Análise dinâmica de vigas longas de concreto protendido pré-moldadas durante operações de içamento

Abstract

The tendency to use slender cross-sections due to the use of high-performance materials makes beams more prone to the phenomenon of lateral instability during transient phases, especially during lifting. Precast concrete beams are lifted during construction by cranes. Several movements must be performed to assemble the structure. However, there is no recommendation for specific operating speeds for long beams, but several collapses have been reported at this stage of construction. In the technical literature so far this problem has only been addressed through static analysis. Therefore, the present research aims to investigate the dynamic behavior of long prestressed beams, identifying critical operating speeds according to crane movements. The eccentricity of the prestressing cables and positioning deviations of the lifting loops were considered in the analysis, as they accentuate the risks of instability. In the research, modal analysis and transient dynamic analysis were performed considering the geometric non-linearity, for a prestressed concrete beam with a span of 40 m. The analysis was performed with a 3D finite element model, and the system with multiple degrees of freedom was tested in upward, downward and lateral movement. The most critical movement observed was the lateral movement, as it mobilizes more the lateral stiffness of the slender beams, exciting oscillation frequencies close to the natural frequency of the beam. Safety against cracking and subsequent failure during vertical and lateral movements was verified for crane operating speeds of 36 cm/s and 17 cm/s, respectively. For the most critical case analyzed, the dynamic analysis showed 12 times increase in tensile stresses compared to the static equilibrium situation of the beam, 80% in compressive stresses and a 5 times amplification in beam rotation.