Shaking table tests on a stone masonry building: modeling and identification of dynamic properties including soil-foundation-structure interaction

This article presents the identification of dynamic properties of a stone masonry building, followed by numerical simulation of its dynamic response accounting for soil-foundation-structure interaction. The first part regards numerical simulations of the earthquake response of a two-story building prototype with timber floors, made of three-leaf stone masonry without laces. This 1:2 scale prototype was tested on a shaking table in its as-built state and after strengthening, at the National Technical University of Athens. Afterward, the building prototype was modeled with flat shell elements and equivalent frames (common frames and macro-elements), for an investigation of its linear and nonlinear seismic response, assuming base fixity. Numerical results were compared to the experimental ones, which yielded conclusions on the considerations of each employed modeling strategy, as well as its efficiency and applicability. The second part considers the effect of soil-structure interaction using appropriately modified foundation stiffness values to account for the foundation soil flexibility. Comparison of the numerical results with and without SSI effects showed how the flexibility of the soil-foundation system and the soil-structure interaction modified the system’s modal characteristics and response within the elastic range, in terms of both seismic loads and deformations, and produced conclusions about its consequences on the overall structural stability.     

故宫灵沼轩的动力特性及抗震性能研究

故宫灵沼轩是我国最早建造的钢铁-砌体组合结构之一,具有重要的历史、艺术和科学价值。为了评估灵沼轩在地震作用下的结构安全状况,建立了灵沼轩结构的三维有限元模型,并对其进行了动力特性和地震时程分析,得出了其固有频率、模态振型、地震位移响应和地震应力响应。结果表明:灵沼轩整体结构布置对称性较高,扭转刚度较大,对抗震较为有利;在8度多遇地震、设防地震和罕遇地震作用下,灵沼轩的金属结构部分及砌体结构部分的顶点位移和层间位移角均符合现行规范要求,砌体结构部分的第三主应力响应均小于材料的抗压强度,不存在压溃风险。在8度多遇地震和设防地震作用下,砌体结构部分的第一主应力响应均小于材料的抗拉强度,结构不会发生拉裂。但在8度罕遇地震作用下,砌体结构的部分位置拉应力超过材料的抗拉强度,这些位置存在开裂危险。最后,综合动力特性和抗震性能分析的结果,提出了灵沼轩的抗震加固建议。     

Shaking Table Testing of a Full-Scale Prefabricated Three-Story Timber-Frame Building

Shake table tests were carried out on a 7 m × 5 m three-story, timber light-frame building (7.5 m height) at the TreesLab laboratory (Eucentre) in Pavia. The aim of the research was to evaluate the seismic behavior of a typical Italian prefabricated timber building and to study the interaction between the individual structural components tested in quasi-static manner in a previous experimental study. The 1979 Montenegro Earthquake ground motion, recorded at the Ulcinj-Hotel Albatros station, was selected as the ground motion for seismic tests. The maximum peak ground acceleration was scaled to 0.07 g, 0.27 g, 0.5 g. 0.7 g, and 1 g in order to evaluate the building’s performance at different levels of seismic input. More than 100 instruments were used to monitor the behavior of the building during seismic tests measuring acceleration, displacement, and forces. The visual inspection shows that the building did not show any damage during all seismic tests. However the data analysis (dynamic identification, capacity spectrum, inter-story drift) confirm that during the 1.00 g test the structure went beyond its linear elastic limit. The results obtained from this experimental study suggest that the design hypotheses commonly adopted in practice for seismic analysis (e.g., in terms of force distributions between the walls, and also the behavior factor q) are not always consistent with the real behavior of timber frame multi-story buildings, and should be backed by more accurate knowledge of the contributions of the individual structural components.     

Theoretical and Numerical Seismic Analysis of Masonry Building Aggregates: Case Studies in San Pio Delle Camere (L’Aquila,Italy)

Masonry building aggregates are large parts of the Italian building heritage often designed without respecting seismic criteria. The current seismic Italian code does not foresee a clear calculation method to predict their static nonlinear behavior. For this reason, in this article a simple methodology to forecast the masonry aggregate seismic response has been set up. The implemented procedure has been calibrated on the results of two FEM structural analysis programs used to investigate three masonry building compounds. As a result, a design chart used to correctly predict the base shear of aggregate masonry units starting from code provisions has been set up.     

