Real Earthquake Motions Research Articles

Seismic stability evaluation of rubble mound breakwater: Shake table tests and numerical analyses

Rubble mound (RM) breakwaters are coastal structures constructed to provide tranquil condition around the port areas. After past earthquakes such as the 2004 Indian Ocean earthquake and the 2011 Great East Japan earthquake, it was found that stability of breakwater not only depends on the wave action but seismic motions also play an important role for this. Very limited studies are available for the stability evaluation of RM Breakwater under earthquake motions by conducting physical model tests. To the end, an attempt has been made in the study to evaluate the stability of RM breakwater subjected to earthquake loadings. A series of shaking table tests conducted to evaluate the seismic behaviour of the RM breakwater. A prototype RM breakwater is modelled on two layers of seabed foundation soil. Different amplitudes of sinusoidal seismic motions (foreshocks and main shock) are provided at the base of the model. Later, the breakwater stability was evaluated for real earthquake motions. Various parameters such as settlement, horizontal displacement, acceleration-time histories and excess pore water pressure were measured during the tests. Deformation pattern was also studied by photos and videos captured during the tests. During the mainshock, the crown wall settled by 111 % more comparable to second foreshock; and the structure laterally displaced by more than 200 % comparable with first foreshock. The peak acceleration of input wave amplified while it was travelling from bottom to the crest of breakwater. The excess pore water pressure was maximum beneath the rubble mound, in loose sand and it was five times more during the mainshock compared to first foreshock. Due to loss in bearing capacity of foundation soil, the breakwater collapsed. Also, the effects like rolling down of armor units, densification and slumping of core material, shear deformation of breakwater body were observed during the main shock. Thus, the breakwater failed during the mainshock. Numerical analyses were also executed for both sinusoidal and real earthquake motions to make clear the mechanism of the breakwater behaviour subjected to the earthquake loadings.

Seismic analysis of Souk Tlata dam behaviour using finite element simulation

In this study, earthquake resistance of Souk Tlata earth dam was examined. It corresponds to a preliminary investigation of the behaviour of the dam under earthquake loading with a dominant frequency close to the fundamental frequency of the dam. The dam was modeled with plane-strain finite elements using Plaxis-2D program. The behaviour of both the fill and core of the dam is described using the simple non associated Mohr-Coulomb criterion. The development of the safety factors as a function of the displacement of the dam crest dam reveals that the static stability of the dam under conditions of drawdown is not assured. Dynamic analyses are conducted under a real earthquake motion, which corresponds to the main shock of May 21, 2003 in Algeria. The results show that the seismic loading, with peak acceleration exceeding 0.10g, induces significant settlement at the crest of the dam and substantial displacement in the upper part of the rip-rap in the upstream side.

Effect of soil-rock interface position on seismic response of subway station structure

For subway stations in a site with thin soil layers on the engineering rock, the lower part of the subway station is embedded in the hard rock mass and the upper part is buried in the soil layers. The presence of the soil-rock interface may lead to significant increase to the seismic responses of the adjacent soil masses and underground structures. In this paper, the effect of the soil-rock interface position on the seismic response of a two-story and three-span subway station structure is investigated by numerical analyses. It is found from the numerical analyses that a most detrimental soil-rock interface position exists and makes the underground structure have maximum responses under a given artificial earthquake motion. For the upper floor, the most detrimental soil-rock interface position is near the middle slab. For the lower floor, the most unfavorable soil-rock interface position is near the bottom slab. Softer site generally leads to greater structural responses under the same intensity of ground motion excitation. On the basis of the above findings, three earthquake motions included above an artificial earthquake motion and other two real earthquake motions and three burial depths of structure are further studied. Same conclusions can be obtained for the structure with different burial depths or subjected to different input earthquake motions. The conclusions drawn from the numerical examples of this paper may be useful for seismic design of subway station structure buried in shallow soil layers on rock.

