Showing papers in "Journal of Earthquake Engineering in 2008"

Experimental Investigation on the Seismic Behavior of Beam-Column Joints Reinforced with Superelastic Shape Memory Alloys

Abstract: Superelastic Shape Memory Alloys (SE SMAs) are unique alloys that have the ability to undergo large deformations and return to their undeformed shape by removal of stresses. This study aims at assessing the seismic behavior of beam-column joints reinforced with SE SMAs. Two large-scale beam-column joints were tested under reversed cyclic loading. While the first joint was reinforced with regular steel rebars, SE SMA rebars were used in the second one. Both joints were selected from a Reinforced Concrete (RC) building located in the high seismic region of western Canada and designed and detailed according to current Canadian standards. The behavior of the two specimens under reversed cyclic loading, including their drifts, rotations, and ability to dissipate energy, were compared. The results showed that the SMA-reinforced beam-column joint specimen was able to recover most of its post-yield deformation. Thus, it would require a minimum amount of repair even after a strong earthquake.

Influence of masonry strenght and openings on infilled r/c frames under cycling loading

Abstract: The influence of masonry infills with openings on the seismic performance of reinforced concrete (R/C) frames that were designed in accordance with modern codes provisions is investigated. Two types of masonry infills were considered that had different compressive strength but almost identical shear strength. Infills were designed so that the lateral cracking load of the solid infill is less than the available column shear resistance. Seven 1/3 – scale, single–story, single–bay frame specimens were tested under cyclic horizontal loading up to a drift level of 40%. The parameters investigated are the opening shape and the infill compressive strength. The assessment of the behavior of the frames is presented in terms of failure modes, strength, stiffness, ductility, energy dissipation capacity, and degradation from cycling. The experimental results indicate that infills with openings can significantly improve the performance of RC frames. Further, as expected, specimens with strong infills exhibited better ...

Eurocode 8 Compliant Real Record Sets for Seismic Analysis of Structures

Abstract: Among all the possible options to define the seismic input for structural analysis, natural recordings are emerging as the most attractive. Easily accessible waveform databases are available and evidence shows that only a relatively limited number of criteria has to be considered in selection and scaling to get an unbiased estimation of seismic demand. Like many codes worldwide, Eurocode 8 (EC8) allows the use of real ground-motion records for the seismic assessment of structures. The main condition to be satisfied by the chosen set is that the average elastic spectrum does not underestimate the code spectrum, with a 10% tolerance, in a broad range of periods depending on the structure's dynamic properties. The EC8 prescriptions seem to favour the use of spectrummatching records, obtained either by simulation or manipulation of real records. The study presented herein investigates the European Strong-Motion Database with the purpose of assessing whether it is possible to find real accelerogram sets complying with the EC8 spectra, while accounting for additional constraints believed to matter in the seismic assessment of buildings, as suggested by the current best practice. Original (un-scaled) accelerogram sets matching EC8 criteria were found, for the case of one-component (P-type) and spatial sets (S-type), for the spectra anchored to the Italian peak acceleration values. The average spectra for these sets tend to be as close as possible to the code spectrum. Other sets, requiring scaling, have been found to match the non dimensional (country-independent) EC8 spectral shape. These sets have also the benefit of reducing, in respect to the un-scaled sets, the record-to-record variability of spectra. Combinations referring to soft soil, stiff soil, and rock are presented here and are available on the internet at http://www.reluis.it/

Experimental Verification of Viscous Damping Modeling for Inelastic Time History Analyzes

Abstract: Inelastic time history analyzes (ITHA) are recognized as the most suitable tool for describing the behavior of structures under seismic actions. It is clear that the reliability of ITHA results is strongly dependent on the modeling choices. One of the aspects still debated, in spite of a large amount of research, is the modeling of viscous damping forces. In the case of a single degree of freedom (SDOF) structure, the damping not represented by hysteretic response is commonly assumed proportional to the square root of the system stiffness: typically in the research papers it is not declared whether the initial or tangent-stiffness is considered. Recent research has shown that initial or tangent-stiffness proportional damping can give very different results in terms of energy dissipation, and peak response displacement. In this work, experimental and numerical tests performed to detect the most appropriate model are described and discussed.

