Comparison of the Behavior of Reinforced Concrete Beams under Actual Loads and Design Loads According to SDM and WSD

Abstract

This research investigates the flexural capacity of reinforced concrete beams designed using the Strength Design Method (SDM), based on the Ministerial Regulation B.E. 2566. The analysis compares actual service loads (DL + LL) with design load combinations (1.4DL + 1.7LL) and (1.2DL + 1.6LL), in order to determine the percentage of the actual bending moment in relation to the ultimate flexural strength of the beam. The study considers beam spans ranging from 4 to 8 meters and live loads from 50 to 800 kg/m², which are typical for residential and commercial buildings. The results indicate that beams designed under SDM are utilized at only 54–67% of their design moment capacity under actual loading, reflecting an appropriate level of safety margin in structural design. The utilization percentage tends to decrease as live load increases, while span length does not significantly affect it. A mathematical model was developed to describe the relationship between live load and allowable usage percentage, demonstrating high accuracy (R² = 0.9860). This enables reliable and safe prediction of beam flexural performance under real service conditions. Additionally, when comparing SDM and Working Stress Design (WSD), it was found that SDM provides a significantly higher flexural capacity, despite using the same cross-sections and design conditions.