Analysis Of Supported Length Of Simply Supported Beam Bridge Under Far-Field,Near-Field And Across Fault Earthquake

The simply supported beam bridge is one of the most used bridges in our country.The failure of the falling beam is a common earthquake disaster in the simply supported beam bridge,and the supported length of the bridge is closely related to the failure of the falling beam.The main reason for the failure of the falling beam is the relative displacement of the bridge’s pier and beam during the earthquake that exceeds its set supported length.As the transportation network radiates to remote areas,there are more and more far-field,near-field and cross-fault simply supported beam bridges.Therefore,it is very important to study the supported length requirements of simply supported beam bridges under far-field,near-field and cross-fault earthquakes.Taking a simply supported beam bridge as the research object,and the elastic-plastic analysis model was established based on the ANSYS platform to study the supported length requirements of this type of bridge under far-field,near-field and cross-fault earthquakes.The main research contents and conclusions are as follows:(1)The anti-drop beam system of bridges and the calculation formula of supported length in domestic and foreign codes are introduced,and the research contents of domestic and foreign scholars on supported length,limit device and connecting beam device are summarized.The parameter analyzes the difference of the calculation formula of supported length in Chinese,American and Japanese codes.Research shows: The calculation of the supported length of the Japanese code is only related to the span,while the calculation of the supported length of the Chinese and American codes comprehensively considers the influence of the pier height and the span.(2)The far-field seismic characteristics are introduced,and 20 far-field measured seismic waves are selected.Through dynamic response analysis,elastic-plastic analysis and seismic capacity analysis,the supported length requirements of simply supported beam bridges under far-field earthquakes is studied,and the relationship between the relative displacement of piers and beams and the peak acceleration of ground motion is analyzed based on the least square method.Research shows: The relative displacement of the pier beam increases with the increase of the peak acceleration of ground motion.The maximum shear-to-shear strength ratio of the plastic hinge region under far-field earthquakes is 0.49.The residual sum of squares and goodness of fit of polynomial fitting are 21.798 and 0.976 respectively.When PGA=0.25 g,the relative displacement of the pier beam accounts for about 37% of the standard supported length.The supported length of simply supported beam bridge under far-field earthquake meets the requirements of the specification.(3)The characteristics of near-field earthquakes are introduced,and 24 near-field ground motions are selected by using the target response spectrum and ground motion intensity index.The influence of the pulse effect of near-field ground motion on the power spectrum and response spectrum is studied,and the correlation between the near-fault ground motion intensity index and the maximum seismic response of a single free system is analyzed.Through dynamic response analysis,elastic-plastic analysis and shear capacity analysis,the supported length demand of simply supported beam bridge under near-field earthquake is studied,and the relationship between the relative displacement of pier beam before plastic hinge failure and the peak acceleration of ground motion is analyzed based on the least square method.Research shows: pulse effect amplify the seismic response spectrum and power spectrum,and also increase the seismic response of the structure.The ground motion peak accelerations of both pulsed and non-pulsed ground motions show strong correlation with the maximum seismic response of a single free system.The maximum shear-to-shear strength ratio of the plastic hinge zone under near-field earthquakes is 0.84.The residual sum of squares and goodness of fit of polynomial fitting are 1626.74 and 0.863 respectively.When PGA=1.0g,the relative displacement of the pier-beam before the failure of the plastic hinge accounts for about 41% of the standard supported length.The supported length requirements of simply supported beam bridges under the action meet the requirements of the specification.(4)The synthetic method and baseline correction method of cross-fault artificial seismic waves are introduced.Six artificial synthetic seismic waves are generated and six measured original seismic waves are baseline corrected.Through dynamic response analysis,elastic-plastic analysis and shear capacity analysis,the supported length demand of simply supported beam bridge under cross fault earthquake is studied,and the relationship between the relative displacement of pier beam before plastic hinge failure and the permanent displacement of ground is analyzed based on the least square method.Research shows: The slip impact effect of cross fault ground motion will increase the seismic response of the structure.The maximum shear strength ratio of plastic hinge area under cross fault earthquake is 0.47.The sum of square residuals and goodness of fit of polynomial fitting are539.91 and 0.984 respectively.When the ground permanent displacement is 1.6m,the relative displacement of pier beam before plastic hinge failure accounts for about 56% of the supported length calculated in the code.Under the condition of small peak acceleration,the supported length of simply supported beam bridge under cross fault earthquake meets the requirements of the code.Through the dynamic response analysis,elastic-plastic analysis and shear capacity analysis of simply supported beam bridges,this thesis studies the requirements of the supported length of simply supported beam bridges under the action of far-field,near-field and cross fault ground motions respectively,so as to provide data reference for the improvement of the supported length calculation in the bridge seismic design code and the drafting of the seismic design code of cross story bridges in the future.