Effects Of Ground Motions Research Articles

Effect of torsional ground motion on the seismic response of highway bridges

Seismological aspects of rotational ground motions can be found in published research, however, studies on implications for the seismic response of structural systems are rare. This preliminary study investigates the effects of torsional component of the ground motions (TGM) on the overall seismic response of bridges and interactions among their components. For this purpose, a series of computational models of bridges were developed. The models assume linear-elastic substructure with continuous rigid deck, sliding supports and rigid abutments, and vary key parameters including skew angle (β), normalized stiffness eccentricity between the center of mass and rigidity (ex/r), gap size between deck and abutments, dominant frequency ratios. However, inelastic impact between the superstructure and abutments is modeled explicitly to ensure that consequential in-plane deck rotations are accounted for. It was shown that larger deck rotations due to TGM lead to larger impact forces, which further amplifies deck rotations. The adverse effects may be more pronounced in symmetric bridges and in bridges with significant skew. All response quantities; deformations and substructure forces, are amplified significantly due to TGMs which suggests that current design codes may underestimate seismic deformation and force demand.

PROPOSITION OF A SPECTRAL AMPLIFICATION MODEL FOR NEAR FAULT DIRECTIVITY EFFECT WITH UTILIZING PROBABILISTIC EARTHQUAKE SCENARIOS

Near fault ground motions with directivity effect show larger spectral demands due to their specific waveform features. Directivity effect occur due to S-wave polarization when the fault rupture propagation and waveform travel direction coincide on each other. In this situation, a pulse arises in the beginning of ground motion displacement and velocity waveforms which is mostly seen in the fault normal direction. The pulse period (Tp) is directly proportional to the earthquake magnitude (moment magnitude, Mw) and its amplitude is inversely proportional to the rupture distance (Rrup) from the site. The other characteristic of the near fault ground motions due to S-wave polarization is known as “Fling Step” in which a residual displacement is seen in the fault parallel displacement waveform. This study summarizes the observations from the effect of near fault ground motions on design spectrum. The results of these observations are used to develop simplified equations for incorporation of near fault effect in design spectrum. Probabilistic seismic hazard scenarios are utilized in this study in order to investigate the effect of fault seismicity, fault length (characteristic magnitude of the fault), pulse period and source-to-site geometry on the level of spectral amplification due to directivity effect. The near fault effect is modelled with assembling two narrow-band directivity models (Shahi and Baker 2011; Chiou and Spudich 2013) on the probabilistic seismic hazard scenarios respectively. In the last part of this study the significance of seismological and geometrical parameters on the level of spectral amplification due to directivity will be shown and discussed. The results are used to propose relationships for the incorporation of near fault directivity effect for different geometrical distributions of the sites around the fault.

Seismic Fragility of Base-Isolated Nuclear Power Plant Considering Effects of Near-Fault Ground Motions

Seismic Fragility of Base-Isolated Nuclear Power Plant Considering Effects of Near-Fault Ground Motions

Timber beam seismic design – A numerical simulation

To seismic design of timber beam, effect of near-fault ground motion on timber beam has numerically been investigated, with refer to small displacement theory. The simulated near-fault ground motion is applied on the fixed base of timber frame model. The beam is located in single span of a timber frame. The beam has two different size of 1.8 and 3.3 meter length. The seismic load-displacement seismic load-strain and strain-displacement have been calculated for all models and graphically depicted. The beam is designed with nonlinear analysis method and same mechanical properties for all models have been used, in numerical analysis. The numerical analysis results indicate that, the inertial interaction, energy dissipation and nonlinear deformation of beam in timber frame have direct relationship with frame span. In beam with smaller length, higher seismic loading is caused lower displacement. The displacement is reduced with reduction length of the beam. The inertial interaction, energy dissipation and nonlinear deformation are changed in respect to length of beam. The novelty of this paper is developing cycling graphs by using ABAQUS, for improve design of timber frame, and adequate explanation of seismic behavior of timber beam.

