The Bridge Inspection Report
An engineer reviews a 40 m steel footbridge. Inspection notes: (1) a visible crack 15 cm long in a main load-bearing weld; (2) surface rust across 30% of the deck; (3) one handrail section loose; (4) the bridge carries up to 200 people at peak, though it was designed for 150. The engineer must classify each finding by risk and recommend action within a maintenance budget that covers only one major repair this quarter.
1. Rank the four findings by risk and justify the ranking using consequence and likelihood.
Read the worked answer
Risk ≈ consequence × likelihood.
- Crack in load-bearing weld - highest. A weld failure can be sudden and catastrophic (collapse), and a 15 cm crack is already propagating. High consequence, non-trivial likelihood.
- Overcrowding (200 vs 150 design) - high. Exceeds design load by 200/150 ≈ 33%, stressing the already-cracked structure. High consequence, high likelihood at peak.
- Loose handrail - moderate. Real injury risk (falls) but localised and not structural.
- Surface rust (30%) - lowest immediate risk, though it signals long-term deterioration. The reasoning: structural, sudden-failure items outrank cosmetic or localised ones.
2. The budget covers only one major repair. What should be done this quarter, and what interim measures accompany it?
Read the worked answer
Fund the weld repair / structural inspection by a certified welder - it addresses the only finding that can cause collapse. Interim measures: impose a load limit (e.g., cap simultaneous users at or below 150 via stewarding or a barrier) to remove the 33% overload while repair proceeds, and close or flag the loose handrail section (cheap, done from the same budget or minor funds). Rust treatment is deferred to next quarter with monitoring. Justification: spend on the catastrophic-risk item, and use no-cost/low-cost controls to reduce the other high risks in the meantime.
3. A colleague argues the rust should be fixed first because it is the most visible. Construct the counter-argument.
Read the worked answer
Visibility ≠ risk. Rust at 30% surface coverage is largely cosmetic/slow-progressing and rarely causes sudden failure; a weld crack can fail without warning, killing people. Prioritising by visibility would spend the limited budget on the least dangerous item while leaving a collapse risk unaddressed - the worst possible allocation. The correct principle is to rank by consequence severity and probability of failure, not by how obvious a defect looks. The counter-argument should also concede that rust must be scheduled and monitored, just not first.