Ground Motion Excitation Research Articles

Seismic fragility analysis of concrete encased frame‐reinforced concrete core tube hybrid structure based on quasi‐static cyclic test

SummaryConcrete‐encased frame‐core tube hybrid structural system has been widely employed in high‐rise buildings. This paper intends to analyze the seismic fragility of this structural system under ground motion excitation. The quasistatic cyclic test on a 1/5‐scaled, 10‐story three‐bay specimen is introduced. Fiber‐based finite element model is developed and integrated with numerical techniques that would be able to simulate the nonlinear response based on the OpenSees program. As the model is verified by the experimental data, a series of incremental dynamic analyses (IDAs) considering different frame‐tube stiffness ratios are carried out. IDA curves are drawn to describe each structural performance state. Fragility curves and probabilistic demand models are proposed for quantifying failure probability. The collapse margin ratio is employed to evaluate the collapse probability. The result shows that the collapse probability under rare earthquake still meets the requirement of Applied Technology Committee‐63 Report. The hybrid structure is proved to perform superior collapse resistance ability. The proper increase in the stiffness of core tube can reduce the collapse probability and enhance the collapse resistance capacity.

A sand-rubber deformable granular layer as a low-cost seismic isolation strategy in developing countries: Experimental investigation

This paper presents experimental investigations on the feasibility of using a sand-rubber deformable granular layer as a low-cost seismic isolation strategy for developing countries. The mechanical characteristics of a potential failure mechanism inside the sand-rubber layer are investigated. Direct shear testing is performed to quantify the angle of friction of three different sand-rubber mixtures subjected to different vertical stress levels. The experimentally derived mechanical characteristics are compared to the corresponding values for pure rubber and pure sand samples. The frictional characteristics of sliding between a sand-rubber layer and a timber interface are identified. Direct shear testing is performed to quantify the quasi-static friction of the same sand-rubber mixtures against a timber interface, that is part of the foundation casting, subjected to alternative vertical stresses. The effect of the shear rate and the saturation of the sand-rubber mixture on the aforementioned mechanical characteristics is presented. A uniaxial shaking table experimental setup is used for the investigation of the dynamics of a rigid sliding block and the quantification of the kinetic friction of different sliding interfaces against two different sand-rubber mixtures for two different sand-rubber layer heights. The rigid sliding block designed to slide against the sand-rubber layer is subjected to both a harmonic ramp loading and earthquake ground motion excitation. The design outcome of this static and dynamic experimental investigation is the determination of the optimum grain size ratio and the height of the sand-rubber layer, that corresponds to the lower (and more favourable from a seismic isolation view point) friction coefficient between the sand-rubber layer and the foundation. The quantification of these fundamental parameters paves the way for a holistic design of a response modification strategy for mitigating seismic damage in developing countries.

Optimal design for high-performance passive dynamic vibration absorbers under random vibration

This work is motivated to determinate the optimal design for various types of Dynamic Vibration Absorbers (DVAs) under the effect of random loads. These devices are the following: DVAs connected in parallel or series; two degree-of-freedom traditional dynamic vibration absorber (2dof-TVA) with translational and rotational motion; and inerter-based DVAs (IDVA-C6, C4, and C3). These types of DVAs were selected by their high effectiveness for suppressing vibration however a comparative study on their dynamic performance has not yet been performed. Two different excitation sources of random loads are studied in this paper which are random ground motion and force excitation. An optimum design for majority of these devices has not yet been computed when subjected to random ground motion excitation, and therefore in this paper are computed. For random force excitation, some numerical solutions for the optimal design of these devices have not yet reported which in this paper are computed in order to compare the dynamic performance for each device with respect to that of the classic DVA. For both random excitation cases, the H2 optimization criteria is used to analytically compute the variance of squared modulus of frequency response of the undamped primary structure, and then nonlinear unconstrained optimization problems are formulated in order to obtain the optimal design. Numerical solutions revealed that the IDVA-C6, C3, and DVAs connected in series presents more than 13% and 10% improvement with respect to the classic DVA for random ground motion and force excitation cases, respectively. These devices can widen the suppression band (SB) from 30% to 40% for mass ratios values from 1% to 10%. It means that devices are more effective and robust for mitigating vibration than the classic DVA. In addition, the rotational inertial double tuned mass damper (IDVA-C6) has the same relative dynamic performance (RDP) and suppression band index (SB) than the double-mass dynamic vibration absorber arranged in series. For practical application where the mounting space of the DVA is extremely reduced, the DVAs connected in series could be more convenient to use than IDVA-C6. The concept of equivalent mass ratio is introduced in order to explain the superiority of these devices with respect to the classic DVA. Finally, in H∞ optimization criteria, the IDVA-C6 presents the same vibration amplitudes at all excitation frequency range and suppression band than DVAs connected in series.

Multi-platform soil-structure interaction simulation of Daikai subway tunnel during the 1995 Kobe earthquake

The multi-platform simulation technique is employed in this study to analyze the Daikai subway tunnel that collapsed during the 1995 Kobe earthquake. The generalized distributed simulation framework (UT-SIM framework), developed at the University of Toronto, is used to perform the analysis. The soil-tunnel system is subdivided into two substructures: the soil domain and the tunnel domain. The open-source finite element analysis program OpenSees is used to model the soil domain subjected to the ground motion excitation, while the state-of-the-art reinforced concrete analysis software package, VecTor2, is used to model the nonlinear behavior of the tunnel. The two modules are integrated with the UT-SIM framework. The integrated soil tunnel models, through the multi-platform simulation technique, replicated the damage of the center column and the actual failure mode of the tunnel. The results of the soil and tunnel models are post-processed and visualized. It is confirmed that the multi-platform simulation method can replicate the crack pattern, as well as the deformation and failure mechanism of the center column.

