Showing papers in "Bulletin of Earthquake Engineering in 2015"

The 2013 European Seismic Hazard Model: key components and results

Abstract: The 2013 European Seismic Hazard Model (ESHM13) results from a community-based probabilistic seismic hazard assessment supported by the EU-FP7 project “Seismic Hazard Harmonization in Europe” (SHARE, 2009–2013). The ESHM13 is a consistent seismic hazard model for Europe and Turkey which overcomes the limitation of national borders and includes a through quantification of the uncertainties. It is the first completed regional effort contributing to the “Global Earthquake Model” initiative. It might serve as a reference model for various applications, from earthquake preparedness to earthquake risk mitigation strategies, including the update of the European seismic regulations for building design (Eurocode 8), and thus it is useful for future safety assessment and improvement of private and public buildings. Although its results constitute a reference for Europe, they do not replace the existing national design regulations that are in place for seismic design and construction of buildings. The ESHM13 represents a significant improvement compared to previous efforts as it is based on (1) the compilation of updated and harmonised versions of the databases required for probabilistic seismic hazard assessment, (2) the adoption of standard procedures and robust methods, especially for expert elicitation and consensus building among hundreds of European experts, (3) the multi-disciplinary input from all branches of earthquake science and engineering, (4) the direct involvement of the CEN/TC250/SC8 committee in defining output specifications relevant for Eurocode 8 and (5) the accounting for epistemic uncertainties of model components and hazard results. Furthermore, enormous effort was devoted to transparently document and ensure open availability of all data, results and methods through the European Facility for Earthquake Hazard and Risk ( www.efehr.org ).

PERPETUATE guidelines for seismic performance-based assessment of cultural heritage masonry structures

Abstract: Ancient monumental masonry buildings are complex structures that were not based on an engineered design, underwent many transformations during their life and often present lack of connections among the structural elements. Earthquakes are the main cause of damage for ancient masonry structures and, in order to reduce their vulnerability with compatible and light interventions, it is necessary to have accurate models for the seismic analysis, able to simulate the nonlinear behavior of masonry, and a well defined performance-based assessment procedure, aimed to guarantee the acceptable level of risk for the occupants and for the conservation of the monument itself. The paper outlines the guidelines that were developed within the PERPETUATE European research project. The wide variety of architectural assets is classified and the related proper modeling strategies are identified; moreover, immovable artistic assets are considered in the assessment. A displacement-based approach is adopted, because these structures crack even for low intensity earthquakes and can survive severe ones only if they have a sufficient displacement capacity. Safety and conservation requirements are proposed by considering distinct sets of performance levels, related to use and safety of people, conservation of the building and of the artistic assets that might be present. Some indications on the seismic hazard assessment are provided, considering the distinctive features of some types of ancient structures. Within the fundamental knowledge phase, sensitivity analysis is proposed in order to address and optimize the in-situ investigation and to define proper confidence factors, aimed to consider epistemic and statistical uncertainties. Different modeling approaches and methods of analysis are considered, depending on the characteristics of the structure; both static pushover and incremental dynamic nonlinear analyses are considered. Related verification procedures are defined to evaluate the seismic intensity measure, and the corresponding return period, which is compatible with each performance level that must be fulfilled.

Bayesian Cloud Analysis: efficient structural fragility assessment using linear regression

Abstract: Cloud Analysis is based on simple regression in the logarithmic space of structural response versus seismic intensity for a set of registered records. A Bayesian take on the Cloud Analysis, presented herein, manages to take into account both record-to-record variability and other sources of uncertainty related to structural modelling. First, the structural response to a suite of ground motions, applied to different realizations of the structural model generated through a standard Monte Carlo, is obtained. The resulting suite of structural response is going to be used as “data” in order to update the joint probability distribution function for the two regression parameters and the conditional logarithmic standard deviation. In the next stage, large-sample MC simulation based on the updated joint probability distribution is used to generate a set of plausible fragility curves. The robust fragility is estimated as the average of the generated fragility curves. The dispersion in the robust fragility is estimated as the variance of the plausible fragility curves generated. The plus/minus one standard deviation confidence interval for the robust fragility depends on the size of the sample of “data” employed. Application of the Bayesian Cloud procedure for an existing RC frame designed only for gravity-loading demonstrates the effect of structural modelling uncertainties, such as the uncertainties in component capacities and those related to construction details. Moreover, a comparison of the resulting robust fragility curves with fragility curves obtained based on the Incremental Dynamic Analysis shows a significant dependence on both the structural performance measure adopted and the selection of the records.

