Showing papers in "Journal of Earthquake Engineering in 2007"

Equivalent Damping in Support of Direct Displacement-Based Design

Abstract: The concept of equivalent linearization of nonlinear system response as applied to direct displacement-based design is evaluated. Until now, Jacobsen's equivalent damping approach combined with the secant stiffness method has been adopted for the linearization process in direct displacement-based design. Four types of hysteretic models and a catalog of 100 ground motion records were considered. The evaluation process revealed significant errors in approximating maximum inelastic displacements due to overestimation of the equivalent damping values in the intermediate to long period range. Conversely, underestimation of the equivalent damping led to overestimation of displacements in the short period range, in particular for effective periods less than 0.4 seconds. The scatter in the results ranged between 20% and 40% as a function of ductility. New equivalent damping relations for four structural systems, based upon nonlinear system ductility and maximum displacement, are proposed. The accuracy of the new ...

Selection and scaling of real accelerograms for bi-directional loading: A review of current practice and code provisions

Abstract: The seismic behavior of asymmetric building structures is often complex and reductions to plane problems which can be analyzed with a single horizontal ground-motion component are often deemed unsatisfactory. As a consequence, dynamic structural analyses with both horizontal ground-motion components become more common, both in research as well as in practice. A review of code provisions regarding selection and scaling of ground motions for bi-directional analysis has, however, revealed that the guidelines provided are frequently inconsistent or are lacking transparency regarding the underlying assumptions. The aim of this study is to shed some light on a number of aspects involved when selecting and scaling records for bi-directional analysis and post-processing results of such analyses.

Seismic Risk Assessment of Transportation Network Systems

Abstract: When evaluating the earthquake risk to transportation system it is important to take into account the integrated effect of ground motion, liquefaction, and landslides on the network components and system. In this article, the risk from earthquakes to a transportation system is evaluated in terms of direct loss from damage to bridges and travel delays in the transportation network. The contribution of site effects to the loss from damage to bridges is estimated using the San Francisco Bay area as a test bed. Damage and loss to bridges from ground shaking and ground displacements (vertical and horizontal) from liquefaction and landslides are computed for a magnitude 7.0 scenario earthquake on the Hayward fault in California. It is found that liquefaction damage is the largest contributor to the repair cost which is used as a measure of the loss from damage. The performance of the transportation network is evaluated in terms of travel delay times. Travel delays resulting from damage due to ground shaking and...

Flexure-Shear Fiber Beam-Column Elements for Modeling Frame Structures Under Seismic Loading — State of the Art

Abstract: Whilst currently existing modeling approaches of reinforced concrete (RC) behavior allow a reasonably accurate prediction of flexural response, the determination of its shear counterpart needs further developments. In the last 20 years, models that predict shear response with acceptable accuracy for conditions of monotonic loading have been introduced, whilst, on the other hand, models capable of predicting the shear response under cyclic loading are less common. Among the approaches used for performing nonlinear analyses of frame structures, fiber beam-column elements have shown high capability in reproducing axial and flexural response, whereas the coupling between normal and shear stresses has been accounted for in very few models only. In this research work, the existing fiber beam-column elements with shear have been reviewed, underlying their most prominent characteristics and shortcomings, where relevant, with the objective of providing those interested in the topic with a relatively comprehensive ...

Seismic Retrofitting of Columns with Lap Spliced Smooth Bars Through FRP or Concrete Jackets

Abstract: The effectiveness of RC jacketing or FRP wrapping for seismic retrofitting of rectangular columns having smooth (plain) bars with 180° hooks lap-spliced at floor level is experimentally investigated. The relatively low deformation capacity and energy dissipation of five unretrofitted columns is found not to depend on lap length, if lapping is not less than 15 bar-diameters. Six columns cyclically tested up to ultimate deformation after RC concrete jacketing demonstrate force and deformation capacity and energy dissipation sufficient for earthquake resistance, regardless of the presence or length of lap splicing in the original column. Another ten columns cyclically tested to ultimate deformation after wrapping of the plastic hinge region with CFRP show that FRP wrapping of the splice region is more effective than concrete jackets for enhancement of the deformation and energy dissipation capacity of old-type columns with smooth bars lap-spliced at floor level, provided that wrapping extends over the member...