An approach for the real-time slip deformation coupled with strain softening of a high rockfill dam subjected to pulse-like ground motions

The dynamic stability of a dam is a major engineering problem, especially since many high dams have been built or designed in near-fault zones. However, few studies have been focused on the dynamic stability of dams subjected to near-fault pulse-like ground motions. The stability analysis of high rockfill dam subjected to pulse-like ground motions is a meaningful topic. In the traditional computing methods for slope stability, the position of potential sliding surfaces can’t be accurate determined. In order to get more accurate results, the finite element method of sliding calculation is proposed to calculate variation process of the dynamic stress of the element. Meanwhile, the strain softening of rockfill materials under low confining pressure is obvious. Hence, the dam stability considering the strain softening of rockfill materials is an urgent problem that must be investigated. In this study, the peak strength and residual strength were obtained, and the normalized residual strength curve considering strain softening was put forward by analyzing consolidated drained triaxial test data that were derived from real projects. The soil slope stability procedure (FEMSTABLE 3.0) was developed by the first author and used to perform a seismic stability analysis of a core-wall dam in the near-fault area. In addition, the more reasonable approach was proposed to calculate the real-time slip of the dam under pulse-like ground motions, and the method was coupled real-time slip deformation and strain softening. The proposed method of sliding calculation may spend much time on computing safety factors and accumulated displacements of a large number of sliding surfaces, therefore the multicore parallel computing technique was used to improve the calculation speed. Moreover, the sliding displacement responses of the dam subjected to pulse-like and non-pulse ground motions were compared. Furthermore, the effects of pulse-like ground motions on the dynamic stability of 300 m core-wall dam considering strain softening were investigated. The results show that the pulse-like ground motion has a considerable effect on sliding displacement and strain softening of the rockfill, ignoring this factor may lead to inaccurate result.

Skew Adjustment Factors for Fragilities of California Box-Girder Bridges Subjected to near-Fault and Far-Field Ground Motions

Past reconnaissance studies revealed that bridges close to active faults are more susceptible to damage and more than 60% of the bridges in California are skewed. To assess the combined effect of near-fault ground motions and skewness, this paper evaluates the seismic vulnerability of skewed concrete box-girder bridges in California subjected to near-fault and far-field ground motions. The relative risk of skewness and fault-location on the bridges is evaluated by developing fragility curves of bridge components and system accounting for the material, geometric, and structural uncertainties. It is noted that the skewness and bridge site close to active faults make bridges more vulnerable, and the existing modification factor in HAZUS cannot capture the variation in the median value of the fragilities appropriately. A new set of fragility adjustment factors for skewness coupled with the effect of fault location is suggested in this paper.

The indirect boundary integral equation method for the broadband scattering of plane P, SV and Rayleigh waves by a hill topography

The indirect boundary integral equation method (IBIEM) is developed to solve the broadband scattering of plane P, SV and Rayleigh waves by a hill topography, incorporated with the technique of domain-partition construction. It illustrates that the semi-analytical IBIEM can accurately and steadily solve the wave scattering around a steep or a gentle hill both for low and high frequency waves. Numerical results indicate that the seismic response characteristics of a hill strongly depend on the incident angle, wave frequency, the medium stiffness and height-width ratio of the hill. In general, the steep hill has more significant amplification effect on the surface displacement amplitude of the hill, and the displacement peak usually located at the top of the hill for vertically incident SV waves but can also appear near the hill bottom for high-frequency P waves. Multiple standing wave points can be observed for high-frequency wave and steep hill due to the interface effect of scattered waves. Moreover, the high hill has significant shielding effect on the Rayleigh waves. In practice, the notable amplification effect and obvious spatial variation of ground motion must be taken into account in the anti-seismic design of engineering structures in mountainous area.

Engineering analysis of strong ground rocking and its effect on tall structures

Two, the most intensive 6-dof ground motion records were selected from a database of a program of acquiring seismic aftershocks at Kefalonia Greek Island as reported by Perron et al. in 2018 [1]. The maximum Modified Mercalli (MM) intensity VII and peak ground velocity 18.4 cm/s reached during these seismic events made it possible to investigate substantial structural response under combined translational and rotational excitations. Respective horizontal-rocking effects are studied first on a simplified cantilever model of a slender tower. Next, the seismic responses of two 10 and 30 story tall buildings and a 160 m, reinforced concrete industrial chimney under the joint rocking and horizontal excitations are investigated in detail. A complicated pattern of translational-rocking seismic effects on slender structures was observed. It was noted that sometimes the presence of seismic rotations about horizontal axis increases the overall response (up to 30%), sometimes it decreases the combined seismic response. The net effect depends primarily on two, the most important structural parameters: fundamental natural period and effective height of the analysed structure. The rocking response spectrum proposed in Eurocode 8 part 6 for slender structures was compared with seismic time history response analyses of the r/c chimney, yielding its substantial conservatism compared with the time history response computations using the chosen records of 6-dof strong ground motion. It is also concluded that the application of the familiar SRSS rule to combine the horizontal and rocking effects may underestimate the overall seismic response. This may be important for future calibrations of the seismic codes including rocking ground motion effects.

