Loading Histories for Cyclic Tests in Support of Performance Assessment of Structural Components

The need for comparable loading histories for seismic acceptance testing is becoming more prevalent as (a) performance-based design, which requires quantification of performance, is becoming a more widely accepted alternative to routine code design, (b) more and more innovative performance enhancement systems (e.g., buckling restrained braces) hit the engineering market, and (c) globalization becomes a widely accepted concept that necessitates globally accepted performance standards. Present codes, standards, and guidelines make reference to the need for performance assessment through testing, but with few exceptions (e.g., AISC 341-05 and testing of base isolation systems in ASCE 7-05) they remain mostly silent on testing and acceptance criteria to be used for this purpose. This paper provides a summary of (a) the background behind several of the presently employed cyclic loading histories, (b) basic concepts that should be considered in the development of loading histories, and (c) recent developments that point towards the need for modifications or expansion of presently employed loading protocols. . WHY DO WE HAVE TO BOTHER WITH LOADING PROTOCOLS All structural elements have limited strength and deformation capacities; and collapse safety as well as damage control depend on our ability to assess these capacities with some confidence. Implicitly, we lump our knowledge of these capacities into response modification coefficients (R-factors) for new structures (ASCE 7-05), and into m-factors (or estimates of plastic deformation capacities) for seismic retrofits (FEMA 273/356 and ASCE 41-06). For some cases our knowledge is adequate to assign reasonable values, for many cases it is not. So, we have to resort to testing (in addition to analytical modeling) to evaluate performance of many conventional components, and particularly of new and innovative components (or systems) that show much promise for enhanced performance. Unfortunately, in earthquake engineering, strength and deformation capacities depend (sometimes weakly and sometimes strongly) on cumulative damage, which implies that every component has a permanent memory of past damaging events and at any instance in time it will remember all the past excursions (or cycles) that have contributed to the deterioration in its state of health. Thus, performance depends on the history of previously applied damaging cycles, and the only reasonable way to assess the consequences of history (short of developing complex analytical models that can be used for damage state predictions) is to replicate, to the best we can, the load and deformation histories a component will undergo in an earthquake (or several earthquakes if this is appropriate). The objective of a loading protocol is to achieve this in a conservative, yet not too conservative, manner. 1 Dept. of Civil and Environmental Engineering, Stanford University; Stanford, CA 2 The need for representative loading histories is becoming more prevalent as performance-based seismic design, which requires quantification of performance, is becoming a more widely accepted alternative to routine code design, and as more and more innovative performance enhancement systems become available. Present codes, standards, and guidelines make reference to the need for performance assessment through testing in various sections. For instance, ASCE 7-05 states in Section 12.2.1, “Seismic force-resisting systems that are not contained in Table 12.2-1 are permitted if analytical and test data are submitted that establish the dynamic characteristics and demonstrate the lateral force resistance and energy dissipation capacity to be equivalent to the structural systems listed in Table 12.2-1 for equivalent response modification coefficient, R, system overstrength coefficient Ω0, and deflection amplification factor, Cd, values.” However, except for testing of isolators and dampers for verifying the properties used in design (Sections 17.8 and 18.9), this standards document remains silent on testing and acceptance criteria to be used for this purpose. CONCEPTS BEHIND DEVELOPMENT OF LOADING PROTOCOLS There is no unique and “best” loading history, because no two earthquakes are alike and because the specimen may be part of many different structural configurations. The overriding issue is to account for cumulative damage effects through cyclic loading. If there is no cumulative damage, there is no need for cyclic loading. The number and amplitudes of cycles applied to the specimen may be derived from analytical studies in which models of representative structural systems are subjected to representative earthquake ground motions and the response is evaluated statistically. Any loading protocol will always be a compromise that will provide deformation histories whose realism will depend on many parameters. For one, actual histories, as experienced in earthquakes, will depend on the intensity and frequency content (magnitude, distance, and soil type dependence) of the ground motion the specific component (or assembly) will be subjected to as part of the structural system. Equally important, the number and amplitudes of cycles the component will experience depend on the configuration, strength, stiffness, and modal properties (periods and participation factors) of the structure and on the deterioration characteristics of the structural systems and its components. Moreover, in most practical cases (except for near-fault ground motions) the actual sequence of cycles (excursions) as experienced in an earthquake has to be rearranged into cycles of increasing magnitude in order to avoid commitment to a single maximum amplitude that may have meaning only for a specific combination of ground motion and structural configuration. For reasons just quoted, there are many decisions and judgmental steps to be taken in order to come up with a compromise loading history that is conservative but statistically representative of the full range of ground motions and structural configurations. These decisions and steps are too elaborate to enumerate here. The reader is referred to the following references for detailed discussions: ATC-24, 1992, Krawinkler et al., 2000-a, and Krawinkler et al., 2000-b. The overriding issue is that cumulative damage concepts (of the type presented in Krawinkler et al., 1983) have to be employed in order to guide the decision process for loading history development.