Archetype Development
4.3 Performance Groups
Index archetype configurations are assembled into performance groups (or bins) that reflect major differences in configuration, design gravity and seismic load intensity, structural period, and other factors that may significantly affect seismic behavior within the archetype design space.
Performance groups should contain multiple index archetype configurations that reflect the expected range of permissible variation in size and other key parameters defined by the archetype design space. For example, each performance group should contain index archetype configurations that cover the range of building heights (up to the limits that can be assessed using the specified ground motions) permitted by the system design requirements.
Binning of index archetype configurations into performance groups provides the basis for statistical assessment of minimum and average collapse margin ratios for performance evaluation in Chapter 7. Performance group
populations should not be made larger than necessary, biased towards certain configurations, or otherwise manipulated to bias the average collapse
statistics for the bin. Binning of index archetype configurations into performance groups should be reviewed and approved by the peer review panel.
4.3.1 Identification of Performance Groups
As illustrated by the generic performance group table shown in Table 4-3, performance groups should be organized to consider: (1) basic structural configuration; (2) gravity load level; (3) seismic design category; and (4) period domain. The number of basic structural configurations will vary by system (i.e., 1 through N), and variation in gravity load levels may (or may not) affect the performance of certain systems. As a minimum, assuming one basic structural configuration and no dependence on gravity loads, all
systems will have at least four performance groups based on combinations of two seismic design levels and two period domains. These parameters should be used as a guide in establishing performance groups, as follows:
Basic Structural Configuration. Changes in the basic structural configuration are intended to capture major variations in the seismic-force-resisting system that are permissible within the design
requirements and are likely to affect the structural response. Examples of alternative configurations that could be separated into performance groups include: (1) variations in bracing configurations (e.g., X-bracing versus chevron bracing) in steel concentrically braced frames; (2) variations in framing spans and story heights in moment frames; and (3) variations in shear wall aspect ratios that may influence whether a wall responds in shear or flexural behavior.
Gravity Load Level. To the extent that the gravity load intensity and distribution of gravity loads affect the response of a seismic-force-resisting system, gravity loads should be varied in the index archetype
Variation in gravity load level is related to the intensity of the gravity load (as affected by the type of gravity framing and use of the structure) and the amount of seismic mass that is tributary to the seismic-force-resisting system in the form of directly applied gravity loads. In moment frames, tributary gravity loads can be distinguished through the use of perimeter frames versus space frame configurations. In wall systems, differences can be distinguished through the use of bearing walls (where most of the gravity loads are directly supported by the walls) versus non-bearing walls (where gravity loads are supported by other means, such as frames).
Table 4-3 Generic Performance Group Matrix Performance Group Summary
Design Load Level Period Domain
Seismic Design Category. In concept, the full range of Seismic Design Categories for which the seismic-force-resisting system will be permitted
should be reflected in the index archetype configurations and separated into different performance groups. Generally, however, it should suffice to check a system for the maximum and minimum spectral intensities of the highest Seismic Design Category (SDC) in which the system will be permitted. For example, systems intended for SDC D should be
designed and assessed for the maximum and minimum spectral acceleration values (SDC Dmax and SDC Dmin) as given in Table 5-1A and Table 5-1B of Chapter 5. Usually, designs for the maximum spectral acceleration of the highest Seismic Design Category will control the collapse performance of the system, which is an indication that
assessment of performance in lower Seismic Design Categories will not be required. If, however, the minimum spectral acceleration of the highest Seismic Design Category controls performance, then the system should also be designed and assessed for the minimum spectral
acceleration of the next lowest Seismic Design Category.
Period Domain. Differences in fundamental period, T, between short-period and long-short-period systems should be reflected in the index archetype configurations and separated into different performance groups. Period domain is described in Chapter 5, and defined by the boundary between the constant acceleration and constant velocity regions of the design spectrum. Since, within a given structural system, building period varies with the building height, bins of short-period and long-period archetypes will typically be distinguished by building height (or number of stories). The range of index archetypes should generally extend from short-period configurations for one-story buildings up to long-period configurations for the tallest practical buildings within the archetype design space. For the potentially limited number of short-period building archetypes, development of index archetypes should consider variations in both the number of stories and the story height.
Each performance group should include at least three index archetypes.
There is no maximum number of archetypes in each performance group, but it is expected that each group will typically have three to six index archetype configurations. This minimum requirement may be waived if it is infeasible to have three alternative designs within a specific performance group. For example, in the case of flexible moment frame systems, it may not be possible to have three distinct index archetype configurations within the short-period domain. Further guidance on the minimum number of performance groups, and index archetype configurations in each group, is provided in Chapter 7.