Farzin Zareian

Rotational Stiffness of Exposed Column Base Connections: Experiments and Analytical Models

AbstractThe rotational flexibility of column base connections is often ignored in structural simulation of moment frames in which the bases are assumed to be either fixed or pinned. Although structural response is sensitive to base flexibility, methods to properly characterize base flexibility are not readily available. Motivated by these issues, a new approach to characterize the rotational flexibility of exposed column base connections is presented in this paper. The method leverages existing design procedures for base connection design, such that the rotational stiffness may be calculated with modest effort once the design has been completed. The method is validated through comparison with nine experiments, which interrogate a range of parameters including base-plate size and thickness, level of axial load, anchor-rod strength, and concrete strength. The comparison between the test and the predicted values indicates that, on average, the method predicts the stiffness with accuracy, such that the averag...

Validation of ground-motion simulations for historical events using SDoF systems

The study presented in this paper is among the first in a series of studies toward the engineering validation of the hybrid broadband ground-motion simulation methodology by Graves and Pitarka (2010). This paper provides a statis- tical comparison between seismic demands of single degree of freedom (SDoF) systems subjected to past events using simulations and actual recordings. A number of SDoF systems are selected considering the following: (1) 16 oscillation periods between 0.1 and 6 s; (2) elastic case and four nonlinearity levels, from mildly inelastic to severely inelastic systems; and (3) two hysteretic behaviors, in particular, nondegrading-nonevolutionary and degrading-evolutionary. Demand spectra are de- rived in terms of peak and cyclic response, as well as their statistics for four historical earthquakes: 1979 Mw 6.5 Imperial Valley, 1989 Mw 6.8 Loma Prieta, 1992 Mw 7.2 Landers, and 1994 Mw 6.7 Northridge. The results of this study show that both elastic and inelastic demands from simu- lated and recorded motions are generally similar. However, for some structural sys- tems, the inelastic response to simulated accelerograms may produce median demands that appear different from those obtained using corresponding recorded motions. The magnitude of such differences depends on the SDoF period, the nonlinearity level, and, to a lesser extent, the hysteretic model used. In the case of peak response, these dis- crepancies are likely due to differences in the spectral shape, while the differences in terms of cyclic response can be explained by some integral parameters of ground motion (i.e., duration-related). Moreover, the intraevent standard deviation values of structural demands calculated from the simulations are generally lower than those given by recorded ground motions, especially at short periods. The assessment of the results using formal statistical hypothesis tests indicates that, in most cases, the dif- ferences found are not significant, increasing the trust in the use of simulated motions for engineering applications.

Effect of Column-Base Flexibility on the Seismic Response and Safety of Steel Moment-Resisting Frames

The effect of column-base flexibility on the response of steel moment frames is assessed through parametric simulation. The response of four frames (2-, 4-, 8-, and 12-story), designed as per current codes, is investigated through static push-over simulations and sophisticated nonlinear response-history simulations, including collapse simulation. For each frame, a range of base fixities is interrogated, including realistic values that are calculated from the designed connections. The results indicate that a reduction in base fixity alters the force distribution and the plastic mechanism, significantly reducing ductility capacity and strength, as well as collapse resilience, while increasing member forces. For the 4-, 8-, and 12-story frames, this trend suggests that the expected response of such frames is worse than is implied by simulations and design approaches that assume a fixed-base condition. However, the trend is beneficial for the 2-story frame, which is analyzed and designed assuming a pinned base.

Reconnaissance of the Chilean Wine Industry Affected by the 2010 Chile Offshore Maule Earthquake

This paper summarizes the EERI reconnaissance team findings on damage to the Chilean wine industry after the 27 February 2010 Offshore Maule Earthquake. Wine production is one of the major industries in Chile, with an annual production of approximately one million metric tons. It is estimated that the total loss to the wine industry is over 125 million liters, with infrastructure damage estimated as high as US

Structural System Parameter Selection Based on Collapse Potential of Buildings in Earthquakes

430. Most of the damage was concentrated in older wineries with collapse of adobe walls and timber roofs or ribbed brick vaults. Damage to steel fermentation tanks was widespread among all wineries visited with the severity of such damage depending on the type of tank anchorage. Local buckling of legs in legged tanks or excessive movement followed by the tank falling off the support pad led to toppling that ruptured piping or valves. Stacked barrels, stored bottles of wine, and production lines were also damaged.

