The moment diagram may be constructed in a, similar manner; one must remember that the bending moment at any section of a beam is the algebraic sum of the moments of all the external forces on either side of. the section. When calculating the bending moment, use the segment (or side) for which the arithmetic will be the simplest. A positive moment is one which tends to cause the beam to be concave on the upper side (top fibers in compression). An easy method of arriving at the correct sign in calculating bending moment is to give plus signs,-to the moments of upward forces and minus signs to the moments of downward forces. This method will give the correct sign to the algebraic sum of the moments, whether the forces to the right of the section or to the left of the section are used.

Examples of shear and moment diagrams for a few of the commoner types of loading are shown in Fig. 6-12.

Figure 6-12

For combined loading the shear and moment diagrams may be constructed by adding (algebraically) the individual shear and moment diagrams due to the individual loadings. This will give the total values for shear and moment for all parts of the beams. Each load will produce the same effect as if it had acted alone; it is unaffected by the other loads. Another method is to calculate the moments at different points on the beam and draw a smooth curve. Care must...