All questions of Solid Mechanics for Civil Engineering (CE) Exam

The maximum positive bending moment in a fixed beam of span 8m and subjected to a central point load of 120 kN is (in kN-m)
  • a)
    240
  • b)
    120
  • c)
    80
  • d)
    None of these
Correct answer is option 'B'. Can you explain this answer?

Tanvi Shah answered
Bending moment at centre of the span
= (W×L)/4      ie (120 kn × 8 m )/4
= 240 kn-m.
Since the load acting in the middle of span. Hence the load will be equally distributed at both endsTherefore max bending moment in fixed beam
= 240 kn-m / 2= 120 kn-m .

A simple supported beam with rectangular cross section is subjected to a concentrated load at mid span. If the width and depth of beam are doubled, the deflection at mid span will be reduced to:-
  • a)
    6.25%
  • b)
    12.5%
  • c)
    50%
  • d)
    25%
Correct answer is option 'A'. Can you explain this answer?

Rhea Reddy answered
Max. deflection at mid span(δ)=5WL4/384EI
Hence,deflection is inversely proportional to the moment of inertia.
For rectangular section,I=bd3/12
Therefore,if width and depth both are doubled,deflection will get reduced to 6.25%.

The stress in a body due to suddenly applied load compared to when it is applied gradually is _______.
  • a)
    Same
  • b)
    Half
  • c)
    Two times
  • d)
    Four times
Correct answer is option 'C'. Can you explain this answer?

  Gradually applied load is given as σ = (F/A) ------ (F is the gradually applied load)

here, work done is given as (F δL) / 2 and strain energy stored = (σ2 / 2E) AL

Work done is equal to the strain energy stored.

(F δL) / 2 = (σ2 / 2E) AL

Therefore, σ = (F/A) ----------- (1)

Suddenly applied load is given as σ = (2F/A), here work done = (F δL)

(F δL) = (σ2 / 2E) AL

Therefore, σ = (2F/A) ------------- (2)

From (1) and (2), it can be concluded that
Hence, suddenly applied load is twice the gradually applied load.

The equivalent length of a column of length L having one end fixed and the other end free is
  • a)
    2L
  • b)
    L
  • c)
    L/2
  • d)
    L/√2
Correct answer is option 'A'. Can you explain this answer?

Effective length of columns under different end conditions are:
1. For both ends hinged: Le = L
2. One end fixed and another end free: Le = 2L
3. Both ends fixed = 
4. One end fixed and other is hinges: 

A steel bar of 5mm is heated from 5°C to 40°C and is free to expand. The bar will induce
  • a)
    No stress
  • b)
    Tensile stress
  • c)
    Shear stress
  • d)
    Compressive stress
Correct answer is option 'A'. Can you explain this answer?

Aarav Kulkarni answered
When a material undergoes a change in temperature, it either elongates or contracts depending upon whether temperature is increased or decreased of the material. If the elongation or contraction is not restricted, i. e. free then the material does not experience any stress despite the fact that it undergoes a strain. 

The shear stress on a beam section is maximum:-
  • a)
    On the extreme bottom surface fibres
  • b)
    At the neutral axis of the section
  • c)
    At the free edges
  • d)
    On the extreme top surface fibres
Correct answer is option 'B'. Can you explain this answer?

Lavanya Menon answered
Shown below is a rectangular beam in pure bending. As shown above, shear stresses vary quadratically with the distance y1 from the neutral axis. The maximum shear stress occurs at the neutral axis and is zero at both the top and bottom surface of the beam. Shear flow has the units of force per unit distance.

Two bars of different materials are of the same size and are subjected to same tensile forces. If the bars have unit elongations in the ratio of 3 : 8, then the ratio of moduli of elasticity of the two materials is:
  • a)
    7 : 4
  • b)
    3 : 8
  • c)
    8 : 3
  • d)
    64 : 9
Correct answer is option 'C'. Can you explain this answer?

Sarthak Menon answered
Solution:

Given,

Unit elongation of bar A to bar B = 3 : 8

Let,

A = Area of cross-section of the bar

L = Length of the bar

F = Tensile force applied on the bar

σA = Stress developed in bar A

σB = Stress developed in bar B

Ea = Modulus of elasticity of material A

Eb = Modulus of elasticity of material B

Formula used:

Stress = Force / Area

Unit elongation = Elongation / Original length

Stress = Modulus of elasticity × Strain

Calculation:

Let us assume that the original length of both bars is L.

Then, the elongation produced in bar A and bar B is 3L and 8L respectively.

