基于体外预应力提载加固钢桁架桥研究

Study on Reinforcement of Steel Truss Bridge Based on External Prestressed Load Lifting

体外预应力概念最早源于法国,是后张预应力体系的重要分支之一,通常采用大直径钢筋、钢绞线、高强钢丝等作为张拉施力工具,对梁体进行预应力处理。此方法可有效减轻结构的应力水平,并能起到加固卸载和改变结构内力分布等作用,同时可提高结构的承载力、抗裂性和刚度。但体外预应力法在加固钢桁架结构中研究较少,相关理论相对缺乏。为了研究不同根数钢绞线的体外预应力对钢桁架桥的加固效果,以跨径为128 m的下承式简支钢桁架桥——奥莫河桥为研究对象,提出3种体外预应力加固方案并对其进行提载分析,运用有限元软件分别建立每束7根、9根、11根的钢绞线加固方案的全桥模型,从杆件强度、结构刚度、疲劳、整体稳定及节点板应力分布5个方面来评价对比3种加固方案的提载效果。分析可知:纵梁、横梁、上弦杆、下弦杆的应力随着钢绞线数量增多而逐渐减小,杆件应力逐步得到改善;钢绞线数量增多,结构的下挠程度逐渐减小,基频逐渐增大,其刚度不断提高;随着加固所用钢绞线数量的增加,除纵梁外,其余杆件的疲劳应力幅值不断降低;临界荷载系数不断增大,稳定性随之增加;重要部件节点板低应力区面积增大,高应力区与次高应力区面积减小,处于潜在撕裂区的螺孔数量不断减少,节点板发生撕裂破坏的概率减小。但随着钢绞线数量增加,会使下弦杆轴力增加,截面稳定安全系数减小,故需控制钢束张拉数量,限制提载幅度。结果表明:3种体外预应力加固方案对此桥均能达到提载效果,且随着所用钢绞线数量的增加,其结构强度、刚度、稳定性呈非线性提高,节点板低应力分布区占比逐步增大;采用每束11根钢绞线进行加固是最实用、合理的加固方案。

The concept of in vitro prestressing originated from France at the earliest and is one of the important branches of post-tensioning prestressing system,which usually adopts large-diameter steel bars,steel strands and high-strength steel wires as tension application tools to prestress the beams.This method can effectively reduce the stress level of the structure and can play the roles of reinforcement unloading and changing the internal force distribution of the structure,and at the same time can improve the load bearing capacity,crack resistance and stiffness of the structure.However,the in vitro prestressing method is less studied in reinforced steel truss structures,and the related theory is relatively lacking.In order to study the effect of in vitro prestressing with different number of strands on the strengthening of steel truss bridges,this paper takes a 128 m span under-bearing simply-supported steel truss bridge over the Omo River as the research object,and proposes three in vitro prestressing strengthening schemes for its load lifting analysis,and uses finite element software to establish the full bridge model with 7,9,and 11 strands per beam respectively,and analyzes the strengthening of steel trusses in terms of rod strength,structural stiffness,fatigue,overall stability,and nodal plate stress distribution.The result analysis shows that:the stress of longitudinal beam,crossbeam,top chord and bottom chord gradually decreases with the increase of the number of strands,and the stress of the bars gradually improves;the number of strands increases,the deflection of the structure gradually decreases,the fundamental frequency gradually increases,and its stiffness continuously improves;with the increase of the number of strands used for reinforcement,the other strands,except for the longitudinal beam,gradually increase.With the increase in the number of strands used for reinforcement,in addition to the longitudinal beam,the fatigue stress amplitude of the rest of the rod is constantly reduced;the critical load factor is constantly increased,and the stability is increased;the area of the low stress area of the important parts of the node plate is increased,and the area of the high stress area and the secondary high stress area is reduced,and the number of screw holes in the potential tearing area is constantly reduced,and the probability of tearing damage of the node plate is reduced.However,as the number of strands increases,the axial force of the lower chord increases and the stability safety factor of the section decreases,so it is necessary to control the number of strands tensioned and limit the magnitude of load lifting.The results show that all the three in vitro prestressing reinforcement solutions can achieve the load lifting effect,and with the increase of the number of strands used,the structural strength,stiffness and stability are non-linearly improved,and the proportion of low stress distribution area of the node plate is gradually increased,and the bridge is reinforced by 11 single-steering bending line shaped in vitro prestressing strands,which is the most practical and reasonable reinforcement solution.