Strength and Serviceability in Steel Beam Design: A Comparative Study of ASD and LRFD

Abstract

The design of steel structures in accordance with modern codes allows for the use of either Allowable Strength Design (ASD) or Load and Resistance Factor Design (LRFD) methods. Although both approaches are assumed to provide comparable levels of safety and economy, their practical implications, particularly in relation to span length and serviceability constraints, remain an area of active investigation. This study aims to compare the structural performance of simply supported steel beams designed using the ASD and LRFD methods across varying span lengths with and without deflection limits. The analysis was conducted on noncomposite laterally braced I-section beams subjected to uniform dead and live loads. Span lengths ranging from 20 to 60 ft were evaluated. For each case, designs were performed using both ASD and LRFD in accordance with SNI 1729:2020, and the resulting section selections were assessed based on demand-capacity ratios. Two scenarios were considered: strength-only design and a design with a deflection limit of L/500. The results show that under strength-only conditions, both methods yield efficient designs with demand-capacity ratios typically between 80% and 100%. LRFD generally resulted in slightly higher utilisation, especially in shorter spans. However, when the deflection constraint was applied, the governing factor shifted from strength to serviceability, causing a significant reduction in the demand-capacity ratio to below 65% for both methods. ASD demonstrated a marginally higher utilisation in deflection-controlled designs. These findings underscore the importance of considering both strength and serviceability in beam design, and provide insight into the conditions under which one design method may be favoured over the other.