Effects of Deep Beam Characteristics on Ultimate Load Capacity, Failure Mode, and Serviceability Performance of Reinforced Concrete Deep Beams

Deep beams are reinforced concrete structural members characterized by a small span-to-depth ratio, in which loads are transferred primarily through arching action rather than conventional flexural behaviour. This study experimentally investigated the influence of deep beam characteristics on structural performance under a centrally applied point load. Four reinforced concrete deep beam specimens with varying effective span-to-depth depth (L__e⁄(h )) and shear span-to-depth ((a/d)) ratios were tested to failure using a 300 kN Universal Testing Machine (UTM). The experimental programme evaluated the first crack load, ultimate load capacity, midspan deflection, final flexural crack width, and observed failure mode. The results showed that decreasing the (L__e⁄(h )) ratio from 2.5 to 1.7 and the (a/d) ratio from 1.4 to 0.9 significantly enhanced structural performance. The first crack load increased from 89.97 kN to 180.79 kN, while the ultimate failure load increased from 124.51 kN to 241.43 kN. Deflection decreased from 0.21 mm to 0.08 mm, indicating increased stiffness, whereas the final crack width increased from 2.54 mm to 3.58 mm due to the higher stress levels sustained before failure. All beam specimens exhibited diagonal cracking followed by shear-compression failure, confirming that shear behaviour governed the ultimate response. The study concludes that reducing the effective span-to-depth and shear span-to-depth ratios significantly improves the strength and stiffness of reinforced concrete deep beams through enhanced arching action. Lower (L__e⁄(h )) and (a/d) ratios are therefore recommended for heavily loaded deep beams, with adequate reinforcement detailing to control crack widths and ensure satisfactory serviceability.