Influence of member depth on the shear strength of concrete beams

The objective of this dissertation was to determine experimentally and analytically whether and how member depth plays a role in the shear strength of normal weight nonprestressed concrete beams without shear and skin reinforcement. The experimental component of this work consisted of tests on eight simply-supported beams that were designed, constructed, and tested to failure. Test results, supplemented with data from selected tests in the literature, formed the basis of the analytical component to address the broader issue of understanding the influence of the effective depth on the shear strength of these members. Findings indicate that varying the effective depth parameter while holding constant other “influential parameters” can lead to differences in predominant mechanisms of shear transfer and influence the relative behavior and shear strength. In particular, the arch action, dowel action, and interface shear transfer mechanisms were shown to be influenced by geometric scale, which can lead to decreasing shear strength with increasing effective depth. Results also suggest that it is inappropriate to compare collections of test specimens that were not geometrically scaled to analyze the shear strength of concrete beams. In practical design of nonprestressed concrete beams, however, it is difficult to decouple the influence of effective depth from the effect of geometric scale (ie. the effect of not holding the geometric proportions constant) because of limitations with respect to geometry and materials such as reinforcing bar sizes, maximum aggregate sizes, and concrete cover. In this study, the observed reduction in shear strength with increasing depth resulted in shear strengths as low as one-half the nominal value given in ACI 318 (2005 and 2008). An evaluation of the requirements for minimum shear reinforcement for beams in ACI 318 (2005 and 2008) identified specific deficiencies with respect to the beams tested

Internal and external transversal reinforcement interaction in RC beams strengthened in shear with externally bonded composites

The overall shear strength of Reinforced Concrete (RC) beams strengthened in shear with Fiber Reinforced Polymer (FRP) and Fiber Reinforced Cementitious Matrix (FRCM) composites is generally computed as the simple summation of the individual contributions of concrete and transversal internal (i.e. steel stirrups) and external (i.e. FRP or FRCM composites) reinforcement. However, the interaction between transversal internal and external shear reinforcement might cause an overestimation of the strength of the strengthened beams when this simplified approach is used for FRP strengthened elements. Although the experimental evidence is still limited, a similar interaction has been reported for beams strengthened in shear with FRCM composites. This paper presents the results of an experimental campaign carried out to investigate the aforementioned interaction. RC beams were strengthened in shear using FRP and FRCM composites and then tested using a traditional four point bending scheme. Interaction between the internal and external transversal reinforcement was observed, as strains measured in the stirrups of the strengthened beams were lower than those measured in the unstrengthened beams at peak load. Although this reduction was more evident for FRP strengthened beams, the presence of the external reinforcement precluded stirrups from yielding for either FRP or FRCM strengthened beams in this study