Attenuation Relationships for Iran

Abstract: The purpose of this study is to derive the attenuation relationships for PGA, PGV, and EPA parameters for areas within the seismic zones of Zagros, Alborz and Central Iran with rock and soil substructures. In order to do so, at first the available scientific data including the methods used for deriving attenuation relationships and the parameters involved have been gathered. Afterwards, all the efforts have been focused on gathering a thorough catalogue of earthquakes occurred in Iran. In this regard, a majority of the available catalogs in Iran have been gathered and corrected through different methods and finally a set of 89 earthquake events including 307 earthquake records with reliable data was chosen. Since in order to derive the attenuation relationships it is essential to extract the parameters from the acceleration records, a great effort was placed on gathering the earthquake acceleration records of Iran. This resulted in building a database of a majority of the earthquake records up to the year...

Seismic Hazard Assessment of the NW Himalayan Fold-and-Thrust Belt, Pakistan, Using Probabilistic Approach

Abstract: The Seismic Hazard Assessment (SHA) based on probabilistic approach has been carried out for the entire seismically active NW Himalayan Fold and Thrust Belt in Pakistan. Additional information in the form of earthquake catalog, delineation of 41 active faults in a structural map, their relationship to the seismicity, and establishment of seismotectonic zones has also been undertaken. From the distribution of the 813 events within the study area, it appears that seismicity (≥4.0 Mw) is associated with both surface and blind faults. The clustering of events in specific parts along the surface faults shows that some fault segments, especially in the hinterland zone, are more active. In parts of the active deformational front, like the Salt Range, southern Potwar and Bannu, lesser seismic activity (≥4.0 Mw) could be due to damping effect of the thick Precambrian salt. A majority of the earthquakes (86%) range in magnitude from 4.0 to 4.9 Mw, followed by 107 events (13%) ranging from 5.0 to 5.9 Mw. The remaini...

Physical and Numerical Approaches for the Optimal Insertion of Seismic Viscous Dampers in Shear-Type Structures

Abstract: This article presents the results of an exhaustive parametric analysis which compares the performances offered by various systems (which lead to both classical and non classical damping matrices) of added viscous dampers in shear-type structures. The aim of the research work here presented is the identification of the system of added viscous dampers which maximizes the dissipative properties under an equal “total size” constraint. The choice of the systems of added viscous dampers considered in the comparison is carried out both using a numerical approach (based upon the use of genetic algorithms) and a physically based approach (based upon the properties of classically damped systems). The comparison is carried out through the numerical evaluation of the dynamic response of representative shear-type structures to both stochastic and recorded earthquake inputs. The results obtained using both approaches indicate that a damping system based upon the mass proportional damping component of the Rayleigh visco...

Stochastic Seismic Response and Reliability Analysis of Base-Isolated Structures

Abstract: Stochastic seismic response analysis and reliability evaluation of base-isolated structures are conducted combining the physical stochastic ground motion model and the probability density evolution method. Three cases are investigated, including the base-isolated structure subjected to frequent earthquake of intensity 8, the identical super structure with fixed base subjected to frequent earthquake of intensity 7, and the base-isolated structure subjected to rare earthquake of intensity 8. Comparative studies reveal and verify some features of performances of base-isolated structures from the point of view of random vibration. The base-isolated super structure behaves like a near rigid body when subjected to small earthquakes while under strong earthquakes the first mode shape dominates the responses with ignorable effects of higher-order mode shapes. The response and earthquake action on the super structure could be reduced by one degree of intensity. Dynamic reliabilities against different response indi...

Seismic Performance of Square Reinforced Concrete Columns under Combined Cyclic Flexural and Torsional Loadings

Abstract: This article presents an experimental investigation on the behavior of reinforced concrete cantilevered columns subjected to combined cyclic bending and torsional loading. Seven columnar specimens with the same structural properties were tested under various loading conditions: cyclic torsion, cyclic uniaxial bending, and combined cyclic torsion and cyclic uniaxial bending with and without a constant axial compression force. Several combinations of cyclic bending and cyclic torsion expressed in terms of the rotation–drift ratio r were applied to study the interaction between torsion and flexural capacity of columns. The result reveals that the flexural capacity decreases and the damage tends to occur upward outside the flexural plastic hinge region as the level of applied cyclic torsion increases. An empirical model for the flexural and torsional hysteretic behavior is proposed based on the experimental result.

