Subsoil Interventions Effect on Structural Seismic Response. Part I: Validation of Numerical Simulations

Subsoil interventions to enhance the static soil resistance and reduce deformations may alter significantly the seismic response of the complex soil-foundation-structure system. The aim of this article is to have an insight in the physics of the problems encountered and to validate an adequate numerical modeling procedure to study these effects of the intervention in the global response of the system. Validation concerns wave propagation, site effects, and dynamic soil-structure-interaction issues including the intervention beneath the foundation. Theoretical models-expressions and experimental results from centrifuge tests have been used. The proposed numerical model is proven very efficient to describe the complex dynamic phenomenon and anticipate the seismic response after the employment of subsoil interventions.     

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.     

Analytical Seismic Assessment of a Tall Long-Span Curved Reinforced-Concrete Bridge. Part II: Structural Response

The seismic assessment of special bridges, even under the hypothesis of full knowledge of site conditions, structural characteristics, and seismic activity at their location, is not an easy and straightforward task due to the complexities and uncertainties related to the finite-element modeling approaches, structural loading scenarios, and seismic analysis methodologies. In this article, a series of nonlinear static and dynamic finite-element analyses on the Mogollon Rim Viaduct are performed with consideration of both uniform and conditionally simulated non-uniform seismic motions. The failure modes of the bridge using different numerical modeling approaches are discussed, and the degree of sensitivity of its response to the different seismic assessment strategies is evaluated. The effect of the multi-component, multi-support and multi-directional excitations of ground motions on the design and response are studied, and the pros and cons of the commonly used structural analysis methodologies of bridges are also addressed. The numerical results of the present study provide a deeper insight into the nonlinear behavior of curved reinforced-concrete bridges, and suggest practice-oriented approaches for their seismic assessment.     

Development and Verification of a New Site Classification System and Site Coefficients for Regions of Shallow Bedrock in Korea

In this article, site response analyses for 124 sites collected in Korea were performed to evaluate earthquake ground motions in regions of shallow bedrock in Korea. Based on the results, a new two-parameter site classification system was developed for use in regions of shallow bedrock. The system incorporates depth to bedrock and mean V_S of soil above bedrock as parameters for site classification instead of V_S30. Soil sites were divided into seven site classes and the corresponding site coefficients were proposed for each site category. Some verification work demonstrates the superiority of the proposed system over the current seismic codes.     

Design Earthquake Ground Motion Prediction for Perth Metropolitan Area with Microtremor Measurements for Site Characterization

Perth is the largest city in Western Australia and home to three-quarters of the state's residents. In recent decades, there have been a lot of earthquake activities just east of Perth in an area known as the South-West Seismic Zone. Previous numerical results of site response analyses based on limited available geology information for PMA indicated that Perth Basin might amplify the bedrock motion by more than 10 times at some frequencies and at some sites. Hence, more detailed studies on site characterization and amplification are necessary. The microtremor method using spatial autocorrelation (SPAC) processing is a useful tool for gaining thickness and shear wave velocity (SWV) of sediments and has been adopted in many previous studies. In this study, the response spectrum of rock site corresponding to the 475-year return period for PMA is defined according to the probabilistic seismic hazard analysis (PSHA) based on the latest ground motion attenuation model of Southwest Western Australia. Site characterization in PMA is performed using two microtremor measurements, namely SPAC technique and H/V method. The clonal selection algorithm (CSA) is introduced to perform direct inversion of SPAC curves to determine the soil profiles of representative PMA sites investigated in this study. Using the simulated bedrock motion as input, the responses of the soil sites are estimated using numerical method based on the shear-wave velocity vs. depth profiles determined from the SPAC technique. The response spectrum of the earthquake ground motion on surface of each site is derived from the numerical results of the site response analysis, and compared with the respective design spectrum defined in the Australian Earthquake Loading Code. The comparison shows that the code spectra are conservative in the short period range, but may slightly underestimate the response spectrum at some long period range.     

