦=857.5× 104 𝑐𝑚4 , Thickness of the web=6.7 mm.

129
QUIZ 2
A hollow cylindrical cast iron column is 4 m long, both ends being fixed. Design the column to
carry an axial load of 250 kN. Use Rankine‟s formula and adopt a factor of safety of 5. The
internal diameter may be taken as 0.8 times the external diameter. Take 𝑓𝑐 = 550 N/mm2
and
𝑎 = 1 /1600
QUIZ 3
A short length of tube having Internal diameter and external diameter are 4 cm and 5 cm
respectively, which failed in compression at a load of 250 kN. When a 1.8 m length of the same
tube was tested as a strut with fixed ends, the load failure was 160 kN. Assuming that 𝜎 𝑐 in
Rankin‟s formula is given by the first test, find the value of the constant 𝛼 in the same formula.
What will be the crippling load of this tube if it is used as a strut 2.8 m long with one end fixed
and the other hinged?
A hollow mild steel tube 6 m long 4 cm internal diameter and 6 mm thick is used as a strut with
both ends hinged. Find the crippling load and safe load taking factor of safety as 3.
Take E = 2 x 105
N / mm2
.
Example:04(discusion)
A simply supported beam of length 4m is subjected to a uniformly distributed load of 30 KN/m
over the whole span and deflects 15 mm at the centre. Determine the crippling load the beam is
used as a column with the following conditions: (i) One end fixed and another end hinged (ii)
Both the ends pin jointed.
A hollow cylindrical cast iron column is 4 m long with both ends fixed. Determine the
minimum diameter of the column if it has to carry a safe load of 250 KN with a factor of safety
of 5. Take the internal diameter as 0.8 times the external diameter. Take σC = 550 N /mm2 and.
a = 1/1600 in Rankine's formul
Example:06
Compare the ratio of the strength of solid steel column to that of the hollow steel column of the
same cross-sectional area. The internal diameter of the hollow column is 3/4th of the external
diameter. The columns have the same length and are pinned at both ends. Use Euler‟s theory.
SOLUTION
Let „d‟ be the diameter of the solid strut and D be the outer diameter of the hollow strut. As the
cross-sectional areas are same,

130
Example:07
A solid round bar of 60 mm diameter and 2.5 m long is used as a strut. Find the safe compressive
load for the strut using Euler‟s formula if (a) both ends are hinged (b) both ends are fixed. Take
E = 2 × 105
N/mm2
and factor of safety = 3.
SOLUTION

131
Example:08
What is the ratio of the strength of a solid steel column of 150 mm diameter to that of a hollow
circular steel column of the same cross-sectional area and a wall thickness of 15 mm? The two
columns have the same length and similar end conditions.
SOLUTION
Diameter of circular column d = 150 mm

132
Example:09
Find the Euler‟s crushing load for a hollow cylindrical cast iron column 120 mm external
diameter and 20 mm thick, if it is 4.2 m long and is hinged at both ends. Take E = 80 kN/mm2.
Compare this load with the crushing load as given by Rankine‟s formula using constants fc = 550
N/mm2 and a = 1/1600. For what length of strut does the Euler‟s formula cease to apply?
SOLUTION
External diameter = 120 mm
Thickness = 20 mm
Internal diameter = 120 – 2 × 20 = 80 mm

133
Example:10
An ISLB 300 section is provided with a flange plate 200 mm × 12 mm for each flange. The
composite member is used as a column with one end fixed and the other end hinged. Calculate
the length of the column for which, crippling loads given by Rankine‟s formula and Euler‟s
formula will be the same. Take E = 210 kN/mm2, fc = 330 N/mm2, a = 1/7500
Properties of ISLB 300 section are:
Overall width = 150 mm, Overall depth = 300 mm,Thickness of flange = 9.4 mm, Thickness of
web = 6.7 mm ,Ixx = 73.329 × 106 mm4
,Iyy = 3.762 × 106 mm4
,A = 4808 mm2

134
Therefore, required actual length for one end hinged and other end fixed column for which
critical load by Rankine‟s formula and Euler‟s formula will be equal is

135
Example:11
A built up steel column, 8 m long and ends firmly fixed is having cross-section as shown in
Figure below. The properties of I-section are Area = 9300 mm2
, Ixx = 3 × 106 min4,
Iyy = 8.4 × 106 mm4. Determine:
(i) The safe axial load the column can carry with a factor of safety of 3.5 using (a) Euler‟s
Formula, (b) Rankine‟s Formula.
(ii) The length of the column for which both formulae give the same crippling load.
(iii) The length of the column for which the Euler‟s formula ceases to apply.
Take E = 2 × 105
N/mm2
, fc = 330 N/mm2
, a = 1/7500

137
Example:12
A hollow circular column of internal diameter 20 mm and external diameter 40 mm has a total
length of 5m. One end of the column is fixed and the other end is hinged. Find out the crippling
stress of the column if 𝐸 = 2 × 105 N/mm2
. Also findout the shortest length of this column for
which Euler‟s formula is valid taking the yield stress equal to 250 N/mm2

138
Example:13
A T-section 150 mm x 120 mm x 20 mm is used as a strut of 4 m long with hinged at its both
ends. Calculate the crippling load if modulus of elasticity for the material be 2.0 x 105
N/mm2
.

STRENGTH OF MATERIALS for beginners

More Related Content

What's hot (20)

Similar to STRENGTH OF MATERIALS for beginners (20)

More from musadoto (20)

Recently uploaded (20)

STRENGTH OF MATERIALS for beginners