GEOTECHNICAL SEISMIC ISOLATION METHOD USING RUBBER-SOIL MIXTURES

Seismic isolation is a method of protecting a building from earthquake-induced deformations by using isolators and devices under the superstructure. The purpose of the seismic isolation method is to reduce the earthquake forces transferred from the ground to the structure by placing energy-absorbing elements between the foundation and superstructure. Especially in developing countries, the "Geotechnical Seismic Isolation (GSI)" system has been proposed as a new isolation method to reduce earthquake-induced damages on buildings. In this study, it is aimed to reduce the effects of earthquakes in a multi-story building with an isolation layer formed with a mixture of rubber-sand (RSM). For this purpose, a 10-story reinforced concrete building was modeled numerically. A seismic energy absorbent RSM layer was placed beneath the foundation of the building. Geosynthetic layers are used to prevent the contact of the RSM layer with the natural soil. The developed model was tested under real earthquake motions with varying amplitudes. The performance of the developed RSM layer has been evaluated in terms of reducing the earthquake effects on the analyzed building model. The analysis was carried out with the dynamic module of the PLAXIS 2D finite element program.

Study on Seismic Spectrum and Amplification Effect of Soft Soil in Tianjin New Coastal Area

Tianjin new coastal area has a large area of soft soil with high salinity and high humidity. This kind of foundation has unique spectral characteristics and amplification effect on ground motion, which is a deep factor to increase earthquake damage. Based on the geotechnical data of Tianjin new coastal, we established a simplified calculation model and adopted a nonlinear method to study the seismic spectrum and amplification effect of three typical soft soil foundation with different thickness in Tianjin new coastal area, which real earthquake motion EL-Centro seismic wave passing through, that is in order to provide a basis for the seismic design of engineering structures in this area. The results show that: The soft soil foundation in Tianjin new coastal area has an obvious amplification effect on EL-Centro wave within 8HZ, while the seismic wave greater than 8HZ rapidly attenuates after filtering through the soft soil, and the amplitude greatly reduced. After the EL-Centro wave passes through the three types of soft soil, the acceleration, velocity and displacement coefficient are always in the state of amplification. The amplification coefficient is the largest in the muddy soil, followed by the muddy clay and the smallest in the silty clay. The softer the soil, the more obvious the amplification effect will be. Except the displacement amplification coefficient does not change much with the increase of soft soil thickness, the acceleration and velocity coefficients increase gradually with the increase of soft soil thickness, and the amplification effect is obvious after reaching the surface.

Rocking Response of Free-Standing Rigid Blocks on Slopes

Structure, equipment, statue or storage cask may experience free-standing rocking on slopes because of the nonuniform settlement of the base, construction error or foundation failure. The free-standing rigid block on a slope subjected to one-sine pulse excitation and earthquake motions is examined in this paper. First, considering the free-standing rigid block on a slope, the overturning acceleration spectrum under one-sine pulse is built. The spectrum covers two overturning modes, i.e. overturning with one impact either before or after the excitation and overturning without impact. Then the influences of the slope on the overturning acceleration spectrum are discussed in depth. The analytical solution is compared with the numerical solution based on both linear and nonlinear formulations. It reveals that the safe region of the minimum overturning acceleration spectrum between mode 1 and mode 2 depends on the angle of the slope. Further, the block on a slope is subjected to the Tianjin, Kobe and Northbridge earthquake motions. By simplifying the main pulse of the motions to one-sine pulse, although the overturning acceleration spectrum fails to predict the overturning acceleration of the block subjected to real earthquake motions, it can provide some guidelines to predict the practical performance of the rocking structures.

2D non-linear seismic response of the Dinar basin,TURKEY

Local geological conditions generate significant amplification of ground motion and concentrated damage during earthquakes. The highly concentrated damages at the edges of the Dinar basin during occurred earthquakes at regions close to rock outcrop bring up the effect of the inclined bedrock effect on the dynamic behavior of the basin with 2D geometry. In this study, first the idealized 2D model of the basin based on the results of the underground explorations and geologic investigations is proposed. Results show that Dinar basin has an asymmetric 2D geometry with two different bedrock angles at edges. Then, a numerical study using finite difference based nonlinear code which utilizes appropriate static and dynamic boundary conditions, and includes hysteresis damping formulation based on the user defined degradation curves is conducted using real earthquake motions of different strength and frequency content. The constructed model is subjected to the collection of 16 earthquakes with different PGA's of 0.1, 0.2, 0.3 and 0.4g, four motions for each PGA. It was seen that the dynamic behavior of the basin is broadly affected by the two dimensional bedrock. The results indicates the higher effect of the 6° bedrock inclination at east part on the amplification with respect to the steeper 20° bedrock slope at the west. Also, the results show the insignificant effect of the bedrock at the depth more than 150m on the amplification of the east edge. While the effect of the 6° bedrock angle at the east part continues until 1500m from the outcrop, it affects the amplification until 700m from the outcrop at the west part with 20° bedrock angle.

