Using prestressed slender beams has become common in Civil Engineering, especially those designed to transpose long spans of bridges and viaduct. It is worrying behavior of these beams due to the lateral stability in the transitional phases and in service. The presence of geometric imperfections aggravates the situation of these beams. Two stages must be evaluated: lifting and when the beam is on provisional supports, i.e., when final connections are not yet executed. These beams, in most cases, are supported on elastomeric bearing pads that, due to its flexibility, provide additional degrees of freedom to the structure that can lead the beam to collapse. Considering the occurrence of collapse of beams in the pre-service phase already registered, this paper has as main objective to evaluate the behavior of concrete beams on elastomeric bearings and to analyze the influence of variables such as: initial imperfections, concrete resistance, with or not wind load and bearing compression stiffness. For the numerical analysis, ANSYS, based on the Finite Element Method, was used. The analyzes show that the influence of the strength of the concrete is significant in the lateral stability of the beam. The wind load represents a considerable increase for the loss of contact (lift off) between the beam and the bearing. The consideration of concrete physical nonlinear behavior shows that in some situations it is not possible to find a position of equilibrium in the numerical model. The roll stiffness of the bearing decreases as the skew angle increases. Finally, the combination of these factors can result in a critical stress situation on the beam, that it is not possible to have equilibrium, causing its toppling.
ResumoThe stability of long and slender precast beams has been associated with structural collapse during the transitory phase of construction. The inevitable deviations regarding the execution between the support cross-section and midspan aggravate the instability problem, thus causing the beams to become even more susceptible to its effects. The focal point of this study is the numerical and behavioral evaluation of concrete beams that present geometric imperfections on elastomeric supports and analyze the influence exerted by the variables of: strength characteristics of concrete, physical non-linearity of the concrete and the compression stiffness of the bearing pad. The numerical analyses were developed on a computer program based on the finite element method (FEM). Among the principle conclusions drawn from the study, one finds that the increase in the characteristic strength of the concrete provides the beam with a higher degree of stability, and the consideration of the physical non-linear behavior of the material did not allow the equilibrium point on the numerical model to be found in some of the cases that were evaluated. The rotational stiffness of the pad decreases as the skew angle increases. Therefore, one needs to remember that in very critical situations, it may not be possible to find the point of equilibrium, thus leading to collapse.
RESUMOUma das características próprias de elementos pré-moldados de concreto é a existência de fases transitórias durante sua vida útil. Os esforços próprios dessas fases podem gerar problemas relacionados à estabilidade lateral especialmente em vigas longas, esbeltas e que apresentam imperfeições iniciais. Nesse sentido pretende-se, neste trabalho, avaliar a influência de variáveis adotadas numa das fases transitórias: o içamento. Conhecendo as características mecânicas e geométricas de vigas padronizadas e também o valor, medido em campo, de suas imperfeições iniciais, analisaram-se: esforços, deslocamentos e rotações, para diferentes comprimentos de balanço e inclinação dos cabos de içamento, utilizando a planilha eletrônica VT Lifting Analysis (2013). As análises mostraram que a suspensão na ausência de balanços interfere consideravelmente nos esforços, deslocamentos e rotações. Verificou-se, ainda, uma tendência no comportamento estrutural das diferentes vigas quando foram analisados em conjunto todos os resultados obtidos. PALAVRAS-CHAVE:Instabilidade lateral. Vigas pré-moldadas. Içamento.
RESUMO: A utilização de vigas pré-moldadas de concreto cada vez mais esbeltas tem se tornado comum na Engenharia Civil, sobretudo aquelas destinadas a transpor longos vãos em pontes, viadutos e obras de arte em geral. Preocupa o comportamento dessas vigas face à estabilidade lateral nas fases transitórias e em serviço. A presença de imperfeições geométricas agrava ainda mais a situação crítica dessas vigas. Duas fases devem ser avaliadas: o içamento e quando a viga encontra-se sobre apoios provisórios, ou seja, quando as ligações definitivas ainda não foram executadas. Na tentativa de estudar o comportamento de vigas esbeltas com relação à estabilidade lateral, este trabalho apresenta a análise de instabilidade feita numa viga de dimensões padronizadas denominada viga platibanda realizada no programa computacional de análise numérica ANSYS. O objetivo é conhecer a carga crítica de instabilidade para cada uma das situações analisadas: içamento com cabos retos e inclinados na presença e na ausência de balanços e quando a viga encontra-se apoiadas sobre apoios tipo garfo deformáveis ou não a torção na presença e na ausência de balanços. A determinação da carga crítica de flambagem permitiu conhecer os casos que conferem mais segurança à estrutura durante as fases transitórias. Dos casos analisados, a suspensão com cabos retos e balanços e a situação da viga sobre apoios indeformáveis a torção mostraram ser os casos mais benéficos para a segurança da estrutura.ABSTRACT: Using prestressed slender beams has become common in Civil Engineering, especially those designed to transpose long spans of bridges and viaduct. It is worrying behavior of these beams due to the lateral stability in the transitional phases and in service. The presence of geometric imperfections aggravates the situation of these beams. Two stages must be evaluated: lifting and when the beam is on provisional supports, i.e., when final connections are not yet executed. In order to study the behavior of slender beams with respect to lateral stability, this work presents the eigenbuckling analysis made in a beam of standard dimensions called “viga platibanda” performed in the computational program of numerical analysis ANSYS. The objective is to determine the critical load of instability for each of the analyzed situations: lifting with straight and inclined cables in the presence and in the absence of overhangs and when the beam is supported by deformable or not deformable supports in the presence and in the absence of overhangs. The determination of the critical buckling load allowed to know the cases that give more security to the structure during the transitional phases. From all the analyzed situations, lifting with straight cables and overhangs and beam on indeformable supports proved to be the most beneficial cases for the safety of the structure.
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