火灾条件下混凝土箱梁梁端预应力衰变规律

针对火灾下混凝土梁桥截面损伤所导致的梁端预应力损失问题，研究了火灾高温传导模式和热传导混合边界条件，设定了预应力混凝土箱梁的火灾场景，给出了混凝土高温强度与刚度的衰减模型和烧损层计算方法；采用热力耦合计算方法和子模型分析方法计算了不同火灾场景中混凝土箱梁梁端区域钢束预应力时程变化曲线；通过工程实例分析了混凝土箱梁梁端截面不同钢束预应力的时变状态，揭示了火灾条件下混凝土箱梁梁端预应力衰变规律；通过曲线的最优与最差拟合及比较分析，提出了混凝土箱梁梁端预应力衰变计算公式。结果表明:处于箱梁梁端腹板上部的钢束预应力变化趋势受梁底部火灾面积的影响，梁底部受火面积较小时，随延火时间的增加逐渐增大，增加趋势平缓，梁底部受火面积增大时，随延火时间的增加平缓衰减；处于箱梁梁端腹板中部的钢束预应力随延火时间的增加始终呈减小趋势，处于腹板下部的钢束预应力随延火时间的增加下降幅度较大，延火至120 min时梁端钢束预应力的衰减终值介于常温下初值的94％~96％；提出的混凝土箱梁梁端预应力衰变计算公式简洁，可为类似预应力混凝土箱梁端部结构的抗火设计提供基础数据。

Decay Law of Prestress at Concrete Box Girder End Under Fire Condition

In view of the problem of prestress loss at girder end caused by section damage of concrete beam bridge under fire, the conduction mode of high temperature in fire and the mixed boundary conditions of heat conduction were studied. The fire scene in prestressed concrete box girder was set, and the decay model for strength and stiffness of concrete in high temperature and calculation method of burning damage layer were proposed. The time history variation curves of steel strands prestress at concrete box girder end in different fire scenes were calculated by means of thermo mechanical coupled method and sub model analysis method. The time varying state of different steel strands prestress of concrete box girder end section was analyzed through engineering example. The decay law of prestress at concrete box girder end under fire condition was revealed. The calculation formula of prestress decay at concrete box girder end was put forward through comparison and analysis of the best and worst fitting curves. The results show that the variation trend of steel strands prestress in the upper part of web at box girder end is affected by fire area. The prestress increases with the increase of fire time when the fire area is smaller at the bottom of beam, and the increase trend is gentle. The prestress gradually decays with fire time extension when fire area enlarges at the bottom of beam. The steel strands prestress in the middle of web at box girder end always decreases with the increase of fire time. The steel strands prestress in the lower part of web decreases obviously with the increase of fire time. The final decay value of steel strand prestress at girder end is 94% 96% of initial value at normal temperature when fire time is 120 min. The proposed calculation formula of prestress decay at concrete box girder end is simple, and can provide basic data for fire resistance design of similar end structure of prestressed concrete box girder.