Response of sedimentary basin to obliquely incident SH waves

This research studies the impact of the incident angle of SH waves on the seismic response of two-dimensional sedimentary basins by using a nonlinear method. At first Ricker wavelet is input for a detailed analysis, followed by a statistical analysis based on a total of 100 real earthquake motions recorded at rock sites. The results show that the incident angle has a significant implication on the basin ground motion. First, the incident angle affects the short-period components more than the long-period ones of the spectral response acceleration, but the dominant period of the spectral response acceleration is insensitive to incident angle and location. Second, the MDIA of a basin is not necessarily 0° (vertical incidence) but in the range of approximately 0°–30°, and hence due attention should be paid to the influence of incident angle in seismic response analysis. Third, basin central areas are seismically preferable to edge regions for short-period buildings located on the basin, while, for long-period buildings, the edge areas become preferable. However, with the increase in incident angle, the difference between edge and central areas diminishes gradually. Finally, given that the dimensions of a basin are perceivable to incidence waves, the slope angle has a considerable impact on the PGA distribution pattern by controlling whether or not peak appears in the edge area. The MDP is most likely to be in the edge area of a basin with small slope angle when subjected to excitation with small incident angle (including vertical incidence).

Seismic Response of Bucket Foundations for Offshore Wind Tower

This study aims to investigate seismic response of bucket foundations for the support of offshore wind tower. The prototype design of the bucket foundation was scaled down with a scaling number of 1/240 to manufacture the model. An additional gravity base foundation (GBF) model was prepared and its diameter is same with the bucket. The dynamic centrifuge tests for both models installed in sandy soil were carried out at 60 G resulting in the 1/4 scale prototype. Two real earthquake motions, Kobe and El Centro earthquakes were applied to the models. The earthquake motion scaled with peak accelerations from 0.02 G to 0.2 G was excited at the base of the soil container. The acceleration at the platform level (i.e. the connection of the tower and foundation) for the bucket foundation is lower than that of the GBF. The types of earthquake affected the responses at the platform. The displacement at the platform level of the bucket was also lower than the GBF. The contact pressure was at the bottom invert of both models and skirt surface of the bucket. The contact pressure at the bottom invert of the GBF significantly increases with increasing excitation while that of the bucket is negligible. The contact pressure at the skirt surface of the bucket significantly increases due to seismic loading.

Stochastic optimization of shape-memory-alloy rubber bearing (SMARB) for isolating buildings against random earthquake

SUMMARY Shape-memory-alloy (SMA) aided isolation bearing has been proposed recently in order to achieve improved performances in several aspects, comparing the traditional bearings for isolation. The behavior of such system has been experimentally demonstrated and numerically simulated under recorded seismic ground motions. The present work deals with the study of optimum performance of SMA-based isolation system under random earthquake. The stochastic response evaluation of the isolated building is performed by nonlinear random vibration analysis via stochastic linearization of load-deformation hysteresis of SMA. The behavior of the stochastic responses indicates the existence of optimal value of transformation strength of SMA to minimize the superstructure acceleration and thereby most efficient isolation. Thus, a stochastic structural optimization problem is taken up in order to obtain the optimal transformation strength. A closed form expression of such optimal parameters are also presented, which may be used for establishing an initial design. The viability of the optimization study is justified by analyzing the response of the optimal system under set of real earthquake ground motion records. The stochastic response behavior of the optimal system is found to be consistent with the responses obtained under real earthquake motions. Copyright © 2014 John Wiley & Sons, Ltd.