Seismic assessment of rocking masonry structures

Abstract: The seismic assessment of rocking masonry structures is a complex task, due to the high sensitivity of the behavior to the characteristics of the input motion. The paper compares the results offered by different models and the reliability of widely used intensity measures. A displacement-based approach based on an incremental limit analysis of rigid blocks is proposed for the performance-based assessment of rocking masonry structures, such as: rocky structures (archeological remains, obelisks, columns, trilithons), arch-piers systems (e.g. triumphal arches, belfries), out-of-plane mechanisms of walls (standing out walls, facades in buildings or churches etc.) or artistic assets prone to overturn (pinnacles, statues etc.). The method is compatible with the format of the PERPETUATE performance-based assessment method for cultural heritage assets

Updated predictive equations for broadband (0.01–10 s) horizontal response spectra and peak ground motions, based on a global dataset of digital acceleration records

Abstract: Presented herein is an updated model for empirical prediction of 5 %-damped elastic response spectra in the period range 0–10 s, peak ground acceleration and velocity, based on a global dataset of digital acceleration records. The predictive model features saturation of the shaking parameters with both magnitude $$M_{W}$$ and distance $$R_{ RUP }$$ , magnitude-dependent distance attenuation, alternative parameterisations of the amplification effects due to local site conditions (based either on ground types or $$V_{S,30}$$ ) and corrective terms for style-of-faulting. The calibration dataset comprises more than $$1{,}880\times 2$$ orthogonal horizontal accelerometer records with $$R_{ RUP }< 150$$ km from 98 global earthquakes with $$4.5\le M_{W}\le 7.9$$ . The processing technique applied to the acceleration data optimises the reliability of the predictions at long periods, as required by displacement-based design techniques. Developed independently of the recent NGA-West2 and RESORCE-based models, the new predictive tool effectively contributes to capturing the epistemic uncertainties associated with the prediction of seismic shaking levels for engineering applications.

A method for the analysis and classification of historic masonry

Abstract: It is known that the mechanical behavior of masonry material depends on many factors, such as compressive or shear strength of components (mortar and blocks), blocks shape, volumetric ratio between components and wall texture, that is the result of applying a series of construction devices which form the “rule of the art”. Taking into account the complexity of the problem due to the great number of variables, a fair assessment of the load carrying capacity of masonries can be made only with in situ test. Being in situ test a semi-destructive methods, not always viable, a numerical estimate of the mechanical parameters of the walls can be made on the basis of a qualitative criteria evaluation. The method here proposed, called Masonry Quality Index method, consists in evaluating the presence, the partial presence, or the absence of certain parameters that define the “rule of the art”, namely a set of construction devices that, if executed during the construction of a wall, provides a good behavior and ensure the compactness and the monolithicity.

Seismic risk assessment for mainland Portugal

Abstract: The assessment of the seismic risk at a national scale represents an important resource in order to introduce measures that may reduce potential losses due to future earthquakes. This evaluation results from the combination of three components: seismic hazard, structural vulnerability and exposure data. In this study, a review of existing studies focusing on each one of these areas is carried out, and used together with data from the 2011 Building Census in Portugal to compile the required input models for the evaluation of seismic hazard and risk. In order to better characterize the epistemic uncertainty in the calculations, several approaches are considered within a logic tree structure, such as the consideration of different seismic source zonations, the employment of vulnerability functions derived based on various damage criteria and the employment of distinct spatial resolutions in the exposure model. The aim of this paper is thus to provide an overview of the recent developments regarding the different aspects that influence the seismic hazard and risk in Portugal, as well as an up-to-date identification of the regions that are more vulnerable to earthquakes, together with the expected losses for a probability of exceedance of 10 % in 50 years. The results from the present study were obtained through the OpenQuake engine, the open-source software for seismic risk and hazard assessment developed within the global earthquake model (GEM) initiative.

