Methods and Challenges for the Seismic Assessment of Historic Masonry Structures

ABSTRACT

Despite the high vulnerability of historic structures to earthquakes, the approaches for evaluating seismic demand and capacity still appear inadequate and there is little consensus on the most appropriate assessment methods to use. To develop an improved knowledge on the seismic behavior of masonry structures and the reliability of analysis tools, two real-scale specimens were tested on a shake table, and several experts were invited to foresee failure mechanism and seismic capacity within a blind prediction test. Once unveiled, experimental results were simulated using multi-block dynamics, finite elements, or discrete elements. This article gathers the lessons learned and identifies issues requiring further attention. A combination of engineering judgment and numerical models may help to identify the collapse mechanism, which is as essential as it is challenging for the seismic assessment. To this purpose, discrete modeling approaches may lead to more reliable results than continuous ones. Even when the correct mechanism is identified, estimating the seismic capacity remains difficult, due to the complexity and randomness of the seismic response, and to the sensitivity of numerical tools to input variables. Simplified approaches based on rigid body dynamics, despite the considerable experience and engineering judgment required, provide as good results as do advanced simulations.     

Numerical Analyses of the Effectiveness of Soft Barriers into the Soil for the Mitigation of Seismic Risk

An approach to mitigate the seismic risk of existing structures by means of the creation of a continuous thin layer of grouted soil at a convenient depth is presented. A parametric numerical analysis is reported using different constitutive models with reference to two geometrical schemes. It is shown that if the grouted layer has a stiffness significantly lower than that of the surrounding soil, it may be effective in reducing the seismic demand. In the parametric analyses, the positive role of yielding is also observed, which indicates that the barrier is more effective with larger input amplitudes.     

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.     

Seismic Behavior of L-Shaped RC Squat Walls under Various Lateral Loading Directions

Considerable progress has been made on the research of non-rectangular reinforced concrete (RC) squat walls over the past decades. However, the experimental data of L-shaped RC squat walls remain limited, especially for their seismic behaviors under non-principal bending actions. This paper presents an experimental and numerical investigation on L-shaped RC squat structural walls with an emphasis on how varying the directions of lateral cyclic loading influences the seismic responses of these walls. Four L-shaped specimens are tested under lateral cyclic displacements and low levels of axial compression The variables are axial loads and lateral loading directions. The performance of specimens is discussed in terms of cracking patterns, failure mechanisms, hysteretic responses, deformation components and strain profiles. Furthermore, three-dimensional finite element models are developed to supplement the experimental results. The direction of lateral loading is found to have a significant effect on the peak shear strength of L-shaped RC squat walls.     

Lightly Reinforced Walls Subjected to Seismic Excitations: Interpretation of CAMUS 2000–1 and 2000–2 Dynamic Tests

In this article a study is presented of the inelastic seismic performance of two 5-story reinforced concrete wall specimens, which were tested in the context of the CAMUS 2000 program. The structure has been sized and detailed following the French PS92 code. To investigate the simplifying assumptions made in design, a 3-D refined nonlinear analysis was conducted. Particular aspects of the behavior of the two tested specimens are presented and then test results are compared with numerical predictions. The experimental-analytical comparisons not only demonstrate the accuracy of the time-history analysis model, but also allow obtaining more detailed information about the behavior of the specimen when it is subjected to seismic excitation. The significant effect of degradation of the stiffness and strength of the wall suggests that it is always important that design procedures are derived from numerical modeling and experimental observations.     

