Seismic Performance Research On Long Span Continuous Rigid Frame Railway Bridge With Novel Column-Slab Hollow Tall Pier In Near Field

As a new type of pier form,the column-slab hollow high pier has become a new method to solve the seismic problem of long-span bridge in high intensity seismic area due to its novel seismic design concept,and has been applied to actual engineering.However,the mountainous area of high intensity earthquake are mostly close to the geological fault zone,and earthquakes occur frequently,under complex sites and geological conditions,what is the seismic response law and seismic performance of this new type of bridge structure? and how about its damage develops after being subjected to multiple earthquakes? For this series of problems,there are few relevant studies.The slabs in the pier is designed as energyconsuming components under strong earthquake,and the problems caused by the post-earthquake repairs are not considered.How to realize the its function of easy repair,easy inspection and quick replacement after earthquake,lacking corresponding research.Therefor,the in-depth study of the above problems is very beneficial to the design and application of this new bridge structure.In this paper,A long-span railway rigid frame bridge with Column-Slab hollow high pier is taken as the engineering background,using the finite element numerical analysis method,and the following aspects are studied:(1)Using general synthetic methods of ground motion and various existing velocity pulse models,A synthetic method for synthesizing near field ground motion is proposed,The low-frequency component of near-field ground motion is simulated by Tian Yuji velocity pulse model,and the high-frequency component is generated by the design response spectrum,The near-field ground motion with directivity effect and permanent ground displacement effect is obtained by superposition of the two components.(2)Based on NGA-WEST2 ground motion attenuation model,the specific method and process of synthesizing mainshock-aftershock sequences earthquake ground motion is proposed.using this model,the PSA(pseudo spectral acceleration)curve of the site under mainshock and aftershock is obtained.and then the ground motion time histories of the mainshock and aftershock are synthesized by using the artificial wave synthetic method,Finally,the mainshock-aftershock sequence ground motions are synthesized by adding time intervals between them.The influence of parameters such as fault distance,site conditions on PGA of mainshock and aftershocks is studied,the result show that the PSA of the site decreases rapidly with the increase of fault distance;when the fault distance is constant,the PSA of the site is proportional to the earthquake magnitude;the PGA of mainshock-aftershocks earthquake sequences will decreases with the increase of fault distance,and the PGA will be significantly enlarged on the soft soil sites.(3)The column-slab component model was established by using nonlinear fiber beam-column element and nonlinear layered shell element.The influence of thickness ratio of slab to columns,reinforcement ratio and axial compression ratio on the seismic performance of column-slab model is studied,and the value of each parameter is suggested: the thickness ratio of slab to column should not be greater than 0.25;The reinforcement ratio of column should not be less than 5%,and the slab can be reinforced according to the structure;in order to improve the ductility and energy dissipation capacity of members,the axial compression ratio should not be greater than 0.15.(4)The seismic demand of Column-Slab hollow pier is studied by using synthetic near-field ground motion.The results show that the there are two kinds of force states for columns of column-Slab hollow pier along the axial direction:compression and tension,The additional dynamic axial force caused by horizontal earthquake should be considered in design.The maximum bending moment demand along column height is the same as that of a common rigid frame pier,which is K-shaped when analyzing along the longitudinal bridge direction,however,there is a large different in transverse direction,the maximum bending moment is usually located near the middle of the column;Affected by the rigidity constraint of the pier top and the high order mode,the curvature along different sections of the column height differently.the maximum curvature demand along the longitudinal direction is located at the top of the column,and the transverse direction is near the middle of the column.(5)Based on the artificially synthetic near-field ground motion,the seismic response law and damage of bridge piers were studied,and the material strain was used as the damage index to evaluate the pier damage.The results show that:the structure seismic response under the near-field earthquake is significantly greater than far-field;the movable pier is attenuated faster than the rigid frame pier by the near-field effect,and the influence distance is shorter than the rigid frame pier,which is basically within 50 km.If the rigid frame pier is damaged under the near-field earthquake,residual internal force will appear after the earthquake,and the permanent ground displacement effect will cause additional residual displacement in the structure.Compared with other rigid frame piers,column-slab piers are less damaged under near-field earthquake and have better seismic performance.However,due to the influence of high-order mode and structural,the damage location and sequence are quite different from those of regular rigid frame piers.The failure type and sequence of the slabs are basically not affected by the near-field effect.Under the strong earthquakes,the slabs have various failure forms such as shear failure,bending failure,and bending-shear failure.Only of them yield and fail before the column,thus avoiding or delaying the premature failure of the columns.(6)The synthetic mainshock-aftershock sequences earthquake ground motion is used to study the cumulative additional damage of the whole bridge,The results show that the failure of the slabs effectively reduces the structural stiffness and prolongs the structural period,Therefor,the aftershock will not cause significant cumulative additional damage in the piers,the structural damage degree of the pier is basically determined by the mainshock,and the aftershock can be ignored in the seismic design,In addition,the amplitudes of mainshock and aftershock in the mainshock-aftershock sequences earthquake have a certain influence on the additional damage degree of the structure.and the aftershock amplitude is more obvious.(7)Research on seismic performance of functional recoverable column-BRB(Buckling Restrained Braces)bridge piers,In order to realize easy replacement and rapid repair after the earthquake,BRB was used to replace the inter slab,and the influence of layout form of BRB on the structure's damping effect was studied.The results show that the diamond-shaped and X-shaped schemes have better damping effect,while the V-shaped scheme has poor damping effect,and even exceeds the non-damping structure in some locations;in addition,the seismic intensity and fault distance will have a certain impact on the damping effect.