Dimitrios G. Lignos

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.

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

Collapse Assessment of Steel Moment Frames Based on E-Defense Full-Scale Shake Table Collapse Tests

AbstractThis paper presents key parameters that affect numerical modeling of steel frame structures for reliable collapse simulations. The collapse assessment is based on experimental data obtained from a full-scale shaking table collapse test of a 4-story steel moment frame and a blind numerical analysis contest that was organized in parallel with the collapse test. It is shown that (1) there is no clear advantage between three-dimensional (3D) and 2D analyses in the prediction of a sidesway collapse mechanism for buildings with a regular plan view as in the case of study; (2) the assumption of Rayleigh damping leads to better predictions of structural response compared with stiffness proportional damping; and (3) accurate prediction of collapse necessitates that P-Δ effects always be considered in the analysis. It is also proven that accurate simulation of steel component deterioration is a key factor for reliable prediction of collapse behavior. On the basis of a synthesis of experimental and analytica...

Computational Approach for Collapse Assessment of Concentrically Braced Frames in Seismic Regions

This paper proposes a computational approach for the collapse assessment of concentrically braced frames (CBFs) subjected to earthquakes. Empirical formulations for modeling the postbuckling behavior and fracture of three main steel brace shapes that are commonly used in CBFs are developed. These formulations are based on extensive calibrations of a fiber-based steel brace model with available information from a recently developed steel brace database. As part of the same computational approach, the representation of strength and stiffness deterioration associated with plastic hinging in steel columns and gusset-plate beam-to-column connections is considered. Through a case study of a 12-story Special Concentrically Braced Frame (SCBF), the influence of classical damping on the collapse capacity of CBFs is investigated. It is demonstrated that when SCBFs attain a negative stiffness during an earthquake, their collapse capacity can be significantly overestimated, if viscous damping is based on a commonly employed Rayleigh assumption with initial stiffness approximation. It is shown that sidesway collapse of CBFs should be traced based on a combination of criteria that associate large story drift ratios and the story shear resistance of a CBF at the corresponding story drift ratios.

Use of Wavelet-Based Damage-Sensitive Features for Structural Damage Diagnosis Using Strong Motion Data

This paper introduces three wavelet-based damage-sensitive features (DSFs) extracted from structural responses recorded during earthquakes to diagnose structural damage. Because earthquake excitations are nonstationary, the wavelet transform, which represents data as a weighted sum of time-localized waves, is used to model the structural responses. These DSFs are defined as functions of wavelet energies at particular frequencies and specific times. The first DSF (DSF1) indicates how the wavelet energy at the original natural frequency of the structure changes as the damage progresses. The second DSF (DSF2) indicates how much the wavelet energy is spread out in time. The third DSF (DSF3) reflects how slowly the wavelet energy decays with time. The performance of these DSFs is validated using two sets of shake-table test data. The results show that as the damage extent increases, the DSF1 value decreases and the DSF2 and DSF3 values increase. Thus, these DSFs can be used to diagnose structural damage. The r...

Dynamic Response of a Chevron Concentrically Braced Frame

Large-scale shake table tests were conducted at E-Defense, Japan, to examine the dynamic response of a steel concentrically braced frame. The specimen was a single-bay, single-story frame with a pair of square hollow structural section braces placed in a chevron arrangement. The gusset plates connecting the brace to the framing elements were provided with an elliptic fold line to accommodate out-of-plane rotation of the brace in compression. The specimen was subjected repeatedly to a unidirectional ground motion with increasing magnitude until the braces buckled and eventually fractured. The bracing connections performed as intended; the gusset plates folded out of plane, and no crack was observed in the gusset plate or in the critical welds. Consequently, the test results demonstrated excellent performance of the bracing connections. Elastic deformation of the beam prevented the braces from developing their full tensile strength. Yielding in the middle of the beam, which was predicted by monotonic loading analysis, did not occur. The specimen response was reproduced by a numerical model using fiber elements. This model was able to predict the occurrence of brace buckling and fracture and thereby accurately trace the dynamic behavior of the frame.

