Cracking Analysis of the Cable Saddle in Extradosed Bridges Based on the Extended Finite Element Method

Abstract

Cable pylons are important components of extradosed bridges. However, cable pylons pose difficulties in design and analysis because of their complicated loading conditions in the anchorage zone. In the present paper, taking the super-large Ningjiang Songhua River Bridge as the reference, a theoretical study of the crack resistance of concrete pylons in serviceability stage, and a numerical simulation analysis of the crack condition and stress distribution below the cable saddle constructed of parallel steel pipe are conducted using extended finite element method (XFEM) and the material nonlinear finite element method. The results obtained could provide a reference for the design and reinforcement of the pylon of similar bridges. Preface The extradosed bridge, also called partial cable-stayed bridge, is a new-type of bridge intervenient the cable-stayed bridge and the continuous beam bridge. Given that its pylon is relatively low and the proportion of load borne by its stay cable is lower than that borne by the stay cable of the cable-stayed bridge, the mechanical properties of the extradosed cable-stayed bridge are closer to those of the continuous beam bridge and the continuous rigid frame bridge. In the course of design, the extradosed bridges are provided with relatively high-rigidity main beams and relatively small amount of stay cables, thus, both the cable force of stay cables and the increment of their cable force under the live load are relatively low. For the convenience of construction and to control the pylon size, layered cable saddle is usually adopt to anchor stay cables onto the pylon. Among more than 30 prestressed concrete extradosed bridges constructed nationwide, most of them adopt the anchoring scheme of setting a cable saddle on the pylon. In the case of the cable saddle-structured pylon due to its relatively small size, it is difficult to configure effective circumferential prestressed reinforcement in pylons, so the pylons are usually designed as reinforced concrete members. However, in contrast to the mature design methods of reinforced concrete beams and columns, the cable saddle has a relatively complicated stress-concentrated local response which is influenced by its size and shape. A mature design and computation method in the cable saddle have not been developed yet. Thus, it is necessary to conduct a detailed nonlinear finite element analysis of the pylon of extradosed bridges in order to study the rule of stress distribution of the concrete below the saddle, particularly the pattern of crack distribution around the saddle, thus providing a theoretic basis for its local reinforcement design. The extended finite element method (XFEM) is a computational method which can simulate the patterns of cracks as well as their initiation and extension along any direction. Moreover, for reinforced concrete structures which are mainly subjected to local tensile failure, computation using XFEM can visually reflect their crack distribution and the extension direction of these cracks properly.