Structural Performance Study Of Step-terrace Frame Structures Considering Foundation Stiffness

Step-terrace frame strucures constructed in mountain areas exist natural vertical and torsional irregularity,causing the loading and deformation features as well as failure patterns different from ordinary moment frames supported on flat ground.The unequal height of the upper and lower ground makes it more affected by the foundation soil.It is conducive to accurately study of the dynamic characteristics and seismic performance of step-terrace frame structure that reasonably considering the influence of foundation stiffness of the step-terrace frame strucure.Choosing a reasonable method to calculate the foundation stiffness that suitable for engineering practice is also the key to facilitate research and engineering application.This paper draws on the calculation method of foundation stiffness in foreign countries,and introduces the foundation stiffness calculation method and soil spring simulation model used in the US codes and technical reports to calculate the foundation stiffness of the step-terrace frame structure.Elastic response spectrum analysis and dynamic time history analysis were conducted to investigate the influence of different foundation stiffness on seismic performance of step-terrace frame structures,using SAP2000 software.Mainly analyzed the representative parameters such as shear force,torsion displacement ratio,inter story drift ratio and hinge distribution.It is summarized that the law of the structure performance of the step-terrace frame structure affected by the foundation stiffness.The main work and conclusions were summarized as follow:(1)The calculation method of foundation stiffness of natural shallow foundation and pile foundation is introduced in detail,including the definition of foundation geometric parameters,calculation of soil characteristic parameters,the impact of foundation embedding depth and dynamic load effects on foundation stiffness,etc.The applicability of the two foundations is compared and the calculation procedure of the foundation stiffness of the pile foundation is sorted out.(2)The natural vibration period,shear force,torsion displacement ratio,inter story drift ratio and hinge distribution of step-terrace frame structure was studied.Combining the results of elastic and elastoplastic analysis,it is found that after considering the stiffness of the structure,although the torsional effect of elasticity is reduced due to the flexibility of the stiffness of the foundation,under the action of seismic waves,after the structure enters elastoplastic and nonlinear,the displacement becomes structural damage control,the torsional effect of the structure increases and the deformation is concentrated and significantly increased to the lower floors,which is unfavorable for the deformation control of the weak layer of the structure.Under rare earthquakes,considering the foundation stiffness,the damage of the upper frame beam is obvious,but the frame columns are relatively safe(3)The natural vibration period,stress and deformation and plastic hinge distribution of step-terrace frame structure are studied when only considering the foundation stiffness of the upper ground layer.Combined with the results of elastic and elastoplastic analysis,it is found that after considering the rigidity of the grounding foundation,the torsional effect of each floor is significantly reduced,and the deformation of the structure is concentrated and significantly increased to the lower floor,which is unfavorable for the deformation control of the weak layer of the structure.Under rare earthquakes,the frame beam of the upper ground layer is obviously damaged after considering the rigidity of the upper ground layer.The column hinges of the falling layer are increased but not damaged,and the plastic hinges of the structure are concentrated to the lower part.After considering the stiffness of the foundation,the plastic hinges of the frame columns are less and there is no damage,but the ground frame beams on the dropped frame structure are seriously damaged,and the upper ground layer and the dropped part of the structure are deformed and concentrated,which involves the safety and applicability of the structure.Therefore it is recommended to reasonably consider the stiffness of the foundation when designing the earthquake-resistant frame structure built on the soil foundation.