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

For the seismic isolation of light structures, the use of laminated rubber bearings is neither economical nor, for most cases, technically suited. For the isolation of this type of structure a new system, consisting of steel balls rolling on rubber tracks, has been developed at TARRC (Tun Abdul Razak Research Centre).

This article presents the results of experimental tests carried out for the characterization of the behavior of this new device. A numerical model is also proposed that can be used to assess the seismic response of structures with this isolation system.

Comparison of the predictions of the numerical model with the experimental data shows that the model is adequate to perform the correct assessment of the seismic response of isolated structures. The results of the experimental campaign of shaking-table tests, as well as the numerical simulations, show that there is an effective reduction of the acceleration levels induced in the isolated structures.     

Temporal change in groundwater level following the 1999 (Mw = 7.5) Chi-Chi earthquake, Taiwan

We examine the post‐seismic change in the groundwater level following the 1999 (M_w = 7.5) Chi‐Chi earthquake in central Taiwan, as recorded by a network of 70 evenly distributed hydrological stations over a large alluvial fan near the epicenter. Four types of post‐seismic responses may be distinguished. In type 1, the groundwater level declined exponentially with time following a coseismic rise. This was the most common response in the study area and occurred in unconsolidated sediments on the Choshui River fan. In type 2, the groundwater level rose exponentially with time following a coseismic fall. This occurred in the deformed and fractured sedimentary rocks in the foothills near the Chelungpu fault that ruptured in the Chi‐Chi earthquake. In type 3, the groundwater level continued to decline with time following a coseismic fall. This also occurred in the deformed and fractured sedimentary rocks near the ruptured fault. Finally, in type 4, the groundwater level, following a coseismic rise, stayed at the same level or even rose with time before it eventually declined. This occurred mostly in unconsolidated sediments along the coast of central Taiwan and along the Peikang Stream. We analyze these post‐seismic responses by using a one‐dimensional model. Together with the results from well test, the analysis show that the type 1 response may be explained by an aquifer model with coseismic recharge and post‐seismic subhorizontal discharge across a length of 500–5000 m; the type 2 response may be explained by a model of coseismic discharge and post‐seismic recharge from surface water; the type 3 response may be explained by a model of coseismic discharge and post‐seismic subhorizontal discharge across a length of 500–5000 m; and the type 4 response may be explained by a model of coseismic recharge and sustained post‐seismic recharge from surface water. The characteristic time for the post‐seismic changes is similar to that for the groundwater‐level decline during dry seasons before the earthquake, suggesting that there was no earthquake‐induced changes in the aquifer properties (i.e. hydraulic conductivity), confirming the earlier results from recession analyses of the post‐seismic streamflow elsewhere after several earthquakes.     

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.     

DIAPHRAGM ACTION OF SANDWICH PANELS IN PIN-JOINTED STEEL STRUCTURES: A SEISMIC STUDY

The paper focuses on the seismic response of steel pin-jointed frames braced by lightweight cladding panels. In particular, with the aim to investigate the performance of such a structural scheme when acting as a dissipative system, a wide numerical study has been developed. It is based upon available shear tests on screwed sandwich panels, whose experimental cyclic responses are properly incorporated into a scope-oriented, computer program. The goal is firstly to check the possibility of using cladding panels as shear diaphragms in seismic areas and then to assess an appropriate design behaviour factor, accounting for their actual hysteretic response. Key findings from the nonlinear dynamic analyses are: (1) a portal frame steel building in a low-medium seismicity zone may be braced by common cladding panels, completely avoiding the use of other bracing systems; (2) this structural solution, if compared with a conventional one, appears to be more efficient and cost-effective, giving rise to a weight saving which, in the case under examination, reaches a value of about 20%; (3) on the basis of the numerical study a design behaviour factor q _d =2 seems to be realistic for such a structural typology.     

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.     

