ANÁLISIS NUMÉRICO DEL ANCLAJE EN ELEMENTOS POSTENSADOS DURANTE LA ETAPA DE TENSADO

Abstract

Post-tensioning of concrete structures is one of the most extended constructive techniques of our time. Nevertheless, one of the most important problems occurring during the post-tensioned beam’s construction is a premature damage in the anchorage zone due to the high concentration of stresses induced by the cable’s strength. In order to evaluate the stress distribution on this region, some simplified linear elastic methods have been developed (analytical, semi-empirical and numerical) that provide different distribution of stresses. In spite of this, these methods might be limited because the real problem is strongly nonlinear, due to the fact that the high concentration of efforts around the cable might activate the inelastic behavior of concrete (cracking). The most reliable methods to study the problem in greater detail should be the experimental and numerical analysis. This research focuses in studying the nonlinear behavior of a concrete girder, post-tensioned at the end zone during the stretching. The response is simulated through the use of the finite element method, and the results are compared to those obtained using simplified elastic methods. To carry out the numerical analysis, the following strategy was adopted in this research: a) Selection of a representative post-tensioned beam, which was evaluated with a typical engineering standard procedure; b) Construction of the end region of the beam using a three-dimensional mesh based on cubic solid elements of 8 nodes; c) Implementation of two non linear material models for concrete based on damage mechanics; d) Comparison of numerical results. With respect to the finite element model, three cases of loading were simuated: i) Traction loads applied over the anchorage plates induced by the cables; ii) additional loading induced by the own weight of the beam; iii) additional loading effects for the strand curvature and eccentricity. In all of the models, some important aspects were taken into account: The holes for the prestressed strands were explicitly modeled; the strand loads were transferred to the concrete body as normal forces applied on the hole’s nodes; the support’s area under the beam was modeled with different sizes. The numerical results obtained from FE simulations show that important tensile stresses appear in all of the three directions inside of the rectangular ends, increasing the possibility of cracking in any direction, which usually are not taken into account in the classical procedures of design. In comparison to the simplified methods of analysis, some important differences about stress distribution were found and they are discussed into the document. Finally, the use of damage models for concrete allows predicting the local failure of concrete in some regions of the beam when this one is subjected to a loading combination higher than the ultimate loading of design: the most important damage appears in a small portion of the end of the beam