Section 6. Energy Principles 6.1. Strain energy of a cantilever 6.2. Potential energy of a beam with built-in edges 6.3. Principle of stationary potential energy 6.4. principle of virtual displacements 6.5. Betti’s theorem 6.6. Castigliano’s theorem 6.7. Deflection of a cantilever 6.8. Rotation from linearly distributed load 6.9. Displacement of a frame 6.10. Rayleigh-Ritz method