Development of Methods for Ground Motion Selection and Incremental Dynamic Analysis

Abstract

Linear and nonlinear response history analyses have become popular in seismic design and seismic performance evaluation procedures. The accuracy of analysis results depends not only on accurate analytic models for structures but also on the proper selection of input ground motions. Several methods were developed to select ground motions matching target response spectrum. However, most existing methods to select ground motions for response history analyses are complex and require enormous computational efforts. The purpose of this study is to develop a computationally efficient and accurate algorithms selecting ground motions considering various types of target response spectra. The proposed procedures for selecting ground motions in this study are conceptually simple and straightforward method, by which a desired number of ground motions are sequentially selected from first to last. Moreover, the selection method is developed that does not employ a simulation algorithm that requires a sophisticated subroutine program. The accuracy and consistency of the proposed method are verified by comparing the selection results of the proposed method and other existing methods. In addition, the ASCE 7-10 ground motion selection criteria with the proposed algorithm is assessed using the results of nonlinear response history analyses of twelve single degree of freedom (SDF) systems and one multi-degree of freedom (MDF) system. The results show that the target seismic demands for all model SDF and MDF systems can be predicted using the mean seismic demands over seven and ten ground motions selected according to the proposed method within an error of 30% and 20%, respectively. Currently, incremental dynamic analyses (IDA) are open used in performance-based earthquake engineering (PBEE) to accurately estimate the seismic demand and collapse capacity of structures. A single set of ground motions are used in conventional IDA. It is, however, noted that important ground motion properties change according to the level of seismicity. In this study, an improved IDA is proposed to account for multi-level seismic hazard levels. In the proposed IDA, multiple sets of ground motions are repeatedly selected using the ground motion selection algorithm proposed in this study, which matches individual seismic hazard levels. Because the proposed IDA does not require large scale factors of ground motions unlike conventional IDA, unbiased IDA curves can be obtained using the proposed method. This study evaluates the collapse strengths of a tri-linear SDF system using improved IDA and conventional IDA. It is observed that conventional IDA can produce inaccurate the seismic performance evaluation results.|선형 및 비선형 시간이력해석은 구조물의 내진설계 및 내진성능평가에서 널리 사용되고 있는 해석방법이다. 시간이력해석의 결과는 구조물의 정확한 해석 모델과 함께 적절한 입력 지반운동의 선정이 매우 중요하다. 목표 스펙트럼에 적합한 지반운동을 선정하기 위하여 다양한 방법들이 개발되어 왔다. 그러나, 개발된 대부분의 지반운동 선정 방법은 절차가 매우 복잡하며 소요시간이 과도하다는 문제점이 있다. 본 논문의 목적은 구조물의 시간이력해석을 위하여, 다양한 유형의 목표 스펙트럼에 적용 가능하고 효율적이며 정확한 입력 지반운동 선정 방법을 제안하는 것이다. 본 논문에서 제안된 지반운동 선정 방법은 절차가 매우 단순하며, 첫 번째 지반운동부터 원하는 수의 지반운동까지 순차적으로 선정하는 방법이다. 그리고 기존의 방법과 달리 지반운동의 시뮬레이션 절차를 포함하고 있지 않으므로 복잡한 프로그램을 필요로 하지 않는다. 기존에 개발된 방법을 이용한 지반운동 선정 결과와 비교하여 본 논문에서 제안된 방법의 정확성과 효율성을 평가하였다. 또한, 제안된 지반운동 선정 방법을 사용하여, 단자유도 및 다자유도 구조물을 대상으로 입력 지반운동 선정을 위한 ASCE 7-10 요구 조건의 타당성에 대하여 평가하였다. 그 결과, 7, 10개의 지반운동을 사용하여 구조물의 비선형 시간이력해석을 수행한 뒤 얻은 평균 응답은 구조물의 목표 응답에 대해 각각 30%, 20% 이내의 오차를 갖는 것으로 나타났다. 최근의 성능기반 내진공학에서는 구조물의 정확한 붕괴성능을 평가하기 위하여 증분동적해석(Incremental Dynamic Analysis)이 널리 사용되고 있다. 이 방법에서는 하나의 지반운동집단을 사용하여 구조물의 시간이력해석을 수행하지만, 기존의 증분동적해석에서는 다양한 지진 재해 수준에서의 지반운동특성(지진의 규모, 거리, 지반조건, 응답 스펙트럼 형상 등)을 고려하지 못한다는 문제점이 있다. 또한, 해석 과정에서 지반운동의 과도한 배율조정계수의 사용으로 인하여 구조물의 지진 응답이 편향(bias)될 수 있다. 따라서, 본 논문에서는 다양한 지진 재해 수준을 모두 고려할 수 있는 개선된 증분동적해석을 제안하였다. 개선된 증분동적해석에서는 고려하고자 하는 각 지진 재해수준에서, 제안된 지반운동 선정 방법을 사용하여 각각 지반운동을 선정하므로 지반운동의 특성을 만족할 수 있다. 또한, 기존의 증분동적해석과 달리 과도한 배율조정계수를 사용하지 않으므로 지진 응답의 편향성(bias)을 피할 수 있다. 3선형 단자유도 시스템을 대상으로 하여 기존의 증분동적해석과 본 연구에서 제안된 개선된 증분동적해석을 수행한 결과, 기존의 증분동적해석은 사용된 지반운동에 따라 해석 결과의 편차가 매우 큰 것으로 나타났다.; Linear and nonlinear response history analyses have become popular in seismic design and seismic performance evaluation procedures. The accuracy of analysis results depends not only on accurate analytic models for structures but also on the proper selection of input ground motions. Several methods were developed to select ground motions matching target response spectrum. However, most existing methods to select ground motions for response history analyses are complex and require enormous computational efforts. The purpose of this study is to develop a computationally efficient and accurate algorithms selecting ground motions considering various types of target response spectra. The proposed procedures for selecting ground motions in this study are conceptually simple and straightforward method, by which a desired number of ground motions are sequentially selected from first to last. Moreover, the selection method is developed that does not employ a simulation algorithm that requires a sophisticated subroutine program. The accuracy and consistency of the proposed method are verified by comparing the selection results of the proposed method and other existing methods. In addition, the ASCE 7-10 ground motion selection criteria with the proposed algorithm is assessed using the results of nonlinear response history analyses of twelve single degree of freedom (SDF) systems and one multi-degree of freedom (MDF) system. The results show that the target seismic demands for all model SDF and MDF systems can be predicted using the mean seismic demands over seven and ten ground motions selected according to the proposed method within an error of 30% and 20%, respectively. Currently, incremental dynamic analyses (IDA) are open used in performance-based earthquake engineering (PBEE) to accurately estimate the seismic demand and collapse capacity of structures. A single set of ground motions are used in conventional IDA. It is, however, noted that important ground motion properties change according to the level of seismicity. In this study, an improved IDA is proposed to account for multi-level seismic hazard levels. In the proposed IDA, multiple sets of ground motions are repeatedly selected using the ground motion selection algorithm proposed in this study, which matches individual seismic hazard levels. Because the proposed IDA does not require large scale factors of ground motions unlike conventional IDA, unbiased IDA curves can be obtained using the proposed method. This study evaluates the collapse strengths of a tri-linear SDF system using improved IDA and conventional IDA. It is observed that conventional IDA can produce inaccurate the seismic performance evaluation results.