A numerical study on the 2D behavior of the single and layered clayey basins

In this numerical study the effects of the stratification and change in material type on the dynamic behavior of the clayey basins are investigated. For this purpose two kinds of single and double layer basin models with the total depth of 100 m and width of 2 km have been selected. The single layer basins consist of soft, medium and stiff clayey materials, while double layer basins consist of a stiff sandy sub-layer overlaid by one of the soft, medium and stiff clayey soil layer. A range of bedrock inclinations at the valley sides from slighter $$10^{\circ }$$ and $$20^{\circ }$$ to steeper $$30^{\circ }$$ and $$40^{\circ }$$ are selected. A numerical study using finite difference based nonlinear code which utilizes appropriate static and dynamic boundary conditions, and includes hysteresis damping formulation based on user defined degradation curves is conducted using real earthquake motions of different strength and frequency content. The results of the 2D analyses show the differences in dynamic behavior of single and double layer basins from the aspect of amount and position of the maximum amplification and resonance period. It was seen that the shallower lateral parts of the basins are sensitive to lower periods while inner parts are sensitive to higher periods. Among other differences, the most important difference between the behavior of single and double layer basins was seen at the resonance period of the inner parts of the basins.

A numerical study on the basin edge effect on soil amplification

In this paper a numerical study on the effects of the basin edge on the dynamic behavior of the model basins are investigated. For this purpose a range of bedrock inclinations at the valley sides from slighter \(10^{\circ }\) and \(20^{\circ }\) to steeper \(30^{\circ }\) and \(40^{\circ }\) are selected. A numerical study using nonlinear code which utilizes appropriate static and dynamic boundary conditions, and includes hysteresis damping formulation based on user defined degradation curves is conducted utilizing two sandy and clayey materials. Using several different real earthquake motions provide opportunity for the assessment of the site response to the variation of the motion intensity. The analyses results are presented in the form of the acceleration and spectral acceleration amplification curves. Also, by conducting 1D analyses along the valley the aggravation curve for every case are evaluated and discussed. It was seen that variation of the bedrock inclination not only affects the peaks of the spectral amplification curves, but also the position of the maximums of the curves on the valley surface are changed. Also, the frequency domain results show that different parts of the valleys are sensitive to different periods. While the lateral parts are sensitive to lower periods, the maximum amplification of the inner parts takes place at higher periods. Based on results the 2D behavior not only is dominant at the latreal parts of the valley, but also affects the behavior of the inner parts. Also, the use of the 1D analyes for the estimation of the 2D behavior remains insufficient. Finally, the results of this research show the important effect of the motion intensity on the 2D behavior of the valley specially on the increase of the resonance period at higher period.

Development of a servo-hydraulic earthquake actuator for the Cambridge Turner beam centrifuge

Dynamic centrifuge modelling has been carried out at Cambridge since the late 1970s. Over this period, three different mechanical earthquake actuators were developed. In this paper the development of a new servo-hydraulic earthquake actuator is described. The basic design principles are explained along with the need to carry out these designs to match the existing services and systems of the 35 year old Turner beam centrifuge at Cambridge. In addition, some of the features of the Turner beam centrifuge are exploited in the design of this new earthquake actuator. The paper also explains the mechanical fabrication of the actuator and the control systems that were developed in order to generate real earthquake motions. Finally, the performance of this new servo-hydraulic earthquake actuator is presented and assessed based on a wide range of earthquake input motions.

Experimental p-y loops for estimating seismic soil-pile interaction

Seismic soil-pile interaction is evaluated in this study based on back-calculated p-y loops constructed from sampled data of pile bending moments. Fundamental properties of p-y loops are implemented to derive distributed springs and dashpots, thereby quantifying soil-pile interaction in the realm of a Beam on Dynamic Winkler Foundation modeling. The procedure is validated by means of well-documented centrifuge tests of a single pile supported structure founded on a two-layer soil profile that comprises of soft clay overlying dense sand. Two shaking levels of a real earthquake motion applied at the base of the soil profile were examined and the generated seismic p-y loops were compared to cyclic p-y curves commonly used in pile design practice. The results demonstrate the strong influence of intensity of the input motion on seismic p-y loops while cyclic p-y curves established for soft clays tend to overestimate soil stiffness under strong excitation. Typical sets of recorded and computed structural response are presented, denoting the ability of the BDWF model related to p-y loops in reproducing adequately fundamental aspects of seismic soil-pile interaction.