Assessing Seismic Behavior of a Masonry Historic Building considering Soil-Foundation-Structure Interaction (Case Study of Arge-Tabriz)

ABSTRACT

Historic heritage buildings are a part of historic basis of each society and an economic resource. Therefore, preserving and maintenance of these buildings are cultural, economic and social demand. This research investigates the seismic performance of a historical building named Arg of Tabriz (Arge Alishah) that dates back to 14th century and is located at the city center of Tabriz (NW of Iran). Static, modal, and finally nonlinear dynamic (time history) analysis were performed by both “Considering Soil-Structure Interaction (SSI)” and “fixed base (ignoring SSI)” Cases.

It is found from the results that, SSI extremely affects mode shapes and their frequencies and depending on the frequency content of the records, can has an incremental or decremental effect on structural responses. As expected, the building of Arg could carry gravity loads easily and despite its stability against earthquake loading in fixed base case, showed a weakness (especially in eastern and western walls direction) and overturned when it was analyzed in SSI case because of yielding of the surrounding soil.     

SYSTEM IDENTIFICATION OF FEI-TSUI ARCH DAM FROM FORCED VIBRATION AND SEISMIC RESPONSE DATA

This paper presents the experimental investigation of the Fei-Tsui arch dam using the forced vibration test and its seismic response data. A forced vibration test was conducted on Fei-Tsui dam, this study presents the identified dynamic properties of the dam from these test data. For the identification of dam properties from seismic response data, in order to consider the nonuniform excitation of the seismic input and to describe the global behavior of the dam, the multiple input/multiple output discrete-time ARX model with least square estimation is applied to identify the dynamic characteristics of the dam. The system modal frequency, damping ratio and frequency response function are identified from both the forced vibration and seismic response data. To verify the accuracy of the identification result, comparison between discrete-time ARX model and a frequency domain conditioned spectral analysis was made. Finally, the spatial variation of ground motion across the free-field canyon surface is also studied.     

Robust Design of a Single Tuned Mass Damper for Controlling Torsional Response of Asymmetric-Plan Systems

A new robust design methodology to control the seismic performance of asymmetric structures equipped with a Single Tuned Mass Damper (STMD) is presented in this article. This design approach aims to control the seismic response of such systems by reducing both flexible-and stiff-edge maximum displacement. The dynamic problem has been investigated in the state space representation showing that the TMD works as a closed-loop feedback control action. A synthetic index to estimate the seismic performance of the main system has been defined by using H_∞ norm. Wide-ranging parametric numerical experimentation has been carried out to obtain design formulae for the STMD in order to minimize such a performance index. These formulae allow for a simple design of STMD position and stiffness to optimally control both translational and rotational motion components, whereas two mass devices are generally considered to improve the seismic performance of asymmetric structural systems The effectiveness and efficiency of the obtained design formulae have been tested by investigating the dynamic behavior of the asymmetric structure after being subjected to different recorded seismic inputs.     

COMPLEX MODE SUPERPOSITION ALGORITHM FOR SEISMIC RESPONSES OF NON-CLASSICALLY DAMPED LINEAR MDOF SYSTEM

This paper deals with a complex mode superposition method for the seismic responses of general multiple degrees of freedom (MDOF) discrete system with complex eigenvectors and eigenvalues. A delicate general solution, completely in real value form, for calculat-ing seismic time history response of the MDOF system which cannot be uncoupled by normal modes, is deduced based on the algorithms of the complex superposition method. This solution comprises of two parts which are in relation to the Duhamel integration to sine and cosine function respectively. The related term of the Duhamel integration to sine function is actually the displacement response of the oscillator with corresponding modal frequency and the damping ratio. The other can be transferred into a combina-tion of the displacement and velocity responses of the same oscillator. In order to meet the practical needs of seismic design based on code design spectra for various kinds of structures equipped by viscous dampers, the complex complete quadratic combination (CCQC) method is deduced following similar procedures such as the well-known CQC method, in which a new modal velocity correlation coefficient, together with a new modal displacement-velocity correlation coefficient are involved besides the modal displacement correlation coefficient in normal CQC formula. The new algorithm of CCQC is not only as concise as that of the normal CQC but also has explicit physical meaning. The results obtained from complex mode superposition approaches are discussed and verified in some examples through step by step integration computation under a prescribed earth-quake motion input. From these examplary analyses, it may be pointed that the CCQC algorithm normally yields conservative outcome and that the forced mode uncoupling approach has good approximation even the discussed examplary structures are strongly non-proportional.     