Modeling Beam-Column Joints in Fragility Assessment of Gravity Load Designed Reinforced Concrete Frames

Abstract: Reinforced concrete (RC) frame structures customarily have been designed in regions of low-to-moderate seismicity with little or no consideration of their seismic resistance. The move toward performance-based seismic engineering requires accurate reliability-based structural analysis models of gravity load designed (GLD) RC frames for predicting their behavior under seismic effects and for developing seismic fragilities that can be used as a basis for risk-informed decision-making. This analytical approach requires particular attention to the modeling of beam-column joints, where GLD frames differ significantly from their counterparts in high-seismic areas. This article focuses on modeling shear and bond-slip behavior of beam-column joints for purposes of seismic fragility analysis of GLD RC frames. The joint panel constitutive parameters are defined to replicate the experimental joint shear stress-strain relationships, while the effect of bond-slip is taken into account through a reduced envelope for the...

Vector-valued Intensity Measures Incorporating Spectral Shape For Prediction of Structural Response

Abstract: Vector-valued ground motion intensity measures (IMs) are developed and considered for efficiently predicting structural response. The primary IM considered consists of spectral acceleration at the first-mode structural period along with a measure of spectral shape which indicates the spectral acceleration value at a second period. For the IM to effectively predict response, this second period must be selected intelligently in order to capture the most relevant spectral shape properties. Two methods for identifying effective periods are proposed and used to investigate IMs for example structures, and an improvement in the efficiency of structural response predictions is shown. A method is presented for predicting the probability distribution of structural response using a vector IM while accounting for the effect of collapses. The ground motion parameter e is also considered as part of a three-parameter vector. It is seen that although the spectral shape parameter increases the efficiency of response predi...

Quasi-Static Cyclic Tests of Two U-Shaped Reinforced Concrete Walls

Abstract: U-shaped or channel-shaped walls are frequently used as lateral strength providing members in reinforced concrete (RC) buildings since their form does not only provide strength and stiffness in any horizontal direction but is also well suited to accommodate elevator shafts or staircases. Despite this popularity, experimental results on the seismic behavior of U-shaped walls are scarce. For this reason a research program with the objective to provide additional experimental evidence for such walls under seismic loading was developed. It included quasi-static cyclic testing of two U-shaped walls at the structural engineering laboratories of the ETH Zurich. The walls were built at half-scale and designed for high ductility. The main difference between the two walls was their wall thickness. The project was chiefly focusing on the bending behavior in different directions and therefore the walls were subjected to a bi-directional loading regime. This article discusses the design of the test units, the test set...

Seismic Overstrength in Braced Frames of Modular Steel Buildings

Abstract: The seismic behavior factor, R, is a critical parameter in contemporary seismic design. In the 2005 edition of the National Building Code of Canada, the R factor consists of ductility related force modification factor, Rd, and overstrength-related force modification factor, Ro. The choice of these factors for design depends on the structural system type. In this investigation, typical braced frames of Modular Steel Buildings (MSBs) are designed and modeled. Nonlinear static pushover analyses are conducted to study the inelastic behavior of these frames. Structural overstrength resulting from redistribution of internal forces in the inelastic range, design assumptions, and strain hardening behavior of steel and displacement ductility are evaluated and their relationships with some key response parameters are assessed. The results show that the reserve strength of MSB-braced systems is greater than that prescribed by the Canadian code for regular braced systems. It also appears that R depends on building he...

Section Analysis and Cyclic Behavior of Post-Tensioned Jointed Ductile Connections for Multi-Story Timber Buildings

Abstract: A new seismic resisting system for multi-story timber buildings has been recently developed and proposed by the authors based on refinement and adaptation of jointed ductile connections, using post-tensioning technology, originally developed for precast concrete construction. Successful implementation of the concept and confirmation of the high structural and seismic performance of such systems has been obtained through extensive experimental testing on system subassemblies carried out at the University of Canterbury in the past few years. In this contribution, a simplified analytical model for the analysis of the section and overall connections behavior is proposed based on the extension of existing procedures proposed and adopted for the design and analysis of jointed ductile precast concrete walls and frames. Particular focus will thus be given to the specific modifications required when dealing with the peculiarity of post-tensioned connections using orthotropic timber engineered wood materials such a...