Effective Cyclic Energy as a Measure of Seismic Demand

Abstract: Structural damage during strong ground shaking is associated with both the seismic input energy and the ability of structural components to dissipate energy through viscous damping and inelastic cyclic response. The correlation of the damage potential of ground motions with seismic energy demand is an important element in developing energy-based design methodologies. This article proposes a new measure of the severity of ground motions by introducing the concept of effective cyclic energy (ECE) defined as the peak-to-peak energy demand (sum of hysteretic and damping energies) imposed on a structure over an effective duration that is equivalent to the time between two zero-crossings of the “effective velocity pulse.” The proposed energy measure, which is dependent on the characteristics of the ground motion, is shown to be well correlated with peak seismic demand for a range of system parameters. The development of ECE also provides a basis for defining a non dimensional response index (γ eff ) to quantify...

Site classification using horizontal-to-vertical response spectral ratios and its impact when deriving empirical ground-motion prediction equations

Abstract: We classify sites based on their predominant period computed using average horizontal-to-vertical (H/V) response spectral ratios and examine the impact of this classification scheme on empirical ground-motion models. One advantage of this classification is that deep geological profiles and high shear-wave velocities are mapped to the resonance frequency of the site. We apply this classification scheme to the database of Fukushima et al. [2003], for which stations were originally classified as simply rock or soil. The calculation of average H/V response spectral ratios permits the majority of sites in the database to be unambiguously classified. Soft soil conditions are clearly apparent using this technique. Ground-motion prediction equations are then computed using this alternative classification scheme. The aleatoric variability of these equations (measured by their standard deviations) is slightly lower than those derived using only soil and rock classes. However, perhaps more importantly, predicted response spectra are radically different to those predicted using the soil/rock classification. In addition, since the H/V response spectral ratios were used to classify stations the predicted spectra for different sites show clear separation. Thus, site classification using the predominant period appears to be partially mapped into the site coefficients of the ground-motion model.

Seismic Tests and Numerical Modeling of a Rolling-ball Isolation System

Abstract: For the seismic isolation of light structures, the use of laminated rubber bearings is neither economical nor, for most cases, technically suited. For the isolation of this type of structure a new system, consisting of steel balls rolling on rubber tracks, has been developed at TARRC (Tun Abdul Razak Research Centre). This article presents the results of experimental tests carried out for the characterization of the behavior of this new device. A numerical model is also proposed that can be used to assess the seismic response of structures with this isolation system. Comparison of the predictions of the numerical model with the experimental data shows that the model is adequate to perform the correct assessment of the seismic response of isolated structures. The results of the experimental campaign of shaking-table tests, as well as the numerical simulations, show that there is an effective reduction of the acceleration levels induced in the isolated structures.

Seismic Microzonation and Damage Assessment of Bam City, Southeastern Iran

Abstract: This article aims to study the dynamic characteristics of soil in Bam, southeastern Iran. Fundamental frequency and amplification factor of soil sediment were estimated by microtremor measurement. This procedure was performed at 49 sites in the city. Two 120-second data were recorded at each site. Segmental cross spectra were applied to calculate spectrum, Nakamura's method (H/V) was used for analyzing data and fundamental frequency and amplification factor values were derived. Iso-frequency and iso-amplification maps of the city were prepared. Results show that soil type in Bam city is mainly stiff, although amplification factor is relatively large. There is a short period zone in northwest to southeast direction. Sediment depth was estimated using a correlation between fundamental frequency and sediment thickness. Results obtained from microtremors were compared to the geotechnical boreholes and it was shown that microtremors can be used for a rough estimation of sediment thickness. Damage distribution ...

Seismic Behavior of RC Wide Beam-Column Connections Under Dynamic Loading

Abstract: In this article, the seismic behavior of RC wide beam-column connections designed primarily for gravity loads is evaluated experimentally. A 2/3 scale model of one exterior and one interior connection is constructed and subjected to seismic simulations on a shaking table until collapse. The results of the tests indicate that: (a) the drift at yield is from 3 to 6 times higher than, for example, the 0.5% admitted by Eurocode 8 to satisfy the “damage limitation requirement;” (b) the beam-column joints do not fail; (c) the torsion in the spandrel beams governs the overall load-displacement relationship of the exterior connections and limits the ultimate strength. Based on the test results, a nonlinear model for predicting the hysteretic behavior and the failure of the connections is suggested. The model can be implemented in a computer code for evaluating the vulnerability of this type of structure through nonlinear dynamic response analyses.

