Influencia de la variación de la fibra neutra en el cálculo de flechas instantáneas en piezas de hormigón armado

Influence of the variation of the neutral axis on the calculation of instantaneous deflections in reinforced concrete beams

Cracking appears when the concrete tensile strength is exceeded in a reinforced concrete flexural member, in which some sections are fully cracked, but the concrete between cracks still carries some tension. Under these conditions the curvature changes from each other section, either because of different steel strain or by the variation of the neutral axis. This paper presents a study, based on the tension-stiffeening effect adopted by the CEB-FIP Model Code 90, to obtain an expressionb of the average curvature representing the local behaviour of the beam and taking into account the variation of the neutral axis. By means of the correesponding integration of curvatures along the beam, an expression of equivalent inertia is obtained. Eith this equivalent inertia, representing the global behaviour of the beam, the instantaneous deflection at midspan can be calculated for each loading state. By comparison with the results obtained with other methods, it can be stated that, in general, the value of the computed instantaenous deflection increases when the variation of the neutral axis is taken into account. Neverthless the remarked differences indeflection values only have importance when the ratio of cracking moment to service moment is close to the unit and also when the short-time loads are an important amount of the total load on the beam.