Use of collision shear walls to minimize seismic separation and to protect adjacent buildings from collapse due to earthquake‐induced pounding

AbstractThe use of collision shear walls (bumper‐type), acting transversely to the side subject to pounding, as a measure to minimize damage of reinforced concrete buildings in contact, is investigated using 5‐story building models. The buildings were designed according to the Greek anti‐seismic and reinforced concrete design codes. Owing to story height differences potential pounding in case of an earthquake will occur between floor slabs, a case specifically chosen because this is when pounding can turn out to be catastrophic. The investigation is carried out using nonlinear dynamic analyses for a real earthquake motion and also a simplified solution for a triangular dynamic force of short duration, comparable to the forces caused by pounding. For such analyses, nonlinear, prismatic beam–column elements are used and the effects of pounding are expressed in terms of changes in rotational ductility factors of the building elements. The local effects of pounding on the collision shear walls are investigated using a detailed nonlinear finite element model of the shear walls and results are expressed in terms of induced stresses. It is found that pounding will cause instantaneous acceleration pulses in the colliding buildings and will somewhat increase ductility demands in the members of the top floor, but all within tolerable limits. At the same time the collision walls will suffer repairable local damage at the points of contact, but will effectively protect both buildings from collapse, which could occur if columns were in the place of the walls. Copyright © 2008 John Wiley & Sons, Ltd.

다양한 진동하중의 액상화 시험을 통한 동적 과잉간극수압에 대한 상세분석

지진시 발생하는 지반 액상화 현상은 지진피해예측시 가장 중용하게 평가해야 하는 항목으로 실내에서 액상화 현상을 시뮬레이션하는 경우, 불규칙성의 지진을 제어하기 어려운 이유로 등가반복형태의 정현파 또는 쐐기파를 이용하는 것이 통상적이었다. 그러나, 90년대 이후 컴퓨터의 발달과 함께한 신호처리 및 제어기술의 급속한 발전은 동역학분야에서의 실험연구에 큰 도움을 주었으며 지반진동분야의 경우, 진동대 및 원심모형기와 같은 대형지진모의시험에서 실지진하중을 이용한 연구사례들이 증가되고 있는 실정이다. 국내에서도 실지진하중재하가 가능한 요소시험결과에 대한 연구사례가 발표되는 등 이에 관한 연구가 활발하게 진행되고 있으며 특히, 실지진하중 시험결과로부터 실지진하중하에서의 과잉간극수압의 변화가 정현하중시험에서의 결과와 큰 차이를 보이고 있다는 분석결과가 발표되어 현재까지 지반동역학분야에서 지진하중의 불규칙성을 크게 고려하지 않았던 통상적 가정사항들이 문제점으로 대두되어 관심을 집중시키고 있다. 본 연구에서는 등가의 정현파 및 쐐기파와 증가형태의 쐐기파, 그리고 실지진파 등 다양한 진동하중을 재하할 수 있는 삼축압축시험기를 이용하여 기존의 유효응력개념에서 사용하는 과잉간극수압관련 매개변수인 소성일, 전단소성 변형률 상각궤도, 그리고 액상화 상태전환선의 기울기를 비교분석하였으며 하중변화에 따른 토립자의 변형과 간극수압의 변화를 상세분석하였다. 연구결과, 토립자의 변형, 간극수의 동적흐름, 동적흐름 속도수두의 압력수두로의 전환, 그리고 과잉간극수압의 증가로 표현되는 새로운 개념의 액상화 발생이론을 제안하였으며 그 타당성 검토를 위해 과잉간극수압과 동적흐름의 압력수두(흐름속도의 제곱값)와의 상관관계를 비교하였다. In most experimental researches on the liquefaction phenomenon, an earthquake as a random vibration has been regraded as a sinusoidal wave or a triangular wave with an equivalent amplitude. Together with the development in the part of signal control and data acquisition, dynamic experimental equipments in the soil dynamics have also developed rapidly and further more, several real earthquakes have been simulated in the large model test such as shaking table tests and centrifuge tests. In Korea, several elementary laboratory tests to simulate the real earthquake load were performed. From these test results, it was reported that the sinusoidal wave cannot reliably reflect the soil dynamic behavior under the real earthquake motion. In this study, 4 types of dynamic motions such as the sinusoidal wave, the triangular wave, the incremental triangular wave and several real earthquake motions which were classified with shock-type and vibration-type were loaded to find something new to explain the change of the excess pore water pressure under the real earthquake load. Through the detailed investigation and comparison on all test results, it is found that the dynamic flow is generated by the soil plastic deformation and the velocity head of dynamic flow is changed the pressure head in the un-drained condition. It can be concluded that the change of the excess pore water pressure is related to the pressure head of dynamic flow. Lastly, a new hypothesis to explain such a liquefaction initiation phenomenon under the real earthquake load is also proposed and verified.

