ABSTRACT
In the introduction, this paper reviews the lessons learned from significant earthquakes that have recently occurred regarding the seismic risks in urban areas, concluding that these risks are increasing rather than decreasing and that one of the most effective ways to reverse such situation in future significant earthquakes that can occur under or near urban areas is through first the development of more reliable seismic standards and code provisions than those currently available, and then their stringent implementation for the complete engineering (design, construction, maintenance, and monitoring of the occupancy) of new civil engineering facilities and for the evaluation of the seismic vulnerability and upgrading of existing hazardous facilities. It is emphasized the need for a comprehensive approach for development and implementation of the next and more reliable generation of standards and codes which include consideration of all the aspects involved in the engineering of the earthquake-resistant facilities. A promising approach for such needed development is what has been proposed as Performance-Based Seismic Engineering (P-B SE). After a brief discussion of what is understood by P-B SE, this paper is devoted to its main objective - that is, first to present an overview of the main issues involved in the development of reliable P-B SE code provisions (that can be implemented effectively) and that the experts from each of the different areas should then discuss and collaborate in finding the solutions or at least address the directions toward their solutions. Emphasis is placed on the main issues involved in the development of new reliable methodologies for Performance-Based Earthquake-Resistant Design (P-B EQ-RD) of new buildings and for the seismic upgrading of existing buildings. To facilitate the attainment of the above objectives, the paper presents a critical review of the first two resource documents that have been published in the USA regarding: (1) the development of a conceptual framework for P-B SE and the different methodologies that have been proposed for the application of such framework to the design, construction, and maintenance of new buildings, with emphasis on P-B EQ-RD of new buildings [ SEAOC Vision 2000 Committee Report, 1995 ]; and (2) on the NEHRP Guidelines for the Seismic Rehabilitation of Buildings [1996 ]. From this review the following main observations are made.
First, there is a need to recognize that the buildings are subjected not only to excitations due to the potential sources of seismic hazards but to other types of excitations which are acting before, during, and after the seismic excitations, thus it is proposed to change P-B SE and P-B EQ-RD with “Performance-Based Engineering (P-BE)” and “Performance-Based Design (P-BD)” when the seismic excitations are significant. The reason for the need to consider at the same time the critical combinations of all possible excitations that can act simultaneously with the seismic excitations is the fact that usually it is necessary to accept damage (inelastic behavior) under moderate and particulary major levels of EQ hazards whose prediction cannot be reliably obtained considering the effects of the different excitations independently. The effects of gravity excitations acting together with severe seismic excitations are discussed and illustrated through an example.
Secondly, there is a need for reaching an agreement regarding: first, on what is understood by P-BE by reviewing the definitions that have already been proposed and improving them until a concise but precise definition of P-BE is formulated and agreed upon; and then on what constitutes an optimal comprehensive conceptual framework for P-BE, and an efficient Performance Design Objective Matrix (PDOM), recommending that these be done in the light of the framework and PDOM proposed by the SEAOC Vision 2000 Committee. It is also pointed out the urgent need of complementing the formulations of the PDOM with a three dimensional chart which indicates the life-cycle costs for each performance design objective.
Thirdly, the role and importance of conducting a thorough and reliable Site Suitability Analysis (SSA) is something that has not been emphasized enough in the seismic codes that are presently enforced as well as in the 2new proposed conceptual framework and guidelines for P-BE. To conduct such analysis it is necessary to obtain the following information and make the following identifications: (I) site location and site conditions (soil profile and topography); (ii) identification of the seismic activity at the regions and site of the facility (locations of the EQ sources, and EQ mechanisms); (iii) information of sources of potential hazards at the site and their damage potential; (iv) identification of the potential seismic hazards that may control EQ-RD at the different desired levels of building performance (limit states of mechanical behavior) [Earthquake Ground Motions(EQGMs), surface fault rupture, soil failures (liquefaction, landslides, lateral spreading, differential compaction, etc.), tsunamis, floods, fires, and other potential hazards that can be created by adjacent facilities]. There is a need to not only develop conceptual guidelines regarding how to conduct the SSA but also numerical methodologies to quantify such guidelines so a reliable decision can be made regarding the suitability of the selected site. Even in the case that the only important potential source of seismic hazards is the EQGMs, the current code methodology of using just a general acceleration response spectrum constructed by plotting mapped response acceleration parameters is not enough. There is a need for deriving response spectra for the Energy Input (EI) and its associated energy parameters, and a more reliable microzonation of urban areas than those which are currently present.
