Shear Moment Diagrams Design Sign Conventions

36 Before we derive the Shear-Moment relations, let us arbitrarily define a sign convention.

37 The sign convention adopted here, is the one commonly used for design purposes4. With reference to Fig. 12.5

Axial Force

Shear Force bending Moment

Figure 12.5: Shear and Moment Sign Conventions for Design

Load Positive along the beam's local y axis (assuming a right hand side convention), that is positive upward.

Axial: tension positive.

Flexure A positive moment is one which causes tension in the lower fibers, and compression in the upper ones. For frame members, a positive moment is one which causes tension along the inner side.

Shear A positive shear force is one which is "up" on a negative face, or "down" on a positive one. Alternatively, a pair of positive shear forces will cause clockwise rotation.

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