Preliminary ranking of alternative scalar and vector intensity measures of ground shaking

Abstract: In Performance-Based Earthquake Engineering, seismic demand in structures is predicted by building probabilistic seismic demand models that link measures of earthquake intensity (IMs) to measures of structural demand. Investigations are carried out herein for evaluating the predictive capability of a wide range of commonly-used scalar and vector-valued IMs for different peak-related demand parameters. To accomplish this goal, both efficiency and sufficiency of the candidate IMs are taken into account. The latter is evaluated with the recently-proposed “relative sufficiency measure”. This measure, which is derived based on information theory concepts, quantifies the amount of information gained (on average) by an IM relative to another about the demand parameter of interest. Evaluation of the IMs, herein, uses two sets of ground motions consisting of ordinary and pulse-like near-fault records. Two-dimensional RC frame structures, both fixed and isolated at the base, are selected. The most suitable IMs for predicting the considered different demand parameters and types of structure are identified in terms of both efficiency and sufficiency. The use of these most informative IMs is suggested to build improved probabilistic demand models.

Exploring the impact of spatial correlations and uncertainties for portfolio analysis in probabilistic seismic loss estimation

Abstract: The significant potential for human and economic losses arising from earthquakes affecting urban infrastructure has been demonstrated by many recent events such as, for example, L’Aquila (2009), Christchurch (2011) and Tohoku (2012). Within the current practice of seismic loss estimation in both academic and industry models, the modelling of spatial variability of the earthquake ground motion input across a region, and its corresponding influence upon portfolios of heterogeneous building types, may be oversimplified. In particular, the correlation properties that are well-known in observations of ground motion intensity measures (IMs) may not always be fully represented within the probabilistic modelling of seismic loss. Using a case study based on the Tuscany region of Italy, the impacts of including spatially cross-correlated random fields of different ground motion IMs are appraised at varying spatial resolutions. This case study illustrates the impact on the resulting seismic loss when considering synthetic aggregated portfolios over different spatial scales. Inclusion of spatial cross-correlation of IMs into the seismic risk analysis may often result in the likelihood of observing larger (and in certain cases smaller) losses for a portfolio distributed over a typical city scale, when compared against simulations in which the cross-correlation is neglected. It can also be seen that the degree to which the spatial correlations and cross-correlations can impact upon the loss estimates is sensitive to the conditions of the portfolio, particularly with respect to the spatial scale, the engineering properties of the different building types within the portfolio and the heterogeneity of the portfolio with respect to the types.

Seismic responses of a large underground structure in liquefied soils by FEM numerical modelling

Abstract: A large underground subway structure could be severely damaged in a strong earthquake, such as the seismic damages of Daikai subway station in the 1995 Kobe earthquake. After the 1995 Kobe earthquake, the effects of earthquakes on underground structure became a hot topic. According to the seismic damage characteristics of the pipelines buried in the liquefiable soils, a large underground structure may be damaged ever more severely by the liquefaction. To examine the dynamic properties of saturated liquefiable soil, an existed constitutive model is revised and implanted into the commercial FEM software. Then the nonlinear seismic responses of large underground subway structures built in liquefiable soils are analyzed by a numerical modeling. The results of the comprehensive numerical analysis indicate that the existing subway station has a significant effect on the liquefaction of the nearby soils that are likely to be liquefied. The around sand soils under the suggested depth 20 m may be also liquefied under the influence of the large subway station, which should be non-liquefiable sand in many related codes of China. The subway station floats up as soon as the nearby soils are liquefied, and the soils accordingly flow from the lateral foundation to the bottom foundation of the subway station. It is also found that the floating of the subway station is completely out of sync in the main vibration stage of the inputted ground motions and the rising stage of the pore pressure, and it also lags behind in the main vibration stage for a considerable time and begins to stabilize only when the vibration weakens sharply and is close to zero. Generally, the dynamic softening soils under the subway station also have great effect on the seismic response of the large subway station.

Performance assessment and optimization of fluid viscous dampers through life-cycle cost criteria and comparison to alternative design approaches

Abstract: The performance assessment and optimal design of fluid viscous dampers through life-cycle cost criteria is discussed in this paper. A probabilistic, simulation-based framework is described for estimating the life-cycle cost and a stochastic search approach is developed to support an efficient optimization under different design scenarios (corresponding to different seismicity characteristics). Earthquake losses are estimated using an assembly-based vulnerability approach utilizing the nonlinear dynamic response of the structure whereas a point source stochastic ground motion model, extended here to address near-fault pulse effects, is adopted to describe the seismic hazard. Stochastic simulation is utilized for estimation of all the necessary probabilistic quantities, and for reducing the computational burden a surrogate modeling methodology is integrated within the framework. Two simplified design approaches are also examined, the first considering the optimization of the stationary response, utilizing statistical linearization to address nonlinear damper characteristics, and the second adopting an equivalent lateral force procedure that defines a targeted damping ratio for the structure. These designs are compared against the optimal life-cycle cost one, whereas a compatible comparison is facilitated by establishing an appropriate connection between the seismic input required for the simplified designs and the probabilistic earthquake hazard model. As an illustrative example, the retrofitting of a three-story reinforced concrete office building with nonlinear dampers is considered.

