Which statement describes the serviceability criteria for deflection, crack width, and vibration in bridges?

• A. Deflection limits are set solely to protect the structure from excessive stresses; crack width is not important; vibration is not considered.
• B. Crack width is the only serviceability criterion; deflection and vibration are ignored.
• C. Crack width governs durability; deflection and vibration are optional.
• D. Deflection limits, crack width, and vibration all constitute serviceability criteria to maintain performance.

Answer: (D)

Serviceability in bridges is about how the structure performs under everyday use, not just whether it can resist ultimate loads. Deflection limits ensure the geometry stays within usable bounds so the bridge remains comfortable, correctly aligned, and free from operational issues like drainage problems or bearing gaps. Crack width matters for durability: narrow cracks help prevent water and chlorides from entering and initiating corrosion, extending the life of the members and reducing maintenance. Vibration limits address ride quality and fatigue: if the bridge vibrates too much, it can be uncomfortable for users and may lead to repeated dynamic stresses that shorten service life.

Because all three factors influence how well a bridge performs during its normal service, they are all considered serviceability criteria. If you ignore any one of them, the bridge could look or feel acceptable at first glance but fail in practice—excessive deflection can affect function, wide cracks can lead to durability problems, and excessive vibration can cause discomfort or fatigue over time. So the statement that deflection, crack width, and vibration are all serviceability criteria best captures the full picture of how a bridge should perform in daily use.