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All questions of Bending & Shear Stress in Beams for Mechanical Engineering Exam

A beam with a square section of 80 mm x 80 mm is simply supported at its ends. A load W is applied at the centre of the beam. If the maximum shear stress developed in the beam section is 6 N/mm2. What is the magnitude of W?
  • a)
    2.56 kN
  • b)
    25.6 kN
  • c)
    -51.2kN
  • d)
    5.12 kN
Correct answer is option 'C'. Can you explain this answer?

Given data:
Dimensions of the square section of the beam = 80 mm x 80 mm
Maximum shear stress developed in the beam section = 6 N/mm2
We need to find the magnitude of the load W.

Formula used:
Maximum shear stress (τmax) = (4/3) x (Shear force (V) / Area of cross-section (A))

Calculation:
Area of cross-section (A) = 80 mm x 80 mm = 6400 mm2
Shear force (V) at the center of the beam = W/2
Substituting the given values in the formula, we get:
6 = (4/3) x (W/2) / 6400
W/2 = 6 x 6400 x 3 / 4
W/2 = 72,000
W = 2 x 72,000
W = -144,000 N
Therefore, the magnitude of the load W is -51.2 kN (Option C).

Explanation:
The given beam is simply supported at its ends and has a square cross-section of 80 mm x 80 mm. The load W is applied at the center of the beam. The maximum shear stress developed in the beam section is 6 N/mm2. To find the magnitude of the load W, we use the formula for maximum shear stress and substitute the given values. On solving the equation, we get the magnitude of the load W as -51.2 kN. The negative sign indicates that the load is acting downwards.

A beam of rectangular cross-section is 100 mm wide and 200 mm deep. If the section is subjected to a shear force of 20 kN, the maximum shear stress is
  • a)
    1 N/mm2
  • b)
    1.125 N/mm2
  • c)
    1.33 N/mm2
  • d)
    1.5 N/mm2
Correct answer is option 'D'. Can you explain this answer?

Ankit Joshi answered
The maximum shear stress in a rectangular beam occurs at NA and is given by,

∴ 
= 1.5 N/mm2  

Hence option (D)  is correct

For studying more on Bending and Shear stress click on the link given below:

A MS beam is subjected to a bending moment, such that a stress of 100 MPa is developed in a layer at a distance of 100 mm from the neutral layer. If E= 200 GPa, what is the radius of curvature of the beam?: .
  • a)
    400 m
  • b)
    200 m
  • c)
    100 m .
  • d)
    50 m
Correct answer is option 'B'. Can you explain this answer?

To find the radius of curvature of the beam, we can use the formula for the stress developed in a layer of a beam under bending:

σ = (E * r) / c

Where:
- σ is the stress developed in the layer
- E is the modulus of elasticity of the material
- r is the radius of curvature of the beam
- c is the distance from the neutral axis to the layer

Given that the stress is 100 MPa, the distance from the neutral layer is 100 mm, and the modulus of elasticity is 200 GPa, we can rearrange the formula to solve for the radius of curvature:

r = (σ * c) / E

Substituting the given values, we have:

r = (100 MPa * 100 mm) / (200 GPa)

Converting the units to Pascal and meters:

r = (100 * 10^6 Pa * 0.1 m) / (200 * 10^9 Pa)

Simplifying:

r = 0.05 m

Therefore, the radius of curvature of the beam is 0.05 m, which is equivalent to 200 m.

A cantilever of uniform strength σ, having rectangular section of constant breadth b but variable depth d is subjected to a UDL throughout its length. If the depth of the section is 150 mm at the fixed end, then what is the depth of the middle of the length of cantilever
  • a)
    150 mm
  • b)
    100 mm
  • c)
    75 mm
  • d)
    125 mm
Correct answer is option 'C'. Can you explain this answer?

Aditi Sarkar answered
A cantilever of uniform strength is a beam that is fixed at one end and extends horizontally, with a cross-sectional area that remains constant throughout its length. This means that the beam has the same strength and stiffness at every point along its length, making it ideal for supporting loads that are distributed evenly across its surface.