Damage spectra of the mainshock-aftershock ground motions at soft soil sites

The soft soil tends to amplify the damage accumulation induced by mainshock-aftershock (MSAS) sequences due to the significant amplification effect of soft soil on the ground motions. This manuscript proposes the damage spectra of MSAS ground motions at soft soil sites to quantify the accumulated damage experienced by the structures under the MSAS sequences. Artificial MSAS ground motions are firstly generated by the revised randomization method (i.e. combing two random records which belong to two different ground motion groups respectively). The effects of period normalization (i.e. the vibration period is normalized by the predominant period of mainshock ground motion) are discussed from the point of view of dispersion and mean. The effects of the aftershock ground motions at soft soil sites on the damage spectra are quantitatively studied. The results indicate that the period normalization can reduce the dispersion of damage spectra of MSAS sequences, but larger intensity of aftershock ground motion can contaminate the efficiency of period normalization. The effects of strong aftershocks at soft soil sites on the damage spectra can exceed 40%. Strong aftershock ground motions at soft soil sites tend to cause larger additional damage than those at firm soil sites. The prediction equations are finally proposed to predict the mean of damage spectra for MSAS sequences at soft soil sites.

Parametric Study of Effects of Vertical Ground Motions on Base Isolated Structures

ABSTRACT Presently, the effects of vertical components of ground motions are investigated on the performance of the base isolated building. The response assessment of the isolated building is carried out by nonlinear dynamic analysis under a set of recorded ground motions, involving both horizontal and vertical components. It is demonstrated that the vertical ground motion increases the isolation efficiency but results in extensively large displacement of the bearing. Two types of isolation bearing, namely, the conventional lead-rubber-bearing (LRB) and shape-memory-alloy-rubber-bearing (SMARB) are considered. The superior performance of the latter is established in circumventing the adverse consequences of the vertical ground motions.

A note on near-field site amplification effects of ground motion from a radially inhomogeneous valley

To improve the understanding of the near-field soil and topographic amplification effects, an analytical solution by the authors for the scattering of plane SH waves by a radially inhomogeneous semi-cylindrical valley is extended to the case of a line source of cylindrical SH waves. Upon confirmation of its accuracy with past exact solutions for a homogeneous and an inhomogeneous semi-cylindrical valley under far-field plane SH waves, the extended solution is used to calculate the ground motion amplification factors for both the homogeneous and inhomogeneous valleys subjected to near-field waves. A comprehensive parametric study is conducted with respect to the location of the wave source, the dimensionless frequency of the incident waves, and the inhomogeneity degree of the covering soil layer. It is found that more amplifications and reductions of ground motions will occur within a certain range in and around the valley as the sources are located further. Consistent with the far-field case, it is confirmed that an increase of the degree of inhomogeneity of the covering soil layer generally amplifies the ground motions significantly.

Equivalent linearization parameters of soil-MDOF structure systems subjected to pulse-like earthquakes

In seismic design codes and documents, the period and damping ratio of a linear fixed–base single degree-of-freedom (SDOF) system are tuned to return the dynamic response of a given nonlinear fixed–base SDOF structure through a so-called equivalent linearization method. The period and damping ratio of the equivalent linear system are referred to as equivalent or effective linear parameters (ELPs). These ELPs may vary due to higher modes of the superstructure, the soil-structure interaction (SSI), or the pulse effects of the earthquake ground motions. Hence, this paper focuses on an extension of the equivalent linearization concept to nonlinear soil-multi-story structure systems under pulse-like earthquakes where the whole system is substituted with an equivalent fixed-base SDOF oscillator. To achieve this goal, the underlying soil is assumed as a homogeneous half-space medium and the superstructure is modeled as a nonlinear multiple degrees-of-freedom (MDOF) shear building assuming bilinear behavior for each story. Non-dimensional frequency and aspect ratio parameters are used to enforce the SSI effects in the system. The seismic responses are determined under an extensive ensemble of 91 pulse-like earthquakes for two different ranges of fixed-base structure-to-pulse period ratios, and the ELPs are determined for a range of soil-structure systems throughout a statistical framework. It is found that higher modes have decreasing effects on the ELPs of soil-MDOF structure systems, while the effects of the strain-hardening ratio of each story are insignificant. In addition, the impulsive nature of the ground motions strongly affects the ELPs, and this influence is far more pronounced for structure-to-pulse period ratios between 0.8 and 1.2, i.e., very close to the resonance of the first mode of the superstructure.

