What is the primary design philosophy difference between LRFD and ASD?

• A. LRFD uses load and resistance factors to ensure uniform reliability; ASD uses allowable stresses with fixed safety factors, often leading to larger member sizes for the same loads.
• B. LRFD uses fixed allowable stresses with safety factors; ASD uses load and resistance factors to ensure reliability.
• C. LRFD and ASD are identical in practice.
• D. LRFD cancels safety factors entirely.

Answer: (A)

The main idea is that LRFD builds safety in by adjusting both what loads you consider and how strong the member must be, so the design aims for a uniform level of reliability across all conditions. In LRFD you multiply each load with a factor that reflects its uncertainty (and you apply a resistance factor to the member’s strength). The design check is that the sum of factored loads is less than the factored resistance: phi times the nominal strength must exceed the sum of gamma-weighted loads. This approach targets a consistent probability of failure regardless of load type or combination.

In contrast, ASD uses fixed allowable stresses tied to the material strength divided by a single safety factor. You compare the service stresses under loads to an allowable stress, rather than balancing factored loads against factored resistance. This can lead to overdesign in some cases, and thus larger member sizes to maintain the same level of safety across all scenarios.

So the key distinction is LRFD’s load and resistance factors to achieve uniform reliability, versus ASD’s fixed allowable stresses with a safety factor that can drive larger members for the same loading.