Explain the difference between LRFD and ASD design philosophies in steel design.

• A. LRFD uses factored loads and resistance factors; ASD uses allowable stresses with safety factors on loads and material strengths.
• B. LRFD uses only material strengths; ASD uses only applied loads.
• C. LRFD uses factored loads and resistance factors; ASD uses allowable stresses with safety factors on loads and material strengths.
• D. LRFD ignores load combinations; ASD requires them.

Answer: (A)

The main idea is how safety is built into the design. In LRFD (Load and Resistance Factor Design), you treat loads as uncertain and multiply them by load factors to form a factored design load. You also apply a resistance factor to the member’s strength. The design check is that the sum of factored resistances must exceed the sum of factored demands. This framework aims for a consistent reliability level by explicitly accounting for uncertainties in both loads and strengths.

In ASD (Allowable Stress Design), you limit the stresses in the member to an allowable value derived from the material strength divided by a safety factor. The stresses produced by service or typical load cases are compared to these allowable stresses, which incorporates safety through factors on material strength and/or loads. The result is a simpler stress-based check rather than multiplying everything by separate factors for load and strength.

So LRFD uses factored loads and resistance factors to compare against factored capacity, while ASD uses allowable stresses with safety factors applied to loads and material strengths. This is why the statement describing LRFD with factored loads and resistance factors and ASD with allowable stresses and safety factors is the correct distinction.