Stochastic Seismic Response of Pipelines with Corrosion

For aging pipelines, corrosion gradually appears inside and outside during in-service time. However, because the uncertainty characteristics of the soil around the pipeline and conveying material in the pipeline, the generation and development processes of the pipeline corrosion would be stochastic processes. This background must be taken into account in the seismic response analysis of the pipeline. In this article, using Markov chain with absorbing barrier, a model simulating the evolution of the pipeline corrosion is presented. This model divides a pipeline into several segments and obtains the distribution probabilities of the cross section areas of the segments. Elastic foundation beam method and random perturbation method are used to obtain the seismic response of the pipeline with corrosion. To validate the proposed method, two pipelines are investigated in detail. The mean and standard deviation of the displacements and stress of the pipeline under earthquake are evaluated by the proposed method. Also, the results of one pipeline are compared with Monte Carlo random simulation. The numerical results show that the proposed model can reasonably predict the seismic response of the pipeline with corrosion.     

HINTS ABOUT SITE AMPLIFICATION EFFECTS COMPARING MACROSEISMIC HAZARD ESTIMATE WITH MICROTREMOR MEASUREMENTS: THE AGRI VALLEY (ITALY) EXAMPLE

We approach from a new standpoint the problem of estimating seismic hazard for some towns and villages located in Val d'Agri area (Southern Italy) that in the past have been affected by several seismic events. The estimates are carried out using a method that is based on the analysis of site seismic history extracted from macroseismic catalogues. To study the influence of site effects two different procedures have been performed: in the first, seismic hazard estimates have been deduced from epicentral data only, in the second, intensity data actually observed at the site are also considered. The difference between the two estimates can be correlated with local variations of seismic response due to local geological features which are responsible for possible cases of amplification. In order to validate the presence of such correlation, seismic hazard estimates have been compared to site amplification measurements obtained by using the HVSR (Horizontal to Vertical Spectral Ratio) technique. Our findings reveal a good correlation between seismic hazard enhancements and the presence of site amplification effects. The application of this kind of analysis to the Val d'Agri area has pointed out that the joint estimates of site seismic hazard enhancement and HVSR measurements could be a helpful tool to identify problems related to seismic microzonation.     

Seismic Reliability of Code-Conforming Italian Buildings

ABSTRACT

This paper presents and discusses some research results related to the seismic failure risk of standard, residential and industrial, buildings designed for damage, and life-safety according to the Italian seismic code, which is somewhat similar to Eurocode 8. The five considered structural typologies are as follows: masonry, cast-in-place reinforced concrete, precast reinforced concrete, steel, and base-isolated buildings. The archetype structures have been designed according to standard practice at three sites, representative of the seismic hazard across the country. Seismic risk is defined here as the annual rate of earthquakes able to cause structural failure in terms of usability-preventing damage and global collapse. For each structure, the failure rates have been evaluated in the framework of performance-based earthquake engineering, that is, via integration of site’s probabilistic hazard and structural fragility. The former has been computed consistently with the official hazard model for Italy that is also used to define design actions in the code. The latter has been addressed via nonlinear dynamic analysis of three-dimensional numerical structural models. Results indicate that, generally, design procedures are such that seismic structural reliability tends to decrease with increasing seismic hazard of the building site, despite the homogeneous return period of exceedance of the design seismic ground-motion.     

TUNING OF SEISMIC HAZARD ESTIMATES BY MEANS OF OBSERVED SITE INTENSITIES

Probabilistic seismic hazard is usually assessed by means of computer codes using seis-mogenic sources, parametric catalogues, seismicity rates and attenuation relationships. All these ingredients are conditioned by expert judgement that influences the final results. Even the attenuation relationships, though strictly based on experimental data, are considered a weak point due to the difficulty of modelling the interaction between seismic energy radiation and site response, and because earthquakes do not usually repeat themselves according to one theoretical model. Recently, methods making wide use of site intensity data have been developed in regions such as Italy, where the observed seismic history at selected sites is quite exhaustive. We analyzed these observations to assess seismic hazard at about 600 sites. We used a probabilistic counting technique, integrating the observations (when necessary), with computed shakings obtained from a logistic-type attenuation model. The results were then compared with the estimates provided by the recent seismic hazard map of Italy, compiled according to the traditional probabilistic seismic hazard approach. The match shows significant differences for some sites. A tentative explanation which seems to point to three alternatives is provided: (1) The mismatch between the two methodologies might appear because the stationary assumption has a poor fit with reality (at least in certain areas); (2) Some sites show a response that is systematically different from the average values predicted using attenuation relationships; (3) The definition of seismogenic zones leads to a bias in the seismicity rate estimate.     

