单箱双室箱梁对称弯曲时的局部扭转效应

为研究单箱双室箱梁在对称竖向荷载作用下的受力特征，基于横截面的荷载等效原理与荷载分解，分析了单箱双室箱梁对称弯曲时的局部扭转效应。基于截面的剪力流平衡和箱室受力分解，得到了局部扭转的等效荷载及应力计算式。通过与有机玻璃模型试验和有限元模拟结果的对比，验证了局部扭转效应计算式的正确性，并获得了单箱双室简支箱梁的局部扭转效应下的应力分布规律。研究表明，单箱双室箱梁在仅中腹板作用竖向荷载及两边腹板作用相等竖向荷载时，均存在纵向弯曲和局部扭转的组合变形模式。局部扭转由约束扭转、畸变和横向弯曲效应组成，在截面引起纵向应力和横向应力。局部扭转效应的理论计算结果与模型试验和板壳有限元分析结果吻合良好，表明基于截面剪力流等效的局部扭转荷载求解方法对双室箱梁是适用的；单箱双室箱梁的局部扭转效应在荷载作用点附近截面最为突出，截面上、下缘纵向应力和横向应力以中腹板为拐点呈折线分布，其应力分布和大小与荷载在横截面上作用的部位紧密关联；对算例箱梁，局部扭转效应产生的纵向应力可达初等梁弯曲应力的25%。

Analysis on local torsional behavior of twin-cell box girder under symmetrical bending

By using force equilibrium and decomposition, the local torsional behavior of twin-cell box girders subjected to a symmetrical loading was presented. The formulas of forces and stresses of local torsion were deduced based on shear flow equilibrium on cross-section. Compared with model test and finite element numerical solutions of a simple-supported twin-cell box girder, the calculation formula of the local torsion was verified, and the stress law of local torsional behavior was analyzed. Results indicate that if the mid-web is subjected to vertical load or the two side-webs are subjected to equal vertical loads, accompanied by vertical flexural bending, the local torsion behavior is induced, including effect of torsion, distortion and lateral bending. The local torsion behavior can cause longitudinal stresses and transverse stresses on cross-section. The present theoretical solutions are in good agreement with the model test and numerical analysis results. Based on the shear flow equilibrium on cross-section, the load decomposition method is efficient for calculation of the local torsional force of twin-cell box girder. Local torsion effect is most prominent near the loading section. On the upper and lower plates of cross-section, the distributions of longitudinal stress and transverse stress show a broken line change where the inflection point is at the mid-web. The distributions and value of stresses are correlated with the position of load on the cross-section. For the example of box girder, the longitudinal stress caused by local torsion is 25% of bending stress calculated by primary beam theory.