Which statement best contrasts the LRFD design philosophy with ASD design in bridge engineering?

• A. LRFD uses load and resistance factors to achieve target reliability across limit states, incorporating uncertainties in loads and strengths; ASD uses fixed safety factors on strengths, applying a simpler, deterministic check without explicit probabilistic reliability targets.
• B. LRFD uses fixed safety factors on strengths; ASD uses load and resistance factors with probabilistic reliability targets.
• C. LRFD and ASD are the same approach, with different naming.
• D. LRFD ignores uncertainty; ASD explicitly considers variability.

Answer: (A)

The main concept here is how design philosophy treats uncertainty and reliability. In LRFD, loads and resistances are treated as random variables, and the design uses load factors and resistance factors that are calibrated to meet a specific target reliability across different limit states. This means the method explicitly accounts for uncertainties in both what loads can occur and how strong the structure is, aiming for a chosen probability of failure that remains roughly constant for strength, serviceability, fatigue, and other limit states.

In contrast, ASD relies on fixed safety factors applied to the material strengths and uses a deterministic check against allowable stresses. There isn’t an explicit probabilistic reliability target baked into the design; it’s more about maintaining stresses below a set allowance under nominal loads.

So the statement that correctly contrasts them is that LRFD uses load and resistance factors to achieve target reliability across limit states, incorporating uncertainties in loads and strengths, while ASD uses fixed safety factors on strengths, applying a simpler deterministic check without explicit probabilistic reliability targets.