Long-Period Ground Motion Simulation and its Impact on Seismic Response of High-Rise Buildings

This paper first critically reviews a seismological model and then a three-segment curve model (in log-log space) to model the Q-f relationship is proposed to overcome the potential biased estimation in the long-period range by the “coda wave” method. The optimal curve-fitting process is performed to determine the Q-f relationship for the Hong Kong region. The calibrated seismological factors are incorporated with the stochastic simulation procedure to generate synthetic ground motions, which are validated through comparison with seismic records. The impact of long-period ground motions on the seismic response of high-rise buildings is finally manifested through a numerical study.     

MODIFICATION OF SITE RESPONSE IN PRESENCE OF LOCALISED SOFT LAYER

Experimental and numerical simulations are performed to evaluate the modification of ground response resulting from either the presence of soft layers or occurrence of partial liquefaction. Results from two densely instrumented dynamic centrifuge tests are presented to show the ambiguous role played by the presence of a soft layer. It was found that the lateral extent of the soft layer has significant influence on the overall response of the layered strata and any structure founded on it. The experimental observations are supported by simplified numerical analysis. The amplification or deamplification of the input motion is found to be a function of the ratio of the width of soft layer to the wave length. Based on the numerical analysis, a general function describing the site amplification is presented which may be used as a guide in seismic design of foundations in such layered strata.     

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.     

SEISMIC SOIL-STRUCTURE INTERACTION: BENEFICIAL OR DETRIMENTAL?

The role of soil-structure interaction (SSI) in the seismic response of structures is reex-plored using recorded motions and theoretical considerations. Firstly, the way current seismic provisions treat SSI effects is briefly discussed. The idealised design spectra of the codes along with the increased fundamental period and effective damping due to SSI lead invariably to reduced forces in the structure. Reality, however, often differs from this view. It is shown that, in certain seismic and soil environments, an increase in the fundamental natural period of a moderately flexible structure due to SSI may have a detrimental effect on the imposed seismic demand. Secondly, a widely used structural model for assessing SSI effects on inelastic bridge piers is examined. Using theoretical arguments and rigorous numerical analyses it is shown that indiscriminate use of ductility concepts and geometric relations may lead to erroneous conclusions in the assessment of seismic performance. Numerical examples are presented which highlight critical issues of the problem.     

Seismic Hazards and Water Supply Performance

This article describes seismic hazards, including fault rupture, liquefaction, landslides, and site amplification, using Los Angeles as a case study. Water supply simulation results are presented for a 7.8 M_W earthquake on the San Andreas Fault. Severe water losses are shown after 24 h, with nearly 2,700 locations of pipeline damage and a 66% decrease in normal water service. The water supply system was modeled with and without reservoirs that have been removed from service to meet water quality standards. The results show that opening the disconnected reservoirs immediately after a serious earthquake is an effective strategy for emergency response.     

DUCTILITY REDUCTION FACTOR OF MDOF SHEAR-BUILDING STRUCTURES

In this paper, the results of recent studies on inelastic seismic response of MDOF shear-building structures are presented. In the last few decades, the concept of response modification factor R has been introduced and developed to account for inelastic nonlinear behaviour of structures under earthquakes. In this paper, an attempt has been made to adjust and extend this concept through introducing a modifying factor R _T . This factor is used for dynamic analysis of MDOF structures, including the calculation of inelastic response spectra. Sensitivity analysis was carried out to identify the parameters that have influence on R _T . It has been demonstrated that R _T is predominantly a function of number of stories, and accordingly a relationship has been suggested. Finally, an approximate approach has been developed for evaluating the seismic strength and ductility demands of MDOF structures.