Control measures for thermal effects during placement of span-scale girder segments on continuous steel box girder bridges

Abstract

In this study, we examined the thermal effects throughout the process of the placement of span-scale girder segments on a 6×110-m continuous steel box girder in the Hong Kong-Zhuhai-Macao Bridge. Firstly, when a span-scale girder segment is temporarily stored in the open air, temperature gradients will significantly increase the maximum reaction force on temporary supports and cause local buckling at the bottom of the girder segment. Secondly, due to the temperature difference of the girder segments before and after girth-welding, some residual thermal deflections will appear on the girder segments because the boundary conditions of the structure are changed by the girth-welding. Thirdly, the thermal expansion and thermal bending of girder segments will cause movement and rotation of bearings, which must be considered in setting bearings. We propose control measures for these problems based on finite element method simulation with field-measured temperatures. The local buckling during open-air storage can be avoided by reasonably determining the appropriate positions of temporary supports using analysis of overall and local stresses. The residual thermal deflections can be overcome by performing girth-welding during a period when the vertical temperature difference of the girder is within 1 °C, such as after 22:00. Some formulas are proposed to determine the pre-set distances for bearings, in which the movement and rotation of the bearings due to dead loads and thermal loads are considered. Finally, the feasibility of these control measures in the placement of span-scale girder segments on a real continuous girder was verified: no local buckling was observed during open-air storage; the residual thermal deflections after girth-welding were controlled within 5 mm and the residual pre-set distances of bearings when the whole continuous girder reached its design state were controlled within 20 mm.

概要

目的

采用整孔安装的连续钢箱梁对施工精度要求极高, 而其在露天存放、环缝焊接和设置支座预偏量等环节会不可避免地受温度变化的影响而产生应力和位移, 因此应引起特别关注.首先, 置于露天场地存放的钢箱梁节段各处受到的日光照射不均匀, 由此产生的截面竖向温度梯度会使梁底临时支墩的反力分布出现巨大变化, 从而影响箱梁节段的局部受力安全; 其次, 因为环缝焊接会使相邻梁段结构体系从简支梁变成连续梁, 所以在焊接过程中若箱梁顶底板因存在温差而引起了变形, 则该变形在焊接后不会随着温差的减小而逐步消除; 最后, 连接在箱梁底板上的滑动支座顶板在支座安装完成后仍会受温度影响而发生位移.为了使永久支座在施工完毕后的顶底板中心线对齐, 在安装支座时需要考虑这些位移并对支座进行预偏.

创新点

1. 发现温度梯度会导致露天存放的大节段钢箱梁下局部支墩反力的大幅度增加; 2. 发现在环缝焊接时箱梁截面温度梯度导致的位移不会在焊接后随着温度梯度的消失而减小, 这是因为箱梁的边界条件发生了变化; 3. 提出了考虑温度效应的支座预偏量公式; 4. 针对这些温度效应提出了应对策略.

方法

1. 根据钢箱梁截面的实测温度数据(图3), 建立连续钢箱梁施工全过程的有限元分析模型; 2. 通过有限元模型的计算结果, 对钢箱梁在露天存放、环缝焊接及设置支座预偏量时的温度效应展开分析研究, 并给出相应的解决方案; 3. 将解决方案应用于实际桥梁施工中, 以验证所提方法的可行性和有效性.

结论

1. 钢箱梁在露天存放、环缝焊接及设置支座预偏量时的温度效应会严重影响钢箱梁的整孔安装施工, 因此必须得到有效控制; 2. 利用钢箱梁截面的实测温度数据建立钢箱梁施工全过程的有限元模型, 可对施工过程中的温度效应进行预测并据此提出控制措施; 3. 钢箱梁的成功施工验证了本文所提出的温度效应控制措施的有效性, 这对同类工程的施工具有借鉴意义和参考价值.