Which collapse pattern occurs when one outer wall fails while the opposite wall remains intact, forming a triangular void underneath?

• A. Lean-to collapse
• B. A-frame collapse
• C. Pancake collapse
• D. Cantilever collapse

Answer: (A)

Lean-to collapse happens when the wall on one side fails while the opposite wall stays upright. With the roof and floor bearing toward the remaining wall, the structure tilts, creating a triangular void beneath the leaning portion. This pattern is identified by the asymmetrical failure: one outer wall collapses and the other remains, directing the collapse toward that standing wall.

This differs from an A-frame collapse, where the roof or framing folds inward toward the center, typically forming a more symmetrical, inverted-triangle shape. Pancake collapse involves floors giving way in a stacked, vertical failure, not a single leaning wall. Cantilever collapse features a supported end and a sagging, extending portion rather than a triangular void formed by an opposite intact wall.