Seismic Fragility Analysis On Seismic-Damaged Double-Deck Viaduct Frame Piers Strengthened With Enveloped Steel Jackets

Double-deck viaduct can realize urban three-dimensional traffic and improve traffic conditions.It has gradually become a popular structural system in urban bridge construction in recent years.Frame piers of double-deck viaducts are subjected to complex forces under earthquake action and are prone to damage.Study on reinforcement of frame pier of double-deck viaduct after earthquake damage is of great importance.Therefore,this paper studies the seismic fragility of seismic-damaged double-deck viaduct frame piers strengthened with enveloped steel jackets.The main research work and results are as follows :(1)Finite element models of seismic performance of seismic-damaged double-deck viaduct frame piers strengthened with enveloped steel jackets were established.Based on the seismic performance test of frame piers of earthquake damaged double-deck viaduct reinforced by steel jacket.The material reduction method is used to simulate the seismic damage of frame piers.The finite element models of a comparison specimen,a specimen strengthened with an enveloped steel jacket,a moderately pre-damaged strengthened specimen,and a severely pre-damaged strengthened specimen were established using Open Sees open source program.Quasi-static test was carried out on the numerical model.Compared with the test hysteresis curve and skeleton curve,the curve was in good agreement.(2)The seismic fragility analysis of seismic-damaged double-deck viaduct frame piers strengthened with enveloped steel jackets.Seismic fragility analysis of frame piers was carried out by using peak ground acceleration(PGA)as ground motion intensity index and displacement ductility ratio as engineering demand parameter.The results indicated that relative to unreinforced pier,under slight,moderate,severe,and collapse damaged states,the median peak ground acceleration of the moderately damaged pier strengthened with an enveloped steel jacket increased by 35.2%,51.7%,67.0%,and73.5%,respectively.The median peak ground acceleration of the severely-damaged pier strengthened with an enveloped steel jacket increased by 32.4%,44.6%,56.1%,and67.9%,respectively.In the severe damage range,the enveloped steel jacket reinforcement restored and improved the seismic performance of seismic-damaged double-deck viaduct frame piers.(3)Parameter analysis of seismic-damaged double-deck viaduct frame piers strengthened with enveloped steel jackets based on fragility.It studied the influence of concrete strength,steel strength,batten plate spacing and steel content of enveloped steel jacket on seismic fragility of bridge piers.The results show that steel content and steel strength have a good effect on the reinforcement effect of steel jacket.Spacing of batten plate has little influence on reinforcement effect of steel jacket.When the steel ratio of 2.86 % ～ 3.63 % and Q345 grade steel is the most cost-effective choice.When the concrete strength increases from C30 to C50,the failure probability of the pier decreases to varying degrees.When the concrete strength exceeds C50,it has little effect on the seismic vulnerability of the pier.