Research On Seismic Reliability Of Reinforced Concrete Column-isolated Footing Sub-structures

The isolated footing under the reinforced concrete column is mostly used in low-rise,multi-layer and small high-rise structures.The reliability of the footing is very important to the building.At the same time,the interaction between the footing and the superstructure has a non-negligible influence on the internal force distri bution of the structure.Therefore,it is very important for us to realize the true reliability level of the structure considering the interaction of soil and superstructure.Consider the five main failure modes of footing failure,column eccentric compression failure,footing shear failure,footing punching failure and footing flexural failure under the conditions of Chinese code.This paper takes the foundation,isolated footing and column as a substructure for seismic reliability analysis.Based on the Sobol method and Monte Carlo reliability analysis principle,the corresponding analysis program was compiled by MATLAB software,and seismic reliability and sensitivity analysis are carried out from two levels of components and systems.Considering various structural forms and load distribution forms,the seismic reliability of five single failure modes is analyzed under 40 different load-effect ratios.It is found that the reliability index of the five failure modes is greater than 1.5 under different load-effect ratios.The load-effect ratio has little effect on the seismic reliability index of the three failure modes of the isolated footing.The random characteristics of the load have a great influence on the seismic reliability index of the column eccentric compression failure.We need to improve the level of seismic reliability.According to the sensitivity analysis of the bearing capacity of five single failure modes,the first-order sensitivity index and global sensitivity index of each random variable under different load-effect ratios are obtained.At the same time,the influence of a single resistance random variable on the seismic reliability ind ex of each failure mode is quantified.When the accuracy requirement of the reliability index is not very high,it can be considered that the concrete compressive strength,concrete tensile strength and footing length are used as the resistance random variables for reliability calculation.The reliability analysis of five failure modes as a series system show s that the correlation of each failure mode has little effect on the reliability of the series system.When the substructure bearing capacity is controlled by the footing shear failure mode.The seismic reliability index of the substructure is the largest.Considering the reliability and ductility of the structure.In the design,the column eccentric compression failure should be used as the control failure mode of the substructure failure.When the width-to-height ratio of footing is greater than 2,the reliability index decreases with the increase of the width-to-height ratio,and the footing design should adopt a smaller width-to-height ratio,and the reliability of the structure is higher.Considering the random characteristics of the load,it is recommended to adjust the adjustment coefficient of seismic bearing capacity and the load partial coefficient.The system reliability index of substructures is greater than 1.9 when the seismic adjustment coefficient of components is increased by 0.1.When the partial coefficient of gravity load and partial coefficient of horizontal seismic action is increased by 0.15,the reliability index of substructure system is just over 1.5,which meets the reliability index requirements of seismic target.