Study On Anchoring Method Of External Prestressing For A Continuous UHPC Box-girder Bridge

A novel long-span continuous UHPC box-girder bridge composed of ultra-high performance concrete(UHPC) was proposed which was based on the excellent tensile and compressive properties of UHPC. In the proposed structure, the wall thickness of the box girder is greatly reduced, and the densely distributed diaphragms are added to reduce the torsional distortion deformation of the box girder and the local stress in the top slab under the wheel loads, as well as to improve the shear performance of the webs and the stability of the bottom plates. As a result, the conventional three-direction prestressed box-girder bridge can be simplified to a unidirectional(longitudinal) prestressed structure. It may provide an integrated solution for two types of common damages(i.e., numerous cracks in the main g irder and excessive mid-span deflection) in the conventional long-span prestressed concrete box-girder bridges. According to the characteristics of a novel continuous UHPC box-girder bridge, the layout of externally prestressed tendons and the anchor block for external tendons on the web was studied. The research in this thesis are summarized as:(1) The layout forms of external prestressing tendons of segmental construction box-girder bridge were summarized. Some specific suggestions were made on the mixed layout forms of internal and external tendons for the proposed continuous UHPC box-girder bridge.(2) In terms of the characteristics of the proposed continuous UHPC box-girder bridge and its prestressing system, the anchor block for external t endons on the web was studied. After a preliminary anchoring scheme model test, some results was found. Since the wall thickness of UHPC box-girder bridge was decreased when comparing with a traditional concrete box-girder bridge, a large tensile stress was appeared on the lateral wall plate at the anchorage zone after an eccentric external anchoring load was applied. There is a great risk of cracking in the wall plate at the anchorage zone with the preliminary scheme.(3) The effect of the plate thickness on the “local bending effect” in anchorage zone was revealed through the analysis of stress in the isolated rectangular anchor block. A simplified plane linkage model, which can reveal the force transfer mechanism in anchorage zone, was proposed through the topology optimization, then two kinds of methods were proposed to strengthen the anchorage zone based on the model. Based on the above research, six different anchoring schemes of the anchor block for the external tendons were designed. After analyzing and comparing the performance of different anchoring schemes, the effects of diaphragms, steering of prestressing tendons and anchoring length of the anchor block on the mechanical behavior of the anchorage zone were analyze d, and a type of reasonable anchor block of external tendons was finally obtained.(4) According to the characteristics of force transfer mechanism and stress distribution in anchorage zone for a novel continuous UHPC box-girder bridge, a strut-and-tie model was proposed for the anchorage scheme of UHPC box-girder bridge. Since the model is in good agreement with the stress distribution in anchorage zone, it can provide a reference for reinforcement design of anchorage zone.