Estimation of the Fundamental Vibration Period of Existing RC Buildings in Turkey Utilizing Ambient Vibration Records

Abstract: Earthquake design codes generally provide empirical formulae or various approximations for determining fundamental periods of buildings for the evaluation of statically equivalent seismic forces. It is thus important to use realistic periods for the design and performance assessment of existing buildings. This study deals with the evaluation of fundamental vibration periods of reinforced concrete (RC) buildings having a frame-type structural system, and considers the effects of infill walls. The problem is evaluated by utilizing experimental and analytical approaches. A height-dependent relationship to estimate the fundamental period of vibration of Turkish RC moment-resisting frames is derived for a fully elastic condition. Furthermore, a constant parameter is defined to magnify the empirical relationship in order to attain code-specified periods obtained by considering moderate-intensity earthquakes.

Plastic Design of Seismic Resistant V-Braced Frames

Abstract: In this article, a new method for designing chevron concentrically braced steel frames is presented. The aim of the proposed method is the design of concentrically braced steel frames able to guarantee, under seismic horizontal forces, a collapse mechanism of global type. This result is of great importance in the seismic design of structures, because local failure modes give rise to a worsening of the energy dissipation capacity of structures and, therefore, to an higher probability of failure during severe earthquakes. With reference to the examined structural typology, the global mechanism is characterized by the yielding of tensile bracing diagonals and by the buckling of the compressed diagonals of all the stories. The proposed method is rigorously based on “capacity design approach” which requires that dissipative zones have to be designed to withstand the internal actions due to the seismic design horizontal forces and the vertical loads acting in the seismic load combination; while non dissipative ...

Multi-Criteria Decision Making for Seismic Retrofitting of RC Structures

Abstract: The upgrading of existing structures that are not adequate to withstand seismic demand is a widely adopted and effective approach aimed at risk reduction. Nowadays, many are feasible retrofit strategies, employing traditional and/or innovative materials, and several options are available to professionals. Each one has different performances in respect to some criteria, i.e., technical and/or economical, by which each alternative can be evaluated. The selection of the most suitable retrofit strategy for a particular structure may be not straightforward since, in many applications, there is no alternative which clearly emerges among others as the best one according to the whole of the criteria considered. Multi-Criteria Decision Making (MCDM) methods are decision-support procedures used in many fields allowing the evaluation and comparison of a set of alternatives when many evaluation criteria are involved. Ranking the alternative solutions leads to the identification of the optimal solution, which better p...

Estimating the Higher-Mode Response of Ductile Structures

Abstract: While the importance of higher-mode actions is appreciated within the engineering community, the affect that ductile nonlinear response has on higher-mode characteristics and the subsequent implications this has for design has received little attention In this article, the manner in which the higher-mode response of frame-wall structures is affected by inelastic behavior is closely examined and a means of accounting for this in design is proposed The work focuses firstly on the characteristics of the higher modes present at the development of peak response and then considers how these characteristics would affect the total forces in the building The study utilizes a series of nonlinear time-history analyses of two different groups of RC frame-wall structures subject to a suite of real records It is shown that a new modal analysis approach that incorporates transitory inelastic modal characteristics gives significantly improved predictions of peak base shear in frame-wall structures than more tradition

Shear Modulus and Damping Ratio of Cohesive Soils

Abstract: This article presents results from a laboratory investigation into the dynamic characteristics of reconstituted and undisturbed cohesive soils by means of resonant-column tests. In particular, results showing the influence of various soil parameters, such as confining stress, overconsolidation ratio, void ratio, plasticity index, calcium carbonate content, and time of confinement on shear modulus and damping ratio at small and high shear strains are presented and then discussed. Relationships for the small-strain shear modulus, the degradation of shear modulus at high strains, and the increase of damping ratio at high strains over its small-strain value are proposed. Finally, the practical implications of the results in the context of seismic site response analysis are discussed.

A Critical Review of the R.C. Frame Existing Building Assessment Procedure According to Eurocode 8 and Italian Seismic Code

Abstract: In the European seismic countries the assessment of existing structures is a priority, since the majority of the building heritage was designed according to out-of-date or even no seismic codes. The uncertainties about the nonlinear behavior of the structures are, therefore, relevant and the nonlinear response should be treated directly, with corresponding strong increase in complexity of the assessment procedure. This issue was taken into account in this work; in particular, the assessment of two irregular reinforced concrete frame buildings was performed, according to both Italian Seismic Code and Eurocode 8 with the following aims: (i) checking the consistency of the results obtained applying the two considered Codes; (ii) identifying and clarifying the aspects which may lead to different possible interpretations of the code rules; (iii) suggesting simplified approaches or improvements for the assessment procedure, concerning the evaluation of both seismic demand and capacity of the structural members.