Analytical investigation of the cyclic behavior and plastic hinge formation in deep wide-flange steel beam-columns

Abstract: This paper investigates the cyclic behavior of deep wide-flange sections, used as columns in steel Special Moment Frames (SMFs), through detailed finite element (FE) analysis. A wide range of wide-flange sections is subjected to symmetric cyclic lateral loading combined with different levels of constant compressive axial load ratios representing the loading conditions of interior steel columns in SMFs. The FE simulations demonstrate that wide-flange beam-columns, with web and flange slenderness ratios near the current compactness limits of seismic design provisions (AISC 341-10), experience rapid cyclic deterioration in flexural strength under high axial load ratios. It is also found that deep wide-flange slender sections shorten axially to about 10 % of their length due to severe flange and web local buckling. Based on the FE simulations, for bottom story columns, where axial load ratios are in the range of 20–35 %, a reduction to about two thirds of the current compactness limit for highly ductile members would achieve a 4 % chord rotation while maintaining a flexural strength larger than 80 % of the expected plastic flexural strength of a steel column. The FE simulation results also suggest that the pre-capping rotation predicted by current modeling recommendations for steel components (PEER/ATC 72-1) is overestimated for sections with high web and flange slenderness ratios undergoing monotonic and/or cyclic lateral loading combined with high axial load levels.

Seismic vulnerability of building aggregates through hybrid and indirect assessment techniques

Abstract: This work approaches the seismic vulnerability assessment of an old stone masonry building aggregate, located in San Pio delle Camere (Abruzzo, Italy), slightly affected by the 2009 April 6th earthquake occurred in L’Aquila and its districts. This building aggregate has been modelled by using the 3muri® software for seismic analysis of masonry constructions. On one hand, static non-linear numerical analyses were performed to obtain capacity curves together with the prediction of damage distributions for the input seismic action (hybrid technique). On the other hand, indirect techniques, based on different vulnerability index formulations, were used for assessing the building aggregate’s behaviour under earthquake action. The activities carried out have provided a clear framework on the seismic vulnerability of building aggregates, as well as aid future retrofitting interventions.

Acquiring reference parameters of masonry for the structural performance analysis of historical buildings

Abstract: The structural performance of historical masonry elements can be understood provided the following factors are known: geometry; the characteristics of its masonry texture and morphology, state of damage and decay, physical, chemical and mechanical characteristics of the components (units, infill, mortar); the characteristics of built masonry as a composite material. In order to quantify the mechanical properties of the masonry both laboratory and in-situ tests are required. However, in the case of cultural heritage assets, the setting up of an effective knowledge procedure is strictly related to the minimization of invasiveness on the structure, with the aim of its conservation, rather than the cost–benefit optimization: thus it is essential to have available reference parameters to be adopted for different masonry types. Within this context, this State-of-the-Art paper on this topic is organized with integrated outcomes from the test campaigns carried out through the PERPETUATE project, that are also briefly presented. Reference parameters for effective seismic assessment are provided both for brick and stone masonry together with their upper and lower bound values for both mechanical parameters and damage limits for which proper limit states (LS) may be associated. Apart from the LS for structural elements (SE), the relevant LS’s for artistic assets attached to the SE are also presented in this paper.

Investigation of the characteristics of Portuguese regular moment-frame RC buildings and development of a vulnerability model

Abstract: A vulnerability model capable of providing the probabilistic distribution of loss ratio for a set of intensity measure levels is a fundamental tool to perform earthquake loss estimation and seismic risk assessment. The aim of the study presented herein is to develop a set of vulnerability functions for 48 reinforced concrete building typologies, categorized based on the date of construction (which has a direct relation with the design code level), number of storeys (height of the building) and seismic zonation (which affects the design of the buildings). An analytical methodology was adopted, in which thousands of nonlinear dynamic analyses were performed on 2D moment resisting frames with masonry infills, using one hundred ground motion records that are compatible, to the extent possible, with the Portuguese tectonic environment. The generation of the structural models was carried out using the probabilistic distribution of a set of geometric and material properties, compiled based on information gathered from a large sample of drawings and technical specifications of typical Portuguese reinforced concrete buildings, located in various regions in the country. Various key aspects in the development of the vulnerability model are investigated herein, such as the selection of the ground motion records, the modelling of the infilled frames, the definition of the damage criterion and the evaluation of dynamic (i.e. period of vibration) and structural (i.e. displacement and base shear capacity) parameters of the frames. A statistical bootstrap method is demonstrated to estimate the variability of the loss ratio at each intensity measure level, allowing the estimation of the mean, as well as 10 and 90 % percentile vulnerability curves.