Cyclic Response of Concrete Beams Reinforced by Plain Bars

There are many reinforced concrete structures throughout the world that have been built in the past decades that lack appropriate seismic details and reinforced by plain bars. To study the behavior of such buildings, seven beams have been tested under cyclic and monotonic load. The specimens include substandard specimens, with deficient seismic details and reinforced by plain bars, specimens designed in accordance with ACI-318-99 but reinforced by plain bars, and standard specimens reinforced by deformed bars. The tests indicate that the substandard specimens sustain relatively large slip of longitudinal bars, separation of specimen relative to foundation and sliding at large deformation phase, low initial stiffness ratio, limited lateral displacement capacity, and loss of nominal yield strength. The specimens reinforced by plain bars in accordance with ACI-318-99 perform almost similar to standard specimens with deformed bars, in terms of elastic stiffness and lateral displacement ductility; but, they sustain larger slip, and smaller yield strength. Failure of all specimens reinforced by plain bars is characterized by flexural cracks without visible shear failure. Residual shear strength of substandard specimens is modeled by dowel action of longitudinal bars to predict a lower limit for lateral strength of the specimens.     

Impact of Modification on Ground Motion Characteristics and Geotechnical Seismic Analyses for a California Site

The impact of different modification techniques on ground motion characteristics and results of seismic geotechnical analyses is investigated for a site in California. Twenty-eight motions were selected and scaled and also modified using both time domain (TD) and frequency domain (FD) techniques. PGV and PGD of the TD-modified motions are found to be larger than their FD-modified counterparts, but slightly less than the scaled ground motion characteristics. Cyclic stress ratios and amplification factors are similar for all sets of motions. Newmark-type slope displacements caused by the scaled and modified ground motions are similar (within 25%) for a variety of sliding masses.     

Effects of Soil Characterization on the Seismic Input

This paper is aimed at determining the effects of the soil characterization on the seismic input to use for seismic assessment. Three different soil profiles have been assembled to represent the stratigraphies found through a proper experimental investigation, carefully described, and alternative seismic site response analyses have been performed. The surface spectra obtained from the seismic site response analysis (SRA) are very different from each other, thus evidencing the importance of carefully describing soil stratigraphy. Furthermore, the comparison among the surface records found for different return periods has shown a limited sensitivity of the SRA to the seismic intensity.     

Experimental and Finite Element Analytical Investigation of Seismic Behavior of Full-Scale Masonry Infilled RC Frames

This article investigates the seismic behavior of masonry infilled RC frames with/without openings. Four full-scale, single-story, and single-bay specimens were tested under constant vertical loads and quasi-static cyclic lateral loads. The experimental results showed that the infill wall was more influential in stiffness than in load-resisting capacity. The opening increased the ductility ratio of the structure due to the uniform distribution and slow propagation of cracks. Finally, simplified micro finite element models are established to simulate the tested specimens, which effectively predict the load-displacement response of the structures and the crack damage of masonry infill wall with acceptable accuracy.     

Early development of late Vistulian (Weichselian) lacustrine sediments in the Żabieniec swamp (central Poland)

The early sedimentological and ecological developmental stages of a late Vistulian lake in the ?abieniec swamp near ?ód? (central Poland) were analysed by radiometric dating, changes in the frequency of specimens and species composition of Cladocera and multivariate statistical analysis (DCA). The longevity of the lake resulted in a fairly complete sedimentary record from approximately 22 ka to 11 ka cal BP. Species composition and the variability in the frequency of Cladocera specimens has made it possible to distinguish eleven zones of their development, which are well correlated with radiocarbon data. The results were compared to those from other sites of north and central Europe. The initial Cladocera development comprises the richest and oldest late Vistulian Cladocera record in Europe. These cladoceran assemblages show that a deep, oligotrophic, moderately cold-water lake was present at the beginning. It appears that the cladoceran development was mainly due to climate change, but also to changes in locally prevailing conditions in the water body. The high frequency of cladocerans, as well as the presence of cladoceran taxa preferring warmer water, was noted before approximately 16 ka BP. The biota suggests that the Oldest Dryas cooling was not severe and fairly variable in terms of humidity and temperature. The changes in Cladocera composition give evidence of what is described as the Intra-Bølling Cold Oscillation. It is also concluded that the Bølling is represented by two phases in the ?abieniec lake sediments.     

