The Research On Mechanical Behavior Of Flexible High-piers With Complex Boundary Conditions

To obtain the effective length of step piers and the second instability ultimate load-carrying capacity of geometrically nonlinear pier studs with nonlinear materials,based on the engineering background of Xuhuai freeway,this paper studied the first stability critical load,the effective length of step piers,the second stability problems of pier,and the long-term impact of lasting factors on pier deflection through the theoretical derivation and numerical simulation.The main contents of this research are presented as follows:(1)Formulas for determining the equivalent length of pile under different pier top boundary conditions were deduced by simplifying the coupling interactions between the piles and soil.Based on the energy method,the effective length of step piers and first stability critical load were discussed,and an analytical method for step piers by equivalent cross-section was proposed.The results show that the hypothetical consolidation point method is applicable to the pier with free or hinge support at the top,and the effective length coefficient of pier is codetermined by the boundary conditions and its anti-push rigidity.With other conditions unchanged,the computing coefficient decreased with pier stiffness decrease.(2)The elastic stable critical load were deduced with free top and fixed bottom under the different force conditions.Meanwhile,the main factors which influence pier elastic-plastic ultimate carrying capacity were analyzed by the plastic damage models of reinforced concrete(RC)pier.The design formulas for compression-bending member moment magnifier of RC were modified by comparing the results of 04 bridge criterion and 10 construction criterion.The results show that:Different mechanical models and boundary conditions do nothing with Euler critical load of component in the elastic stability;Slenderness ratio and boundary conditions have a great influence on the elastic-plastic stability of abutment;As for RC compression-bending member,the accurate response to the change of different boundary conditions cannot be obtained with the only one parameter--effective length;The changes of boundary condition will lead to the calculation difference of bending moment magnification factor;Thus,when the pier is hinged top and fixed bottom,the design formula should be calculated with relevant theoretical formula in this paper.(3)Based on monitoring data about the pier deflection in Xuhuai freeway,the reasons of deflection causes on juncture piers and fixed supporting piers were discussed,and the long-term impacts on pier deflection due to concrete creep were analyzed.Finally,the vertical angle of deflected piers were monitored for a long time.The results show that:The fundamental element of deflection is the mismatch between the beam and its contact.Concrete creep contributes to deflection extension.Monitoring data shows that the deflection remains stable after bonded reinforcement and piers work well after correction.