Performance-based Seismic Fragility Assessments Of High-rise Diagrid Structures Under Long-period Ground Motion Inputs

With rapid development of social economy and people’s longing for a better life in China,the concept of earthquake prevention and disaster mitigation also tends to move towards new opportunities and face new challenges.Society also puts forward higher requirements for earthquake prevention and disaster mitigation.As a new type of structure rising recently,diagrid structure is often used in landmark building structural systems such as high-rise and super high-rise buildings,and its seismic performance has also attracted much attention.In particular,under the action of long-period ground motions,what kind of seismic performance does the diagrid structure used by high-rise buildings have,which is worthy of further exploration.To this end,current dissertation focuses on the new diagrid structure rising in recent years,concentrates on three main aspects of earthquake engineering research,namely long-period ground motion(i.e.input)— — system performance of diagrid structure(i.e.system)—structural seismic fragility(i.e.output).Two types of long-period ground motions,far-field harmonic ground motion and near-field pulse-like ground motion,are used as inputs.The quantitative evaluation of seismic fragility of diagrid structure based on performance under long-period ground motion input is carried out.It forced the aspects of the separation of long-period components of ground motion,the identification of long-period ground motion,and the seismic fragility of diagrid structure under long-period ground motions in high-rise buildings.It includes the following four parts and conclusions:(1)Analysis on long-period components.By defining the average wavelength in the time-domain of ground motion,a method for separating the long-period component of far-field harmonic ground motion in the time-domain is proposed,which can effectively extract the long-period component of ground motion.However,this method is only applicable to far-field harmonic ground motions.By introducing the Modified Ensemble Empirical Mode Decomposition method into the processing of ground motion signals,the problem of mode aliasing in the traditional Empirical Mode Decomposition method is improved.Based on this method,a way and process of separation and reorganization of long-period components in long-period ground motion are further proposed.It is clear that the long-period large pulse in the long-period ground motion is the reason that has a great impact on the engineering structures.(2)Input preprocessing for long-period ground motion.On the premise of combing all kinds of ground motion characteristic parameters,the representative characteristic parameters are selected as the learning objects.In the field of artificial intelligence machine learning,Support Vector Machine,a supervised learning identification method based on statistical learning theory,linearizes the nonlinear problem,and solves the original nonlinear problem through the transformed linear problem.By solving the linear inseparable problem in the original space,the Ground motion identification results are obtained,and the accuracy is 93.18%.In addition,a long-period ground motion spectrum matching method suitable for considering high-order modes of structures is proposed.The results show that the variation coefficient of response results under multiple earthquakes is smaller by the matching method.(3)Performance level quantification for fragility assessment of diagrid structures.Firstly,the diagrid structure with polygon plane is divided into “elements” and “modules” by geometric method,and the stress analysis and deformation calculation of diagrid structure are further carried out.The vertical stiffness and lateral stiffness of diagrid structure with arbitrary polygon plane under vertical load and horizontal load are derived,respectively.Through the corresponding octagonal scale steel diagrid structure model,the corresponding laboratory experimental research and numerical simulation analysis are carried out.The calculation formula of equivalent lateral stiffness of diagrid structure is proposed.Beyond that,in order to obtain the lateral deformation performance of diagrid structure,the quasi-static tests of four plane steel diagrid structure specimens are carried out,the stiffness degradation of diagrid structure is analyzed,and a simplified restoring force model is established.The quantitative indexes of three main components under different failure states are obtained through statistics,and a case is used to evaluate and analyze the performance.(4)Seismic fragility analysis of diagrid structure with improved intensity measures.By establishing two kinds of nonlinear Single Degree of Freedom systems,the correlation degrees between ground motion Intensity Measure and Engineering Demand Parameter are compared,and the corresponding correlation coefficient spectra are given.According to the analysis results,the acceleration spectrum intensity is preliminarily selected as the basic intensity measure,and the new ground motion intensity measure suitable for diagrid structure is proposed in combination with the high-order vibration mode of the diagrid structure.The robustness of the two new ground motion intensity measures is analyzed and verified.In addition,the seismic risk analysis is carried out based on the design response spectrum in China.The Confidence Level Method which can effectively reduce the calculation of elastio-plastic time-history analysis is innovatively proposed,and the seismic fragility of diagrid structures is analyzed.It is found that when the 50-year exceedance probability of diagrid structure is 63% and 10%,the probability of Normal Operation and Immediate Occupancy of the diagrid structure under the action of far-field harmonic and near-field pulse-like ground motions exceeds 86%.When the exceedance probability of50-years is 0.5%,the probability of being in Life Safety exceeds 57%.The diagrid structure has large anti-collapse margin.