轨道交通U形梁横向预应力的设计研究

国内已建轨道交通U形梁底板横向均为允许开裂的普通钢筋混凝土结构。为避免底板出现裂缝,提高结构刚度和耐久性,进行U形梁横向预应力的应用研究。针对U形梁为开口薄壁结构、腹板和底板平滑过渡的构造特点,横向预应力采用管道尺寸小的扁锚后张法有黏接预应力,沿两侧腹板和底板呈环向布置。利用空间三维实体有限元模型对轨道梁进行应力分析,计算表明,对于B型列车荷载,当横向预应力筋规格为BM15-2、纵向按1 m的间距布置时,底板应力分布均匀,并满足允许混凝土出现拉应力但不允许开裂的受力要求。横向预应力筋构造布置避开纵向预应力筋、声屏障预埋件等。后张法横向预应力施工不增加张拉台座的施工时间,不影响工期。采用单端张拉、相邻钢束交错张拉的方式,可减少张拉工作量。该U形梁已应用于宁波机场路南延工程,并完成设计、施工。

Research on the Design of Transverse Prestress for U-Shaped Girder of Rail Transit

The bottom slab of U-shaped girder of rail transit in China is currently a normal reinforced concrete structure with allowable transverse cracking. To avoid transverse cracks in the bottom slab, improve the stiffness and durability of the structure, this study investigates the design of transverse prestress for the U-shaped girder. In view of the structural characteristics of the U-shaped girder, which is an open thin-wall structure with smooth transition between the web and the bottom slab, transverse prestress is designed using small size flat anchors of adhesive prestress, which are arranged in a circular direction along the web and the bottom slab on both sides. The stress of the girder is analyzed with a three-dimensional solid finite element model. The computational results show that, under the train load of type B and following the transverse prestress specification BM15-2 and the longitudinal spacing 1 meter for the prestress, the stress distribution in the bottom slab is uniform and meets the requirement that the tensile stress of the concrete may occur but not result in cracking. Additionally, the arrangement of transverse prestress tendon should be away from the longitudinal prestress tendon and embedded parts of acoustic barriers. As the post-tensioning construction of the transverse prestress does not increase the time of using longitudinal stressing bed, the construction period is not affected. To reduce the work load, tensions in the single side and cross tension for the adjacent bundle reinforcement are adopted. The U-shaped girder has been successfully applied to the south extension project of Ningbo airport and the design and construction of the project have been completed.