Internal forces and structural diagrams

What this module covers

• Explain the difference between an external load and an internal action
• Use the cut-section method to find internal forces at any point
• Identify normal force, shear force and bending moment on a free body
• Apply a consistent sign convention and read sagging and hogging correctly
• Sketch the shape of a shear force and bending moment diagram before calculating
• Draw both diagrams for point loads, UDLs and overhangs
• Use the relationships between load, shear and moment
• Find the maximum moment by locating zero shear
• Identify points of contraflexure and say why they matter

Lessons

1. 3.1

   Cutting the member open

   How internal forces arise, and how to find them at any section.

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2. 3.2

   Sign conventions, sagging and hogging

   Getting signs right, and knowing which face of the beam is in tension.

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3. 3.3

   Building the diagrams

   Predict the shape, then calculate the values.

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4. 3.4

   Relationships, overhangs and contraflexure

   The calculus behind the shapes, and what happens when a beam hangs over its support.

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