Study On Stiffness Degradation Of CFST Arch Bridge Under Earthquake

In recent years,because of its unique advantages,concrete-filled steel tube(CFST)arch bridges are increasing in number and the span keeping increasing,however,the current CFST arch bridge specification is only limited to the static design and construction verification,so far,there is no special seismic code for CFST arch Bridges,even there are many papers on the seismic behavior of CFST arch Bridges,but,it is rare to study its seismic performance from the view of stiffness degradation,therefore,this paper focuses on stiffness degradation,taking the pseudo-static test of CFST column as the breakthrough point,by means of numerical analysis about stiffness degradation of CFST arch Bridges under earthquake action,and the influence on dynamic characteristics and seismic response of arch bridge of stiffness degradation.The main work completed in this article is as follows:(1)By looking up a large amount of literatures,this paper summary the research status of structural stiffness degradation at home and abroad,the seismic research of concrete-filled steel tube arch bridge is briefly described,and determined the research method flow of this paper.(2)The fiber model theory is described briefly,according to sort out available literature,the damage calculation method of CFST based on fiber model method is inferred,giving the MATLAB program of calculating steel and concrete damage.(3)According to the existing pseudo-static test of the CFST column,the base shear-displacement curve confirms that under the action of non-constant axial force and non-constant bending moment,the changes in the shear capacity and ductility of the column no longer have a single law,and It will not increase(or decrease)monotonously with the increase of axial force or bending moment.It is extended to the CFST arch bridge.The axial bending moment of the main arch ring is constantly changing under the action of earthquake.For such a spatiotemporal variability situation,its stiffness degradation can no longer be analyzed according to the traditional bending moment-curvature curve,It is necessary to consider that the accumulation of steel damage and the tensile and compressive damage of concrete separately.(4)By means of numerical simulation,the stiffness degradation of CFST column under pseudo-static test is calculated.It was found that the concrete and steel at the foot of the column were the most seriously damaged and the stiffness was also the most degraded.After eliminating the error,the results are in good agreement with the experimental phenomena and results,and it also verifies the feasibility of using the fiber model method to calculate the damage to consider the stiffness degradation.(5)Taking a through CFST arch bridge as the actual engineering background,the corresponding finite element model is established,four different representative seismic waves were selected for time-history analysis,extract the stress and strain of the fiber,and use MATLAB to find the damage of the critical section element,the results:Under the unified PGA,the arch ring stiffness degradation has little correlation with the Near-field and far-field seismic waves,which can be ignored,but has a great correlation with the seismic wave frequency,low-frequency seismic waves are more likely to damage the arch ring and cause stiffness degradation.Adjust the arch ring stiffness according to the calculated arch ring stiffness degradation value,analyze and consider the impact of stiffness degradation on the performance of the arch bridge,the results show that:(1)Considering the effect of stiffness degradation on the natural vibration period,degradation of stiffness leads to an increase in the natural vibration period of the structure,the structure is more "flexible",and with the increase of stiffness degradation,the time history displacement of the arch ring increases and the acceleration decreases.(2)Considering the effect of stiffness degradation on deflection under constant load,as the degree of stiffness degradation increases,the deflection value of the arch ring under constant load also increases.(3)Stiffness degradation also affects the shear capacity of the arch ring,stiffness degradation results in a structure with a lower base shear value at the same target displacement,a shorter curve elastic section,and a structure that is more likely to enter the elastoplastic phase,the curvature ductility ratio is used to quantify the damage state of the arch ring.