A Bayesian approach to rapid seismic vulnerability assessment at urban scale

The seismic vulnerability of city centers is commonly assessed by extending the study methods applied to single buildings to urban aggregations. This approach is not always applicable at territorial scale, as it is uneconomical in terms of time and costs. An innovative method provides reasonable large-scale a priori estimation of parameters not directly evaluable from the exterior of buildings by elaborating values which can be measured from the outside. Those parameters are treated as continuous variables, by assigning them a suitable probability density function. The Bayesian approach is adopted, which allows the update of initial hypotheses by using new data gathered during on-site surveys. In this regard, a rapid survey form for the on-site data collection is proposed. An example of its application to a façade of a building structural unit in Santo Stefano di Sessanio in L’Aquila province (Italy) is proposed, showing promising preliminary results for buildings belonging to Italian historical centers.     

Seismic Response of a Four-Story Miniature Building with Replaceable Plastic Hinges

To emphasize on linear and nonlinear seismic behavior of building systems in education, a four-story miniature moment-resisting frame steel building was designed, built, and tested in a shaking table at the Structures Laboratory of the Department of Civil Engineering at the University of Canterbury, New Zealand. A prominent feature of the building is the incorporation of elements designed to form plastic hinges that can be easily replaced after a test with minimum effort and at a very low cost. This model is mainly aimed at education in undergraduate and graduate structural dynamics/earthquake engineering courses and it has also been used to support research. This article describes in detail the main features of the building, its design, and discusses the response of the building to two input ground motions. Because the use of pushover analyses is becoming an industry standard, the some relevant results will be compared with those predicted by such kind of analyses. This article is written in very simple terms and is aimed at the undergraduate and graduate student, at educators in structural design and at structural engineers involved in seismic design of building structures. This article covers many aspects that are seldom highlighted in building behavior to earthquake excitation and that are not always covered in design codes or guidelines.     

Seismic Performance and Modeling of a Half-Scale Base-Isolated Wood Frame Building

The concept of base isolation is a century old, but application to civil engineering structures has only occurred over the last several decades. Application to light-frame wood buildings in North America has been virtually non existent with one notable exception. This article quantitatively examines issues associated with application of base isolation in light-frame wood building systems including: (1) constructability issues related to ensuring sufficient in-plane floor diaphragm stiffness to transfer shear from the superstructure to the isolation system; (2) evaluation of experimental seismic performance of a half-scale base-isolated light-frame wood building; and (3) development of a displacement–based seismic design method and numerical model and their comparison with experimental results. The results of the study demonstrate that friction pendulum system (FPS) bearings offer a technically viable passive seismic protection system for light-frame wood buildings in high seismic zones. Specifically, the amount and method of stiffening the floor diaphragm is not unreasonable, given that the inter-story drift and accelerations at the upper level of the tested building were very low, thus resulting in the expectation of virtually no structural, non structural, or contents damage in low-rise wood frame buildings. The nonlinear dynamic model was able to replicate both the isolation layer and superstructure movement with good accuracy. The displacement-based design method was proven to be a viable tool to estimate the inter-story drift of the superstructure. These tools further underscore the potential of applying base isolation systems for application to North America's largest building type.     

Seismic Assessment of a Monumental Building through Nonlinear Analyses of a 3D Solid Model

ABSTRACT

The paper analyzes the static behavior and the seismic vulnerability of the “San Francesco ad Alto” building in Ancona (Italy), which is currently used as a Regional Headquarter of the Marche Region by the Italian Army and was formerly a monastery. The global static structural behavior and the dynamic properties have been evaluated using the Finite Element modeling technique, in which the nonlinear behavior of masonry has been taken into account by proper constitutive laws. The concepts of homogenized material and smeared cracking are used to evaluate the capacity of the monastery to withstand lateral loads together with the expected demands resulting from seismic actions (N2 method), using a nonlinear static analysis (pushover). The comparison of seismic demand and capacity confirms the susceptibility of these types of buildings to extensive damage and collapse, as frequently observed in similar buildings. This paper aims to point out that advanced numerical analyses can offer significant information on the understanding of the actual structural behavior of historical buildings. It is believed that the methodology and the overall conclusions of this case study are valid for many historical monasteries in Europe.     

