Estudo sobre as reduções de forças cortantes preconizadas pela norma ABNT NBR 6118 para vigas de concreto armado

Abstract

An integral part of the structure of a building, beams are the structural elements responsible mainly for supporting transverse loads from slabs and floors and transmitting these loads to the pillars. Dimensioning it in the right way to resist the forces required is essential to ensure the safety of the structure. The shear force is an example of these internal forces acting on the reinforced concrete beam. The Brazilian standard ABNT NBR 6118 deals with the design of reinforced concrete beams and recommends that the shear forces can be reduced close to the beam supports, according to the type of loading. This work sought to evaluate the effects of this reduction in the design of beams. For the development of this study, a literature review was carried out on some of the main structural models for beam analysis: Euler-Bernoulli Model, Timoshenko Model and Morsch Trellis Model. The automatic dimensioning was performed using electronic spreadsheets, by the calculation models I and II of the standard, evaluating the dimensioning without and with the reduction of the shear force calculation value Vsd. A bi-supported beam of fixed height and variable length was considered for the study, thus obtaining different span and height relationships. With that, the study analysed the influence that this reduction of the shear force close to the supports exerts on the design of the transverse reinforcement. It was also sought, through the review, to understand why the standard allows such a reduction. From the results obtained, it was seen that the reduction of the shear force allowed the design of cases of beams, for which it would not be possible without considering the reduction of the force. It was also seen that, in relation to the variation of the forces, the beams submitted to concentrated loads presented a greater variation of the required steel area, for example, in relation to the beams under distributed loading, when the reduction is considered.