Effects of vertical ground motions on seismic vulnerabilities of a continuous track-bridge system of high-speed railway

A continuous bridge in high-speed railway is close to several known faults in China. Those faults, respectively at different distances from the bridge site, will produce different ground motions with the different ratios of vertical component to the horizontal component at the bridge site. It is necessary to identify the influence of vertical ground motions on the seismic responses and vulnerabilities of the track-bridge system. This paper solved this problem by carrying out an incremental dynamic analysis (IDA) and a further seismic fragility analysis on a widely used continuous bridge in China. The results show that the damage probabilities of most bridge and track components increase along with the increase of vertical part in ground motions. This trend is significant for the sliding layer of track part in the longitudinal direction and the piers of bridge part in any direction, however, insignificant for the bearings of bridge part in any direction. Moreover, this trend is more significant for the track part across the girder gap due to the different seismic responses of adjacent bridges. The seismic design of track-bridge system should rigorously take the vertical part of ground motions into account.

Assessment of Adequate Margin to Liquefaction for Nuclear Power Plants

Design of nuclear power plant shall provide an adequate margin to protect items ultimately necessary to prevent an early large radioactive release in the case of earthquakes exceeding those considered in the design. An essential question is how large the margin should be to be accepted as adequate. In the practice, depending on the country regulation, a plant margin of at least 1.4 or 1.67 times the design basis peak ground acceleration is required to be demonstrated. The catastrophe at the Fukushima Daiichi Nuclear Power Plant revealed the fundamental experience that the plants designed in compliance with nuclear standards can survive the effects of the vibratory ground motion due to disastrous earthquake but may fail due to effects of phenomena accompanying or generated by the earthquakes. Liquefaction is one of those secondary effects of beyond-design basis earthquakes that should be investigated for NPPs at soil sites. However, the question has not been investigated up to now, whether a “margin earthquake”, vibratory effects of which the plant can withstand thanks to design margin, will not induce liquefaction at soil sites and will not result in loss of safety functions. In the paper, a procedure is proposed for calculation of the probability and margin to liquefaction. Use of the procedure is demonstrated on a case study with realistic site-plant parameters. Criteria for probability for screening and acceptable probabilistic margin to liquefaction are proposed. The possible building settlement due to margin earthquake is also assessed.

The capacity loss of a RCC building under mainshock-aftershock seismic sequences

Reinforced concrete containment (RCC) building has long been considered as the last barrier for keeping the radiation from leaking into the environment. It is important to quantify the performance of these structures and facilities considering extreme conditions. However, the preceding research on evaluating nuclear power plant (NPP) structures, particularly considering mainshock-aftershock seismic sequences, is deficient. Therefore, this manuscript serves to investigate the seismic fragility of a typical RCC building subjected to mainshock-aftershock seismic sequences. The implementation of the fragility assessment has been performed based on the incremental dynamic analysis (IDA) method. A lumped mass RCC model considering the tri-linear skeleton curve and the maximum point-oriented hysteretic rule is employed for IDA analyses. The results indicate that the seismic capacity of the RCC building would be overestimated without taking into account the mainshock-aftershock effects. It is also found that the seismic capacity of the RCC building decreases with the increase of the relative intensity of aftershock ground motions to mainshock ground motions. In addition, the effects of artificial mainshockaftershock ground motions generated from the repeated and randomized approaches and the polarity of the aftershock with respect to the mainshock on the evaluation of the RCC are also researched, respectively.

Analysis of influence factors of P-Δ effect considering vertical ground motion

With the development of structure optimization, building materials and construction technology, the study of the P-Δ effect is becoming more and more important in the pursuit of layers, flexibility and light quality. Based on the research status of P-Δ effect at home and abroad, this paper selects thenegative property fictitious member method to calculate the P-Δ effect of the multi-layer frame structure under the combined effect of horizontal and vertical ground motion. This paper discusses the influence of the stiffness-to-weight ratio, structural damping, and the vertical acceleration response of the structure. The results show that the damping and structural acceleration responses have a significant effect on the structural P-Δ effect. Under the premise of considering both, the stiffness-to-weight ratio of the structure, that is, the frequency of the structure, has a greater influence on the P-Δ effect of the structure under earthquakes. With the increase of the stiffness-to-weight ratio, the effect of P-Δ effect on structural response is increased and decreased.