Use of Cumulative Ductility Spectra Within a Deformation-Control Format for Seismic Design of Ductile Structures Subjected to Long Duration Motions

Due to the large economical losses derived from recent seismic events, design methodologies that are based in the explicit control of the dynamic response of structures have been formulated. This article introduces, within the framework of performance-based design, a numerical methodology for the seismic design of structures that are expected to undergo severe cumulative plastic demands. The methodology explicitly considers and integrates: (1) the structural and non-structural performance of the building; (2) the life safety limit state; and (3) the prevention of low cycle fatigue.     

Analytical Seismic Assessment of a Tall Long-Span Curved Reinforced-Concrete Bridge. Part I: Numerical Modeling and Input Excitation

The seismic response of bridges is affected by a number of modeling considerations, such as pier embedment, buried pile caps, seat-type abutments, pounding, bond slip and architecturally flared part of piers, and loading considerations, such as non-uniform ground excitations and orientation of ground motion components, which are not readily addressed by design codes. This article addresses a methodology for the nonlinear static and dynamic analysis of a tall, long-span, curved, reinforced-concrete bridge, the Mogollon Rim Viaduct. Various modeling scenarios are considered for the bridge components, soil-structure interaction system, and materials, i.e., concrete and reinforcing steel, covering all its geotechnical and structural aspects based on recent advances in bridge engineering. Various analysis methodologies (nonlinear static pushover, time history response to uniform and spatially variable seismic excitations, and incremental dynamic analyses) are performed. For the dynamic analyses, a suite of nine earthquake accelerograms are selected and their characteristics are investigated using seismic intensity parameters. A recently developed approach for the generation of non-uniform seismic excitations, i.e., spatially variable simulations conditioned on the recorded time series, is used. Methods for the evaluation of structural performance are discussed and their limitations addressed. The numerical results of the seismic assessment of the Mogollon Rim Viaduct are presented in the companion article (Part II). The sensitivity of the bridge response to the adopted modeling, loading and analyzing strategies, as well as the correlation between structural damage and seismic intensity parameters are examined in detail.     

Nonlinear Static and Dynamic Behavior of a 16-Story Torsionally Sensitive Building Designed According to Eurocodes

A 16-story building under construction in Bucharest has been designed according to the provisions of EC2 and EC8, using elastic spectral modal analysis. Considering that the building is torsionally sensitive in the nonlinear range, it was further checked and verified using nonlinear dynamic and static procedures, using a detailed space-frame model. Specifically, time-history analysis for seven different excitations, as well as respective inelastic static analysis taking into account torsional effects were performed. The results are examined regarding structural (global) and member (local) response and various issues concerning the adequacy of the original elastic design and the applicability of advanced analysis methods are discussed.     

TORSIONAL EFFECTS IN THE PUSHOVER-BASED SEISMIC ANALYSIS OF BUILDINGS

The general trends of the inelastic behaviour of plan-asymmetric structures have been studied. Systems with structural elements in both orthogonal directions and bi-axial eccentricity were subjected to bi-directional excitation. Test examples include idealised single-storey and multi-storey models, and a three-storey building, for which test results are available. The response in terms of displacements was determined by nonlinear dynamic analyses. The main findings, limited to fairly regular and simple investigated buildings, are: (a) The amplification of displacements determined by elastic dynamic analysis can be used as a rough, and in the majority of cases conservative estimate in the inelastic range, (b) Any favourable torsional effect on the stiff side, which may arise from elastic analysis, may disappear in the inelastic range. These findings can be utilised in the approximate pushover-based seismic analysis of asymmetric buildings, e.g. in the N2 method. It is proposed that the results obtained by pushover analysis of a 3D structural model be combined with the results of a linear dynamic (spectral) analysis. The former results control the target displacements and the distribution of deformations along the height of the building, whereas the latter results define the torsional amplifications. The proposed approach is partly illustrated and evaluated by test examples.     

