STRUT AND TIE MODEL ANALYSIS OF SIMPLY SUPPORTED DEEP BEAMS CONSTRUCTED WITH LOCALLY SOURCED MATERIALS

Abstract

This research studied the strut and tie model analysis of simply supported deep beams constructed with locally sourced materials. A beam whose depth is large compared to its span is known as a deep beam and these structural elements belong to disturbed (D) region where the strain distributions are nonlinear, which have traditionally been designed using empirical formulae or using past experience. Strut and tie method (STM) has been found as a realistic approach and more flexible option for designing structural elements of Bernoulli (B) and Disturbed (D) regions. This research focuses on the strut and tie method recommended by ACI 318-8 and uses experimental results to compare and investigate the range of effective span to effective depth L e ⁄ d ratios in which strut and tie method STM can be used for analysis in the design and construction of deep beams using the locally sourced materials. Four different reinforced concrete (RC) simply supported deep beams with different effective span to effective depth L e ⁄ d ratios of 2.9, 2.5, 2.1, and 1.8 with shear span to effective depth a ⁄ d ratios of 1.4, 1.3, 1.1 and 0.9 which were within the ACI 318-08 code recommendation and provision of L e ⁄ a d less than 4 and ⁄ d less than 2. The specimens had a varying cross section of 110 x 220 mm, 125 x 250 mm, 143 x 286 mm and 165 x 330 mm and a total length of 1000 mm respectively. They were experimentally tested under a point load set up under 300 kN load, using universal testing machine (UTM) to determine the ultimate load and shear capacity at first and maximum cracks of deep beams obtained from STM recommended by ACI 318-8 and the experimental results were compared with the estimated results from analysis of section method and strut and tie method (STM). An investigation on failure modes, deflection and crack propagation in the reinforced concrete (RC) deep beams were also done. The decrease in shear span to effective depth a ⁄ d ratios 1.4, 1.3, 1.1 and 0.9 respectively and the crack width at failure load increases as 2.54, 2.87, 3.16 and 3.58 respectively which indicate that the cracking behaviour and failure mode of all the deep beams under point loading failed by shear–compression. The shear span-to-depth ratio 1.4, 1.3, 1.1 and 0.9 for various shear strength of beam were ultimate load capacity of 124.51 kN, 162.13 kN, 192.43 kN, and 241.43 kN respectively. The ultimate load capacity for the beams using strut and tie method (STM) were 120.00 kN, 154.70 kN, 202.44 kN and 269,52 kN respectively and the ultimate load capacity for the beams using method of section (MOS) were 110.85 kN, 150.86 kN, 210.77 kN and 304.43 kN respectively. This research, showed that the experimental ultimate load capacity values for the deep beams, and the ultimate load capacity from strut and tie method (STM) are more accurate than that of results from the method of section (MOS). As the ultimate shear force for the beams also increased with respect to their shear span-to-depth ratio, the beams deflected 0.21, 0.17, 0.12 and 0.08 mm and their flexural capacities 35.08, 31.13, 24.68, and 20.15 respectively also decreased. The relationship of the deep beam shear span-to-depth ratios and the beams deflection showed that deep beams constructed with locally sourced materials can be analyzed using strut and tie model.