Near-source Earthquakes Research Articles

Influence of earthquake vertical excitations on Sloshing-Created P-Δ effect in elevated water Tanks: Experimental Validation, numerical simulation and proposing a modification for Housner model

Seismic sloshing in elevated water tanks creates some additional base moment, which simultaneous with the effect of strong vertical acceleration of near-source earthquakes may increase the P-Δ effect, as an important factor in seismic design of these structures. This study was performed to investigate the extent of this combined effect. First, an experiment was conducted to provide a set of results for validating the finite element analysis (FEA) modeling, conducted as the second part of the study. The test tank was cylindrical with a diameter of 60 cm, installed on an innovative simple shaking table, capable of creating simultaneous horizontal and vertical harmonic excitations. Different frequencies and water depths were used in the tests. For verification, the maximum sloshing height was measured and compared with FEA results. The verified FEA process was used to simulate elevated tanks of actual sizes for various water height to tank diameter and also tank height to tank diameter ratios, using three-component accelerograms of seven selected earthquakes, mostly having strong vertical components, creating totally 126 dynamic FEA cases. In the last stage, a modification was done on the Housner spring-mass model for using it in 3D state with consideration of vertical excitations. Numerical results indicate that the sloshing height and the base moment could increase in average about 20% and 10%, respectively, due to the effect of earthquake vertical excitations. Also, comparison of FEA results with those obtained by using the proposed 3D modified Housner model shows a good agreement of around 90% on average.

BB-SPEEDset: A Validated Dataset of Broadband Near-Source Earthquake Ground Motions from 3D Physics-Based Numerical Simulations

ABSTRACTThis article introduces a strong-motion dataset of near-source broadband earthquake ground motions from 3D physics-based numerical simulations—named BB-SPEEDset—obtained by the code SPEED (SPectral Elements in Elastodynamics with Discontinuous Galerkin)—developed at Politecnico di Milano, Italy. Taking advantage of the earthquake ground-motion scenarios produced so far by SPEED, in most cases validated against earthquake recordings, the main objective of this work is to construct and validate a dataset of simulated broadband waveforms to be used as a support for characterization and modeling of near-source earthquake ground motions. To pursue this objective, the following steps were necessary, namely: (1) the implementation of an effective workflow suitable to process in an homogeneous format various SPEED simulations; (2) the generation of broadband time histories using a technique based on artificial neural networks, trained on strong-motion records; (3) the creation of a flat file collecting, for each simulated scenario, the most relevant metadata (fault rupture scenario, site response proxies, source-to-site distances) as well as a comprehensive set of ground-motion intensity measures of the processed broadband waveforms (peak ground acceleration, velocity and displacement, spectral ordinates, duration, pulse period, etc.). Finally, a comprehensive set of consistency checks is made to verify the absence of any systematic bias in the trend of the BB-SPEEDset results with respect to the NEar-Source Strong-motion (NESS) version 2.0 near-source recorded ground-motion dataset. Indeed, the main features of near-source ground motion in BB-SPEEDset, ranging from the statistical distributions of peak and integral measures both at short and long periods, the ground-motion attenuation with distance, to the features of impulsive ground motions and directionality effects, are in substantial agreement with those from NESS.

Effect of Non-Uniform Vertical Excitations on Vertical Pounding Phenomenon in Continuous-Deck Curved Box Girder RC Bridges Subjected to Near-Source Earthquakes

ABSTRACT The geometry of skewed and curved bridges can cause complexity of their dynamic response and difficulty of analysis against earthquakes. In addition, when vibration occurs in the curved bridges, due to their specific geometry, the vertical, flexural, and torsional vibrations of the deck are coupled. The present study was carried out aiming to investigate the effect of non-uniform vertical excitation of the bridge piers with various vertical/horizontal acceleration ratio (V/H) on the pounding phenomenon in concrete curved bridges subjected to near-source earthquakes. For this purpose, the analytical method was employed using the expansion of the functions of structural dynamics on a two-span continuous concrete bridge model, consisting of continuous flexural/torsional beam elements for the deck, springs for the rubber bearings, and rods as the bridge piers. The non-uniform vertical excitations of the piers were induced to the whole system by the harmonic functions in the form of transient wave transfer functions, and then the equations of the entire system were solved using the Eigen functions. The induced waves can cause separation of the box girder deck from the rubber bearing along with the vertical and horizontal displacement as well as the torsion of the box girder around the longitudinal axis of the bridge. After each separation, the deck returns back and hit the rubber bearing, resulting in a vertical pounding force on the bearing and the pier. The analyses results show that when the curved beam is subjected to a non-uniform vertical excitation with a various V/H, the lateral displacement of the deck are up to 21% larger than when the bridge is excited by considering a value of V/H = 2/3. The results also show when the excitation frequencies are different in different piers of the bridge, the pounding force can be increased up to three times, for some specific curvature angle, comparing to the case of equal excitation frequency in all piers.

