BEHAVIOUR OF UNDER AND OVER REINFORCED CONCRETE SLENDER BEAMS AT FAILURE

Abstract

The structural elements of the collapsed reinforced concrete buildings were designed as under-reinforced structural elements, however majority of these collapsed buildings collapsed suddenly without giving any warning, hence they collapsed without exhibiting the property of an under-reinforced section, which is supposed to exhibit yielding of tension reinforcement first, accompanied by large deformation, thereby giving warning of the impending failure of the structure. Hence, the focus of this research is to investigate the mode of failure of both over-reinforced and under-reinforced concrete slender beams. The total number of the over-reinforced beams were ten, with the provision of the following percentage reinforcements: 4.2%, 4.53%, 5.28%, 5.8% and 6.0%. Also the total number of under-reinforced concrete slender beams were ten, with the provision of the following percentage reinforcements: 1.0%, 1.51%, 2.0%, 2.6% and 3.0%. The beams were loaded with point loads at the centre, with shear span/depth ratio of 3.8. The estimated moment of resistance for under-reinforced slender beams were 5.83 kNm, 13.15 kNm, 15.29 kNm, 17.73 kNm and 19.51 kNm for beams with percentage area of reinforcement of 1.2%, 1.8%, 2.4%, 3.1% and 3.6% respectively, while the actual ultimate moment of resistance were 8.22 kNm, 11.48 kNm, 11.56 kNm, 12.52 kNm and 13.38 kNm respectively. From the above, the estimated ultimate load is higher than the actual ultimate load by 13% to 32% for some under reinforced beams. Also, the estimated ultimate moment of resistance for over-reinforced slender beams were 25.45 kNm, 26.96 kNm, 30.37 kNm, 33.25 kNm and 33.93 kNm with percentage area of reinforcement of 4.9%, 5.4%, 6.3%, 6.9% and 7.2% respectively, while the actual ultimate moment of resistance was 9.40 kNm, 9.44 kNm, 10.35 kNm, 9.23 kNm and 9.37 kNm respectively. From the above, the estimated ultimate moment of resistance is higher than the actual ultimate moment of resistance by 63% to 73%. From the above, the existing formulae for predicting moment of resistance of under-reinforced beam is not adequate and needed to be modified, so as to give more accurate results. The under reinforced concrete slender beams B1, B2 and B3 have the capacity to deform under constant loads of 44.06 kN, 61.2 kN, and 61.67 kN respectively for a longer period of time because it took 10.0 secs, 12.5 secs, and 11.5 secs respectively before their ultimate failure, hence, these beams can sustain large rotations. For beams B4 and B5, although failed at higher loads of 66.77 kN and 71.36 kN respectively, they have limited rotation capacity, hence plastic method of analysis and design cannot be used for these beams because the beams failed immediately after attaining their ultimate load. The over-reinforced concrete slender beams B6, B7, B8, B9 and B10, have no capacity to deform under constant loads of 50.14 kN, 50.34 kN, 55.17 kN, 49.22 kN, 49.96 kN, respectively because the beams failed immediately after attaining their ultimate loads, hence these beams cannot sustain large rotations before failure; therefore plastic method cannot be applied in the analysis and design of these beams. Lastly, all the over-reinforced slender beams failed in a brittle manner without giving any warning of failure, hence this type of beam should be discouraged in design and construction.