Building damage assessment after the 2009 Abruzzi earthquake

Abstract: Soon after the earthquake that damaged L’Aquila municipality and its Province in the Abruzzi Region, Italy, on the 6th of April 2009, a widespread damage and usability assessment of buildings was launched. In 3 months, about 70,000 buildings were inspected, with the AeDES form, by public servants and professionals coordinated by the Italian Civil Protection Department. The paper, after describing the procedures and the form that were used for the assessment, discusses the time evolution of the inspections and analyses the data on building type and seismic damage. The empirical damage distribution conditional upon seismic intensity and building type is provided and the role of several vulnerability factors, such as the quality of masonry, the construction year, the number of stories, and the pre-existing damage, is highlighted. Lastly the damage consequences, such as the immediate occupancy conditional upon building damage and building type, are reported.

Experimental testing of engineered masonry infill walls for post-earthquake structural damage control

Abstract: The paper presents the results of an experimental campaign on the behaviour of engineered masonry infill walls subjected to both in- and out-of-plane loading. The aim of the research was to develop a design approach for masonry infill walls capable of solving their vulnerability and detrimental interaction with the frame structure when exposed to seismic excitation. Tests on two large-scale specimens and sub-assemblies were performed in order to evaluate the infill deformation capacity, the damage associated with different drift levels, and the mechanical properties of the components. A design solution with sliding joints to reduce the infill-frame interaction and ensure out-of-plane stability, which was proposed in a previous study, was developed and refined with focus on construction details. The aim of sliding joints is to ensure a predetermined mechanism in the infill wall, which is governed by hierarchy of strength and is capable of ensuring ductility and energy dissipation that can be taken into account in the design practice, thanks to the predictability of the response. The two infill wall specimens, one of them including an opening, reached up to 3 % in-plane drift with very little damage and supported an out-of-plane force equivalent to a horizontal acceleration four times the acceleration of gravity. The force-displacement hysteretic curve, sliding at the joints and crack pattern show the efficiency of the construction technique, based on affordable and tradition-like construction processes and materials. The technique, presented here for hollow fired-clay masonry units, can be extended to different masonry infill typologies.

Diagonal compression tests on masonry walls strengthened with a GFRP mesh reinforced mortar coating

Abstract: The paper presents the results of a broad experimental investigation conducted through diagonal compression tests on masonry specimens strengthened with a mortar coating applied on both surfaces of the wall and reinforced with a glass fiber reinforced polymer (GFRP) mesh. Four types of masonry, three different types of masonry mortar and five diverse GFRP meshes for the reinforcement were considered. In particular, solid brick masonry 250 and 380 mm thick, two-leaf brick masonry with rubble conglomerate infill and rubble stone masonry were tested. The diagonal compression tests, performed on 60 square masonry specimens with loading-unloading cycles up to the collapse, evidenced a good effectiveness of the strengthening technique in terms of both resistance and ductility. Results also showed the resistance increment in reinforced samples is generally greater for weaker masonry types and, referring to a single masonry type, for specimens built with a weaker mortar. Furthermore, the different GFRP meshes influenced very little the resistance of specimens, but higher reinforcement contents induced a lower decrease of the diagonal load after the cracking.

Prediction of the additional shear action on frame members due to infills

Abstract: Infill masonry walls in framed structures make a significant contribution to the response under seismic actions. With special regard to reinforced concrete (RC) structures, it is known that internal forces modifications caused by the frame–infill interaction may be not supported by the surrounding frame because of the additional shear forces arising at the ends of beams and columns. Such additional forces may lead to the activation of brittle collapse mechanisms and hence their prediction is basic in capacity assessment, especially for structures that disregard the details for seismic zones. In this paper a parametric study is carried out addressed to the prediction of the shear forces mentioned before. The results of this study can be used as a support when the simplified model is adopted consisting in the substitution of infill with an equivalent pin jointed concentric strut, because in this case the structural analysis fails in the prediction of the shear forces in question. Through the paper, in which existing RC infilled frames designed only for vertical loads are discussed, analytical laws, depending on the level of the axial force arising in a concentric strut equivalent to infill, are proposed, the above analytical law allowing to correct the local shear forces in the frame critical sections, which are not predictable in the case of substitution of infill with an equivalent concentric strut.

