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For their calculation and the subsequent construction of the diagrams, the following rule of signs is adopted:
The internal transverse force Q is assumed to be positive (ie, Q> 0) if it tends to rotate the cut off part of the beam clockwise.
In the case of plane transverse bending, an internal transverse force Q and an internal bending moment M arise in the beam sections.
When compiling the equilibrium equations for the cut off parts of the beam, the rule of signs for external loads (for example, the concentrated force F) is defined similarly. Video by subject (contains English subtitles)
In other words, external forces and distributed loads striving to rotate the cut off part of the beam relative to the cross-section in question along the clockwise direction are considered positive, and vice versa.
The internal bending moment M is assumed to be positive (that is, M> 0) if it tends to compress the upper layers of the cut off part of the beam in the section under consideration.
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For external concentrated moments and moments of forces, the rule of signs is analogous, i.e. Positive external moments, concentrated forces and distributed loads compressing the upper layers of the beam are considered positive.