Research On Design Method To Anchor Blocks Of Externally Prestressed Bridges Based On Strut-and-Tie Model

The externally prestressed tendons are associated with externally prestressed concrete bridges only by anchors and deviators. The external prestress anchorage is one of the most important parts for transferring the tension of externally prestressed tendons to the whole structure uniformly and safely. The loading behaveior of anchor structure are very complex, and also exist stress centralization phenomena, but there aren't corresponding design specifications in domestic now. The structural types and mechanical properties of anchor structure are analyzed by means of the theoretical study and finite element numerical simulation in this paper, and the mainly research contents and results are obtianed as follows:(1)The application and research status on segmental precast construction of externally prestressed concrete bridges and anchor structure are summarized at home and abroad, existing main problems at this stage are pointed out and the main research contents are proposed.(2)The mechanical characteristics and typical structure of anchor structure are simply introduced, and design specifications for local zone at home and abroad are made a comparative analysis. Modeling method, rationality evaluation criterion, designing and calculating program of spatial strut-and-tie model are detaily introduced, and the strut-and-tie angle formula is derived. The method of tensile stress-based reinforcement is described.(3)Rationality evaluation criterion of spatial strut-and-tie model——the method of minimum complementary energy is an effective method to determine the action positions of compression bars and tie rods, and it is the reasonable means to solute the uncertainty of spatial strut-and-tie model. With the reinforcement design of the side anchor and single anchor, when tie rods are perpendicular to axises of the box girder bottom slab and the web plate respectively, the strut-and-tie model is the most reasonable.(4)The method of spatial strut-and-tie model based on topological optimization technique is intuitive obviously, load transmission mechanism clearly. According to optimization results, design formulas of bottom centeral anchorage, side anchor, and single and double anchor with chamfer are derived in the paper.The thickness of anchor block directly affects the values of tension and compression, the methods including the analysis results based on the finite element, the strut-and-tie angle formula, Saint-vinant principle and so on are described in this paper, and the thickness of anchor block under common prestressed loads are gived.(5)Through comparative models are established by ANSYS, the factors of reinforcement ratio and steel strain in web plate, and the compressive ultimate strength of concrete to yield strength of reinforcement of webs are analyzed, we can obtain the results:with the increase of steel strain and the reinforcement ratio in web plate and the reduction of the compressive ultimate strength of concrete, the curves of the minimum strut-and-tie angle are showing an upward trend.(6)The main factors of anchor blocks and the change of the oblique angle of obliqued web plate are comprehensively analyzed, the analysis results show:the maximum tensile stress is mainly concentrated in anchorage 10cm inside, and the maximum compressive stress is mainly concentrated in the anchorage 20cm inside, with the increase of the size of thickness, transverse section and vertical section, the maximum tensile stress of key section is reduced. With oblique angle of oblique webs increases, the values of tensile and compressive stress all reduce, when the oblique angle is about 10°, and the change trends obviously, the largest decrease scope is 6.46%.The study provides an effective method of calculation and analysis to the anchor block of segmental precast construction of externally prestressed concrete bridges and the valuable conclusions can provide some good advices for structural designs of anchorage zone.