Use of Ambient Noise: From Spectral Amplitude Variability to H/V Stability

Abstract: The study of the variation over time of both spectral amplitudes and H/V curves, has been performed on three different sites, two close to cities and one in the countryside, during periods varying from week to over a month. It demonstrates the robustness of the H/V technique to give consistent peak frequency values. In particular, H/V peak frequencies, either fundamental (f0) or natural (fx, x•1), are not affected by weather nor the level of human activity. However, while fundamental H/V peak amplitudes are stable, they proved rather unstable for natural (secondary) peak. Spectral amplitude curves are very variable but follow human activity cycles from week-week end and day-night variations down to a very small scale, such as lunch breaks. Finally, the frequency limit between anthropic noise and natural noise, commonly taken at 1 Hz, is not straightforward and is varying from site to site from 0.7–0.8 Hz up to 2–3 Hz.

Behavior Factor for Performance-Based Seismic Design of Plane Steel Moment Resisting Frames

Abstract: Simplified expressions to estimate the behavior factor of plane steel moment resisting frames are proposed, based on statistical analysis of the results of thousands of nonlinear dynamic analyses. The influence on this factor of specific structural parameters, such as the number of stories, the number of bays, and the capacity design factor of a steel frame, is studied in detail. The proposed factor describes the seismic strength requirements in order to restrict maximum storey ductility to a predefined value. Interrelation studies between maximum storey ductility and the Park-Ang damage index are also provided for the damage-based interpretation of the performance levels under consideration. Realistic design examples serve to demonstrate the ability of the proposed factor to convert conventional force-based design to a direct performance-based seismic design procedure.

A New Seismic Intensity Parameter to Estimate Damage in Buried Pipelines due to Seismic Wave Propagation

Abstract: A new seismic intensity parameter to estimate damage in buried pipelines due to seismic wave propagation is proposed. This parameter depends on the peak ground velocity (PGV) and the peak ground acceleration (PGA). It is shown that PGV2/PGA is related to displacement, a parameter directly related to ground strain, which is the main cause of buried pipeline damage. For the case of Mexico City, this parameter exhibits higher correlation with damage than PGA or PGV alone. Finally, we presented intensity-damage relations for the Mexico City's primary water system using PGV2/PGA as the measure of seismic intensity.

Seismic Fragility Curves for Un-Anchored On-Grade Steel Storage Tanks: Bayesian Approach

Abstract: In this article, Bayesian approach is used for estimation of seismic fragility of un-anchored on‐grade steel storage tanks based on historical data and American Lifeline Alliance tanks database. The approach properly accounts for epistemic as well as aleatory uncertainties. The point and interval estimates of the fragility are formulated based on structural reliability method that implicitly or explicitly reflects the influence of epistemic uncertainty. The fragility curves developed herein are compared to corresponding relations currently available in the technical literature, the comparison suggest that actual tank performance is better than that predicted in the literature. Finally, application of fragilities in loss estimation methodology is investigated and showing a considerable scatter in the results corresponding to different fragilities.

On-Line Seismic Hazard Data for the New Italian Building Code

Abstract: The probabilistic seismic hazard of Italy was assessed in 2004 to match the requirements of the new seismic provisions. This such map, now recognized as the official reference for design according to the building and administrative issues, is the result of a comprehensive seismic hazard model that takes into account the variability in seismicity, seismogenic potential, and propagation in different areas of Italy. Since 2004, we have computed seismic hazard in terms of peak ground acceleration and spectral acceleration values for varied annual probabilities of exceedance, including a measure of their variability. These data allow as to: (1) compute site-specific seismic hazard curves and uniform hazard spectra; (2) anchor the elastic response spectra; and (3) set seismological constraints on the limit states. These seismic hazard data are stored in a database, freely accessible to all end-users via the web, where they can be downloaded or consulted through a WebGIS tool.