Computational numerical models for seismic response of structures isolated by sliding systems

The problem of sliding structures is discontinuous one as different sets of equations of motion with varying forcing functions are required for the sliding and non-sliding phases. This is inconvenient for the numerical integration of the governing equations for the response of sliding structures. To overcome such difficulties continuous hysteretic models of the sliding systems have been presented in the past. In the present study, a comparison of the response of structures (i.e. multi-storey buildings and bridges) isolated by sliding systems with conventional and hysteretic models of the frictional force of the sliding system is carried out to investigate the comparative performance and computational efficiency of the two models. The seismic response of isolated structures is obtained by solving the governing equations of motion using a step-by-step method under single and two horizontal components of real earthquake motions. For comparative study, the seismic response of a multi-storey building obtained by the conventional model is compared with the corresponding response by the hysteretic model under different sliding isolation systems, numbers of storeys and values of the fundamental time period of the superstructure. It is found that the conventional and hysteretic models of sliding systems predict a similar seismic response for isolated structures. However, the difference in the response between the two models is relatively more for the pure friction system as compared with the sliding systems with restoring force. Further, the conventional model is relatively more computationally efficient as compared with the hysteretic model. Copyright © 2004 John Wiley & Sons, Ltd.

THREE DIMENSIONAL NONLINEAR BUILDING POUNDING WITH FRICTION DURING EARTHQUAKES

The three dimensional pounding phenomenon of two adjacent buildings during earthquakes with aligned rigid horizontal diaphragms is investigated for the linear and nonlinear structural response. The developed formulation takes into account three dimensional dynamic contact conditions for the velocities and accelerations based on the impulse-momentum relationship, using the coefficient of restitution e and the ratio μ, of tangential to normal impulses, which corresponds to the coefficient of friction under certain conditions. The contact points are determined geometrically from the displacements of the diaphragms' centre of mass. The results of the proposed formulation are compared with those obtained with the Lagrange multipliers approach. Test results are performed for two sets of multi-storey adjacent buildings subjected to real earth-quake motions with elastic and inelastic structural response.

Three-Dimensional Simulation of Structural Pounding during Earthquakes

A three-dimensional model is developed for the simulation of the pounding response of two or more adjacent buildings during earthquakes. The contact-impact problem is treated by the Lagrange multiplier method. The structures are modeled as multiple-degree-of-freedom systems with finite elements and with the possibility of contact between slabs or slabs and columns. Contact points are not known a priori, and are located through a searching procedure among the candidate contact surfaces. The dynamic equilibrium equations of contact follow an iterative formulation at each time step to account for the current contact conditions. With this three-dimensional idealization, the effect of various in-plan configurations of adjacent buildings as well as the induced deformations due to torsion during pounding are investigated for two real earthquake motions.

A Study on Energy Absorption Capacity of Structural System with Hysteretic Restoring Force.

In this paper, an analytical method for estimating the capacity of energy absorption of a structural system with a hysteretic restoring force subjected to seismic excitation is proposed. Using a single-degree-of-freedom system with bilinear restoring force, the conventional equation for calculating the capacity of energy absorption has been presented by applying the method of equivalent linearization, which represented the reduction of input energy due to the absorbed energy of the hysteretic restoring force. Finally, it is clear that the calculated capacity of energy absorption by the proposed method almost agrees wih that obtained through the response analysis using several real earthquake motions.