What best describes bridge fatigue and how design details mitigate?

• A. Fatigue is caused by random environmental temperature changes.
• B. Fatigue is caused by repeated fluctuating stresses that lead to crack initiation and growth.
• C. Fatigue is caused by repeated fluctuating stresses that cause crack initiation and growth; design details reduce stress concentrations, ensure smooth transitions, and provide durable welds.
• D. Fatigue only occurs in cables, not in steel girders.

Answer: (C)

Fatigue in bridges arises from repeated fluctuations in stress due to traffic and other loading, causing cracks to initiate at sensitive features and then grow with each load cycle until failure. The best description combines both the mechanism and the practical mitigation: design details that lower stress concentrations, provide smooth transitions, and ensure durable welds all help keep the stress field in every cycle more uniform and less prone to crack initiation and fast crack growth. That’s why this answer is correct—the explanation reflects the actual fatigue process and how careful detailing and workmanship reduce the likelihood and severity of fatigue damage. Incomplete statements—such as attributing fatigue to temperature changes alone or omitting mitigation, or claiming fatigue only affects cables—don’t capture the real mechanism or its prevention in bridge design.