In 'plastic analysis', the 'plastic hinge' in a steel beam forms when:
Choose the correct answer
First yield occurs at extreme fibre (elastic limit)
Entire cross-section yields — moment = Mp = Zp×fy (section cannot carry more moment, rotates freely)
Member buckles laterally
Shear yield occurs at neutral axis
Correct Answer
B. Entire cross-section yields — moment = Mp = Zp×fy (section cannot carry more moment, rotates freely)
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Plastic hinge: when extreme fibre stress reaches fy, the elastic moment capacity Me = Ze×fy is reached. With further loading, yielding progresses inward until the entire section yields → plastic moment Mp = Zp×fy (Zp = plastic section modulus). At Mp, the section can rotate freely (hinge) with constant moment Mp. Shape factor = Mp/Me = Zp/Ze (= 1.5 for rectangle, 1.12 for I-section).
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