The strength of a cantilever beam is determined by its cross-sectional area, material properties, and length. The longer the beam, the greater the bending moment that it can withstand before it fails. The material properties of the beam, such as its modulus of elasticity and yield strength, also play a significant role in determining its strength.

A cantilever beam of uniform strength is often used in construction applications, such as in the design of bridges, buildings, and other structures. These beams can be made from a variety of materials, including steel, concrete, and wood, depending on the specific requirements of the project.

In summary, a cantilever of uniform strength is a strong and stable beam that can support loads evenly across its surface, making it a popular choice for many construction applications.

The modular ratio of the materials used in the flitched beam is 10 and the ratio of the allowable stresses is also 10. Four different sections of the beam are: shown in the given figures. The material shown hatched has larger modulus of elasticity and allowable stress than the rest.
Which one of the following statements is true for the beam under consideration ?
  • a)
    All the given sections would support the same magnitude of load.
  • b)
    Sections’ll, III and IV would support equal loads which is more than what section I would support
  • c)
    Sections I and II would support equal loads which is more than what section III and IV would support.
  • d)
    Section II would support greatest load.
Correct answer is option 'D'. Can you explain this answer?

In the flitched beams there are two principles to be followed in analysis,
(i) strain diagram is linear
(ii) The radius of curvature of both materials is same at all the points.
  • Allowable stress in weak material = f
  • Allowable stress in strong material = 10 f
  • Modulus of elasticity of weak material = E
  • Modulus of elasticity of strong material = 10E
  • Maximum strain in weak material = f/E
  • Maximum strain in strong material =10f/10E = f/E
  • The hatched material can be replaced by equivalent weak material when its width is multiplied by modular ratio. So the equivalent sections are:



It is very clear that section modulus of section-ll is more than that of section-Ill as more area is close to the neutral axis in section-lI. Section-IV is unequal and its section modulus in tension and compression will be different. Since the strain is (f/E) at topmost and at bottom most fibre it is less than f/E.
So its capacity will be less than that of section-ll. Comparing section-l and section-ll, more area is away from the neutral axis in section-ll, so the moment capacity of section-ll is more than that of section-l. Thus section-ll has higher capacity.

A l-section is subjected to transverse shear force. At which layer maximum shear stress is developed?
  • a)
    Neutral axis
  • b)
    At top edge of flange
  • c)
    At bottom edge of fiange
  • d)
    None of these
Correct answer is option 'A'. Can you explain this answer?

Bijoy Mehra answered
Introduction:
In structural engineering, an L-section is a type of beam cross-section that resembles the shape of the letter 'L'. When an L-section beam is subjected to a transverse shear force, the shear stress distribution across the section can be analyzed. The question asks about the layer in the L-section where the maximum shear stress is developed.

Understanding Shear Stress:
Shear stress is a measure of the force per unit area acting parallel to the surface of a material. In the case of an L-section beam, the shear stress distribution can be visualized as a shear flow across the section.

Shear Stress Distribution:
When a transverse shear force is applied to an L-section beam, the shear stress distribution is triangular in shape. The shear stress is maximum at the neutral axis of the section and decreases linearly towards the top and bottom edges of the flange.

Explanation:
The neutral axis is the axis in the L-section beam where there is no tension or compression. It is the location where the bending stress is zero. In an L-section beam, the neutral axis is located at the centroid of the section.

Since shear stress is directly related to the bending stress, the shear stress distribution follows a similar pattern as the bending stress distribution. Therefore, the maximum shear stress is developed at the neutral axis of the L-section beam.

At the top edge of the flange, the shear stress is lower than at the neutral axis because the distance from the neutral axis is greater. Similarly, at the bottom edge of the flange, the shear stress is also lower than at the neutral axis due to the larger distance from the neutral axis.

Conclusion:
In an L-section beam subjected to a transverse shear force, the maximum shear stress is developed at the neutral axis of the section. This is because the neutral axis is the location where the bending stress is zero, and shear stress distribution follows a similar pattern as the bending stress distribution.

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