Effect of SMA Braces in a Steel Frame Building

Abstract: Pull-back and shaking table test results on a simple model of a three-storey structure that includes shape memory alloys (SMA) copper-based dampers are presented and discussed. The model corresponds to a rigid-framed steel structure and the dampers to austenite CuAlBe wires inserted as bracing at each story. The inclusion of the dissipators in the structure increases the percentage of critical damping from 0.59% for the bare case to 5.95% for the braced system. At the same time, the structural stiffness increases making the first fundamental frequency change from 2.5–3.7 Hz (0.4–0.27s). The net effect of these two factors is a 30–60% reduction of peak relative displacements compared to the ones obtained without dissipation devices when the structure is subjected to earthquake records. Depending on records frequency contents, a reduction of the peak accelerations to near 58% also can be obtained. Additionally, a crude nonlinear analytical model has been studied that can predict the earthquake responses rea...

Testing of Superelastic Recentering Pre-Strained Braces for Seismic Resistant Design

Abstract: Over the past decade, the use of shape memory alloys (SMAs) in passive control devices has been explored. Nevertheless, some aspects in regards to the cyclic behavior of SMAs and the effect of pre-straining need to be clarified. In this study, small-scale shake table tests have been performed to explore the effectiveness of SMA bracing systems as compared to steel bracing systems. The reduced-scale experimental results imply that SMAs used in braces are more effective in controlling the response of a steel frame compared with a traditional bracing system. A finite element model (FEM) of the frame is developed in order to compare the analytical results with the shake table tests. Further, the effect of pre-straining the SMA braces is evaluated through both experimental and analytical studies. The results show that pre-straining improves the performance of the frame compared to the nonpre-strained case. However, as the level of pre-straining increases above approximately 1.0% to 1.5%, the benefits of pre-st...

A Fast and Efficient Simulation of the Far-Fault and Near-Fault Earthquake Ground Motions Associated with the June 17 and 21, 2000, Earthquakes in South Iceland

Abstract: The two Mw 6.5 earthquakes on June 17 and 21, 2000, respectively, in the populated South Iceland Seismic Zone (SISZ) significantly augmented the Icelandic database of strong ground motions, and several strong velocity pulses were recorded at near-fault sites. The strong motions are interpreted via the Specific Barrier Model (SBM) and a mathematical model of near-fault velocity pulses. The data indicates self-similar source scaling and significantly greater attenuation of seismic waves than in other interplate regions. Through inversion of the data a new attenuation function for the SISZ has been adopted, which results in unbiased simulations. For the first time, the characteristics of the recorded near-fault pulses have been identified and compared to the worldwide database of such records. The SBM and the near-fault pulse model combine naturally in a fast and efficient synthesis of realistic, broad-band strong ground motions in the far-fault and near-fault region. Such simulations are showcased for the J...

Nonlinear Static Methods vs. Experimental Shaking Table Test Results

Abstract: Three different Nonlinear Static Methods (NSM's), based on pushover analysis, are applied to a 3-story, 2-bay, RC frame They are (i) the Capacity Spectrum Method (CSM), described in ATC-40, (ii) the Displacement Coefficient Method (DCM), presented in FEMA-273 and further developed in FEMA 356, and (iii) the N2 Method, implemented in the Eurocode 8 Pushover analyses are conducted with DRAIN-3DX by using four different lateral force distributions, according to the acceleration profile assumed along the height of the structure: uniform, triangular, modal-proportional, and multimodal fully adaptive In the numerical model, RC members are modeled as fiber elements The numerical predictions of each method are compared to the experimental results of the shaking table tests carried out on two similar 1:33-scale structural models, with and without infilled masonry panels, respectively The comparison is made in terms of maximum story displacements, interstory drifts, and shear forces All the NSM's are found t

The 4th February 1997 Bojnurd (Garmkhan) Earthquake in NE Iran: Field, Teleseismic, and Strong-Motion Evidence for Rupture Directivity Effects on a Strike-Slip Fault

Abstract: The Bojnurd region of NE Iran experienced a Mw 6.4 earthquake on February 4, 1997. By combining results from teleseismic body-waveform analysis, field observations of structural damage, coseismic deformation, geomorphology, and analysis of the resulting strong ground-motions, we build a coherent picture of the faulting associated with this earthquake. The earthquake resulted from almost pure right-lateral strike-slip motion (0.5–1.0 m), which ruptured a ∼15 km long section of fault, striking ∼340° at its northern end, which changes to ∼320° at its southern end. This fault can be seen clearly on the geological map and satellite imagery. The village of Sheikh lies ∼10–15 km SE of the fault rupture, yet was severely damaged during the earthquake. Analysis of strong-motion records, recorded by the Building and Hazard Research Center of Iran, particularly their significant duration, the polarization of the fault-normal component, and the velocity pulse, indicates a probable directivity effect, in which the rup...