The effect of curvature angle of curved RC box-girder continuous bridges on their transient response and vertical pounding subjected to near-source earthquakes

Among highway bridges, curved ones are very common at urban intersections and, generally, in limited urban spaces. However, such bridges are often involved in multiple vibrational behaviors, making them more sensitive to vertical components of ground motion due to their irregular geometry. The primary objective of this study has been to develop a theoretical approach using the relationships of structural dynamics to evaluate the effect of central curvature angle on the seismic response of curved bridges with rubber bearings subjected to strong vertical ground motions of near field earthquakes. In this regard, the continuous flexural/torsional beam-spring-rod model, has been used to calculate vertical force and the effect of this force on the dynamic response of curved bridge components. The wave propagation in the form of harmonic functions is induced by changing the phase angle for piers, and the wave transfer functions are solved by using eigenfunctions. The results of numerical analyses show that the wave transient behavior leads to separation of the girder from the bearing along with the pounding of the girder returning back onto the bearing, and that in the curved bridges with different central angles, the phenomenon of vertical pounding would occur in the range of the bridge natural vibration period, and pounding intensity would be strongly dependent on seismic excitation scenarios. The results also show that with increase in the curvature angle of the girder, the girder's lateral displacement does not necessarily increase, while the increase in the girder’s torsion is related to its curvature angle.

The correlation between the ground motion intensity measure parameters of earthquakes

In performance-based seismic engineering, the choice of ground motion goes hand in hand with dynamic analysis. This paper presents the relationship between the various intensity measures (IMs) parameter of ground motions that can be used in evaluating the behavior of structures. This is through the relationship between them and the ratio of the peak ground acceleration to the peak ground velocity (PGA/PGV). The PGA/PGV ratio has been used an empirical parameter to estimate ground motion frequency content and categorizes ground motion suites for performing nonlinear time history analyses of structures. Regression analyses are performed on data obtained from ground motion records with different PGA/PGV ratios. The regression analyses indicate that the best correlation IMs parameter with the PGA/PGV ratio in both near- and far-source earthquakes is the mean period (Tm). Also, the relationships between PGA/PGV ratio and some IMs parameters in near-source earthquakes are stronger than it in far-source earthquakes.

Overview of the development of smart base isolation system featuring magnetorheological elastomer

Despite its success and wide application, base isolation system has been challenged for its passive nature, i.e., incapable of working with versatile external loadings. This is particularly exaggerated during near-source earthquakes and earthquakes with dominate low-frequency components. To address this issue, many efforts have been explored, including active base isolation system and hybrid base isolation system (with added controllable damping). Active base isolation system requires extra energy input which is not economical and the power supply may not be available during earthquakes. Although with tunable energy dissipation ability, hybrid base isolation systems are not able to alter its fundamental natural frequency to cope with varying external loadings. This paper reports an overview of new adventure with aim to develop adaptive base isolation system with controllable stiffness (thus adaptive natural frequency). With assistance of the feedback control system and the use of smart material technology, the proposed smart base isolation system is able to realize real-time decoupling of external loading and hence provides effective seismic protection against different types of earthquakes.

Effect of near-field earthquake excitation on seismic behavior of knee-braced moment frames

Inelastic behavioral characteristics of knee-braced moment-resisting frames having 3, 8 or, 12 stories were investigated under near-source earthquake excitation using initially pinned frames that were subsequently substituted with rigid knee elements. These frames were designed so that the knee braces would yield and buckle under seismic loading. Inelastic time-history analysis was carried out to assess the structural performance of the buildings by evaluating the maximum axial forces of the columns, vertical displacement of internal beams, roof horizontal displacement, and maximum base shear of columns in the building members using PERFORM-3D software. The nonlinear behavior of the frames was investigated by comparing the results of pinned and rigid knee elements subjected to near-field earthquakes. The results indicate that rigid-to-pinned connections for knee elements can increase the axial forces of columns by nearly 15% for a 3-story building and about 7% for 8- and 12-story buildings. The vertical displacement of the beams was noticeable, especially for the three-story building. The horizontal displacement of the roof and base shear of columns using pinned connections for knee elements were generally greater than for the rigid connections.