Inelastic wide-column models for U-shaped reinforced concrete walls

Abstract: Although core structures are often used in reinforced concrete buildings as members providing lateral strength and stiffness, experimental and numerical studies on their inelastic behavior are scarce. In an experimental program recently completed at the ETH Zurich, two U-shaped walls were subjected to a bi-directional quasi-static cyclic loading regime. In this article, inelastic wide-column models for these two test units are developed. The wide-column analogy was chosen because it combines the merits of representing the U-shaped wall as a three-dimensional structure with inelastic properties while still being relatively simple and easy to set up when compared to shell or solid element models. It is therefore a tool which is not only available to researchers but also to design engineers. The article commences with the analysis of wide-column models that have been built according to recommendations found in the literature. Since these recommendations had been derived from analyzes of elastic systems, they are then revisited in a sensitivity study in which the effects of different modeling assumptions on the inelastic behavior of wide-column models are investigated. Finally, comparing the numerical results with the experimental evidence from the tests, the article concludes with practical recommendations for setting up wide-column models of U-shaped walls subjected to large inelastic deformations.

Failure mode control of x-braced frames under seismic actions

Abstract: In this article, a new method for designing concentrically braced steel frames is presented. The aim of the proposed method is the design of concentrically braced steel frames able to guarantee, un...

Earthquake Loss Assessment for Integrated Disaster Risk Management

Abstract: Understanding probable losses and reconstruction costs due to earthquakes creates powerful incentives for countries to develop planning options and tools to cope with risk, including allocating the sustained budgetary resources necessary to reduce those potential damages and safeguard development A specific catastrophic risk model has been developed to evaluate, building by building, the probabilistic losses and pure premiums of different portfolios, taking into account the seismic microzonation of cities This model has been used to evaluate the fiscal contingency liabilities of the government and to build an optimal structure for risk transfer and retention, considering contingent credits, reserve funds, insurance/reinsurance, and cat bonds Lastly, the model allows the evaluation of an exceedance probability curve of benefit-cost ratio, providing an innovative and ground-breaking tool for decision makers to analyze the net benefits of the risk mitigation strategies, such as earthquake retrofitting and

Vulnerability Analysis for Gravity Load Designed RC Buildings in Naples – Italy

Abstract: Damage scenario and risk analyses are helpful tools for the local administrators for mitigation of potential earthquake losses at the urban-regional level. One of the main issues in developing such scenarios is the choice of proper capacity functions expressing the effective seismic supply for the existing building classes and the convolution with demand in the so-called fragility analysis. This article, as a further implementation of a recently developed mechanical based procedure for class scale quantitative risk evaluation, presents the derivation of class representative capacity curves and the relative fragility curves for slight, moderate, extensive, and complete damage states as defined by the well-known HAZUS methodology. Starting from an extensive building survey of Arenella district in Naples, southern Italy, statistics on main model input parameters are obtained for selected building classes of existing and/or pre-code RC buildings. Accordingly, a number of building models is simulated designed and analyzed in order to determine building class capacity. Fragility curves are computed simulating the fraction of ‘‘failures’’ within a capacity spectrum method framework. These capacity and fragility curves have been used in a companion paper by Lang et al. [2008] for the computation of damage scenarios in Arenella.

Experimental Dynamic Behavior of Free-Standing Multi-Block Structures Under Seismic Loadings

Abstract: This article describes the dynamical behavior of free-standing block structures under seismic loading. A comprehensive experimental investigation has been carried out to study the rocking response of four single blocks of different geometry and associations of two and three blocks. The blocks, which are large stones of high strength blue granite, were subjected to free vibration, and harmonic and random motions of the base. In total, 379 tests on a shaking table were carried out in order to address the issues of repeatability of the results and stability of the rocking motion response. Significant understanding of the rocking motion mechanism is possible from the high quality experimental data. Extensive experimental measurements allowed to discuss the impulsive forces acting in the blocks and the three-dimensional effects presented in the response.

