Service pipes and ducts used for water supply, wastewater system and computer networks are necessary for multi-storied RC buildings. The utility pipes placed under the floor beams increase the height of the headroom and the opening placed inside the floor beam reduce the beam's strength and causes cracks around the opening. In this case, strengthening the circular openings should be done to improve the beam behaviour by using steel plates in the flexural region. In this study, totally seven beams were cast; one control beam, three beams with unstrengthened circular openings of 150mm diameter in the flexural zone at the top (compression zone), centre and bottom (tension zone) and three beams with circular openings strengthened with steel plate around the opening. Two-point loading was used for testing the beams. The effect of using openings in beams with and without steel plates at different locations along the depth of the beam is investigated. In the beam with opening provided in the compression zone of the beam, the ultimate load decreased by 22% compared to the control beam and sudden failure occurred as the concrete in the compression zone was reduced due to the opening. The ultimate load decreased by 8.7% when the opening was provided in the tension zone of the beam at the midspan. Strengthening the opening with steel plate decreased the ultimate load capacity and the failure mode was shear failure with cracks in the shear zone instead of flexural failure mode. RC beam with circular opening provided in the tension zone is preferred to the beam with opening provided in the compression zone, because reduction in ultimate load capacity is only marginal compared to the control beam.

Keywords: Circular openings; Flexural zone; Strengthening; Steel plate; Ultimate load.