Analysis Of Seismic Calculation Parameter For Underground Subway Structures At Loess Area

Subway projects in Chinese western loess area gradually increased in recent years,seismic performance and safety evaluation of subway underground structures at the loess area was closely watched.This field is quite backward in our country, Code for Seismic Design of Urban Rail Transit Structures(GB 50909-2014) which began in December 2014 made provisions for subway underground structures,but the seismic calculation methods and seismic parameters didn’t consider the influence of the regional special geological conditions,not easy to operate.At present, the calculation methods which are suitable to guide the calculation methods for subway underground structure at loess area reliably are still blank.This paper takes xi ’an flying road subway underground station as an example,used theoretical analysis, experimental research and numerical simulation method,studied applicability of seismic calculation method for subway underground structures i at the loess region.comprehensive recommended formula of subgrade reaction spring stiffness which considered multifactor impact and practical table of level relative displacement of soil at the loess areas wrer put forward,then response displacement method was improved.Specific content is as follows1.The applicability analysis of seismic design method for subway underground structure at loess areathe basic principle、calculation steps and characteristics of earthquake coefficient、esponse displacement method and response acceleration method Detaily;taking xi ’an flying road subway station for example, compared the calculation results of the three kinds seismic calculation method with the time history analysis method. Study shows that internal force result of response acceleration method was closest to time history analysis,then response displacement method,the result of seismic coefficient method was too small.2 Comprehensive recommended formula of subgrade reaction spring stiffnessbased on the finite element method, analyzing relationship between subgrade reaction spring stiffness and several key factors such as the elastic modulus,poisson’s ratio,cohesive force, internal friction angle of the soil and width,height of section,buried depth,distance between floor and bedrock.Then formula about subgrade reaction spring stiffness was fitted.Comprehensive recommended formula which considered multifactor impact was put forward.Through comparing with earthquake damage phenomenon of shaking table test and result of time history analysis method,the reliability of comprehensive recommended formula was proved.3.The practical table of level relative displacement of soil at the loess areasbased on the finite element method,practical table which consider parameters fully such as loess soil parameters, the ground motion characteristics,earthquake intensity was proposed and it is suitable for the loess region.Used speed response spectrum method,allow stress method,earthquake intensity method and practical table to calculate level relative displacement of loess soil, Through comparing with result of time history analysis method,the reliability of practical table was proved.4.The improving of response displacement methodStudies show that ignoring structure bottom vertical soil pressure and vertical seismic soil pressure is the main cause of small axial force results of response displacement method. In this paper, the structure bottom vertical soil pressure and vertical seismic soil pressure in response displacement method was add.Through comparing with result of time history analysis method,the reliability of improved response displacement method was proved.Improved response displacement method provides an easy,quick and accurate seismic design method for seismic design work of subway underground structure at loess area.