Seismic Reliability Evaluation of Structural Systems for Different Soil Conditions

Introduction

After major earthquakes, structures in the affected areas suffer a considerable amount of damage in most cases. The economic consequences of the seismic events in the adjacent areas can be catastrophic. Some of the structures suffer minor damage, others more severe requiring repair before re-occupation, and few others collapse or become unusable. In some parts of the world, for example in Mexico City, structures suffer more damages in a very limited area compared to just outside certain boundaries. Experts believe that it is due to the fact that Mexico City is located above a dry lake bed. During the mid-90s, the devastating damages caused by the Northridge earthquake in the U.S., Kobe earthquake in Japan, and some other parts of the world prompted the profession to critically evaluate the seismic design procedures practiced at that time. The comprehensive evaluations produced several important outcomes. Protecting human lives was the major design consideration during this period. For the brevity of discussion, it will be denoted as the pre-Northridge design criterion. Considering an enormous amount of economic consequence, in the post-Northridge era, the professionals decided to alter the design criterion from the safety of human life to minimizing loss of economic consequences, at least in the U.S. The effort was initially financed by the Federal Emergency Management Agency (FEMA). Several major research and experimental studies were conducted. One of the first publications of this report was FEMA 273. Other major findings were published as a series of reports denoted as FEMA- 350, 351, 352, 353, 355C, and 355F. These studies prompted FEMA to advocate for the major change in the seismic design of structures. It is commonly described as Performance-Based Seismic Design (PBSD). The PBSD concept was proposed by the joint venture of SAC, consisting of Structural Engineers Association of California (SEAOC), Applied Technology Council (ATC), and California Universities for Research in Earthquake Engineering (CUREE).

The PBSD concept is expected to be incorporated in the next generation seismic design guidelines. It essentially allows to design a structure by considering a predefined amount of damage the stakeholders are willing to accept and the corresponding economic consequences. Naturally, this gives options to design structures satisfying different performance levels. FEMA 350 defined three performance levels; they range from immediate occupancy to collapse prevention. Immediate Occupancy (IO) represents the performance of a building that has sustained minimal or no damage to its structural elements and only minor damage to its nonstructural components. Life Safety (LS) level represents a structure with extensive damage; the risk to life is low, but repairs may be required before re-occupancy. Collapse Prevention (CP) level is represented by a building that has suffered extensive structural damage; total collapse is avoided, but structural repairs are uneconomical. PBSD criteria are usually established on the basis of multiple target performance levels. At present, the performance level is defined in terms of serviceability; the overall and the inter-story drift. FEMA 350 proposed the performance levels for CP, LS, and IO in terms of earthquake return period, probability of exceedance, and allowable drifts. The information is summarized in Table 1. The allowable drift is a function of H; it represents the total height of the building for the overall allowable drift at the top of the roof. For the allowable inter-story drift, H represents the story height.