--- title: "Causes and crack patterns of out-of-plane failure" subject: "Concrete Technology and Masonry Structure" chapter: 9 key: "semester_5/concrete_technology_and_masonry_structure:out-of-plane-failure-causes" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/out-of-plane-failure-causes site: BCT Engineering --- # Causes and crack patterns of out-of-plane failure Concrete Technology and Masonry Structure · Chapter 9: Masonry structures under lateral loads - Asked in: 2080 Baisakh, 5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/out-of-plane-failure-causes - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-9 ## Question What are the causes of out-of-plane failure of masonry? Give any two examples of effects (crack pattern) seen in masonry structure due to this type of failure. ## Short answer Causes: low flexural strength, high slenderness, missing lateral support or ties, heavy roofs, poor construction. Two crack patterns: horizontal crack at mid-height from vertical bending, and vertical corner separation crack when bending horizontally between cross walls. ## Answer #### Causes of out-of-plane failure Out-of-plane failure occurs when the lateral (inertia) force acts perpendicular to the wall plane and the wall bends. The main causes are: 1. **Low tensile and flexural strength** of masonry and weak mortar bond. 2. **High slenderness:** large height-to-thickness or length-to-thickness ratio. 3. **No lateral support:** missing or weak connection to cross walls, floors and roof; flexible roof not acting as diaphragm. 4. **No bands or ties** at floor and roof; weak connection at wall corners. 5. **Large openings** and long unsupported spans, parapets and gables unbraced. 6. **Heavy roof or floor** that increases inertia force; poor construction (poor bond, thin walls). 7. **Cantilever or free-standing parts** (parapets, boundary walls). #### Two typical crack patterns 1. **Horizontal crack at mid-height (or base) of the wall:** the wall bends vertically between floor and roof supports and breaks like a beam; parapets overturn with a horizontal crack at the base. 2. **Vertical crack at wall corners or junctions (corner separation):** the wall bends horizontally between cross walls, and the two walls separate and fall outwards. In walls supported on four sides diagonal cracks from corners to the centre appear. ```text Horizontal crack Corner crack | | | | |~~~~| <- mid-height -+ +- | | crack ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a principal cause of out-of-plane failure? A. Low tensile strength together with high slenderness B. High compressive strength C. Thick walls D. Many cross walls Answer: A. Slender walls bend easily. (Key point: Main cause) 2. What lack increases failure risk? A. Rigid diaphragm B. Missing connection to cross walls, floors and roof C. Bands D. Small openings Answer: B. No lateral supports leave the wall free. (Key point: Support) 3. What produces a horizontal mid-height crack? A. Horizontal bending between cross walls B. Diagonal shear C. Wall bending vertically between floor and roof like a beam D. Settlement Answer: C. One-way vertical span. (Key point: Mid-height crack) 4. What produces a vertical crack at corners? A. Vertical beam action B. Rocking C. Toe crushing D. Wall bending horizontally between cross walls and separating Answer: D. Poor corner connections. 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