- Practice · 6 marks
Seismic behaviour of unreinforced and reinforced masonry
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).
Explain the seismic behaviour of unreinforced masonry (URM) buildings. How does reinforced masonry perform better? Differentiate between them.
Seismic behaviour of unreinforced masonry
- Heavy and brittle: the mass attracts large inertia force () 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 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 (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 | 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.
✓ Answer checked by Claude Sonnet 5.5
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