An urban viaduct has served for 22 years .It was originally designed as an S-shaped reinforced concrete continuous box girder bridge with capping beams and column piers .The in-spection showed that cracks have developed in a large portion of all spans of the viaduct and the in -tact condition of the viaduct was identified as grade D .Due to the fact that a projected subway tun-nel will underpass the bridge and the foundation of pier No .4 of the viaduct is possibly to be an ob-stacle ,the viaduct needs to be reconstructed .The specific reconstruction scheme involves the fol-lowing items .Firstly ,the pier No .4 should be dismantled and rebuilt ,the other piers should be remained for use after the deteriorations are remedied ,and the existing concrete girder should be replaced with the steel box girder .The arrangement of the bearings for the new steel box girder , the stress and deflection of the box girder and the anti-overturning stability of the overall viaduct should be analyzed and calculated in details .The transverse restraint of the pier No .1 near the in-flection point of the S-shaped plane should be released to reduce the horizontal force in the trans-verse direction ,so as to improve the load bearing condition of the overall viaduct .Inside the steel box girder sections on top of piers ,the plain concrete is cast as counterweight to ensure the anti-overturning stability of the overall viaduct .%某城市高架桥已通车运营22年，原设计为 S 形钢筋混凝土连续箱梁桥，盖梁柱式墩。检测发现该桥上部各跨梁体存在大范围的裂缝，完好状态鉴定为 D 级。因拟建地铁隧道从桥下经过且与第4号墩桩基冲突，需对该桥进行改造。改造方案为：对4号墩拆除重建，其余墩台进行病害整治后予以保留利用，并将既有混凝土梁更换为钢箱梁。对新建钢箱梁的支座布置、箱梁的应力与挠度及全桥抗倾覆稳定性进行详细的分析与计算；采用释放 S 形平面曲线反弯点附近1号墩横向约束的方式减小横向水平力，改善全桥的受力状态；采用全部墩台顶部钢箱梁内浇注素混凝土压重的方式保证全桥的抗倾覆稳定性。
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