Effect of Large Openings on Bending Strength of Reinforced Concrete Beam

Abstract

At the design stage of beams, openings for service ducts are rarely taking into consideration. The introduction of openings for service ducts have negative effects on the strength characteristics of beams, hence the focus of this research is to investigate the effect of large service openings on the ultimate load of reinforced concrete beams produced from locally available materials. A total of ten beams were cast using 20.8 N/mm² concrete grade. The cross-sectional dimensions of the beams were 100mm x 150mm and 1000 mm length, with effective span of 750 mm. The tested beams consisted of two control beams. The experimental beams consisted of eight beams, four of the beams were with 40 mm service openings (with two beams having openings at the centre and two beams having openings at the supports) and the other four beams had 50 mm service holes (with two beams having openings at the centre and two beams having openings at the supports). The reinforced concrete beams were cured by covering it with wet cloths and tested at 28th day. The control beams and experimental beams were subjected to point loads at the beam centre to study the effect of the openings on the ultimate load. The average actual ultimate load for the experimental beams with 40 mm openings at the supports (TII) and centre were (TIII) were 49.48 kN and 63.28 kN respectively, representing 28% increase in the ultimate load when opening is moved from support to the centre of the beam. Also there is 45% increase in the average actual ultimate load for the experimental beams with 50 mm, when the opening is moved from supports (TIV) to the centre (TV). The average actual ultimate load for the control beams (TI) was 63.88 kN, while the estimated ultimate loads for control and the experimental beams was 70.13 kN, which shows that the estimated ultimate load is about 10% higher than the actual ultimate load. The openings placed at the supports of beams have more strength reducing effect on the ultimate load, when compared with the openings placed at the centre of the beams.