Performance Evaluation of Gravity Dams Subjected to Near- and Far-Fault Ground Motion Using Euler Approaches

Ground motions in near source region of large crustal earthquakes are significantly affected by rupture directivity and tectonic fling. These effects are the strongest at longer periods, and they can have a significant impact on engineering structure. This paper focuses on the effects of long-period pulse of near-fault ground motions on the structural performance of concrete gravity dams. Three sets of near- and far-fault ground motion records, which have approximately the same peak ground acceleration, are selected from 1987 Superstitn Hills (B), 1989 Loma Prieta and 1994 Northridge earthquakes. As a case study, Sariyar concrete gravity dam located on the Sakarya River, which is 120 km to the northeast of Ankara, is selected to investigate the near-fault ground motion effects on dam responses. The finite element models of the dam are constituted considering dam-reservoir-foundation interaction using ANSYS software. The behavior of reservoir is taken into account by using Euler approach. To determine the structural response of the dam, the linear transient analyses are performed using above-mentioned ground motion records. In the analyses, element matrices are computed using the Gauss numerical integration technique. The Newmark method is used in the solution of the equation of motions. Rayleigh damping is considered. At the end of the analyses, dynamic characteristics, maximum displacements, maximum and minimum principal stresses and maximum and minimum principal strains are attained and compared with values obtained from analyses under far-fault ground motions recorded far away from the same sites at above-mentioned earthquakes with approximately same peak ground acceleration.

Effect of seismic wave velocity on the dynamic response of multi-story structures on elastic foundation

Traveling wave effects are generally considered with three main cases: (i) Wave passage effect that results with time delay in earthquake motion. (ii) incoherence effect which is defined as loss of coherency in the ground motion due to the reflection and refraction of waves, and (iii) local site effects. For multi-story structures whose supports are close to each other, the incoherence and local site effect may be omitted. In this case, traveling waves result only in a pure time delay in the earthquake motion (wave passage effect). Due to the wave passage effect of vertical and/or horizontal ground motion, the superstructure needs to be analyzed by multi-support excitation. Raft foundations cannot constrain vertical deformations and/or rotations, but they cause a diaphragm effect in the horizontal direction which results in uniform excitation. In this study, the effect of vertical earthquake motions onto multi-story buildings on elastic soil is investigated. Multi support excitation is considered by using displacement loading, which defines the equivalent seismic loads in terms of the ground displacement. According to the performed simulations of the selected structures, it is shown that structural height has a direct influence that results in member force magnifications with slow traveling wave effect. Among these, the ground floor column axial forces are most affected.

An improved defining parameter for long-period ground motions with application of a super-tall building

This paper focuses on defining long-period ground motions and analyzing the effect of them on building structures. Increasing attention has been paid to the effect of long-period ground motions on building structures. However, long-period ground motions are not quantitatively defined due to the lack of numerous ground motion investigations. In this paper, based on the statistical analysis of 39,744 ground motions, an improved frequency-domain defining parameter, namely βl, is firstly proposed to identify long-period ground motions without the necessity of seismological information. Then, based on the definition, long-period ground motions are selected for the time history analysis of a super-tall building structure. The structural responses are obtained under three scenarios: the original ground motion record, the ground motions scaled to minor earthquakes and major earthquakes. To assess the validity of the proposed parameter βl in identifying long-period ground motions, correlation between the structural response and the defining parameter is obtained.

Seismic Response of a Base Isolated Cable-Stayed Bridge Under Near-Fault Ground Motion Excitations

Nowadays, development of cable-stayed bridges is increasing around the world. The mitigation of seismic forces to these bridges are obligatory to prevent damages or failure of its structural members. Herein, this paper aimed to determine the near-fault ground motion effect on an existing cablestayed bridge equipped with lead-rubber bearing. In this context, Shipshaw cable-stayed bridge is selected as the case study. The selected bridge has a span of 183.2 m composite deck and 43 m height of steel tower. 2D finite element models of the non-isolated and base isolated bridges are modelled by using SAP2000. Three different near-fault ground motions which are Tabas 1978, Cape Mendocino 1992 and Kobe 1995 were subjected to the 2D FEM models in order to determine the seismic behaviour of the bridge. The near-fault ground motions were applied to the bridge in the longitudinal direction. Nonlinear dynamic analysis was performed to determine the dynamic responses of the bridge. Comparison of dynamic response of nonisolated and base isolated bridge under three different near-fault ground motions were conducted. The results obtained from numerical analyses of the bridge showed that the isolation system lengthened the period of bridge and minimised deck displacement, base shear and base moment of the bridge. It is concluded that the isolation system significantly reduced the destructive effects of near-fault ground motions on the bridge.