Seismic Design Evaluation of Reinforced Concrete Buildings for Near-Source Earthquakes by Using Nonlinear Time History Analyses

Seismic design codes mostly claim that their requirements lead to Life Safety (LS) Performance Level (PL) for buildings. This is while many buildings, designed based on the current codes have shown unacceptable performance, and even have collapsed in some recent earthquakes, particularly near-source events. On this basis, it seems that the code provisions still need further improvement to create sufficient confidence in the engineering community. This study has been conducted to find out how IBC 2009 And ACI 318-2014 codes are effective in providing the LS PL in reinforced concrete multi-story regular buildings with special moment frame lateral load bearing system. For this purpose, a set of multi-story buildings up to 16 stories were considered in the highest seismic hazard zone of Tehran, and were designed based on the codes. Then, a set of near-source three-component accelerograms were employed and scaled according to the code, and a series of nonlinear time history analyses were conducted for all buildings. Roof displacement and acceleration, and base shear forces were calculated, and also the formation trend of plastic hinges and their distribution in the structures were investigated for evaluating the seismic performances. Results show that for some earthquakes the buildings performance exceed LS PL, and even in some cases they reach collapse level.

The 3D numerical simulation of near-source ground motion during the Marsica earthquake, central Italy, 100 years later

In this paper we show 3D physics-based numerical simulations of ground motion during one of the most devastating earthquakes in the recent Italian history, occurred on Jan 13, 1915, Marsica, Central Italy. The results provide a realistic estimate of the earthquake ground motion and fit reasonably well both the geodetic measurements of permanent ground settlement, and the observed macroseismic distribution of damage. In addition, these results provide a very useful benchmark to improve the current knowledge of near-source earthquake ground motion, including evaluation of the best distance metrics to describe the spatial variability of the peak values of ground motion, the relative importance of fault normal vs fault parallel components, the conditions under which vertical ground motion may prevail, as well as the adequacy of 1D vs 3D modelling of site amplification effects.

Maximum damping forces for structures with viscous dampers under near-source earthquakes

This paper examines the inelastic response behaviour of structures with supplemental viscous dampers under near-source pulse-like ground motions. It is well known that the design of dampers requires the effective evaluation of maximum seismic velocities or maximum damping forces. In order to avoid complicated methods, such as the dynamic inelastic analysis, this study proposes a simple and effective evaluating method for these maximum values using the inelastic velocity ratio. This ratio is a modification factor which allows the evaluation of the maximum inelastic velocity or damping force from their corresponding elastic counterparts. The paper focuses on structures. Extensive parametric studies are conducted to examine the influence of characteristics of structure (period of vibration, post-elastic stiffness, force reduction factor), of supplemental damping (equivalent viscous damping ratio) and of ground motion (type of earthquake fault) on the maximum seismic velocities and damping forces.

Response spectra for differential motion of structures supports during earthquakes in Egypt

Differential motions of ground supports of stiff structures with large plan dimensions and separate foundations under earthquakes were studied by researchers during the last few decades. Such a type of structural response was previously underestimated. The importance of studying such a response comes up from the fact that usually the structures affected are of strategic importance such as bridges. During their expected life, structures may experience vibrations excited by ground waves of short wavelengths during near-source earthquakes, or during amplified earthquake signals, during explosions, or during vibrations induced from nearby strong vibration sources. This is the case when the differential motion of supports becomes considerable. This paper aims to review the effects of seismic signal variations along the structures dimensions with emphasis on Egypt as a case study. The paper shows some patterns of the damage imposed by such differential motion. A replication of the differential motion in the longitudinal direction is applied on a frame bridge model. The resulting straining actions show the necessity for considering the differential motion of supports in the design of special structures in Egypt. Finally, response spectra for the differential motion of supports, based on the available data from previous earthquakes in Egypt, is derived and proposed for designers to include in the design procedure when accounting for such type of structural response, and especially in long-span bridges.