According to the question,

Unit elongation of bar A to bar B = 3 : 8

∴ 3L / L = Ea / Eb

∴ Ea / Eb = 3 / 8

Stress developed in bar A = F / A

Stress developed in bar B = F / A

From the formula of stress,

Stress = Modulus of elasticity × Strain

∴ σA / Ea = 3L / L

∴ σA = 3Ea

Similarly,

σB / Eb = 8L / L

∴ σB = 8Eb

Now,

σA / σB = F / A × Eb / F × A / Ea

∴ σA / σB = Ea / Eb

From the above calculation,

Ea / Eb = 3 / 8

∴ σA / σB = 3 / 8

Therefore, the ratio of moduli of elasticity of the two materials is 8 : 3.

Hence, the correct option is (c) 8 : 3.

A point within the cross sectional plane of a beam through which the resultant of the external loading on the beam has to pass through to ensure pure bending without twisting of the cross-sectional of the beam is called:
  • a)
    Moment center
  • b)
    Centroid
  • c)
    Shear center
  • d)
    Elastic center
Correct answer is option 'C'. Can you explain this answer?

Bijoy Chauhan answered
Shear center is the correct answer.

Explanation:
The shear center is a point within the cross-sectional plane of a beam where the external loading on the beam must pass through in order to ensure pure bending without twisting of the cross-section. It is an important concept in structural engineering, particularly in the design of beams and other structural elements.

In order to understand the significance of the shear center, it is important to first understand the concepts of bending and twisting in a beam.

Bending refers to the deformation of a beam under load, which causes the beam to curve. This bending is typically caused by a moment applied to the beam. When a beam bends, the top and bottom surfaces of the beam experience tension and compression, respectively.

Twisting, on the other hand, refers to the deformation of a beam that causes it to rotate about its longitudinal axis. Twisting can occur when a shear force is applied to the beam, causing the top and bottom surfaces of the beam to experience shear stresses.

When a beam is subjected to both bending and twisting, it is important to ensure that the resulting deformation is pure bending without any twisting. This is particularly important in structural applications where twisting can lead to instability or failure of the structure.

The shear center is the point within the cross-sectional plane of the beam where the external loading must pass through in order to eliminate twisting and ensure pure bending. It is the point where the shear forces and bending moments due to the external loading are in equilibrium, resulting in pure bending without any twisting.

By ensuring that the external loading passes through the shear center, engineers can design beams and other structural elements that are resistant to twisting and have predictable and controllable behavior under load.

In summary, the shear center is the point within the cross-sectional plane of a beam where the external loading must pass through to ensure pure bending without twisting. It is an important concept in structural engineering and plays a critical role in the design of beams and other structural elements.

A rectangular beam is 24 cm wide and 50 cm deep, its section modulus is given by:
  • a)
    1000 cm3
  • b)
    50000 cm3
  • c)
    10000 cm3
  • d)
    100000 cm3
Correct answer is option 'C'. Can you explain this answer?

Aniket Mehta answered
Section modulus is defined as the ratio of moment of inertia of a beam about its C.G to the maximum distance of extreme x-section of the beam (Ymax).

The major and minor principal stresses at a point are 3Mpa and -3Mpa respectively. The maximum shear stress at the point is
  • a)
    Zero
  • b)
    3Mpa
  • c)
    6Mpa
  • d)
    None of these
Correct answer is option 'B'. Can you explain this answer?

Anshu Kumar answered
Explanation:

In the theory of elasticity, the stress at a point can be resolved into three principal stresses: the major principal stress (σ1), the intermediate principal stress (σ2), and the minor principal stress (σ3). The major and minor principal stresses are the maximum and minimum values of stress at the point, respectively.

Given:
Major principal stress (σ1) = 3 MPa
Minor principal stress (σ3) = -3 MPa

To find the maximum shear stress (τmax), we can use the following formula:

τmax = (σ1 - σ3) / 2

Substituting the given values:

τmax = (3 MPa - (-3 MPa)) / 2
= (3 MPa + 3 MPa) / 2
= 6 MPa / 2
= 3 MPa

Therefore, the maximum shear stress at the point is 3 MPa.

Rankine theory is applicable to the ________.
  • a)
    Short strut/column
  • b)
    Long column
  • c)
    Both short and long column
  • d)
    None of these
Correct answer is option 'C'. Can you explain this answer?

Anmol Roy answered
For short or long columns Rankine’s Formula is used.
Where P is crippling load by Rankine formula;  PC is crushing load; PE is crippling load by Euler’s formula.
For Short column: PE is very large so
 will be very small and negligible as compared to   so 
For Long column: PE is small so   will be large as compared to   
Hence the value of   can be neglected. So
Hence the crippling load by Rankine’s formula for long column is approximately equal to the crippling load by Euler’s formula.

Modulus of rigidity is defined as the ratio of ________.
  • a)
    Longitudinal stress and longitudinal strain
  • b)
    Volumetric stress and volumetric strain
  • c)
    Lateral stress and lateral strain
  • d)
    Shear stress and shear strain
Correct answer is option 'D'. Can you explain this answer?