Estimation of Probabilistic Seismic Losses and the Public Economic Resilience—An Approach for a Macroeconomic Impact Evaluation

Abstract: The Disaster Deficit Index (DDI) measures country risk from a macroeconomic and financial perspective, according to possible catastrophic events. The DDI captures the relationship between the demand for contingent resources to cover the maximum probable losses and the public sector's economic resilience; that is, the availability of internal and external funds for restoring affected inventories. For calculating potential losses, the model follows the insurance industry in establishing a probable loss, based on the critical impacts during a given period of exposure, and for the economic resilience the model computes the country's financial ability to cope with the situation taking into account: the insurance and reinsurance payments; the reserve funds for disasters; the funds that may be received as aid and donations; the possible value of new taxes; the margin for budgetary reallocations; the feasible value of external credit; and the internal credit the country may obtain. Access to these resources has l...

Can Earthquake Loss Models be Validated Using Field Observations

Abstract: The occurrence of a damaging earthquake provides an opportunity to compare observed and estimated damage, provided that detailed observations of the earthquake effects are made in the field. A question that arises is whether such comparisons can provide the basis for validation of an earthquake loss model. In order to explore this issue, a case study loss model for the northern Marmara region has been set up and the losses have been calculated for various ground-motion fields that arise when different assumptions are made about the ground-motion variability. In particular, the influence of removing the inter-event variability for a scenario earthquake and modeling spatial correlation among ground motions is studied. Further analyses are conducted assuming that a number of accelerograms are available within the region and that knowledge of spatial correlations among ground motions can therefore be used to better predict the motions at sites in the vicinity of the recording stations. The results demonstrate...

Neural Network Based Attenuation of Strong Motion Peaks in Europe

Abstract: Artificial Neural Network (ANN) is used in this article to develop attenuation relationships for three peak ground motion parameters, namely, peak ground acceleration (PGA), peak ground velocity (PGV), and peak ground displacement (PGD). This article demonstrates the capability of ANN to capture the key physical aspects of seismic wave attenuation and region specific earthquake characteristics. Limited strong ground motion data and no particular functional form except for few constraints are used in the development of ANN based attenuation relationships. The database consists of 358 records (2 horizontal components of ground acceleration at each station) from 42 European shallow earthquakes. The surface magnitude (Ms), distance of site from surface projection of the rupture (R), and broad categories of soil type (soft soil, stiff soil, and rock formation) are the three input parameters. The Ms ranges from 5.5–7.9 and R ranges from 3 – 260 Km. The model is trained using 75% (134 data points) of the total d...

A Comparative Study of European Earthquake Loss Estimation Tools for a Scenario in Istanbul

Abstract: A damage estimation exercise has been carried out using the building stock inventory and population database of the Istanbul Metropolitan Municipality and selected European earthquake loss estimation packages: KOERILOSS, SELENA, ESCENARIS, SIGE, and DBELA. The input ground-motions, common to all models, correspond to a “credible worst case scenario” involving the rupture of the four segments of the Main Marmara Fault closest to Istanbul in a Mw 7.5 earthquake. The aim of the exercise is to assess the applicability of the selected software packages to earthquake loss estimation in the context of rapid post-earthquake response in European urban centers. The results in terms of predicted building damage and social losses are critically compared amongst each other, as well as with the results of previous scenario-based earthquake loss assessments carried out for the study area. The key methodological aspects and data needs for European rapid post-earthquake loss estimation are thus identified.

A Detailed Dynamic Model of a Six-Axis Shaking Table

Abstract: This article describes the modeling of a 5 m × 5 m 6 degree-of-freedom (DOF) shaking table and the subsequent computer simulation of the system's dynamic characteristics. The simulation model is required to aid the development of a closed-loop controller with a bandwidth specified up to 120 Hz, and thus high order dynamics including valve response and structural effects are included. Significant nonlinearities associated with the hydraulic and mechanical components are also included (e.g., spool slew rate and saturation limits, valve overlap, manifold and valve body pressure losses, friction, and geometric nonlinearities). Some model parameters are found from physical knowledge; others are determined experimentally, and methods for estimating key parameters from experimental data are developed. The simulation is implemented using Simulink® and its multi-body mechanical simulation tool SimMechanics®. The simulated response compares well with the measured response of the table.