Seismic hazard analyses for Taipei city including deaggregation, design spectra, and time history with excel applications

Given the difficulty of earthquake forecast, Probabilistic Seismic Hazard Analysis (PSHA) has been a method to best estimate site-specific ground motion or response spectra in earthquake engineering and engineering seismology. In this paper, the first in-depth PSHA study for Taipei, the economic center of Taiwan with a six-million population, was carried out. Unlike the very recent PSHA study for Taiwan, this study includes the follow-up hazard deaggregation, response spectra, and the earthquake motion recommendations. Hazard deaggregation results show that moderate-size and near-source earthquakes are the most probable scenario for this city. Moreover, similar to the findings in a few recent studies, the earthquake risk for Taipei should be relatively high and considering this city's importance, the high risk should not be overlooked and a potential revision of the local technical reference would be needed. In addition to the case study, some innovative Excel applications to PSHA are introduced in this paper. Such spreadsheet applications are applicable to geosciences research as those developed for data reduction or quantitative analysis with Excel's user-friendly nature and wide accessibility.

Reduction Measure for 3-D structure During Near-Source Ground Motions

In this study an approach for reducing the influence of the strong long-period pulses as well as the effect of the simultaneous ground excitation on a three-dimensional (3D) frame structure is presented. The considered ground excitation is the 1995 Kobe earthquake and the 1999 Turkey earthquake. The investigation reveals that the common base isolators are not reducing the response of the structure in the case of the near-source earthquakes with long period impulses. Even they can amplify the response of the structure. By modifying the fundamental natural mode of the structure, we can reduce the effect of the strong pulses of the horizontal ground motion. The improvement of the dynamics of the structure is achieved with the combination of the lateral elastic spring and the viscous damper at the first story level of the structure and the base isolation. The proposed approach reduces not only the displacement but also the activated forces in the structural members.

Seismic Isolation with No Strain Energy – Research on New Seismic Isolation System with No Resonance Characteristics –

Since the safety of seismically-isolated buildings during earthquakes depends mainly on the largedeformation stability of the isolation devices, it is necessary to either provide large isolator deformation capacity or to reduce seismic response deformation to ensure enhanced building safety. From the viewpoint of earthquake demand, it must be recognized that, the isolator response deformation may exceed the allowable capacity when the isolation system resonates with strong, near-source earthquake ground motions. This paper first establishes the problem of the damping capacity of conventional isolation systems that a system with strong restoration spring causes large elastic strain energy to accumulate in largely deformed isolators. Then, a new hysteresis behavior is proposed to reduce the strain energy developed in isolators. Based on studies of the fundamental characteristics of the proposed hysteresis behavior, this paper proposes a new isolation system concept called “Seismic Isolation with No Strain Energy (NSE)” which does not result in resonance because it eliminates the strain energy stored in deformed isolators, even if the period of the isolation system coincides with predominant period of the input ground motions. The superior performance of NSE Seismic Isolation is confirmed by the results of dynamic response analyses for strong, near-source earthquake ground motions.

Assessment of Reduction Measure for Seismicresponse of Bridge Decks during near-Source Earthquakes

During the strong near-source ground motions, adjacent bridge decks often collide each other because of the insufficient separations. Pounding can cause severe bridge damage, or even result in unseating if the seating length is not enough. In this study, the bridge structure is modeled by a finite element method. The subsoil is represented by steel springs and viscous dampers. The considered reduction measure consists of steel springs with additional friction element. The nonlinear analysis is performed in the time domain. The considered earthquakes are the 1994 Northridge earthquake and the 1995 Kobe earthquake. The reduction of the relative displacement and pounding forces of the two bridge decks is investigated. The investigations reveal that the proposed reduction measure can be used to reduce the seismic responses between two bridge decks with their neighboring abutments.

Seismic reliability of steel framed buildings

Abstract This study evaluates the seismic reliability of eccentrically braced frame (EBF) and buckling-restrained braced frame (BRBF) systems in comparison with that of moment-resisting frame (MRF) systems. In detail, a series of three-dimensional nonlinear time-history analyses was carried out for six example buildings. The effectiveness of braced frames was assessed in 6- and 20-story steel office buildings. Two ensembles of ordinary and near-source earthquake ground motions were evaluated with a vector-valued intensity measure. The results show that the effectiveness of braced frames can vary significantly with ground motions and building rises. The added braced frames help to reduce story drifts, decreasing the probability of failure against ordinary ground motions. The effect is particularly obvious for low-rise buildings. On the other hand, the pulse-like feature of near-source ground motions decreases the drift demands on MRF systems but increases the drift demands on EBF and BRBF systems. As a result, the added braced frames can help to resist near-source ground motions, but only to a limited extent.

