Damage of structures can be significantly reduced through robust prediction of possible future earthquakes that can occur during the life-time of the structure and through accurate modeling of the nonlinear behavior of the structure under seismic loads. Modern seismic codes specify natural records and artificially generated ground accelerations as input to the nonlinear time-history analysis of the structure. The advantage of using natural records is the inclusion of all important characteristics of the ground motion (fault properties, path effects and local soil condition) in the design input. This option requires selecting and scaling a set of proper accelerograms from the available records. However, the site under consideration may have limited or scarce earthquake data. In such case, numerically simulated ground motions can be employed as input to the dynamic analysis of the structure. This chapter deals with the damage assessment of inelastic structures under numerically simulated critical earthquakes using nonlinear optimization, inelastic time-history analysis, and damage indices.
Top1. Introduction
Damage indices describe the state of the structural damage and correlate well with actual damage displayed during earthquakes. The critical ground motion for a give structure is estimated by solving an inverse nonlinear dynamic problem in time domain using constrained nonlinear optimization techniques. The critical excitation method relies on the high uncertainty associated with the occurrence of the earthquake phenomenon and on the safety requirements of important and lifeline structures. The earthquake input is taken to maximize the damage index of the structure while satisfying predefined constraints that are quantified from the earthquake data available at the site. Numerical illustrations for damage assessment of one-storey and two-storey plane frame structures under possible future earthquakes are presented.
Earthquakes continue to claim thousands of lives and to damage structures every year (Comartin et al, 2004). Each earthquake brings out new surprises and lessons with it. In fact, the unexpected loss of lives and the severe damage of infrastructures and buildings during past strong earthquakes (e.g., 1994 Northridge, 1995 Kobe, 2010 Haiti and the most recent 2011 Tohoku earthquakes) have raised significant concern and questions on life safety and performance of engineering structures under possible future earthquakes. The occurrence of strong earthquakes in densely populated regions, especially in developing countries with vulnerable building stock and fragile infrastructure, could lead to catastrophic consequences. A notable example is the 2010 Haiti earthquake that killed 250,000 people and left a long-term suffering for the residents of this developing country (USGS/EERI 2010). On the other hand, the severe damage caused by the 2011 Tohoku earthquake and associated tsunami in Japan has raised significant challenges to one of the most developed countries as well (Takewaki et al, 2011). Hence, the assessment of seismic performance of structures under strong ground motions is an important problem in earthquake engineering. Structures need to resist unknown future earthquakes which adds more complexity to the problem (Moustafa 2011, 2009, Moustafa & Takewaki 2010a, Abbas & Manohar 2007, Takewaki 2002a, 2007). The consideration of the earthquake inherent uncertainty, the variability in the structure parameters and modeling the nonlinear behavior of the structure is essential for the accurate prediction of the actual response of the structure. Earthquake uncertainties include time, location, magnitude, duration, frequency content and amplitude, referred to as aleatory uncertainties.
The earthquake-resistant design of structures has been an active area of research for many decades (e.g., Penelis & Kappos 1997). The structural engineer aims to ensure safe performance of the structure under possible future earthquakes while maintaining optimal use of the construction material. The design objectives in current seismic building codes are to ensure life safety and to prevent damage of the structure in minor and moderate frequent earthquakes, and to control local and global damage (prevent total collapse) and reduce life loss in a rare major earthquake. This can be achieved through: (1) robust prediction of expected future strong ground motions at the site, (2) accurate modeling of the material behavior under seismic loads, and (3) optimal distribution of the construction material.