Seismic Response of the Central Part of Indo-Gangetic Plain

In spite of high seismic risk, there is no numerical model of Indo-Gangetic Plain. In this paper, seismic response of the central part of Indo-Gangetic Plain, i.e., Ganga Plain is studied using a two-dimensional plane strain finite element model. Seismic source is assumed to be located beneath the Himalayas near Himalayan Frontal Thrust. Basin response is simulated for a hypothetical M_w 8.0 Himalayan earthquake. Ground motion amplification and sensitive frequency band of the basin, obtained from the numerical simulation, show that the sediment depth and epicentral distance at a site play significant role in the seismic response of the site.     

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.     

传统方法加固馆藏浮放文物抗震性能试验

为探索馆藏浮放文物的有效抗震加固方法,采用振动台试验手段,研究了传统方法加固浮放文物的抗震性能。基于故宫博物院某展柜原型尺寸,制作了1:1比例陈列柜模型。在陈列柜内浮放一轻质陶瓷文物,分别考虑采用砂子、塑料卡、鱼线、橡皮泥及磁铁加固文物,进行了振动台试验。通过白噪声激励,获得了陈列柜基频分布;通过输入不同强度、不同类型的地震波,获得了不同方法加固条件下陈列柜和文物的位移响应、加速度响应和和动力放大系数,评价了不同方法的加固效果。结果表明:陈列柜的基频与输入地震波相差较大,这是陈列柜震害不明显的主要原因之一;传统方法加固文物后,输入地震波的加速度仍有一定放大,但是与加固前文物相比,文物的位移和加速度响应均有所减小,且文物因摇晃幅度而产生倾覆的可能性减小。此外,从减小文物加速度响应峰值角度考虑,橡皮泥和鱼线加固文物效果优于其它方法。试验结果可为文物防震保护研究提供较全面的参考。     

Experimental Evaluation of Seismically and Non-Seismically Detailed External RC Beam-Column Joints

An experimental investigation was undertaken to study the seismic performance of external reinforced concrete (RC) beam-column joints having representative details for mid-rise RC frame buildings in developing countries such as Iran that were designed and constructed prior to the 1970s. Three half-scale external RC beam-column joints were tested by applying lateral cyclic loading of increasing amplitudes. Tested specimens were comprised of one unit having seismic reinforcement detailing in accordance with the seismic requirements of ACI 318-11, and two units having non-seismic reinforcement detailing in accordance with the 1970s construction practice in many developing countries, such as Iran. Two typical defects were considered for the non-seismic units, being the absence of transverse steel hoops and insufficient bond capacity of beam bottom reinforcing bars in the joint region. Test results indicated that the non-seismically detailed specimens had a high rate of strength and stiffness degradation when compared to the seismically detailed specimen, which was attributed primarily to the joint shear failure or bond failure of the beam bottom bars. The non-seismically detailed specimens also showed a 30% reduction in both average strength and ductility and a 60% loss of energy dissipation capacity in comparison to the seismically detailed specimen.     

THE SHADOW-ZONE OF LARGE ITALIAN EARTHQUAKES. EARLY JOURNALISTIC SOURCES AND THEIR PERCEPTION OF 17TH-18TH CENTURIES SEISMICITY

From their earliest beginnings in the 16th century, journalists never failed to find earth-quakes interesting. As a result, early journalistic sources offer a huge hoard of original earthquake data, mostly untapped to this day for seismological purposes [Camassi and Castelli, 2003]. In the aftermath of large earthquakes, the journalists of the 17th and 18th centuries tended to react in two ways. Either their interest for seismic news would increase, leading them to hunt for more news of the same kind, or it would focus exclusively on the “big one” to the detriment of lesser contemporary earthquakes. In the first case, it was possible that more earthquakes than usual would be given coverage; but in the second case more earthquakes than usual were likely to be overlooked and shadowed by the larger event. Through a comprehensive cross-check of Italian early journalistic sources, this paper attempts to highlight the “shadow-zones” of some major 17th–18th centuries Italian earthquakes.     