Real-Time Ground Motion Forecasting Using Front-Site Waveform Data Based on Artificial Neural Network

Real-time earthquake information made available by the Japan Meteorological Agency (JMA) publicly since October 2007 is intended to dramatically reduce human casualties and property damage following earthquakes. Its current limitations, however, such as a lack of applicability to near-source earthquakes and the insufficient accuracy of seismic ground motion intensity leave much to be desired. The authors have suggested that the forward use of front-site waveform data leads to improve accuracy of real-time ground motion prediction. This paper presents an advanced methodology based on artificial neural networks (ANN) for the forward forecasting of ground motion parameters, not only peak ground acceleration and velocity but also spectral information before S wave arrival using the initial P waveform at a front site. Estimated earthquake ground motion information can be used as a warning to lessen human casualties and property damage. Fourier amplitude spectra estimated highly accurately before strong shaking can be used for advanced engineering applications, e.g., feed-forward structural control. The validity and applicability of the proposed method have been verified using Kyoshin Network (K-NET) observation datasets for 39 earthquakes occurring in the Miyagi Oki area.

Incorporating Source Rupture Characteristics into Ground-Motion Hazard Analysis Models

Source rupture characteristics have significant effects on the level and the distribution of near-source earthquake ground motions. The present state of knowledge concerning the impacts of source characteristics on ground motions makes a quantitative analysis of the effect of fault rupture on the near-source ground motions possible. To simplify incorporating source effects into seismic hazard analysis models, they may be classified into two groups: rupture directivity and rupture heterogeneity. Rupture directivity reflects the effects of the direction of rupture on the fault as viewed from the site. This characteristic is important even in the case of homogeneous rupture, which corresponds to uniform seismic energy release over the rupturing surface. The state of understanding of rupture directivity, particularly that derived from recordings of many recent earthquakes, warrants incorporation of homogeneous fault rupture effects into ground-motion hazard analysis methodologies. However, fault rupture, especially in large earthquakes, is often far from homogeneous. This is due to irregularities in the geometry of the fault, non-uniformities in the mechanical properties of the materials involved, and variability in the regional and local stresses. A major contributor to rupture heterogeneity is the existence of asperities, that is, zones with strength and deformation characteristics significantly different from the rest of the fault. The existence of asperities contributes to heterogeneous rupture of the fault, characterized by non-uniformities in slip, slip-rate, stress-drop, rupture velocity, post-earthquake residual stress, and after-shock activities. The state of understanding of source heterogeneities is not adequately developed to allow incorporation of this factor into seismic hazard analysis methodologies for design and decision-making. However, information on the nature of asperities, slip distributions, and other characteristics accumulated by the study of recent earthquakes can be tested to determine their effects on ground-motion hazards. One way to include source rupture characteristics—homogeneous and heterogeneous—into seismic hazard analysis models is through …

630 直下型地震評価用実大振動試験体の動的挙動解析

The great Hanshin-Awaji earthquake has brought a large number of structural failures, and some of them of them are considerably different from the failures in the past earthquakes. One of them is the brittle fracture of steel box column of a high building. In this study, to clarify the fracture process in large members such as the steel box column and H-steel beam subjected to severe land movements caused by strong near-source earthquake, dynamic behavior of full-scale shaking table specimen was analyzed. In the analysis, specimen was modeled in detail in order to compare analytical results with the experimental results. Through the comparisons, an effective earthquake-proof construction was discussed.

Influence of soil-structure interaction on pounding response of adjacent buildings due to near-source earthquakes

A numerical approach to analyse pounding responses of adjacent buildings on subsoil to earthquakes is presented. The nonlinear calculation of the soil-structure system is performed subsequently in the Laplace and the time domain. The adjacent buildings and the subsoil are described by finite elements and boundary elements, respectively. In the numerical investigation the effect of Kobe, Northridge and Chi-Chi near-source earthquakes is considered. The result reveals that both the subsoil and long-period pulses in the ground motions can increase the pounding potential of buildings. In addition, poundings can amplify the induced floor vibrations. In contrast, soil-structure interaction has reduction effect on the induced vibrations. In order to estimate the distance required to prevent pounding the influence of the soil-structure interaction is significant.