A Study On Seismic Rotation Mechanism And Displacement Control Of Three-span Simply-supported Skewed Girder Bridges

The seismic responses of the skewed bridges show their particular and complex characteristics: the deck has tendency of rotating about the vertical axis.In this paper,software Midas/Civil and OpenSees were used to establish dynamic calculation models of three-span simply-supported skewed girder bridges(SSGB).The seismic rotation mechanism of three-span SSGB was revealed through the modal analysis and the time-history analysis.The main works as follows:(1)By consulting abundant documents in home and abroad,the seismic researches statuses on skewed bridges were reviewed.The superstructures and pounding models were summarized in detail and the calculation methods of the shear keys and rubber bearings' stiffnesses were summarized.(2)The comparison was made between the results of a simplified single beam model and a multi beam model of three-span SSGB from two aspects of vibration modes and dynamic responses.The results show that both the simplified single beam model and the multi beam model can reasonably reflect the actual seismic responses of structures when ignoring pounding effect.When considering pounding effect,the calculation accuracy of multi beam model is generally higher than single beam model.The calculation accuracy of single beam model could be improved by an average of 8.7% through modifying the joint mass distribution of girders.(3)The three-span SSGB coupling mechanism was revealed by the modal analysis.The results show that the differences of model in side and middle span were caused by the divergences of the structure supporting which shows that the rotation amplitude of the side span is larger than that of the middle span and the translation amplitude is smaller than that of the middle span.Skew angle,width-span ratio and bearings' stiffness have a greater impact on the model vibration period,the longitudinal and transverse coupling translation will exacerbate with the increase of skew angle and width-span ratio,at the same time,the deck rotation mode will forward with the increase of skew angle.(4)Based on the modal analysis,the functional mechanism of all factors on the rotation of three-span SSGB from the aspect of seismic response was studied in two cases including ignoring pounding effect and considering both longitudinal and transverse pounding effect by nonlinear time-history analysis.The results show that the seismic responses of the side span and the middle span for the three-span SSGB were decided by the dynamic characteristics of structure itself without pounding effect,at this time,the change of bearings' stiffness will have a greater impact on the seismic response of the structure,and the change of skew angle will only have a greater impact on the seismic response of the side span,but have little influence on the middle span.The deck rotation of the middle span was attributed to the dynamic characteristics of structure itself while that of the side span was attributed to the dynamic characteristics and pounding moment when considering both longitudinal and transverse pounding effect,at the same time,the variation of skew angle and longitudinal and transverse initial gaps will have a great influence on the seismic responses of the structure.The deck rotation and the longitudinal translation will be overestimated when ignoring the role of bridge abutment backfill.(5)The deck rotation and the longitudinal translation of the bridge will reduce when adding the restrainer or the X-shaped steel hysteretic dampers.Adding the restrainer have more obvious effect to the longitudinal translation and adding the X-shaped steel hysteretic dampers have more obvious effect to the deck rotation of the bridge.