--- title: "Seismic behaviour of unreinforced and reinforced masonry" subject: "Design of Timber and Masonry Structures" chapter: 7 key: "semester_5/design_of_timber_and_masonry_structures:seismic-behavior-urm-versus-reinforced" source: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/seismic-behavior-urm-versus-reinforced site: BCT Engineering --- # Seismic behaviour of unreinforced and reinforced masonry Design of Timber and Masonry Structures · Chapter 7: Seismic Design and Strengthening of Masonry Buildings - Asked in: year not given, 6 marks - Video: [Earthquakes and Unreinforced Masonry (URM) Buildings](https://www.youtube.com/watch?v=9fFw8EaYdbM) — Portland Bureau of Emergency Management - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/seismic-behavior-urm-versus-reinforced - Whole chapter: https://www.bctengineer.com/md/semester_5/design_of_timber_and_masonry_structures/chapter-7 ## Question Explain the seismic behaviour of unreinforced masonry (URM) buildings. How does reinforced masonry perform better? Differentiate between them. ## Short answer URM buildings are heavy and brittle, with low tensile strength, leading to brittle shear or out-of-plane failure and low ductility (R≈1.5). Reinforced masonry uses steel bars to carry tension, providing box action, ductile flexural failure, and higher ductility (R≈3). ## Answer #### Seismic behaviour of unreinforced masonry - **Heavy and brittle**: the mass attracts large inertia force ($F = ma$) while the masonry has low tensile and shear strength, so cracks form early. - **Load path**: seismic forces from the roof pass through the floor diaphragm to the walls parallel to the force (shear walls); walls normal to the shaking bend out of plane. - **Typical damage**: diagonal cracks in piers, corner separation, out-of-plane collapse of walls and gables, and delamination of wall leaves. - **Weak connections** between walls and floors, and no continuous ties, lead to collapse of walls one by one rather than box action. - URM has very small ductility; the response reduction factor $R$ used in design is small (about 1.5). #### Seismic behaviour of reinforced masonry - Vertical bars (in grouted cores) and horizontal bars (in bond beams) carry the tension caused by bending and shear, giving good strength and ductility. - Walls act as a box, with continuous bond beams tying walls and floors, and out-of-plane strength is higher. - Cracks are fine and distributed; energy is dissipated by yielding of steel, and the wall retains vertical load capacity after cracking. - Because of its ductility, a larger $R$ (about 3) is allowed, so the design force is smaller. #### Comparison | Point | Unreinforced masonry | Reinforced masonry | |---|---|---| | Tensile strength | Very low | Provided by steel | | Failure | Brittle shear / out-of-plane | Ductile flexure (if designed) | | Energy dissipation | Poor | Good | | Ductility | About 1–1.5 | Moderate to high | | Design $R$ | Low | Higher | | Post-cracking capacity | Drops quickly | Retained | | Cost | Lower | Higher (steel, grout, labour) | | Height permitted | Low (1–3 storeys) | More storeys | URM can be made safer by confining with RC bands, tie columns and vertical bars at corners. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does URM suffer severe seismic damage? A. It is light and flexible B. It is heavy, so it attracts large inertia force, but is brittle with low tensile and shear strength C. It has high ductility D. It has continuous steel ties Answer: B. Large mass gives large force (F = ma) while strength in tension and shear is low, so cracks form early. (Key point: URM weakness) 2. How is seismic force resisted in a masonry building? A. All walls resist equally in bending B. Only foundation resists C. Force goes through the roof directly to the ground D. Roof force passes via the diaphragm to walls parallel to shaking (shear walls), while walls normal to it bend out of plane Answer: D. Parallel walls act as shear walls; normal walls are loaded out of plane. (Key point: Load path) 3. How does reinforced masonry perform better? A. Steel carries tension, bond beams give box action, ductile energy dissipation and retained capacity B. It is lighter and cheaper C. It has no cracks at all D. It needs no connection to floors Answer: A. Vertical and horizontal bars give strength and ductility, with fine distributed cracks and retained vertical capacity. (Key point: Reinforced masonry advantage) 4. How do the design R values compare? A. URM about 3 and reinforced about 1.5 B. Both are the same C. URM about 1.5 and reinforced about 3, giving smaller design force for reinforced D. URM has the larger R Answer: C. Greater ductility permits a larger R and so a smaller design force for reinforced masonry. (Key point: Response reduction factor) ## Related in Design of Timber and Masonry Structures - [Ductile behaviour of reinforced and unreinforced masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aductility-reinforced-unreinforced-masonry) — Design of Timber and Masonry Structures, chapter 6 - [Seismic strengthening techniques for masonry buildings](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Astrengthening-retrofitting-masonry-buildings) — Design of Timber and Masonry Structures, chapter 7 - [Seismic design principles for masonry buildings](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aseismic-design-principles-masonry) — Design of Timber and Masonry Structures, chapter 7 - [Seismic bands and vertical reinforcement in masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aseismic-bands-and-vertical-reinforcement) — Design of Timber and Masonry Structures, chapter 7 - [In-plane and out-of-plane behaviour of masonry walls](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Ain-plane-out-of-plane-behavior) — Design of Timber and Masonry Structures, chapter 6 - [Typical damage patterns in masonry buildings](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Adamage-patterns-masonry-lateral-loads) — Design of Timber and Masonry Structures, chapter 6 ## Related in other subjects - [Reinforced and unreinforced masonry; rat-trap bond](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Areinforced-unreinforced-rat-trap) — Concrete Technology and Masonry Structure, chapter 7 - [Improving seismic performance of masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aseismic-performance-improvement) — Concrete Technology and Masonry Structure, chapter 9 - [In-plane and out-of-plane behaviour of masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ain-plane-out-of-plane-behaviour) — Concrete Technology and Masonry Structure, chapter 9, asked 9× - [In-plane, out-of-plane; ductile behaviour](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ain-out-plane-ductile-behaviour) — Concrete Technology and Masonry Structure, chapter 9