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THERMO

Unit - 5

Shear Stresses


1. Shear stresses:

The shear force at the cross section of a beam is defined as the algebraic sum of all the unbalanced vertical forces either left or right of the section.

SF = (∑ Fy) L = (∑ Fy) R

Sign convention for shear force:

An upward force to the left of section or downward force to the right of a section will be considered as a positive shear force.

When the force is downward to the left side of section and upward on the right of a section, will be considered as a negative shear force.

2. Derivation of formula

Consider a small portion ABCD of length dx of a beam with bending moment varying from m to m=dm due to udl over its length dx

The distribution of compressive stresses due to bending moment shown in fig

Let

M = bending moment at AB

M + dM = bending moment at CD

F = shear force at AB

F+ dF = shear force at CD

Consider element strip EF at a distance y from the neutral axis as shown in fig.

Let      

σ = bending stress across AB at a distance y from neutral axis

σ + d σ = bending stress across

I = moment of inertia

By using flexural formula

On face AB, 

On face CD,

Face acting across AB     

Similarly, force acting across CD, 

Net unbalanced force on the strip 

The load unbalanced force F above neutral axis can be found out by integrating from 0 to d/2

But ∫ y x dA= first moment of area under consideration from neutral axis= Ay

F = dM/ I x Ax y

This unbalanced force is balanced by a shearing stress 𝜏 acting along the length dx and width b,

Shear stress = shear force/ shear area

But

 

Where,

S = shear force at the section under consideration

A = area above or below the layer under consideration

y = distance of centroid of area under consideration from neutral axis

b = width at the layer under consideration

I = moment of inertia of the section.

Key takeaway:

  • The shear force at the cross section of a beam is defined as the algebraic sum of all the unbalanced vertical forces either left or right of the section.
  • An upward force to the left of section or downward force to the right of a section will be considered as a positive shear force.
  • When the force is downward to the left side of section and upward on the right of a section, will be considered as a negative shear force.
  •  


    1. Shear stress distribution over a circular section:

    Consider a circular section of diameter d as shown in fig.

    Diagram

    The shear stress on a layer AB at a distance is given as,

    Width of strip AB in a circular section

    Where, r= radius of circular section

     Area of shaded strip =

    Moment of area about neutral axis =

     The moment of whole shaded area about neutral axis

    But

    Again, 

    Squaring both side

    2b db = 4(-2y) dy

    Substituting the value of y. dy in equation (i)

    We know that, when y= r. and b = 0

    The limit of integration may be changed from y to r, from b to 0 in equation (i)

    Now substituting Ay in shear stress formula

    Observations from above equation

    From Equation (iii) shear stress 𝜏 increases as y decreases

    At   y = 𝜏              𝜏 = 0  

    At y = 0                  𝜏 is maximum

    Put r= d/2

    put I = 𝛑/64 d4

    Therefore,

    2. Shear stress distribution over a rectangular section:

    Consider a beam of a rectangular section of width b and depth d as shown in fig.

    Shear stress at any layer CD of beam at a distance y from the neutral axis

    Where,

    S = shear force at the section

    A = Area of beam above y

    y = Distance of centroid of shaded area from neutral axis

    I = Moment of inertia of the section

    b = Width of section CD

    Area of shaded portion

    Distance of centroid of area from neutral axis

    Moment of inertia  

    Shear stress at any layer CD     

     

           

    From above equation we can observe that,

    𝜏 increases as y decrease

    Variation of 𝜏 is parabolic as the equation in of second degree

    When   y = d/2, 𝜏 = 0

    When   y= 0, 𝜏 is maximum

    The variation of shear stress across the depth of the beam is parabolic.

    Shear Stress

    At top and bottom, y= d/2

    At neutral axis, y= 0

    Where       

    3. Shear stress distribution over a triangle section:

    Consider a beam of triangular section ABC of base b and height h as shown in fig Shear stress on a layer DE at a distance y from neutral axis.

    Consider a section DE at distance x from apex A

    Width of the trip DE (say b)

    By similarity of triangle

     

    Area of shaded portion ADE

    Distance of centroid of area above layer DE

    Shear stress at DE

    Observation from above equation the variation of shear stress with respect to x is parabolic

    At x= 0, 

    At bottom x=h, 

    At a neutral axis x=2/3 h

    For maximum shear stress

    h-2x = 0

    x= h/2

    Maximum shear stress occurs at h/2 for triangular section

    For triangle section maximum shear stress occurs at h/2 and is on neutral axis

    4. Shear stress distribution over a T section:

    Draw shear stress distribution on a T section with flange 150 x 15 deep and flange 200x 20 wide. The section is symmetric at vertical axis the shear force applied is 110KN

    Solution:

    Given:

    T section

    Shear force S= 110 KN

    To find: Shear stress distribution

    Position of neutral axis

    A1= 15x 150= 2250 mm2

    y1 = 150/2 = 75 mm

    A2 = 200x20 = 4000mm2

    y2 = 150+ 10 = 160mm

    y = 129.4 mm

    moment of inertia

    I = Ixx1+ Ixx2

    [

    = 14.756x106 mm4

    Shear stress at extreme top and bottom is zero

    Shear stress at junction

    Area above junction A = 200x 20 = 4000mm2

    C.G of area above junction wrt.NA

    y = (170-129.4)-20/2 = 30.6 mm

    Width of junction b1= 200mm and b2 = 15mm

     

    References:

  • Timoshenko, S. and Young, D. H., “Elements of Strength of Materials”, DVNC, New York USA.
  • Kazmi, S. M. A., “Solid Mechanics” TMH, Delhi, India.
  • Hibbeler, R. C. Mechanics of Materials. 6th ed. East Rutherford, NJ: Pearson Prentice Hall, 2004
  • Crandall, S. H., N. C. Dahl, and T. J. Lardner. An Introduction to the Mechanics of Solids 2nd ed. New York, NY: McGraw Hill, 1979
  • Laboratory Manual of Testing Materials - William Kendrick Hall
  • Mechanics of Materials - Ferdinand P. Beer, E. Russel Jhonston Jr., John T. DEwolf – TMH 2002.

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