Influência das armaduras complementares na resistência ao cisalhamento de vigas com estribos treliçados pré-fabricados

Abstract

In the design of structures under great loads, conflicts with the assembly of longitudinal and transverse reinforcement may happen. To avoid this type of occurrence, internal shear reinforcement may be a good option as they are inserted between longitudinal bars. However, if an anchorage failure occurs, a secondary effect caused by the use of these bars may cause a horizontal failure called (delamination). In order to prevent this failure, NBR 6118 (2014) states that stirrups anchorage must be secured by the use of hooks or welded longitudinal bars. Currently, few researches evaluate the influence of complementary reinforcement (hooks), and these are limited to an overall analysis of the specimens. Besides conflict with the assembly of densely reinforced members, another factor that may compromise their mechanical performance is the positioning angle of the transverse reinforcement. According to Eurocode 2 (2004), the use of reinforcement bars at an angle of 90º does not present fully satisfactory behavior, which indicates that different angles would result in more efficient responses. This research experimentally analyzes the influence of the complementary reinforcement anchorage, fixed in the compression and tensile zones of reinforced concrete beams, as a reinforcement for the stirrups. Seven RC beams using pre-fabricated truss internal stirrups were used, assessing the influence of the complementary reinforcement and verifying the behavior of stirrups at an angle of 90º or 60º in relation to the horizontal axis of the specimen. Some variations of the complementary reinforcement (hooks) ratio and the spacing between stirrups were made. As conclusions, it was observed that the use of complementary reinforcement to the pre-fabricated truss stirrup resulted in an increase of nearly twice the shear strength in relation to the reference beam, avoiding delamination until the failure load. The specimens with complementary reinforcement on both faces presented higher strength and ductility when compared to beams with complementary reinforcement only in the lower face. Strains increased from 2.46‰ in beam Wc-0.4-60b1 to 3.20‰ in beam Wc-0.4-60a1, showing that its use results in transferring stresses to shear reinforcement. On the behavior of beams Wc-04-90, by reducing the spacing between stirrups in 40 mm, they presented superior performance in terms of ductility and shear strength compared to beams Wc-04-90 with spacing of 100 mm.