--- title: "Plastic hinge versus elastic hinge" subject: "Theory of Structures II" chapter: 5 key: "semester_5/theory_of_structures_ii:define-plastic-hinge-compare-elastic" source: https://www.bctengineer.com/q/semester_5/theory_of_structures_ii/define-plastic-hinge-compare-elastic site: BCT Engineering --- # Plastic hinge versus elastic hinge Theory of Structures II · Chapter 5: Introduction to plastic analysis - Asked in: 2074 Asoj, 4 marks - Video: [Plastic Hinges in Simply Supported Beam ( Part-3)](https://www.youtube.com/watch?v=Re_Q4w6UrKg) — EduceTube - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/theory_of_structures_ii/define-plastic-hinge-compare-elastic - Whole chapter: https://www.bctengineer.com/md/semester_5/theory_of_structures_ii/chapter-5 ## Question Define plastic hinge. Also compare plastic and elastic hinges of a structural system. ## Short answer A plastic hinge is a yielded section where moment equals Mp and rotation occurs at constant moment; an elastic hinge is a real pin carrying zero moment. A plastic hinge forms by yielding at maximum moment, transmits Mp, and locks on unloading, while an elastic hinge is a fixed zero-moment connection. ## Answer A **plastic hinge** is a section at which the bending moment has reached the plastic moment $M_p$, the whole section has yielded, and the member rotates at a constant moment $M_p$ (like a hinge that still transmits the moment $M_p$). An **elastic (ordinary) hinge** is a real pin in the structure: it cannot transmit any moment. | Point | Elastic (ordinary) hinge | Plastic hinge | |---|---|---| | Moment carried | Zero | Constant, equal to $M_p$ | | Cause | Provided by construction (a pin or a connection) | Formed by yielding of the section when $M = M_p$ | | Rotation | Free, in either direction | Only in the sense of the applied moment; locks when the moment reverses/unloads | | Position | Fixed, chosen by the designer | Located at points of maximum moment, depends on the loads | | Length | Zero (a point) | Finite zone of yielding (hinge length) | | Behaviour on unloading | Not affected | Elastic recovery; the hinge becomes a rigid section again | | Effect on the structure | Reduces the degree of indeterminacy by 1 | Also reduces the indeterminacy by 1, and the structure becomes a mechanism when enough hinges form | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which best defines a plastic hinge? A. A real pin that carries no moment B. A section that has fully yielded and rotates at constant moment Mp C. A section where shear is maximum D. A support that allows rotation freely Answer: B. A plastic hinge is a fully yielded section that rotates while still carrying Mp. (Key point: Definition of plastic hinge) 2. How does the moment carried differ between an elastic (ordinary) hinge and a plastic hinge? A. Both carry zero moment B. Elastic hinge carries Mp, plastic hinge carries zero C. Elastic hinge carries zero, plastic hinge carries Mp D. Both carry Mp Answer: C. An ordinary hinge is a pin and transmits no moment; a plastic hinge transmits a constant Mp. (Key point: Moment carried) 3. Which statement about rotation is correct? A. A plastic hinge rotates only in the sense of the applied moment and locks on reversal B. A plastic hinge rotates freely in both directions C. An elastic hinge locks when the moment reverses D. Neither can rotate Answer: A. An ordinary pin rotates freely either way, whereas a plastic hinge rotates only with the moment and recovers elastically on unloading. (Key point: Rotation direction) 4. Where is the position of a plastic hinge decided? A. By the designer at construction B. Always at mid-span C. At supports only D. By the loading, at points of maximum moment Answer: D. Plastic hinges form where moment is greatest, depending on the loads; an elastic hinge is fixed by the designer. 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