Shear Force and Bending MomentsPart 1

Example Problem

 Fig. 1

A beam is loaded and supported as shown in Fig. 1. For this beam

1. Draw complete shear force and bending moment diagrams.
2. Determine the equations for the shear force and the bending moment as functions of x.

Solution

Overall Equilibrium

 Fig. 2

We start by drawing a free-body diagram (Fig. 2) of the beam and determining the support reactions. Summing moments about the left end of the beam

 MA = 7RC - 2 [ 4 × 10 ]- 4(16) - 9(19) = 0 (1a)
gives
 RC = 45 kN (1b)
Then, summing forces in the vertical direction
 F = RA + RC - 4 × 10 - 16 - 19 = 0 (2a)
gives
 RA = 30 kN (2b)

Drawing the Shear Force Diagram

Sometimes we are not so much interested in the equations for the shear force and bending moment as we are in knowing the maximum and minimum values or the values at some particular point. In these cases, we want a quick and efficient method of generating the shear force and bending moment diagrams (graphs) so we can easily find the maximum and minimum values. That is the subject of this first part of the problem.

 Fig. 3

Concentrated Force

The 30-kN concentrated force (support reaction) at the left end of the beam causes the shear force graph to jump up (in the direction of the force) by 30 kN (the magnitude of the force) from 0 kN to 30 kN.

 Fig. 4

The downward distributed load causes the shear force graph to slope downward (in the direction of the load). Since the distributed load is constant, the slope of the shear force graph is constant (dV/dx = w = constant).

The total change in the shear force graph between points A and B is 40 kN (equal to the area under the distributed load between points A and B) from +30 kN to -10 kN.

We also need to know where the shear force becomes zero. We know that the full 4 m of the distributed load causes a change in the shear force of 40 kN. So how much of the distributed load will it take to cause a change of 30 kN (from +30 kN to 0 kN)? Since the distributed load is uniform, the area (change in shear force) is just 10 × b = 30, which gives b = 3 m. That is, the shear force graph becomes zero at x = 3 m (3 m from the beginning of the uniform distributed load).

 Fig. 5

Concentrated Force

The 16-kN concentrated force at B causes the shear force graph to jump down (in the direction of the force) by 16 kN (the magnitude of the force) from -10 kN to -26 kN.

 Fig. 6

Since there are no loads between points B and C, the shear force graph is constant (the slope dV/dx = w = 0) at -26 kN.

 Fig. 7

Concentrated Force

The 45-kN concentrated force (support reaction) at C causes the shear force graph to jump up (in the direction of the force) by 45 kN (the magnitude of the force) from -26 kN to +19 kN.

 Fig. 8

Since there are no loads between points C and D, the shear force graph is constant (the slope dV/dx = w = 0) at +19 kN.

 Fig. 9

Concentrated Force

The 19-kN concentrated force at D causes the shear force graph to jump down (in the direction of the force) by 19 kN (the magnitude of the force) from +19 kN to 0 kN.

Drawing the Bending Moment Diagram

Since there are no concentrated moments acting on this beam, the bending moment diagram (graph) will be continuous (no jumps) and it will start and end at zero.

 Fig. 10

Decreasing Shear Force

The bending moment graph starts out at zero and with a large positive slope (since the shear force starts out with a large positive value and dM/dx = V ). As the shear force decreases, so does the slope of the bending moment graph. At x = 3 m the shear force becomes zero and the bending moment is at a local maximum (dM/dx = V = 0 ) For values of x greater than 3 m (3 < x < 4 m) the shear force is negative and the bending moment decreases (dM/dx = V < 0).

The shear force graph is linear (1st order function of x ), so the bending moment graph is a parabola (2nd order function of x ).

The change in the bending moment between x = 0 m and x = 3 m is equal to the area under the shear graph between those two points. The area of the triangle is

 M = (1/2)(30 × 3) = 45 kN·m
So the value of the bending moment at x = 3 m is M = 0 + 45 = 45 kN·m. The change in the bending moment between x = 3 and x = 4 m is also equal to the area under the shear graph
 M = (1/2)(-10 × 1) = -5 kN·m
So the value of the bending moment at x = 4 m is M = 45 - 5 = 40 kN·m.

 Fig. 11

Constant Shear Force

Although the bending moment graph is continuous at x = 4 m, the jump in the shear force at x = 4 m causes the slope of the bending moment to change suddenly from dM/dx = V = -10 kN·m/m to dM/dx = -26 kN·m/m.

Since the shear force graph is constant between x = 4 m and x = 7 m, the bending moment graph has a constant slope between x = 4 m and x = 7 m (dM/dx = V = -26 kN·m/m). That is, the bending moment graph is a straight line.

The change in the bending moment between x = 4 m and x = 7 m is equal to the area under the shear graph between those two points. The area of the rectangle is just M = (-26 × 3) = -78 kN·m. So the value of the bending moment at x = 7 m is M = 40 - 78 = -38 kN·m.

 Fig. 12

Constant Shear Force

Again the bending moment graph is continuous at x = 7 m. The jump in the shear force at x = 7 m causes the slope of the bending moment to change suddenly from dM/dx = V = -26 kN·m/m to dM/dx = +19 kN·m/m.

Since the shear force graph is constant between x = 7 m and x = 9 m, the bending moment graph has a constant slope between x = 7 m and x = 9 m (dM/dx = V = +19 kN·m/m). That is, the bending moment graph is a straight line.

The change in the bending moment between x = 7 m and x = 9 m is equal to the area under the shear graph between those two points. The area of the rectangle is just M = (+19 × 2) = +38 kN·m. So the value of the bending moment at x = 7 m is M = -38 + 38 = 0 kN·m.

Determining the Shear Force and Bending Moment Equations

Sometimes we are not so much interested in the graphs of the shear force and bending moment as we are in knowing the equations. In particular, we need to integrate the equation for the bending moment to determine the shape of beam and how much the beam will bend as a result of the loads. That is the subject of the second part of this problem.

 Comments and suggestions should be sent to: email: sturges@iastate.edu phone: 515-294-6242.