REINFORCING STRUTS AND TIES IN CONCRETE RING DEEP BEAMS BEHAVIOUR AND STRENGTH

Abstract

ABSTRACT The current research work aims at studying the behavior and strength of reinforced concrete ring deep beams when replacing the conventional reinforcement by reinforcing the struts and ties only. The paths of the compressive struts and tensile ties are defined through adopting the STM of Chapter 23, ACI 318M-19. The experimental program contained casting and testing eight specimens divided into three groups, A, B and C. Group A and B contained the specimens that had a ring diameter of 1000 mm, a height of 400 mm, a width of 120 mm, strut-tie angle of 30°, and they rest on 3 supports. While group C contained the specimens that had a ring diameter of 1000 mm, a height of 400 mm, a width of 100 mm, strut-tie angle of 38°, and they rest on 4 supports. More specifically, Group A contained three specimens in which the first one was the conventionally reinforced control ring deep beam. The second specimen was solid ring deep beam in which only struts and ties are reinforced, while the third specimen was proposed frame that took its geometry and dimensions from the stress paths of STM, ACI 318M-19. The first specimen in Group B was a solid control ring deep beam in which only ties were reinforced, while the second specimen was the proposed frame in which only ties were reinforced. Finally, group C contained three 4- supported specimens that had a width of 100 mm. The first one was the solid conventionally reinforced control specimen. The second specimen was the proposed frame in which only struts and ties are reinforced, while the third one was the proposed frame in which only ties are reinforced. All specimens were tested using increasing monotonic static single midspan concentrated force. Abstract vi The experimental ultimate capacity load, load-deflection response, loadlateral displacement response, first crack load, deflection at first crack, cracks characteristics such as type, width and propagation, average strain values in concrete surface, strain in steel bars, deflection at ultimate load capacity in addition to failure modes and failure loads are recorded and studied. It was concluded that the behavior of both the reference and proposed specimens is approximately similar except at the final loading stages. The proposed specimens did not exceed the reference specimens in terms of experimental ultimate capacity, but in all cases, they exceeded the theoretical STM, ACI 318M-19 design load. Wherefore, the specimens in groups A, B, and C showed a decline in the experimental ultimate capacity compared with their reference specimens by about 10.7-31%, 13.4% and 12.9-15.6%, respectively, but in all cases, their theoretical design loads of STM, ACI 318M-19 remained less than the experimental ultimate capacity by about 2.5-32.9%, 22.4-32.8% and 13.6-33.1%, respectively. That is why, it is possible to conclude that the STM of ACI 318M-19 is very effective in analyzing reinforced concrete ring deep beams, despite its conservatism. It is worth to mention that the proposed specimens were less in weight and cost in addition to providing opening for services by about 10.28- 29.61%, 4.04-20.01%, and 23.48-30.16%, respectively in comparison with the solid reference specimens. It is also worth to mention that increasing number of supports from 3 to 4, i.e. decreasing shear span/effective depth ratio (a/d) by 25%, leads to increase load capacity by about 13.6-47.6%.