Design Methodology for Seismic Upgrading of Substandard Reinforced Concrete Structures

Abstract: This article presents a design methodology for seismic upgrading of existing reinforced concrete (RC) buildings The methodology is based on the modification of the deflected shape of the structure so as to achieve a near-uniform distribution of interstorey drift along the building height, thereby eliminating damage localization Yield Point Spectra are utilized for the definition of demand and a direct displacement-based design approach is implemented The fundamental steps of the method are described in detail, including a systematic evaluation of assumptions and limitations A full-scale tested structure is used as a case study for assessment and verification of the proposed methodology Alternative retrofit scenarios are set according to target response and performance levels The role of the target deflected response shape and its influence on the outcome of the retrofit strategy is investigated The viability of the alternative retrofit scenarios is studied for different ground motions including nea

Hysteretic Energy Demands for SDOF Systems Subjected to Narrow Band Earthquake Ground Motions. Applications to the Lake Bed Zone of Mexico City

Abstract: This study proposes a way to estimate the hysteretic energy demands for oscillators subjected to narrow-band earthquake ground motions using an equivalent sinusoidal pulse. The parameters required to define the equivalent pulse are: the peak ground acceleration, the maximum of the Fourier amplitude spectrum, and the predominant period of the site. The equivalent pulse is utilized to compute the hysteretic energy demands associated to the design elastic pseudoacceleration spectra prescribed by the Mexico City Building Code and to construct damage index maps for systems designed according to this code and subjected to a seismic event with M = 8 and focal distance of 300 km.

The Estimation of Peak Ground-motion Parameters from Spectral Ordinates

Abstract: Relationships are derived between peak ground-motion parameters and the peaks in the corresponding 5% damped response spectrum. The relationships are based on a study of the smoothed spectra from a database of over 1,000 horizontal ground-motion records. A more consistent, but more complex, relationship is also derived between peak ground velocity (PGV) and an integration of the 5% damped spectrum, using random vibration theory. A recent proposal to estimate PGV from the 0.5 sec spectral ordinate is found to give less accurate results than the methods derived in this study, when compared with values derived from the strong ground-motion database; in particular, it consistently underestimates PGV for records with unusually long or short predominant periods. However, when compared with predictive equations for PGV, this method gives results comparable to the other two and is also much the simplest. Relationships between spectral ordinates and PGV are useful for the purpose of estimating PGV from a smoothed ...

The Role of Inelastic Torsion in the Determination of Residual Deformations

Abstract: Recent developments in performance-based seismic design and assessment approaches have emphasized the importance of properly assessing and limiting the residual (permanent) deformations typically sustained by a structure after a seismic event, even when designed according to current code provisions. The performance-based design framework for residual deformations, previously developed by the authors for 2-D regular structures, is further extended to the behavior of 3-D irregular (asymmetric in-plan) buildings. The seismic response of a set of single-story systems, comprising of seismic resisting frames, and modeled to represent alternative materials (concrete or steel), is investigated under uni-directional earthquake loading excitations. Different layouts in plan, leading to either torsionally unrestrained or restrained systems, are considered. The influence of varying torsional restraint is investigated to define how residual diaphragm rotations and center-of-mass displacements are affected by changing ...

Constant and Variable Amplitude Cyclic Behavior of Welded Steel Beam-to-Column Connections

Abstract: This article presents a study on welded beam-to-column joints of moment-resisting steel frames. The main features of the joint specimens are summarised, in order to identify key parameters influencing the joint response as well as their low-cycle fatigue endurance. The cyclic behavior and the low-cycle fatigue strength of the connections were initially assessed by cyclic quasi-static testing, carried out at the Technical University of Milan. Analysis of the results has been carried out in order to verify the validity of a linear damage accumulation model combined with a low-cycle fatigue approach based on S-N lines concept. Moreover, a criterion to predict the type of failure and a procedure of appraising the fatigue endurance are presented and their validity proved by the results of variable amplitude tests.