The LRFD and ASD design methods are two different approaches to designing steel structures, and they differ in their use of load and resistance factors and area requirements.
The Load and Resistance Factor Design (LRFD) method is a more recent design philosophy, and it uses a set of load factors and resistance factors to determine the required strength of a steel member. In this method, the loads are multiplied by load factors, which account for the uncertainties in the loads, and the strengths are divided by resistance factors, which account for the uncertainties in the strength of the material and the connections. The LRFD method is intended to provide a more consistent and reliable design approach, as it takes into account the variability in both the loads and the strengths.
On the other hand, the Allowable Stress Design (ASD) method is an older design philosophy that uses a safety factor to determine the required strength of a steel member. In this method, the loads are multiplied by a safety factor, which is usually around 1.67, and the allowable stresses are multiplied by a resistance factor, which is usually around 0.6. The ASD method is a more conservative approach, as it assumes a constant safety factor for all loads and strengths.
In terms of area requirements, the LRFD method generally requires a smaller cross-sectional area of steel than the ASD method, as it takes into account the variability in both the loads and the strengths. The ASD method, on the other hand, requires a larger cross-sectional area of steel to account for the constant safety factor.
In summary, the LRFD and ASD design methods differ in their use of load and resistance factors and area requirements. The LRFD method is a more consistent and reliable approach, while the ASD method is a more conservative approach. The LRFD method generally requires a smaller cross-sectional area of steel than the ASD method.