A Tutorials on Shear and Bending Moment Diagram? How To Make Shear and Bending Moment Diagram

In order to plot the shear force and bending moment diagrams it is necessary to adopt a sign convention for these responses. A shear force is considered to be positive if it produces a clockwise moment about a point in the free body on which it acts.

A negative shear force produces a counterclockwise moment about the point. The bending moment is taken as positive if it causes compression in the upper fibers of the beam and tension in the lower fiber. In other words, sagging moment is positive and hogging moment is negative.

The construction of these diagrams is explained with an example given in Figure 2.4.

The section at E of the beam is in equilibrium under the action of applied loads and internal forces acting at E as shown in Figure 2.5. There must be an internal vertical force and internal bending moment to maintain equilibrium at Section E.

The vertical force or the moment can be obtained as the algebraic sum of all forces or the algebraic sum of the moment of all forces that lie on either side of Section E.

The shear on a cross-section an infinitesimal distance to the right of pointAisC55 k and, therefore, the shear diagram rises abruptly from 0 to C55 at this point. In the portion AC, since there is no additional load, the shear remainsC55 on any cross-section throughout this interval, and the diagram is a horizontal as shown in Figure 2.4. 

An infinitesimal distance to the left of C the shear is C55, but an infinitesimal distance to the right of this point the 30 k load has caused the shear to be reduced to C25. 

Therefore, at point C there is an abrupt change in the shear force from C55 to C25. In the same manner, the shear force diagram for the portion CD of the beam remains a rectangle. In the portion DE, the shear on any cross-section a distance x from point D is 
               S = 55 − 30 − 4x D 25 − 4x
which indicates that the shear diagram in this portion is a straight line decreasing from an ordinate of C25 at D to C1 at E. 

The remainder of the shear force diagram can easily be verified in the same way. It should be noted that, in effect, a concentrated load is assumed to be applied at a point and, hence, at such a point the ordinate to the shear diagram changes abruptly by an amount equal to the load.

In the portion AC, the bending moment at a cross-section a distance x from point A isM D 55x. Therefore, the bending moment diagram starts at 0 at A and increases along a straight line to an ordinate of C165 k-ft at point C. 

In the portion CD, the bending moment at any point a distance x from C is M D 55.x C 3/ − 30x. Hence, the bending moment diagram in this portion is a straight line increasing from 165 at C to 265 at D. In the portion DE, the bending moment at any point a distance x from D is M D 55.x C 7/ − 30.X C 4/ − 4x2=2. 

Hence, the bending moment diagram in this portion is a curve with an ordinate of 265 at D and 343 at E. In an analogous manner, the remainder of the bending moment diagram can be easily constructed.

Bending moment and shear force diagrams for beamswith simple boundary conditions and subject to some simple loading are given in Figure 2.6.

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