In order to characterize seismic performance while formulating results to address specific decision making needs of various stakeholders, three types of building-specific evaluations are conducted:
Scenario-based; Intensity-based;
Time-based assessments.
A flowchart of the general BSPE procedure, as well as that corresponding to the scenario- based, intensity-based and time-based evaluations, is illustrated in Figure 3-3. While the overall flowchart and performance metrics, which are direct outputs of the numerical modelling approaches previously described, are similar, the results are conditioned on the occurrence of different events, as described in more detail in the following sections.
NUMERICAL MODELLING TECHNIQUES DEVELOPMENT OF REPRESENTATIVE ARCHETYPES EXISTING TALL BUILDING STOCK DATABASE Economic Loss Downtime Recovery Functions Building Characteristics Geographical Distribution
UBC Archetype Building (Old)
IBC Archetype Building (New)
Structural Response
Figure 3-3: Flowchart for building specific evaluations including general evaluation procedure, scenario-based, intensity-based and time-based evaluations. GENERAL EVALUATION PROCEDURE SCENARIO BASED EVALUATION INTENSITY BASED EVALUATION TIME BASED EVALUATION
Seismic Hazard & Ground Motion (GM) Selection Structural Response Simulation via NLRHA Building Performance Evaluation Building Performance Metrics HayWired Earthquake Scenario (3D GM Simulation)
San Francisco & Oakland 40- & 20- Storey Buildings Demand Parameters Loss Downtime Performance under “Expected” Earthquake Shaking Design Level Earthquake & Associated GM Suite 40-Storey Buildings: -Archetype Building -Conceptual Retrofits Demand Parameters Loss Downtime Performance under “Design-Level” Earthquake Shaking Range of Earthquake Intensities & Associated GM Suites 50-Story Buildings: -UBC Archetype (Old) -IBC Archetype (New)
Demand Parameters Loss & Downtime Recovery Functions
Range of Risk Metrics (annual rates of collapse, loss, etc.)
BSPE: Scenario-based assessment
Scenario-based assessments evaluate the expected performance of a building subjected to a user-specified earthquake event, consisting of a specific magnitude earthquake occurring in a specific seismic source, at a specific location (distance) relative to the building site. In general, it is easier for communities at risk and policymakers to relate to the results of a scenario-based assessment than to other evaluations which express earthquake occurrence probabilistically (SPUR 2012). Scenario-based assessments are useful for buildings located in close proximity to active faults and can also be used to evaluate performance under a historic earthquake (Whittaker et al. 2007). The HayWired scenario, a Mw7.0 earthquake on the Hayward fault, developed to study impacts on the San Francisco Bay area (Detweiler and Wein 2017) is selected for this evaluation. The hypothetical HayWired earthquake is used to examine the well- known earthquake hazard of the Hayward Fault, with a focus on newly emerging vulnerabilities (Hudnut et al. 2017). The focus of the assessment is in the evaluation of response parameters such as peak transient and residual drifts in each storey, peak storey accelerations and inelastic deformations in fracture-prone beam-to-column moment connections. These response parameters are selected, as described in Section 2.2.3, because of how well they correlate with damage predictions. A summary of the economic loss and downtime estimates associated with the computed response parameters is also provided as part of this assessment. The scenario- based assessment provides an initial understanding of the expected performance of tall WSMRF buildings in the San Francisco Bay area under a realistic earthquake, highlighting the likely impacts on these structures such that steps can be taken, if necessary, to change negative outcomes and reduce future risk (objective 1, as outlined in section 1.2).
This scenario-based evaluation provides an understanding of seismic performance conditioned on a single event developed to provide science for decision-making, and to engage potential users of the information throughout the scenario development process (Hudnut et al. 2017). Furthermore, the results of this evaluation provide a reference point against more comprehensive risk-based evaluations such as intensity-based or time-based assessments. The scenario earthquake ground motions are developed through a 3D numerical simulation. The 3D model is conditioned on one particular hypocentre, one realization of slip distribution, and one particular simulation of high-frequency motion (Porter 2017). It aims to provide a single outcome in terms of shaking. This method is different from the more common approach to study earthquake scenarios by means of ground motion prediction equations (GMPEs), which is also be utilized later in this study (Chapter 8).
BSPE: Intensity-based assessment
Intensity-based assessments evaluate the expected performance of a building conditioned on a specified intensity of ground shaking. For instance, performance can be evaluated under a shaking intensity, defined by a target response spectrum, representative of the expected shaking under a specified return period, e.g. 475 years. These evaluations are frequently used in the engineering community to evaluate the seismic performance of existing buildings or for new designs. This type of assessment can be used to evaluate performance under the design earthquake shaking specified within a building code. The objective of this evaluation is to enable an understanding of the expected behaviour of existing WSMRF buildings under a ground motion shaking intensity consistent with the design earthquake hazard level defined in modern building codes (IBC 2012) (objective 2, as outlined in section 1.2). The results of such evaluation enable understanding whether expected performance complies with the objectives implicit in code-prescriptive design standards. In order to influence decision making, the results report the expected consequences in terms of direct economic losses, and downtime. Additionally, a number of strategies to achieve increased levels of resilience, including seismic improvements to the structural system, enhancement of non-structural components and systems, as well as mitigation measures to minimize recovery times are evaluated under the same intensity of ground motion shaking (objective 3, as outlined in section 1.2). The focus of the chapter is on the explicit consideration of downtime in the assessment methodology, going beyond damage and direct economic losses to consider repair and recovery times for an existing archetype building and an array of retrofit interventions. These results help design practitioners understand how different types of intervention influence the seismic performance of existing tall WSMRF buildings.
BSPE: Time-based assessment
Time-based assessments are the most comprehensive of the approaches, considering all earthquakes affecting a site and their risk of occurrence over a specified period of time. The period of time is generally one year (results indicate the annual rate of exceedance of a performance measure) or the design life of the building (50 to 100 years). This type of assessment is more comprehensive than a scenario or intensity-based assessment, as it evaluates performance over a range of ground motion levels, i.e. it consists of an array of intensity-based assessments. In this study, a time-based seismic performance assessment of two archetype tall buildings is carried out: a WSMRF designed following the requirements of the 1973 Uniform Building Code (UBC 1973), and a WSMRF designed following the 2012 International Building Code (IBC 2012). The goal of this work is to benchmark the performance of older existing tall
WSMRF buildings against modern designs (objective 6, as outlined in section 1.2). Furthermore, this work intends to verify compliance with the life-safety objective of modern codes under extreme events and provide an understanding of expected performance at other earthquake intensities. The study aims to evaluate performance at an array of earthquake intensities from levels that cause no damage up to levels that trigger collapse (objective 4, as outlined in section 1.2) in order to provide more advanced risk metrics, such as collapse rates, average annual loss (AAL) or average annual downtime (AAD) (objective 5, as outlined in section 1.2), than those obtained under a scenario-based or intensity-based assessment. Generally, the output of these evaluations are useful to catastrophe modellers and the insurance market. However, certain metrics inferred from such assessment, such as the annual rate of collapse (λc), can also provide valuable information to policy makers. Time-based assessments are also referred to as risk-based assessments (NEHRP 2011). In this study, the term time-based is preferred over risk-based because it is believed that the other BSPEs considered, such as the scenario-based or the intensity-based, are also risk-based assessments. If adopting the NEHRP (2011) nomenclature, the term ‘comprehensive risk-based’ assessment rather than simply risk- based assessment is believed to be more accurate when used to describe time-based assessments.