Aniket Mehta answered
The ratio of tensile stress or compressive stress to the corresponding strain is a constant. This ratio is known as Young’s Modulus or Modulus of Elasticity and is denoted by E.
The ratio of shear stress to the corresponding shear strain within the elastic limit, is known as Modulus of Rigidity or Shear Modulus. This is denoted by C or G or N.

When the shear force diagram is a parabolic curve between two points, it indicates that there is a
  • a)
    point load at the two points
  • b)
    no loading between the two points
  • c)
    uniformly distributed load between the two points
  • d)
    uniformly varying load between the two points
Correct answer is option 'D'. Can you explain this answer?

Jay Menon answered
The general relationship between the shear force diagram, bending moment diagram and loading diagram will be:
1. Shear force diagram is 1o higher than loading diagram.
2. Bending moment diagram is 1higher than shear force diagram.
3. For the uniformly varying load on the beam, the shear force diagram is parabolic in nature.
4. For the uniformly varying load on the beam, the bending moment diagram is also parabolic but is 1o higher than shear force diagram.

Match the following beams with their bending moment diagrams:
  • a)
    A-1, b-3, c-4, d-2
  • b)
    A-3, b-4, c-2, d-1
  • c)
    A-4, b-3, c-1, d-2
  • d)
    A-1, b-2, c-3, d-4
Correct answer is option 'B'. Can you explain this answer?

Divya Mehta answered
Bending moment for two point load in simply supported beam will be trapezoidal.
Bending moment diagram for UDL in simply supported beam will be parabolic.
BMD for UDL in cantilever beam as shown in figure (ii)

A tensile test is performed on a round bar. After fracture it has been found that the diameter remains approximately same at fracture. The material under test was
  • a)
    Mild steel
  • b)
    Cast iron
  • c)
    Wrought iron
  • d)
    Copper
Correct answer is option 'B'. Can you explain this answer?

Alok Iyer answered
In Brittle materials under tension test undergoes brittle fracture i.e there failure plane is 90° to the axis of load and there is no elongation in the rod that’s why the diameter remains same before and after the load. Example: Cast Iron, concrete etc
But in case of ductile materials, material first elongate and then fail, their failure plane is 45° to the axis of the load. After failure cup-cone failure is seen. Example Mild steel, high tensile steel etc.

The correct relation between Modulus of elasticity (E), Shear Modulus (G) and Bulk modulus (K) is:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Sahil Chawla answered
Relation between Modulus of elasticity (E), Shear Modulus (G) and Bulk modulus (K) is given by:
Now, rewriting the equation as follows:
3KE + GE = 9KG
3KE – 9KG = - GE
K(3E – 9G) = - GE

Maximum Shear strain Energy theory was postulated by:
  • a)
    Tresca
  • b)
    St Venant
  • c)
    Rankine
  • d)
    Von-mises
Correct answer is option 'D'. Can you explain this answer?

1. Maximum Principal stress theory was postulated by Rankine. It is suitable for brittle materials.
2. Maximum Principal strain theory was postulated by St Venant. This theory is not accurate for brittle and ductile materials both.
3. Maximum Shear stress theory was postulated by Tresca. This theory is suitable for ductile materials. Its results are the safest.
4. Maximum shear strain energy theory was postulated by Von-mises. Its results in case of pure shear are the accurate for ductile materials.

The maximum deflection due to a uniformly distributed load w/unit length over entire span of a cantilever of length L and of flexural rigidity EI, is
  • a)
    wL3/3EI
  • b)
    wL4/3EI
  • c)
    wL4/8EI
  • d)
    wL4/12EI
Correct answer is option 'C'. Can you explain this answer?

Aarav Sharma answered
Formula Used:
The equation for the maximum deflection due to a uniformly distributed load w/unit length over the entire span of a cantilever is given by:

δmax = (5/384)wL^4/EI

where,
δmax = maximum deflection
w = uniformly distributed load
L = length of the cantilever
E = modulus of elasticity
I = moment of inertia

Solution:
Given, a cantilever of length L and of flexural rigidity EI, subjected to a uniformly distributed load w/unit length over the entire span.

Using the formula mentioned above, we can calculate the maximum deflection as:

δmax = (5/384)wL^4/EI

On solving this equation, we get:

δmax = wL^4/8EI

Therefore, the correct answer is option C, i.e., wL^4/8EI.

Conclusion:
The maximum deflection due to a uniformly distributed load w/unit length over the entire span of a cantilever of length L and of flexural rigidity EI is given by the equation δmax = wL^4/8EI. This equation can be easily derived using the basic principles of mechanics and is useful in determining the maximum deflection of cantilever beams under different loading conditions.

The Mohr’s circle given below corresponds to which one of the following stress condition.
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Manasa Sen answered
Radius of Mohr’s circle = 100 MPa
And centre of Mohr’s circle is at a distance,
from the origin. Here it is in origin.
 both the normal stress may be zero which is a pure shear case or opposite in nature which don’t exist in any of the options. So it is the pure shear case, where the radius is

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