Differentiate between the ultimate limit state and serviceability limit state in bridge design.

• A. ULS ensures safety against collapse; SLS ensures usability, comfort, durability, and crack control during service life.
• B. ULS addresses cosmetic aspects; SLS addresses only aesthetics.
• C. ULS is for serviceability; SLS is for ultimate.
• D. ULS and SLS are identical.

Answer: (A)

The main idea is that bridge design separates safety under extreme conditions from everyday performance. Ultimate limit state checks ensure the structure has enough strength and stability to avoid collapse or instability under extreme loads (maximum live load, wind, earthquake, or during construction), typically with appropriate safety factors. Serviceability limit state checks focus on how the bridge behaves during regular use: it should be usable, comfortable, and durable, with acceptable deflections, crack widths, and vibrations, and tolerable effects on durability and aesthetics over the service life. This means controlling crack widths in concrete to limit corrosion, keeping deflections within service criteria like L/250 to L/400, and ensuring vibrations don’t cause discomfort or annoyance.

So the option that pairs ULS with safety against collapse and SLS with usability, comfort, durability, and crack control during service life best captures the intended distinction. The other statements mischaracterize the roles (cosmetic focus, reversed roles, or claiming they are identical), which does not reflect how limit states guide design.