As noted in the overview page, the FEMA P-2139 report series documents a large body of new knowledge related to the response behavior and collapse performance of short-period buildings. Below are some key findings and recommendations. Readers are encouraged to review the published reports for more complete discussions of these and other findings and recommendations.
- Modern, short-period buildings (i.e., fundamental periods less than 0.5 seconds) have performed well in past earthquakes, generally with collapse rates much less than the 10 percent collapse-safety objective of ASCE/SEI 7-16 for Risk Category II structures with MCER ground motions defined by “high-seismic” criteria (i.e., SMS = 1.5g).
- Why this matters: The collection and analysis of short-period building collapse performance data from past earthquakes provide significant evidence supporting the notion that modern short-period buildings have achieved, and often far exceeded, the collapse-safety objective of ASCE/SEI 7 when subjected to high-seismic MCER ground motions.
- With improved numerical modeling and representative building archetypes, computed MCER collapse probabilities decrease as the design period shortens, which is a reversal of the trend observed in prior collapse studies.
- Why this matters: The short-period building seismic performance paradox has been resolved through improved numerical modeling and representative building archetypes. Short-period buildings do not have systematically increased collapse probabilities as previously suggested.
IMPLICATIONS FOR SEISMIC DESIGN
- Incorporation of strength from elements not considered part of the designed SFRS has a significant effect on computed collapse probabilities for some systems.
- Why this matters: This observation came mainly from the wood light-frame archetypes but likely applies to other systems, like cold-formed steel. This observation points to a fundamental disconnect between the way in which these kinds of systems are designed and the way in which they actually behave in terms of collapse performance.
- Systems require larger displacement capacities to achieve the 10 percent collapse-safety objective of ASCE/SEI 7 for Risk Category II structures than those currently used by ASCE/SEI 7 to check deformation compatibility of components not part of the SFRS.
- Why this matters: This observation could point to a fundamental shortcoming in ASCE/SEI 7, leading to the design of some systems that are not able to achieve the 10 percent collapse-safety objective of ASCE/SEI 7.
- Allowing the design base shear to be reduced when soil-structure interaction and foundation flexibility are considered in the design of short-period buildings could lead to worse collapse performance.
- Why this matters: Current national standards, such as ASCE/SEI 7 and ASCE/SEI 41, allow for reduced design base shear when soil-structure interaction and foundation flexibility are considered, but the findings from this project suggest that this may not be justified.
INSIGHTS FOR FUTURE ANALYTICAL COLLAPSE STUDIES
- Archetype model maximum strength, displacement capacity, and fundamental period have a significant influence on computed collapse probability.
- Why this matters: Similar to previous analytical studies, this study found these parameters to be of critical importance in the computation of archetype collapse probability. This finding should inform the development of future analytical collapse studies and future developments of performance-based earthquake engineering.
- Increasing archetype model overstrength, defined here as the maximum base shear of a pushover curve divided by the design base shear for an archetype, decreases computed collapse probability.
- Why this matters: Similar to previous analytical studies, this study found this parameter to be of critical importance in the computation of archetype collapse probability. This finding should inform the development of future analytical collapse studies and future developments of performance-based earthquake engineering.
- Past analytical collapse studies of short-period buildings using single-degree-of-freedom models that relied on ductility-based collapse displacement limits were flawed. Drift-based collapse displacement limits are more realistic and lead to more reliable results for short-period buildings.
- Why this matters: This finding, which is documented in FEMA P-2139-1, contributed to solving the short-period building seismic performance paradox and should inform future analytical collapse studies of short-period buildings using simplified models.
NEED FOR FURTHER STUDY
- “Collapse surfaces” could provide a convenient means for determining the amount of strength, collapse displacement capacity, or both required to achieve a specific collapse performance objective, or conversely, to verify adequate collapse performance given building strength and displacement capacity.
- Why this matters: “Collapse surfaces” are an important new concept for earthquake engineering, first proposed and documented in FEMA P-2139-1. With more development, they could provide a valuable tool for performance-based earthquake engineering in the future.
- Archetypes designed and subjected to “very high-seismic” ground motions (i.e., SMS = 2.25g) show a consistent trend of computed collapse probabilities that exceed the 10 percent collapse-safety objective of ASCE/SEI 7 for Risk Category II structures.
- Why this matters: This study is not the first one to identify this issue, but the findings significantly expand the analytical evidence suggesting that buildings in “very high seismic” regions might not meet the ASCE/SEI 7 collapse safety objectives.
- Much of the existing body of physical test data fails to adequately characterize behavior up to deformations that are associated with system collapse. There is a need to develop test protocols that quantify necessary component and system deformation demands through collapse that can be used to calibrate and validate numerical models for accurate collapse simulations.
- Why this matters: The current lack of these test protocols could hinder future advances in analytical predictions of collapse performance and, more broadly, system behavior when subjected to strong ground motions.