Knowledge-Based Performance Assessment of Existing RC Buildings

One of the most challenging aspects of the seismic assessment of existing buildings is the characterization of structural modeling uncertainties. Recent codes, such as Eurocode 8, seem to synthesize the effect of structural modeling uncertainties in the so-called confidence factors that are applied to mean material property estimates. The confidence factors are classified and tabulated as a function of discrete knowledge levels acquired based on the results of specific in-situ tests and inspections. In this approach, the effect of the application of the confidence factors on structural assessment is not explicitly stated. This work presents probabilistic performance-based proposals for seismic assessments of RC buildings based on the knowledge levels. These proposals take advantage of the Bayesian framework for updating the probability distributions for structural modeling parameters based on the results of tests and inspections. As structural modeling parameters, both the mechanical material properties and also the structural detailing parameters are considered. These proposals can be categorized based both on the amount of structural analysis effort required and on the type of structural analysis performed. An efficient Bayesian method is presented which relies on simplified assumptions and employs a small sample of structural model realizations and ground motion records in order to provide an estimate of structural reliability. As an alternative proposal suitable for code implementation, the simplified approach implemented in the SAC-FEMA guidelines is adapted to existing structures by employing the efficient Bayesian method. This method takes into account the effect of both ground motion uncertainty and the structural modeling uncertainties on the global performance of the structure, in a closed-form analytical safety-checking format. These alternative proposals are demonstrated for the case study structure which is an existing RC frame. In particular, it is shown how the parameters for the safety-checking format can be estimated and tabulated as a function of knowledge level, outcome of tests, and the type of structural analysis adopted.     

A Novel Procedure for Simplified Nonlinear Numerical Modeling of Structural Units in Masonry Aggregates

ABSTRACT

Historical centers are always composed of masonry building aggregates often designed without respecting seismic design criteria. The current seismic Italian code does not foresee a clear calculation method to predict their static nonlinear behavior. For this reason, in this article the seismic response of structural units into masonry aggregates has been predicted through a simplified modeling approach. The implemented procedure has been calibrated on the results of a numerical model performed by using the Equivalent Frame Method (EFM), implemented within a Finite Element Method (FEM) calculation program, used to investigate a basic building compound representative of the constructive techniques developed in the past decades in the Southern Italy.

First, the whole aggregate has been modeled and analyzed in the nonlinear static field in order to evaluate the seismic behavior of both intermediate and head structural units.

Later on, the seismic response of these structural units, considered as isolated structures, has been assessed by considering in simplified way their position in the aggregate, as well as the influence of other constructions.

Finally, the achieved results on the above single analysis cases have been compared with those deriving from the investigation of the whole building compound, allowing to confirm the effectiveness of the proposed novel analysis procedure.     

Simplification and Assessment of Modal-Based Story Damage Index for Reinforced Concrete Frames Subjected to Seismic Excitations

Different relations have been represented for the local damage index of structures to date, while the same application is defined for them as can be an indicator of relative sustained damage by the components or stories. Since different force-resisting systems subjected to the ground motions can behave differently, some well-known story damage indices are evaluated for the reinforced concrete frames with regards to their operation during nonlinear time history analysis. Two general concepts of story damage determination are selected for this purpose. SDI is a modal-based story damage index, which is calculated by the modal frequency and mode shapes. The behavior of this local index is evaluated during the seismic excitations. The results were compared with Park-Ang and modal flexibility story damage indices. Based on analytical study on seismic responses of some RC frames subjected to a suit of earthquake records a new story damage index has been developed. It has been derived from a simple global damage equation (softening index) using a normalized ratio of inelastic story shear to its drift. A procedure is recommended to use the proposed equation without any requirement to perform nonlinear dynamic analysis, which can significantly reduce the computational efforts. Distribution of the new represented SDI along the structural height shows a good agreement with damaged state of the RC frames after seismic excitations.     

SEISMIC RISK ASSESSMENT OF RC STRUCTURES WITH THE “2000 SAC/FEMA” METHOD

The applicability of a new, fully probabilistic approach to seismic design and assessment of reinforced concrete (RC) structures is investigated. Fundamental advantages of the method are mathematical simplicity and comparatively light computational effort. The original formulation, which was developed for steel structures, is first illustrated; ah extension which allows consideration of multiple failure mechanisms, typical of RC structures, is then proposed. The applicability of the method is demonstrated through an example: the seismic risk of a four storey RC building that was not designed for seismic resistance is evaluated. Three failure mechanisms are considered: joint failure, column shear failure and drift failure.