MODELLING OF THE GROUND MOTION AT RUSSE SITE (NE BULGARIA) DUE TO THE VRANCEA EARTHQUAKES

An approach, capable of synthesising strong ground motion from a basic understanding of fault mechanism and of seismic wave propagation in the Earth, is applied to model the seismic input at a set of 25 sites along a chosen profile at Russe, NE Bulgaria, due to two intermediate-depth Vrancea events (August 30, 1986, M _ω=7.2, and May 30, 1990, M _ω=6.9). Accordingly to our results, once a strong ground motion parameter has been selected to characterise the ground motion, it is necessary to investigate the relationships between its values and the features of the earthquake source, the path to the site and the nature of the site. Therefore, a proper seismic hazard assessment requires an appro-priate parametric study to define the different ground shaking scenarios corresponding to the relevant seismogenic zones affecting the given site. Site response assessment is provided simultaneously in frequency and space domains, and thus the applied procedure differs from the traditional engineering approach that discusses the site as a single point. The applied procedure can be efficiently used to estimate the ground motion for different purposes like microzonation, urban planning, retrofitting or insurance of the built environment.     

Cape Rock Lobster (Jasus lalandii) Remains from South African West Coast Shell Middens: Preservational Factors and Possible Bias

Cape rock lobster (Jasus lalandii) remains in South African west coast shell middens are represented by calcareous mandibles, and by fragments of exoskeleton when preservation is good. Quantification of MNIs and observations on ody size of Cape rock lobsters have been based on counts and measurements on the surviving mandibles. Little is known of the importance of this resource in precolonial hunter-gatherer subsistence, and even less is known about the possible preservational bias affecting the recovery of representative quantitative data from Cape rock lobster mandibles. The latter problem is crucial to resolve in order to understand issues about coastal hunter-gatherer subsistence. Left and right mandibles are not exact mirror images of each other, with left mandibles being consistently larger and thicker than the right mandibles. Moreover, small mandibles measuring between 4·5 and 7·5 mm are much thinner and delicate than large specimens. Due to their small size, however, small mandibles can escape attrition by falling within protective spaces, such as those created under large whole shells. In order to ascertain whether or not left mandibles survive in larger number than right ones, and whether or not smaller mandibles (left and right) break more frequently than larger ones, we conducted basic statistical procedures with quantitative data (frequency and mean size of mandibles) from a variety of depositional contexts. The results show that left and right mandibles break with the same frequency, and that breakage does not bias measurements of mandibles towards the larger or smaller end of the size range. Similar studies will need to be conducted when recovering Cape rock lobster mandibles from depositional contexts different to the ones encountered in the study area.     

Probabilistic Seismic Performance Assessment of Classes of Buildings Using Physics-Based Simulations and Ground-Motion Prediction Equations

We discuss the applicability of two different near field representations of seismic input within the capacity-demand diagram method for seismic performance assessment of classes of buildings in urban areas. The two representations of seismic input used are: (1) synthetic accelerograms obtained from 3D wave simulations of random rupture scenarios are used to estimate seismic risk, through the Monte-Carlo approximation; and (2) random realizations of elastic spectral displacement obtained through ground-motion prediction equations. The implementation of the proposed formulation in evaluating the seismic performance of two classes of buildings in the town of Sulmona, Italy, indicates that empirical ground-motion prediction equations, combined with non-iterative methods for estimating the inelastic seismic demand, can produce results comparable to the physics-based simulations.     

Seismic Behavior of Reinforced Concrete Interior Wide-Beam Column Joints

In this article, experimental and finite element (FE) numerical investigations on interior wide-beam column joints are presented. The experimental research consisting of three full-scale interior wide-beam column specimens was carried out at Nanyang Technological University, Singapore to study the seismic behavior. Details of the test results are discussed to understand the specimens' seismic performance in terms of general behavior, hysteresis loops response, and strain profiles of longitudinal reinforcement. In the FE numerical study, the three-dimensional (3D) model developed is validated by comparing the analysis results with the experimental test results, which has shown a good agreement. A parametric study is performed to elucidate more information and to understand the influence of critical parameters affecting the joint behavior such as column axial load, beam anchorage ratio, and wide beam participation.