预制UHPC模壳增强RC桥墩设计方法与抗震性能分析

采用预制超高性能混凝土（Ultra-high Performance Concrete，UHPC）永久模壳增强普通混凝土（Reinforced Concrete，RC）桥墩，可提高其抗震能力和耐久性能，同时加快桥梁施工速度。为研究预制UHPC永久模壳对桥墩抗震性能的影响，提出了预制模壳的设计方法，分析了其对桥墩的主动增强及被动约束机理；通过参数敏感性分析，研究了UHPC永久模壳关键参数对桥墩抗震性能的影响，包括UHPC抗压和抗拉强度等材料性能参数及模壳高度和厚度等几何参数。研究结果表明：永久模壳设计厚度由UHPC抗拉强度及桥墩截面尺寸控制，核心区混凝土浇筑温度及速度对其有一定影响，浇筑温度与模壳设计厚度呈逆相关，当浇筑温度从0℃上升到30℃时，模壳厚度约减小43%，而浇筑速度与模壳厚度呈正相关，当浇筑速度从0.5 m·h^-1增加到4 m·h^-1时，模壳厚度约增加30%；预制模壳的主动增强和被动约束作用可提高RC桥墩最大承载力和耗能能力15%以上，残余变形可减小17%以上；UHPC抗压和抗拉强度对新型桥墩初始刚度、最大承载力、耗能能力等性能指标影响较小，变化量均低于6%，提高UHPC抗压强度可有效降低新型桥墩的残余变形；预制UHPC模壳厚度和高度等几何参数主要影响新型桥墩的初始刚度和残余变形，对其耗能能力和最大承载力无显著影响；研究成果可为预制UHPC永久模壳增强混凝土桥墩的设计及抗震分析提供参考依据。

Design Method and Seismic Performance Analyses of Reinforced Concrete Bridge Piers Confined with Precast UHPC Permanent Formwork

Precast ultra-high performance concrete (UHPC) permanent formwork can improve the seismic performance and durability of reinforced concrete (RC) bridge piers, as well as reduce its construction speed. The effects of the UHPC formwork on the seismic performance of bridge piers were investigated in this study. First, a design method for the UHPC formwork was proposed. The mechanisms backing the active enhancement and passive confinement imposed by the formwork on bridge piers were studied. Subsequently, parametric analyses were conducted to determine the sensitivity of the seismic performance of bridge piers on the material properties and the geometries of the formwork, including its thickness and height, as well as the ultimate tensile and compressive strengths of the UHPC. The results indicate that:i) the design thickness of the formwork is governed by the ultimate tensile strength of the UHPC formwork and the cross-sectional dimensions of the bridge piers. In addition, the temperature and speed while casting concrete at the core of bridge piers have certain effects on it. The casting temperature exhibits a negative influence on the design thickness of the tube. When the temperature ranges from 0℃ to 30℃, the thickness decreases by approximately 43%. However, the casting speed positively affects the design thickness of the tube. When the speed increases from 0.5 m·h^-1 to 4 m·h^-1, the thickness increases by approximately 30%; ii) the active enhancement and passive confinement on bridge piers imposed by the formwork can increase the ultimate bearing and energy dissipation capacities by more than 15%, and reduce the residual drift by at least 17%; ⅲ) the ultimate tensile and compressive strengths of the UHPC formwork indicate minor effects on the initial stiffness, ultimate bearing capacity, and energy dissipation capacity of bridge piers with a variation of less than 6%. However, the residual drift of bridge piers can be effectively reduced by increasing the ultimate compressive strength of the UHPC formwork; iv) the thickness and height of the UHPC formwork primarily affect the initial stiffness and residual drift of bridge piers. Their influences on the ultimate bearing and energy dissipation capacities of bridge piers are negligible. The findings of this study provide references for the design and analysis of earthquakes of bridge piers confined with precast UHPC permanent formwork.