Helmut Krawinkler

Pros and cons of a pushover analysis of seismic performance evaluation

The static pushover analysis is becoming a popular tool for seismic performance evaluation of existing and new structures. The expectation is that the pushover analysis will provide adequate information on seismic demands imposed by the design ground motion on the structural system and its components. The purpose of the paper is to summarize basic concepts on which the pushover analysis can be based, assess the accuracy of pushover predictions, identify conditions under which the pushover will provide adequate information and, perhaps more importantly, identify cases in which the pushover predictions will be inadequate or even misleading.

Deterioration Modeling of Steel Components in Support of Collapse Prediction of Steel Moment Frames under Earthquake Loading

Reliable collapse assessment of structural systems under earthquake loading requires analytical models that are able to capture component deterioration in strength and stiffness. For calibration and validation of these models, a large set of experimental data is needed. This paper discusses the development of a database of experimental data of steel components and the use of this database for quantification of important parameters that affect the cyclic moment-rotation relationship at plastic hinge regions in beams. On the basis of information deduced from the steel component database, empirical relationships for modeling of precapping plastic rotation, postcapping rotation, and cyclic deterioration for beams with reduced beam section (RBS) and other-than-RBS beams are proposed. Quantitative information is also provided for modeling of the effective yield strength, postyield strength ratio, residual strength, and ductile tearing of steel components subjected to cyclic loading.

CUMULATIVE DAMAGE IN STEEL STRUCTURES SUBJECTED TO EARTHQUAKE GROUND MOTIONS

Abstract Experimental data are presented from low cycle fatigue tests of structural steel components. In these tests the failure modes of local buckling in beam flanges and fracture at weldments were studied in detail. Cumulative damage models are proposed which permit a life prediction for arbitrary cyclic loading histories. For the local buckling failure mode a series of linear damage models is used to predict deterioration threshold and deterioration, whereas a single model is used for the fracture mode. Crack propagation at weldments is modeled with a crack growth rate model based on the plastic strain range. Adequate predictions of lives were obtained from the analytical models.

Seismic drift and ductility demands and their dependence on ground motions

Implementation of performance-based earthquake engineering necessitates the probabilistic evaluation of engineering demand parameters that can be related to variables, such as monetary losses, on which quantitative seismic performance assessment can be based. The purpose of this paper is to identify relevant demand parameters, quantify these parameters for regular frame structures, and illustrate how statistically representative relationships between these parameters and ground motion intensity measures can be established. Emphasis is on the development of such relationships for ordinary ground motions, and on issues that have to be addressed in order to establish relationships between demand parameters and near-fault ground motions.

Estimation of seismic drift demands for frame structures

A process is outlined and evaluated for the estimation of seismic roof and storey drift demands for frame structures from the spectral displacement demand at the first mode period of the structure. The spectral displacement demand is related to the roof drift demand for the multi-degree-of-freedom (MDOF) structure using three modification factors, accounting for MDOF effects, inelasticity effects, and P-delta effects. Median values and measures of dispersion for the factors are obtained from elastic and inelastic time history analyses of nine steel moment resisting frame structures subjected to sets of ground motions representative of different hazard levels. The roof drift demand is related to the storey drift demands, with the results being strongly dependent on the number of stories and the ground motion characteristics. The relationships proposed in this paper should prove useful in the conceptual design phase, in estimating deformation demands for performance assessment, and in improving basic understanding of seismic behaviour. Copyright

Development and Utilization of Structural Component Databases for Performance-Based Earthquake Engineering

Performance-based earthquake engineering necessitates the development of reliable nonlinear analysis models that are able to sim- ulate the behavior of structures from the onset of damage through collapse. Thesemodels provide engineering demand parameters that are then related with damage measures and describe the damage of a building and its components. To accurately simulate dynamic response up to col- lapseofstructures,itisimportanttomodelstrengthandstiffnessdeteriorationofstructuralcomponentsinadditiontoP-D effects. These models require theuse of large setsof experimental datafor calibration of theirdeterioration parameters. This paperdiscusses thedevelopment of three databases on experimental data of steel W-beams, tubular hollow square steel columns, and RC beams. These databases are used for quantification of important parameters that affect the cyclic moment-rotation relationship at plastic hinge regions in steel and RC components. Emphasisisplacedonthepredictionofcollapseofbuildingscausedbyearthquakes.Theutilizationandimportanceofthethreedatabasesinthe contextofperformance-basedearthquakeengineeringisdemonstratedthroughacasestudyofa4-storysteelbuilding.Itsseismicperformanceis

Cyclic loading histories for seismic experimentation on structural components

In order to utilize results obtained from quasi-static cyclic load tests on structural components for a general performance evaluation, the need exists to establish loading histories that capture critical issues of component capacity as well as seismic demands. In inelastic seismic problems capacity and demands cannot be separated since one may strongly depend on the other. Because of cumulative damage issues the capacity depends on the number of inelastic excursions and the magnitude of each excursion (not just the largest one). These two parameters depend on the frequency content of the ground motion, the period(s) of the structure, and the strength and inelastic deformation characteristics of the structure. The paper presents procedures how these characteristics can be considered in the selection of suitable loading histories.

Strength Demand Issues Relevant for the Seismic Design of Moment-Resisting Frames

This paper deals with the evaluation of strength demands relevant for the seismic design of columns that are part of moment-resisting frames. Regular frames with fundamental periods from 0.3 sec. to 3.6 sec. and number of stories from 3 to 18 are investigated. An evaluation of the relationships between strength demands (e.g., story shear forces, story overturning moments, and moments in columns), ground motion intensity, fundamental period, and number of stories is the focus of this paper. The results from this study demonstrate that the magnitude and distribution over the height of maximum axial and shear forces in columns exposed to severe earthquakes often are not adequately estimated by current seismic design and analysis procedures (e.g., the nonlinear static pushover). Moreover, the potential of plastic hinging in columns is high for regular frames designed according to the strong-column/weak-beam requirements of current code provisions, and more stringent strong-column/weak-beam criteria appear to be called for. The presented results are intended to provide guidance for improvement of seismic design provisions to avoid brittle failure modes in columns of moment-resisting frames.

Design Concepts for Controlled Rocking of Self-Centering Steel-Braced Frames

AbstractThe self-centering rocking steel-braced frame is a high-performance system that can prevent major structural damage and minimize residual drifts during large earthquakes. It consists of braced steel frames that are designed to remain elastic and allowed to rock off their foundation. Overturning resistance is provided by elastic post-tensioning, which provides a reliable self-centering restoring force, and replaceable structural fuses that dissipate energy. The design concepts of this system are examined and contrasted with other conventional and self-centering seismic force resisting systems. Equations to predict the load-deformation behavior of the rocking system are developed. Key limit states are investigated including desired sequence of limit states and methods to help ensure reliable performance. Generalized design methods for controlling the limit states are developed. The design concepts are then applied to a six-story prototype structure to illustrate application of the rocking steel fram...

Performance Assessment of Steel Components

This paper presents a methodology for the assessment of cumulative damage in structural steel components subjected to cyclic inelastic loading histories of the type experienced in earthquakes. The methodology is based on low-cycle fatigue concepts and the hypothesis of linear damage accumulation. It will be shown that seismic performance of a component depends on two structural performance parameters and on the number and amplitudes of all inelastic excursions and not only on the maximum excursion. Experimental and analytical procedures for obtaining the parameters needed for a performance assessment are suggested in the paper.