强震下高墩大跨刚构桥箱梁开裂及地震反应分析

Cracking damage and seismic response of large-span rigid frame bridges with high piers under strong earthquakes

刚构桥主梁设计时一般不考虑地震荷载,因主梁与桥墩为固结形式,实际上主梁会参与承担水平和竖向地震作用。2008年汶川地震中百米高墩的庙子坪大桥主桥为3跨连续刚构桥,出现了箱梁严重开裂等震害。为研究高墩大跨刚构桥箱梁开裂及其地震反应情况,以包括庙子坪大桥主桥在内的3座不同墩高、不同跨径的刚构桥为例,建立从施工阶段到最终成桥全过程分析模型,输入汶川地震等强震记录,接续主梁初始应力进行时程分析。结果表明:在强地震下边跨和中跨合龙段区域处顶板、底板拉压应力较大,边跨1/5~2/5区域及跨中1/4~3/4区域处腹板的主拉、主压应力也较高,局部区域已超出混凝土抗拉强度标准值,易于开裂。墩高对刚构桥的主梁的应力影响最大,跨度其次,主墩越高,跨度越大,主梁的(主)拉、(主)压应力越大。主梁开裂发生时,支座存在破坏可能,桥墩墩底也易开裂及可能屈服,这些与庙子坪大桥主桥震害情况相符。

The seismic loadfor main girders is generally neglected in the design of rigid frame bridges,but the girder really resistant the horizontal and vertical earthquake forcesdue to main girder and the pier are rigid together.The main bridge of Miaoziping Bridge,as the seismic damage case of 3-spancontinuousrigid frame bridges with high piers over 100min 2008 Wenchuan earthquake,the box girder was cracked seriously.In order to study the cracking damage and the seismic response of large-span rigid frame bridges with high piers,considering the cracking damage of girders with the box sections,three rigid frame bridges with different pier heights and spans are taken as example.Then the whole analysis model is established from construction stage to the final completion,and the strong seismic ground motions like Wenchan earthquake are subjected to the bridges.So the initial stresses of main girders of the bridges are superimposed to the bridges responses for time history analysis.The results show that the tension stress and compressive stress are relatively larger in the top and bottom of the closure section of the side span and the middle span,and the principle tension stress and principle compressive stress are relatively larger in the web near the 1/5 to 2/5 side spanand,1/4 to 3/4the middle span,when the strong seismic actions are considered.Especially,some tension stress demands have exceeded the limits of the characteristic value of concrete tensile stress at the some parts of the bridge girders,and caused cracking easily to occur.The height of piers has the main influence on the stress of the main girders of the rigid frame bridge,and the span is secondary influence factor.The main piers and spans of the bridge are larger,the(principle)tensile and(principle)compressive stress are greater.As the main girders cracks,the bridges bearings can be destroyed at the same time,and the piers may also crack and even yield at the bottom.The findings are consistent with the earthquake damage of Miaoziping Bridge.