Validation of Simulated Earthquake Ground Motions Based on Evolution of Intensity and Frequency Content

This paper attempts to provide insight into the sensitivity of the collapse capacity of moment-resisting frame and shear wall structures to variation in basic structural parameters, and the choice of an appropriate ground motion intensity measure, based on probabilistic estimation of the collapse capacity of a structural system. The effects of fundamental period and base shear strength and of deformation and deterioration properties of structural components on the collapse capacity of frame and wall structures are quantified. It is shown that the collapse potential of moment-resisting frames is highly sensitive to the ratio of column to beam strength; increasing this parameter from 1.2 to 2.4 will increase the median of collapse capacity by up to 90%. Using a scalar ground motion intensity measure for estimating the collapse capacity can lead to underestimation of median collapse capacity by up to 50%, compared to using a vector-valued intensity measure. The provided information can be used to assist in the selection of a suitable structural system and associated parameters in design for collapse safety. Closed-form solutions are formulated using a database of collapse fragility curves developed for the sensitivity study. Application of these closed-form solutions for design decision making is illustrated through a comprehensive example.

A Steel Component Database for Deterioration Modeling of Steel Beams with RBS under Cyclic Loading

Abstract Simulated earthquake ground motions can be used in many recent engineering applications that require time series as input excitations. However, applicability and validation of simulations are subjects of debate in the seismological and engineering communities. We propose a validation methodology at the waveform level and directly based on characteristics that are expected to influence most structural and geotechnical response parameters. In particular, three time-dependent validation metrics are used to evaluate the evolving intensity, frequency, and bandwidth of a waveform. These validation metrics capture nonstationarities in intensity and frequency content of waveforms, making them ideal to address nonlinear response of structural systems. A two-component error vector is proposed to quantify the average and shape differences between these validation metrics for a simulated and recorded ground-motion pair. Because these metrics are directly related to the waveform characteristics, they provide easily interpretable feedback to seismologists for modifying their ground-motion simulation models. To further simplify the use and interpretation of these metrics for engineers, it is shown how six scalar key parameters, including duration, intensity, and predominant frequency, can be extracted from the validation metrics. The proposed validation methodology is a step forward in paving the road for utilization of simulated ground motions in engineering practice and is demonstrated using examples of recorded and simulated ground motions from the 1994 Northridge, California, earthquake.

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

In recent years much effort has been devoted to the development of reliable analytical tools that model component deterioration and can be used to predict global collapse of structural systems under seismic excitations. This paper focuses on development of a steel component database that can serve as the basis for validation and improvement of analytical models that explicitly model deterioration in structural steel components and can be used in collapse assessment of steel moment resisting frames. Relationships that associate deterioration model parameters with geometric and material properties that control deterioration of steel beams with reduced beam sections (RBS) are proposed. The relationships are based on calibration and evaluation of steel beams with RBS subjected to monotonic and cyclic bending moments. The use and importance of deterioration modeling based on the proposed relationships for collapse prediction is demonstrated on a case study of a 20–story steel building designed based on current seismic provisions and evaluated based on FEMA P695 (ATC-63) performance methodology that requires explicit modeling of structural collapse.

Sensitivity of Collapse Potential of Buildings to Variations in Structural Systems and Structural Parameters

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.

Seismic Evaluation of Steel Moment Frames and Shear Walls Using Nonlinear Static Analysis Procedures

This paper discusses the sensitivity of collapse potential of buildings to variation of structural parameters in different structural systems. In this study, collapse capacity of a building subjected to a single ground motion is defined as the scalar ground motion intensity measure at which the building will become dynamically unstable. The process for obtaining the collapse capacity using Incremental Dynamic Analysis is illustrated. For a given building, collapse capacities of a number of ground motions are used to estimate the collapse fragility curve for the building. Two types of structural systems are considered in this study, moment-resisting frames, and shear walls. Simple mathematical models denoted as “generic structures” are devised to model moment-resisting frames and shear walls. A comprehensive database of collapse fragility curves (assuming a lognormal distribution for building collapse capacity) for a wide-range of combination in structural parameters of generic moment-resisting frames and generic shear walls are developed. Using this database, closed-form equations for median and dispersion of building collapse capacity curves are developed. Such equations not only facilitate the design and assessment processes, but also help in understanding the major trends and importance of certain parameters in changing the collapse potential of a structural system. Comparison between estimates of median collapse capacity using the close-form equations and data show that the presented equations are in good agreement with the data. It is concluded that the primary causes of collapse are severe cyclic deterioration and small plastic hinge rotation capacity of structural components. Furthermore, it is shown that P-Delta effect is the major cause of collapse especially in moment-resisting frames with a small yield base shear coefficient. Another parameter that greatly affects the collapse potential of moment-resisting frames is the ratio of column to beam strength. It is shown that increasing this parameter from 1.2 (ACI suggestion) to 2.4 could increase the median of collapse capacity by up to 90%. In this study, effect of incorporating a vector-valued ground motion intensity measure for estimating the collapse capacity is investigated. It is shown that using the scalar ground motion intensity measure for defining the collapse capacity can lead to underestimation of median collapse capacity by 50%, compared to using a vector-valued intensity measure for this purpose. At the end, a brief discussion of the methods for design for collapse safety is presented.