Restoring capability of friction pendulum seismic isolation systems

Abstract: The restoring (or re-centring) capability is an important feature of any isolation system and a fundamental requirement of current standards and guideline specifications for the design of seismically isolated structures. In this paper, the restoring capability of spherical sliding isolation systems, often referred to as friction pendulum systems (FPSs), is investigated through an extensive parametric study involving thousands of non-linear response history analyses of SDOF systems. The dynamic behavior of the isolation system is described with the visco-plastic model of Constantinou et al. (J Struct Eng 116(2):455–474, 1990), considering the variability of the friction coefficient with sliding velocity and contact pressure. Numerical analyses have been carried out using a set of approximately three hundred natural seismic ground motions recorded during different earthquakes and differing in seismic intensity, frequency content characteristics, magnitude, epicentral distance and soil characteristics. Regression analysis has been performed to derive the dependency of the residual displacement from the parameters governing the dynamic response of FPS. The influence of near-fault earthquakes and the accumulation of residual displacements due to real sequences of seismic ground motions have been also investigated. Finally, the restoring compliance criteria proposed in this study are compared to the lateral restoring force requirements of current seismic codes. Based on the results of this study, useful recommendations for a (more) rational design of FPSs are outlined.

Experimental testing on emulative connections for precast columns using grouted corrugated steel sleeves

Abstract: Seismic analysis of precast concrete structures requires specific information regarding the behaviour of the connections under large deformation cycles. If a connection is located within the critical region of a structural element, its energy dissipation capacity needs to be confirmed by experimental testing. The use of emulative connections for precast elements is attractive for designers because common conventional design methodologies and assumptions developed for cast in place structures may be readily used. The results of a structural testing program for precast columns connected using grouted corrugated steel sleeves are reported in this paper. A comparison with reference “cast in place” specimens is made and conclusions regarding the emulative characteristic of the connection are drawn. Tests were carried out on scaled concrete columns using the reaction frame at the Technical University of Civil Engineering of Bucharest. Four precast specimens and two reference specimens were tested. Analysis of the recorded data showed that the precast specimens have similar hysteretic response and energy dissipation capacity as the reference ones.

Shaking table tests on a three-arch type subway station structure in a liquefiable soil

Abstract: A series of large-scale shaking table tests were performed to investigate the damage mechanisms of a three-arch type subway station structure in a liquefiable soil experiencing strong motions. Methods to measure the displacement included the vision-based displacement test and the fiber Bragg grating test to measure the strain of the galvanized steel wire. Sand boils, waterspouts, ground surface cracks and settlements, and buoyancy movement of the model structure were observed. When the peak excess pore pressure ratios dramatically increased, the Arias intensity also dramatically increased. The peak acceleration of the model soil also almost coincided with liquefaction of the model soil. The seismic responses of the model structure and the soil were shown to be more sensitive to input motions with larger low-frequency components, the phenomenon of high frequency filtering and low frequency amplification effect of the liquefied soil were observed. The peak tensile strain located at the top and bottom of the center pillars was larger than that obtained at the subarch, while the peak tensile strain at the atrium arch was the smallest. The peak strain at the primary and secondary observation sections were remarkably affected by the spatial effect. The results can provide valuable insight into the seismic investigation of these subway structures.

Seismic behaviour of traditional timber frame walls: experimental results on unreinforced walls

Abstract: Timber frame buildings are well known as an efficient seismic resistant structure and they are used worldwide Moreover, they have been specifically adopted in codes and regulations during the XVIII and XIX centuries in the Mediterranean area These structures generally consist of exterior masonry walls with timber elements embedded which tie the walls together and internal walls which have a timber frame with masonry infill and act as shear walls In order to preserve these structures which characterize many cities in the world it is important to better understand their behaviour under seismic actions Furthermore, historic technologies could be used even in modern constructions to build seismic resistant buildings using more natural materials with lesser costs Generally, different types of infill could be applied to timber frame walls depending on the country, among which brick masonry, rubble masonry, hay and mud The focus of this paper is to study the seismic behaviour of the walls considering different types of infill, specifically: masonry infill, lath and plaster and timber frame with no infill Static cyclic tests have been performed on unreinforced timber frame walls in order to study their seismic capacity in terms of strength, stiffness, ductility and energy dissipation The tests showed how in the unreinforced condition, the infill is able to guarantee a greater stiffness, ductility and ultimate capacity of the wall

