In LRFD design, which statement best describes how reliability targets are achieved?

• A. By using fixed safety factors on material strengths only.
• B. By neglecting uncertainties in loads.
• C. By using probabilistic load combinations with no consideration of resistance.
• D. By applying load and resistance factors to loads and resistances to meet target reliability across limit states.

Answer: (D)

Reliability targets in LRFD are reached by balancing what the structure can resist with what it may have to carry, all under uncertainty. You apply factors to the loads to reflect the range and probability of demand (dead, live, wind, etc.) and you apply factors to the resistances (material strengths and connections) to account for variability in strength and construction. The design check then requires that the factored resistance exceed the factored demand for all relevant limit states, so the probability of failure stays within the specified reliability target. This calibrated interaction across loads and resistances across limit states is what ensures uniform reliability.