Displacement-based earthquake loss assessment for an earthquake scenario in Istanbul

Abstract: DBELA is a Displacement-Based Earthquake Loss Assessment methodology for urban areas which relates the displacement capacity of the building stock to the displacement demand from earthquake scenarios. The building stock is modeled as a random population of building classes with varying geometrical and material properties. The period of vibration of each building in the random population is calculated using a simplified equation based on the height of the building and building type, whilst the displacement capacity at different limit states is predicted using simple equations which are a function of the randomly simulated geometrical and material properties. The displacement capacity of each building is then compared to the displacement demand obtained, from an over-damped displacement spectrum, using its period of vibration; the proportion of buildings which exceed each damage state can thus be estimated. DBELA has been calibrated to the Turkish building stock following the collection of a large database of structural characteristics of buildings from the northern Marmara region. The probabilistic distributions for each of the structural characteristics (e.g. story height, steel properties etc.) have been defined using the aforementioned database. The methodology has then been applied to predict preliminary damage distributions and social losses for the Istanbul Metropolitan Municipality for a Mw 7.5 scenario earthquake.

Fragility and Hazard Analysis of a Welded Steel Moment Resisting Frame

Abstract: With the move towards performance and consequence-based design and assessment of structures under seismic loading, the engineering community is becoming increasingly convinced that design practices need to be developed and checked using probabilistic methods. In this article, a methodology for the probabilistic assessment of low-rise steel buildings is presented and applied to a welded Moment Resisting Frame (MRF). In light of recent field experience for this form of construction, emphasis is given to the modeling of connections, particularly with respect to fracture characteristics. The seismic behavior of the building is assessed by means of nonlinear dynamic time history analyses, using a set of ground motions scaled according to spectral acceleration. Randomness related both to structural properties and earthquake excitation is explicitly taken into account. Fragility curves are generated using the Monte Carlo (MC) simulation method coupled with the Latin Hypercube Sampling (LHS) technique, and the fa...

Enhanced Seismic Performance of Hybrid Bridge Systems: Comparison with Traditional Monolithic Solutions

Abstract: Remarkable accomplishments have been observed in seismic engineering in the recent past with the definition and development of high-performance seismic resistant systems, able to sustain major ground motions with limited level of structural damage Following the introduction and further developments of jointed ductile connections for the seismic design of precast concrete buildings, the concept of hybrid system, where self-centering and energy dissipation capacity are adequately combined by using unbonded post-tensioned techniques and alternative sources of dissipation, has been recently proposed as a viable and efficient solution for an improved seismic performance of bridge systems In this contribution, based on extensive inelastic time-history analyses using a 3-D lumped plasticity model, a numerical comparison of the seismic response of multi-span bridge systems with hybrid jointed ductile and monolithic connections is presented The seismic performance of the two alternative solutions is evaluated w

Architectural Overview of MAEviz – HAZTURK

Abstract: MAEviz is a broadly extensible, open source platform for earthquake hazard risk management. MAEviz is a model cyberenvironment that provides practical capabilities for researchers through decision-makers to model earthquake events, develop risk reduction strategies, and implement mitigation plans to minimize the impact of earthquake disasters while also providing a pathway for researchers to quickly add new algorithms and data to assure that decisions are based on state-of-the-art engineering understanding. While MAEviz is capable of interacting with remote data and computational sources, it is also fully capable of running analyses locally so research scientists and decision-makers can generate information when a catastrophic event occurs and provide first-responders result information. This article describes MAEviz's overall layered architecture, its foundation in the widely used Eclipse Rich Client Platform (RCP), and use of open-source middleware and geographic information system (GIS) components. MAE...

Recent Developments in the Treatment of Ground-Motion Variability in Earthquake Loss Models

Abstract: This article summarizes recent work investigating the sensitivity of loss estimates for the northern Marmara region to various representations of ground-motion variability. Issues that are discussed include the partitioning of the total ground-motion variability into inter- and intra-event components, macrospatial correlations among ground motions, and the propagation of the ground-motion variability into damage and loss calculations. The issue of whether an earthquake loss model can be validated using field observations is also considered. Recent findings suggest that, regardless of how one treats the ground-motion variability when conducting loss estimation analyses, unless a close network of accelerograms and a high spatial resolution of exposure data exists, the ground-motion variability precludes the validation of loss models.