Estimation of the cyclic capacity of beam-to-column dowel connections in precast industrial buildings

Abstract: The behaviour of precast systems depends on the performance of the specific connections between the precast elements. In European precast design practice, the most common type of connection between beams and columns is a dowel connection. Such connections are subject to the following types of potential failure mechanism: (a) local failure characterized by the simultaneous yielding of the dowel and crushing of the surrounding concrete, and (b) global failure, characterized by spalling of the concrete between the dowel and the edge of the column or the beam. In this paper both types of failure of dowel connections are studied, although somewhat more attention is paid to the less investigated global failure. The local failure mechanism has been relatively well investigated, and the results have been presented in several studies. Thus only some minor changes are proposed in connection with the prediction of the related strength. On the other hand, the majority of existing procedures for the estimation of global strength are over-conservative since they neglect the influence of stirrups, or else only take them into account implicitly. None of these methods explicitly take into account the fact that the global failure of the dowel connection is changed by the presence of stirrups from brittle to ductile. In the paper, a new procedure for the estimation of resistance against global failure is proposed. Taking into account an appropriate strut and tie model of the connections, the influence of stirrups on this resistance as well as on the type of the failure is taken into account explicitly. Comparisons that were performed between the analytically calculated strength and the experimental results obtained have clearly shown that both of the proposed procedures for the estimation of resistance against local and global failure agree very well with the experimental results.

A method for the direct determination of approximate floor response spectra for SDOF inelastic structures

Abstract: Floor response spectra, which are used for the seismic design of equipment, are often based on the assumption that the behaviour of a structure and its equipment is linearly elastic. Significant reductions in the peak values of floor acceleration spectra can be achieved if inelastic behaviour of the structure is taken into account. This paper presents the most important results of an extensive parametric study of floor acceleration spectra, taking into account inelastic behaviour of the structure, and linear elastic behaviour of the equipment. The structure and the equipment were modelled as single-degree-of-freedom systems. The influences of the input ground motion, ductility, hysteretic behaviour and the natural period of the structure, as well as that of damping of the equipment, have been studied. A simple practice-oriented method for direct determination of floor acceleration spectra from an inelastic spectrum for the structure and an elastic spectrum for the equipment is proposed and validated. In this method, the floor response spectra in the resonance region, where the natural period of the equipment is close to the natural period of the structure, are based on the empirical values obtained in the parametric study, whereas the spectra in the pre- and post-resonance regions are based on the principles of dynamics of structures. The method is intended for a quick estimation of approximate floor acceleration spectra.

Seismic behavior of three-leaf stone masonry buildings before and after interventions: Shaking table tests on a two-storey masonry model

Abstract: The aim of this work is to reach a better understanding of the seismic behaviour of historic three-leaf stone masonry buildings with timber floors, before and after interventions. For this purpose, shaking table tests were performed on a building model (scale 1:2). Initially, the dynamic characteristics of the model were identified. Subsequently, biaxial earthquake tests were performed with the base acceleration increased step-wise until the occurrence of repairable damages. Afterwards, the masonry was strengthened by means of grouting and the diaphragm action of the floors was enhanced. Then, the strengthened model was re-tested. The comparison of the performance of the model under earthquake actions before and after strengthening shows that the selected intervention techniques significantly improved the seismic behaviour of the structure.

Seismic assessment of existing and strengthened stone-masonry buildings: critical issues and possible strategies