Fourthly, for successful practical implementation of P-B EQ-RD and therefore P-BE, it is necessary first to improve not only the way that the PDOM is defined but also the reliability with which: the different performance levels (limit states) are quantified; and the so-called levels of design EQs are established and their demands on the entire building system are estimated. For the case that the main potential source of potential EQ hazards are due to the ground shaking (EQGMs) and that these can have different types of time history (from impulsive to periodic with long duration), it is of the utmost importance to recognize that these different types of EQGM time histories can result in completely different performance demands. These demands cannot be obtained from using just one specified general linear elastic pseudo-acceleration response spectrum from which the design spectra for all the different levels of design EQs can be obtained by just modifying (scaling) the given general response spectrum as it has been recommended in the NEHRP Guidelines. The need for considering the different types of time histories of the EQGMs and the use of energy concepts to formulate reliable design EQs are discussed in detail. Secondly, to facilitate the proper selection of the minimum performance design objectives (which should be made by the client in consultation with the designer or team of designers), it is highly desirable to prepare three-dimensional charts where the total expected life-cycle cost for each of the performance design objectives is plotted. Furthermore, it has to be made clear that for achieving a proper P-B EQ-RD it is necessary to consider for each EQ hazard (design) level, a desirable performance and no matter how the design is conducted (for one, two, three, or four performance design objectives), the acceptability checks (analyses) should be conducted for all the selected minimum performance design objectives. The proper selection of the performance design objectives is the key step in the conceptual framework for P-B EQ-RD because such selection sets the design criteria that should be followed throughout the entire design process.
Fifthly, there is a need to emphasize the importance of starting the EQ-RD process with an efficient conceptual overall design of the entire building system. To facilitate the achievement of such a conceptual design, it is necessary to formulate reliable guidelines. Although the Vision 2000 report contains certain guidelines, there is a need to improve them, particularly including guidelines that have been derived from stochastic studies of the importance of designing structures with overstrength, ductility, and redundancy and of their interdependence. From analysis of the results obtained in some of the studies that have been conducted, it is shown that in view of the large Coefficient of Variation (COV) in the estimation of the demands (particularly due to the uncertainties in predicting the time history of the critical EQGMs) to take advantage of the redundancy against the effects of the EQGMs it is necessary to provide the structure with high overstrength and ductility and/or to reduce as much as possible the COVs in the prediction of the demands. It is also highly desirable to formulate more reliable guidelines on how to select the configuration, structural layout, and particularly structural system, foundation, and non-structural components, considering from the start the advantages and disadvantages of using the conventional (or traditional) strategies or approaches to EQ-RD as well as the innovative strategies of controlling the response of the entire facility. It is recommended that these selections be based on Energy Concepts through the use of the Energy Balance Equation. A flowchart of the different approaches (strategies) and of the techniques and devices available is proposed to guide the selection of a proper strategy (system) and technique according to the type of critical EQGM controlling the design at each desired performance level.
Sixthly, there is a need for improving the quantification of the comprehensive conceptual approach for P-B EQ-RD proposed in the Vision 2000 Report so it can be used to carry out studies for reliable calibration of the simplified methods that have already been proposed (or need to be developed) to be included in future seismic 3codes. There is an urgent need to find out what are the restrictions in siting, configuration, types of excitations, structural layout, and structural systems that have to be specified when such simplified methods are used.
Finally, after a brief discussion of the author’s thoughts on the future directions regarding the formulation and implementation of simple but reliable seismic code provisions based on P-BE and of the research, education, and implementation needs to attain and apply such reliable codes in a short time, the following pleas are made: Firstly, to all of the experts in the different disciplines involving the problem of reducing the seismic risks in our urban areas and who are participating in this workshop, that they collaborate closely toward a timely solution to this problem by a timely development of reliable seismic model code based on P-BE taking advantage of the already published resource documents; and secondly to each participant, that he/she devote efforts to identify among the research and/or development results that he/she has obtained those that can be immediately applied in the field to reduce future EQ hazards.