Fragility Assessment of Reduced Beam Section Moment Connections

This paper presents fragility functions to estimate the probability of reaching or exceeding different damage states in reduced beam section (RBS) beam-to-column moment connections of steel moment resisting frames. The fragility functions are developed using results from 71 experimental tests that have been conducted on RBS connections during the past 14 years. The main sources of uncertainty considered are specimen-to-specimen variability of the interstory drifts associated with the various damage states and the epistemic uncertainty arising from using a limited number of experimental data and from interpreting experimental results. Quantitative measures for each of these two kinds of uncertainty were developed using statistical procedures. For a given peak interstory drift ratio the fragility functions developed herein permit the estimation of the probability of experiencing five different levels of damage in RBS moment connections.

Damage to Steel Buildings Observed after the 2011 Tohoku-Oki Earthquake

A joint U.S.–Japan reconnaissance team examined the damage to steel building structures caused by the 2011 Tohoku-oki earthquake. In the city of Sendai, where the peak horizontal ground acceleration exceeded 1 g, the majority of steel buildings performed well. Buildings that used older cladding systems for external finish sustained damage to their claddings even if their structural performance was excellent. Damage to a few braced frames offer insight into the seismic design of bracing connections. In areas attacked by the violent tsunami, many steel buildings stood upright after the tsunami subsided, although these buildings lost much of their external and internal finishes along with their contents. These steel buildings did not provide safe shelter for tsunami evacuation when the building submerged under the tsunami wave. A number of buildings suffered foundation failure, which was likely caused by scouring or liquefaction or a combination of multiple effects.

Reliability of a 4-Story Steel Moment-Resisting Frame Against Collapse Due to Seismic Excitations

This paper illustrates a process for estimating the uncertainty in estimation of collapse capacity of buildings in seismic excitation using Monte Carlo simulation and FOSM method. The structure used in this study is a 4-story steel moment-resisting frame designed based on current seismic provisions whose collapse prediction has been validated through a collapse test of a 1:8 scale model structure at the University of Buffalos NEES Equipment Site. It is shown that the uncertainty in estimation of collapse capacity due to uncertainty in estimation of deformation parameters of beams and columns that control component nonlinear behavior is moderately dependent on the correlation between these parameters. For the 4-story steel moment-resisting frame used in this study the uncertainty in estimation of collapse capacity due to component modeling uncertainties varies between 0.25 and 0.35 for correlation between these parameters ranging from 0.3 to 1.0. This is due to the fact that for the 4–story structure, the P-Δ effect is the major reason for collapse rather than building component deterioration. Results of this research show that the probability of collapse for the 4–story structure at the 2/50 hazard level can increase from 3% to 18% once a confidence of 90% is sought for the collapse probability.

EARTHQUAKE LOSS ASSESSMENT OF STEEL FRAME BUILDINGS DESIGNED IN HIGHLY SEISMIC REGIONS

In recent years, there is an increasing need to quantify earthquake-induced losses throughout the expected life of a building in order to evaluate alternative design options and to minimize repairs in the aftermath of an earthquake. For this reason, the next generation of performance-based earthquake engineering evaluation procedures has formalized procedures that assess several metrics of seismic performance including economic losses. This paper discusses an analytical study that quantifies the expected earthquake-induced losses in typical office steel buildings designed with perimeter special moment frames or perimeter concentrically braced frames at various ground motion intensities. These buildings are designed in urban California in accordance with today’s seismic design provisions in North America. The expected economic losses associated with repair are computed based on a refined performance-based earthquake engineering framework developed within the Pacific Earthquake Engineering Research (PEER) center. This framework integrates site-specific seismic hazard, state-of-the-art nonlinear models that incorporate complex deteriorating phenomena of the structural components of a steel frame building, fragility curves of structural and nonstructural components that express the probability of being or exceeding a specific damage level, and the resulting repair costs. The effect of residual deformations along the height of steel frame buildings on their earthquake losses is also examined. It is shown that repair costs in the aftermath of earthquakes vary significantly depending on the employed lateral load resisting system, as well as the analytical model representation of the steel frame building itself.