Abstract: The seismic performance of stone masonry buildings is known to be generally poor with respect to other structural typologies. However, significant differences can be observed for different architectural configurations, structural details and masonry mechanical properties. In particular, the seismic vulnerability of existing stone masonry structures is often governed by local failure modes, typically consisting of out-of-plane overturning of structural portions or crumbling of outer wythes in multi-leaf walls. In buildings with an adequate masonry quality, an overall behaviour controlled by the in-plane capacity of walls can develop and govern the global failure mode, provided that proper connections between perpendicular walls and between walls and floors are effective in contrasting the activation of early local failures. In these cases, the in-plane stiffness of diaphragms (typically vaults and timber floors/roofs) can play a significant role in coupling the response of the different walls, hence controlling the global building capacity. Recent experimental testing campaigns carried out in different laboratories have focused on several aspects of the seismic response of stone masonry buildings and on the effect of several strengthening techniques. The availability of such experimental results allowed validation and improvement of analysis tools and procedures for the assessment of the seismic capacity of existing stone masonry structures. In order to make them available to all practitioners, the research achievements need to be incorporated in codes and guidelines for the assessment and strengthening of existing stone masonry buildings. The procedures currently proposed in several codes are already based on a rational approach, which starts from the acquisition of an adequate structural knowledge level and allows for using nonlinear analysis procedures. They could straightforwardly include new research findings and practical developments.

Displacement-based design procedure of damped braces for the seismic retrofitting of r.c. framed buildings

Abstract: The insertion of damped braces proves to be very effective for enhancing the performance of a framed building under seismic loads. For a widespread application of this technique suitable design procedures are needed. In this paper a design procedure which aims to proportion damped braces to attain a designated performance level of the structure, for a specific level of seismic intensity, is proposed. In particular, a proportional stiffness criterion, which assumes the elastic lateral storey-stiffness due to the braces proportional to that of the unbraced frame, is combined with the displacement-based design, in which the design starts from a target deformation. To check the effectiveness and reliability of the design procedure, a six-storey reinforced concrete plane frame, representative of a medium-rise symmetric framed building, is considered as primary structure. This, designed in a medium-risk seismic region, has to be retrofitted as in a high-risk seismic region by the insertion of braces equipped with either metallic-yielding dampers or viscoelastic ones. Nonlinear dynamic analyses of unbraced and damped braced frames are carried out, under real (set A) and artificially generated (set B) ground motions, by a step-by-step procedure. Frame members and hysteretic dampers are idealized by bilinear models, while the viscoelastic dampers are idealized by a six-element generalized model describing the variation of the mechanical properties depending on the frequency, at a given temperature.

Time-dependent seismic demand and fragility of deteriorating bridges for their residual service life

Abstract: Coastal bridges that are exposed to marine environments always suffer serious corrosion, which degrades their structural performance during the long-term service period. This paper investigates the time-dependent seismic demand and fragility of coastal bridges for their residual service life. First, the equal exceeding probability method was established to consider the reduction in the seismic hazard level for the residual service life of the structures. A finite element model of sound and aging structures was built by analyzing the different corrosion characteristics of bridge columns in the atmospheric zone, splash and tidal zone and submerged zone due to chloride penetration in concrete. The incremental dynamic analysis method was adopted to analyze the seismic demand and establish the fragility curves of the sound and aging bridges under both the design spectral acceleration and the reduced earthquake inputs for comparison. The results of this analysis indicate that the seismic demand of the structure significantly decreases, and the probability of failure of the structural system does not support the common opinion of increasing in the residual service period, if the shortening of the service period is considered in the analysis.

Real-time analysis and interpretation of continuous data from structural health monitoring (SHM) systems

Abstract: The dynamic characteristics of a structure are commonly defined by its modal properties: modal frequencies, damping ratios, and mode shapes. Significant changes in modal properties of a structure after an extreme event, such as an earthquake, or during its service life can be strongly related to damage in the structures. This makes it crucial that the modal properties are accurately estimated and continuously tracked to detect any changes by the structural health monitoring (SHM) system. This paper introduces an algorithm and a MATLAB-based software that includes modules for real-time data processing, modal identification, damage detection, and stakeholder warnings for vibration-based SHM systems. The data processing and modal identification techniques used are based on the classical and stochastic techniques, and utilize running time windows to keep track of time variations in real-time data. The damage detection algorithm makes use of inter-story drifts to detect and locate damage. Since the calculation of inter-story drifts involves double integration and subtraction of acceleration signals, it is extremely hard to get accurate values of inter-story drifts in real-time monitoring. To improve the accuracy, inter-story drifts are calculated for each mode of the structure separately, and then combined synchronously. The displacements at non-instrumented floors are estimated by assuming that the mode shapes can be approximated as a linear combination of those of a shear beam and a bending beam. A software package, REC_MIDS, is developed for this purpose, and it has been operating in a large number of different structures with SHM systems in Turkey (tall buildings, suspension bridges, mosques, museums), and in seven high-rise buildings in UAE.