Parametric Variations in Reinforced Concrete Beam Relative to Utility Ducts

ABSTRACT

This article investigated the structural behaviour of reinforced concrete (RC) beams incorporating transverse service ducts of varying sizes, shapes, and locations. A total of thirty-six simply supported RC beams were tested under four-point bending, including a control beam (CTB) without ducts and beams with square, circular, and diamond ducts of 25, 40, and 50 mm diameters positioned at different fractions of the span (one half, one third, and one fifth from the support). All beams were designed to fail in flexure, with longitudinal and shear reinforcement provided in accordance with BS EN 1992 (2004). Load versus deflection responses, first crack, yield, and failure loads, as well as ductility indices, were systematically evaluated. Results indicate that the presence of service ducts reduces first crack, yield, and failure loads, with the magnitude of reduction primarily governed by duct size and location. Beams with larger ducts or ducts positioned near supports exhibited the greatest reductions in strength, while duct shape had a minor effect. Despite reductions in stiffness and ultimate load, ductility remained largely preserved, with only minor decreases in specific cases. The study highlights that careful design of service ducts, minimizing size and avoiding critical shear or tensile zones, can mitigate adverse effects on beam performance. Service ducts in RC beams should be kept small relative to beam depth and preferably positioned within the middle third of the span to minimize strength reduction. Where larger openings are unavoidable, additional shear reinforcement or localized strengthening should be provided, and appropriate design modification factors should be considered to account for duct size and location. These findings provide guidance for the design of

ducted RC beams in serviceable structures.

Keywords — Service ducts, Shape, Size, Ductility, Stiffness, Load, Ductility index