Athanasia K. Kazantzi

Performance-Based Seismic Design: Avant-Garde and Code-Compatible Approaches

AbstractCurrent force-based codes for the seismic design of structures use design spectra and system-specific behavior factors to satisfy one or two predefined structural limit states . In contrast, performance-based seismic design aims to design a structure to fulfill any number of target performance objectives, defined as user-prescribed levels of structural response, loss, or casualties to be exceeded at a mean annual frequency less than a given maximum. First, a review of recent advances in probabilistic performance assessment is offered. Second, the salient characteristics of methodologies that have been proposed to solve the inverse problem of design are discussed. Finally, an alternative approach is proposed that relies on a new format for visualizing seismic performance, termed yield frequency spectra (YFS). YFS offer a unique view of the entire solution space for structural performance of a surrogate single-degree-of-freedom oscillator, incorporating uncertainty and propagating it to the output r...

A NEXT GENERATION SCALAR INTENSITY MEASURE FOR ANALYTICAL VULNERABILITY STUDIES

To assess the seismic performance of a structural system within an analytical context, we need, among others, to specify a ground motion Intensity Measure (IM). The wary IM selection is undoubtedly an important step towards the successful implementation of a risk assessment, since insufficient and/or inefficient IMs can induce unwanted bias and variance in the vulnerability estimates. Supplementary issues related to practicality, necessitate the use of IMs for which ground motion prediction relationships exist, such as the elastic response spectral values (i.e. acceleration, velocity and displacement). Several past studies suggested as an improvement the use of IMs defined as the geometric mean of spectral acceleration values computed over a period range. The latter range may span between periods that are below, at or above the fundamental one. Some of these choices were proven to significantly improve both efficiency and sufficiency of the IM compared to more commonly used counterparts. This study investigates the efficiency and sufficiency of a newly developed scalar IM that combines the geometric mean IM concept with the significant duration of the ground motions. Improving the geometric mean IMs via including the significant duration of the ground motions, was driven by recent findings suggesting there is a strong tie between the collapse capacity of a structure and the ground motion duration. Hence, the performance of the proposed next generation IM is addressed in detail by means of comparisons and statistical significance tests. The testing is performed at specific levels of local engineering demand parameters that are closely related to losses, using a testbed capacity-designed steel moment-resisting frame. It was demonstrated that ground motion duration is closely related to the collapse capacity whereas its effect at lower demand levels is insignificant. Hence, the proposed IM may be employed to improve the estimates in collapse assessment studies. Nevertheless, at least for steel moment-resisting frame buildings that exhibit moderate cyclic degradation rates and sustain most losses prior to the global collapse state, the significant duration is anticipated to only minimally affect the evaluated vulnerability and consequently may be disregarded. Athanasia K. Kazantzi and Dimitrios Vamvatsikos

The Hysteretic Energy as a Performance Measure in Analytical Studies

Hysteretic energy dissipation is often employed as a measure of performance for systems subjected to earthquake excitation. This mainly stems from quasi-static cyclic tests where fuller hysteresis loops (i.e., higher energy absorption) are taken to indicate better performance when comparing systems with similar strength under the same cyclic loading protocol. However, seismic loading offers a different proving ground, where energy absorption is strongly correlated with energy input, while the nonstationary loads imply that the beneficial hysteretic effects observed in a cyclic test may never be realized. Given the current state of art in models and methods of performance-based earthquake engineering, we ask whether earthquake records at a given seismic intensity will cause peak/residual displacements or accelerations that favor models having fuller hysteresis. Using incremental dynamic analysis on story-level oscillators with varying hysteretic characteristics, it is demonstrated that hysteretic energy dissipation does not consistently correlate with seismic performance.

DEVELOPMENT OF FEMA P-58 COMPATIBLE STORY LOSS FUNCTIONS: STEEL OFFICE BUILDINGS IN HIGH SEISMICITY REGIONS

Since their release in 2012, the FEMA P-58 guidelines for seismic performance assessment of buildings have been regarded as the state-of-the-art paradigm for buildingspecific risk assessment and loss estimation. The latter is carried out through a rigorous component-by-component procedure that requires a complete component inventory, along with fragility and repair cost information. A fully compatible story-based approach is investigated herein as a simplified alternative that can potentially reduce the required input data with only a minor drop in accuracy. As an example, a set of story loss functions relating story repair cost with story-level engineering demand parameters are derived for standard inventory makeups of low/midrise steel office buildings. Preliminary results for a 4-story steel building show a promising balance between accuracy and simplicity. 1843 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 1843-1856

Analytical seismic vulnerability assessment for a class of modern low-rise steel frames

A set of guidelines was developed for the Global Earthquake Model (GEM), aiming to offer a practical, yet sufficiently accurate, analytical method for assessing the relationship between the ground shaking and the repair cost for a building class. The present work illustrates the methodology for a class of modern low-rise steel moment-resisting frames (SMRFs). The structural analysis is performed using Incremental Dynamic Analysis (IDA). The selection of a single Intensity Measure (IM) to parameterize IDA results and, eventually, vulnerability curves is being tackled through an extended IM comparison study across the entire structural response range considering both interstory drifts and peak floor accelerations. It is demonstrated that scalar IMs, defined as the geometric mean of spectral accelerations values Sagm(Ti) estimated at several periods Ti can have an overall satisfactory performance. Once the uncertain structural response is determined, the methodology proceeds to the vulnerability estimation and consequently to loss assessment that is built upon a simplified componentbased FEMA P-58 style methodology. The end product of this study is a high-quality set of vulnerability curves whose weighted moments are taken as the uncertain vulnerability function of the investigated building class.

SEISMIC FRAGILITY AND VULNERABILITY ASSESSMENT USING SIMPLIFIED METHODS FOR THE GLOBAL EARTHQUAKE MODEL

The Global Earthquake Model (GEM) has commissioned the preparation of analytical vulnerability guidelines for general use. Within this framework, a distinct modeling and analysis method hierarchy has been proposed, whereby both detailed and reduced-order models can be analyzed using nonlinear static or dynamic methods. Each subsequent reduction in complexity increases the speed of application, yet generates additional error that needs to be considered in the form of epistemic uncertainty. The available choices represent different levels of compromise between the accuracy achieved and the associated effort needed, meant to suit users having different levels of expertise and resource availability. Our particular focus will be on the middle path that is expected to become the most popular choice, combining (a) a simplified stick model of the structure with (b) a static pushover analysis with accurate record-to-record dispersion information. The entire procedure is cast within an appropriate probabilistic framework that can effortlessly incorporate all the epistemic and aleatory uncertainty sources to become a viable path for evaluating structural fragility for a building class.