What is plastic analysis and how does it differ from elastic analysis?

Plastic analysis considers the ability of structures to redistribute moments after yielding, allowing higher load capacity than elastic analysis predicts. A plastic hinge forms when a section reaches its full plastic moment Mp (= fy * Zp). The structure collapses when sufficient hinges form to create a mechanism. Shape factor (Zp/Ze) indicates plastic strength reserve, typically 1.15 for I-sections. Plastic design is more economical but requires ductile materials.