Understanding Shear Force and Bending Moment Diagrams

When analyzing the internal forces and moments within a structural element like a beam, engineers often plot diagrams to visualize how these forces and moments change along the length of the element. These diagrams are known as Shear Force Diagrams (SFD) and Bending Moment Diagrams (BMD).

Axes in Shear Force and Bending Moment Diagrams

These diagrams are plotted on a coordinate system with two main axes:

• The Horizontal Axis: This axis represents the length of the beam or structural element. It shows the different locations or points along the member from one end to the other.
• The Vertical Axis: This axis represents the magnitude of the internal force or moment at each corresponding point on the horizontal axis. For the Shear Force Diagram, the vertical axis plots the shear force. For the Bending Moment Diagram, the vertical axis plots the bending moment.

The question asks about the common axis used in plotting both SFD and BMD. Based on the description above, the horizontal axis is the one that is common to both diagrams, and it represents the location along the beam.

Analyzing the Options

Let's look at the given options in the context of what is plotted on the axes of SFD and BMD:

• Mass of the section: Mass is a property related to the material and geometry but is not plotted on the diagram axes directly representing the internal forces or moments distribution.
• Orientation of the section's normal force: Normal force (axial force) can exist, but its orientation isn't plotted on the common axis. SFD plots shear force, and BMD plots bending moment.
• Position of the section: This refers to the specific location or distance along the beam from a reference point (usually one end). This is precisely what is represented on the horizontal axis common to both SFD and BMD.
• Elasticity of the section: Elasticity is a material property (like Young's Modulus) and is not plotted on the axes of SFD or BMD, although it influences the beam's deformation.

Therefore, the common axis in plotting Shear Force Diagrams and Bending Moment Diagrams represents the position along the section of the beam.