The analysis procedures in early 20th century were essentially based on the use of simplified structural models subjected to simplified loading types. For example, for the purpose of structural design, the seismic load was historically idealized as a simple mass-proportional lateral static loading. Later, with the increasing applications of modal analysis and the formulation of the RSA procedure, the role of vibration modes and natural periods in understanding and controlling the seismic demands was recognized. With the advent of computer programs and dynamic analysis solvers in mid-1960s and 1970s, and with the increasing availability of more ground motion records, the use of detailed dynamic analysis procedures based on the direct integration solution of the governing dynamic equations of motion was established. This also started the use of nonlinear modeling for a relatively better structural idealization as compared to the linear elastic models.
In late 1980s and 1990s, the importance of nonlinear modeling and analysis increased significantly with the emergence of performance-based seismic engineering (PBSE) as a well-accepted methodology for the seismic evaluation and design of building structures (ATC 40, 1996). This methodology uses the predicted structural performance to equip the decision-makers with the key information regarding structural safety and risk. The performance is primarily characterized in terms of expected damage to various structural and nonstructural components and building contents. Since the structural damage implies an inelastic behavior, the traditional design and analysis procedures that are based on linear elastic behavior can only implicitly predict the performance. By contrast, the objective of nonlinear seismic analysis procedures is to directly estimate the magnitude of inelastic seismic demands.
The generic procedure for the determination of nonlinear seismic demands involves a number of key steps as shown in Figure above. The engineer is first required to set up a computer model which is expected to mimic the behavior of the actual structure. The anticipated seismic shaking is characterized after a seismic hazard analysis while accounting for various site-specific phenomenon. A suitable analysis procedure is then applied to the structural model considering all important loading scenarios. This results in the predictions of engineering demand parameters (EDPs) which can be subsequently compared with an acceptance criteria (generally prescribed by the seismic evaluation guidelines) to determine the seismic performance of the building. The EDPs normally comprise of global displacements (e.g. at roof or at any other reference point), inter-story drifts, story forces, component distortions, and component forces (FEMA 440, 2005). The level of complexity in this overall process may vary widely depending upon the choice of modeling scheme and analysis procedure, as well as the required degree of accuracy.
Although the seismic evaluation guidelines allow the use of approximate analysis procedures for conventional low- to mid-rise structures, the detailed NLRHA procedure is still recommended as a final check especially for the structures having extraordinary importance or those with special features.
The analysis procedures in early 20th century were essentially based on the use of simplified structural models subjected to simplified loading types. For example, for the purpose of structural design, the seismic load was historically idealized as a simple mass-proportional lateral static loading. Later, with the increasing applications of modal analysis and the formulation of the RSA procedure, the role of vibration modes and natural periods in understanding and controlling the seismic demands was recognized. With the advent of computer programs and dynamic analysis solvers in mid-1960s and 1970s, and with the increasing availability of more ground motion records, the use of detailed dynamic analysis procedures based on the direct integration solution of the governing dynamic equations of motion was established. This also started the use of nonlinear modeling for a relatively better structural idealization as compared to the linear elastic models.
In late 1980s and 1990s, the importance of nonlinear modeling and analysis increased significantly with the emergence of performance-based seismic engineering (PBSE) as a well-accepted methodology for the seismic evaluation and design of building structures (ATC 40, 1996). This methodology uses the predicted structural performance to equip the decision-makers with the key information regarding structural safety and risk. The performance is primarily characterized in terms of expected damage to various structural and nonstructural components and building contents. Since the structural damage implies an inelastic behavior, the traditional design and analysis procedures that are based on linear elastic behavior can only implicitly predict the performance. By contrast, the objective of nonlinear seismic analysis procedures is to directly estimate the magnitude of inelastic seismic demands.
The generic procedure for the determination of nonlinear seismic demands involves a number of key steps as shown in Figure above. The engineer is first required to set up a computer model which is expected to mimic the behavior of the actual structure. The anticipated seismic shaking is characterized after a seismic hazard analysis while accounting for various site-specific phenomenon. A suitable analysis procedure is then applied to the structural model considering all important loading scenarios. This results in the predictions of engineering demand parameters (EDPs) which can be subsequently compared with an acceptance criteria (generally prescribed by the seismic evaluation guidelines) to determine the seismic performance of the building. The EDPs normally comprise of global displacements (e.g. at roof or at any other reference point), inter-story drifts, story forces, component distortions, and component forces (FEMA 440, 2005). The level of complexity in this overall process may vary widely depending upon the choice of modeling scheme and analysis procedure, as well as the required degree of accuracy.
Although the seismic evaluation guidelines allow the use of approximate analysis procedures for conventional low- to mid-rise structures, the detailed NLRHA procedure is still recommended as a final check especially for the structures having extraordinary importance or those with special features.