The seismic responses and seismic properties of large section mountain tunnel based on shaking table tests

Based on the engineering prototype of new Jinjishan Tunnel in Fuzhou 2nd ring road, the 1/30 downscale ground-lining model was fabricated. Then the earthquake-simulating tests were carried out on the shaking table apparatus to explore the seismic responses and seismic properties of large section mountain tunnel. The time-domain analyses on the test results show that the PHA (peak horizontal acceleration) amplification coefficients of ground increase non-linearly with elevation. The elevation effect of ground takes “S” form, which distinguishes itself from other regular ones. The PCP (peak contact pressure) oscillation coefficients at the sidewall and the arch of lining increase almost linearly with the seismic intensity, which implies that the sidewall and arch carry a majority of seismic loading under large ground motion excitation. The frequency-domain analyses on the test results show that the existence of large section tunnel changes the seismic property of original ground significantly. The 1st and 2nd dominant frequencies reflect the seismic properties of original ground and lining structure respectively, and both of them decrease gradually with the repeated excitations. Meanwhile the frequency components around the 2nd dominant frequency are augmented significantly when the seismic wave passing through the model from bottom to surface.

Seismic response of loess-mudstone slope with bedding fault zone

The characteristics of dynamic responses and the failure mechanism of a loess-mudstone slope with an embedded fault zone subjected to seismic loadings were investigated by shaking table test on a scaled slope model and numerical simulation. The model was subjected to seismic ground motion excitations of increasing peak accelerations. The results demonstrated clear amplification effects to the ground acceleration at the slope surface and fault zone, which directly aggravated slope failure. Similar amplification effects were observed at the hanging wall, and this became very apparent when the peak ground acceleration exceeded 0.3 g. The dynamic response of vertical earth pressure was positively correlated with the thickness of the overburden strata under all peak accelerations. The dynamic responses of horizontal earth pressure were strongest at the top of the fault zone under peak accelerations of all levels. The dynamic responses of acceleration and earth pressure in slope both increased with rising peak accelerations. The mode of failure of the loess-mud slope with a dip bedding fault zone under the action of seismic waves was characterized as a shock slip failure. The width of the fracture distribution area in the loess-mud slope induced by seismic waves on the hanging wall was 2.5 times that of the footwall of the fault zone, which was further evidence that a slope with a dip bedding fault zone experiences an amplification effect on the hanging wall of the fault zone.

Seismic Fragility Assessment of Steel SMRF Structures under Various Types of Near and Far Fault Ground Motions

In this paper, the seismic collapse probability of special steel moment-resisting frame (SSMRF) structures, designed to 4th edition of Iranian seismic design code, under near fault pulse-like and far fault ordinary ground motions is evaluated through fragility analysis. For this purpose, five sample frames with 3 to 15 stories are designed and imposed to the ground motion excitations with different characteristics. Fragility curves are derived for the sample frames using the results of incremental dynamic analyses. Three sets of near fault ground motion records with different range of pulse period and one set of far fault ordinary records are used in dynamic analyses. Each record set involves ten acceleration time histories on soil type III. Based on the obtained results, it was found that pulse-like motions with medium- and long-period pulses are significantly more destructive than other types of ground motions. Fragility analysis reveals that the average collapse probability for the case study frames under the far and near fault ground motions at the intensity of 0.35g equals to 4.3% and 10.3%, respectively. These values are 15.9%and 38.6%, for PGA of 0.53g. It is also found that the increase in the height, leads to increase in higher modes effect to transfer drift demands toward upper stories.

Nonlinear random vibrations of 3D cable-moored floating structures under seismic and wave excitations

A numerical model is constructed for carrying out the nonlinear random vibrations of 3D floating structures moored by cables under seismic and wave excitations. The floating structure is moored to the seabed by four cables that distribute symmetrically around the floating structure. The 3D nonlinear elementary stiffness matrix of cable, rather than the tangent elementary stiffness matrix of cable, is derived based on the extended Hamilton principle. The nonlinear equations of motions of the mooring cables are formulated using the 3D cable elements. The floating platform is modeled as a rigid body with six degrees of freedom. The effects of added mass and nonlinear hydrodynamic drag forces for both the floating platform and mooring cables are taken into consideration. The connection conditions which represent the relationships between the displacements of the floating platform and cables are formulated. The equations of motions of both the mooring cables and floating platform are formulated separately and then assembled as a whole system through the connection conditions. Eventually, the whole system under both horizontal seismic ground motion and wave excitation is analyzed numerically using Monte Carlo simulation method. The mean values, standard deviations and probability density functions of the responses of the floating structure and the maximum tensile force in the cables are presented and studied under various conditions.

Comparative studies of vibration control effects between structures with particle dampers and tuned liquid dampers using substructure shake table testing methods

This paper compares the vibration control performance of particle dampers (PDs) and tuned liquid dampers (TLDs) by using the up-to-date substructure shake table testing (SSTT) method. In implementing the SSTT, three representative model-based integration algorithms, whose algorithmic parameters are functions of the complete model of the structure system to enable unconditional stability to be achieved within the framework of an explicit formulation, are adopted. The experimental procedures of the SSTT methods using the three integration algorithms are presented. The SSTT system of the single-degree-of-freedom (SDOF) structures with PDs/TLDs is constructed and verified by comparing the experimental results of the SSTTs and the corresponding complete structure shake table tests. The vibration reduction effects of PDs and TLDs with the same mass ratio are investigated and compared by conducting a series of SSTTs of the SDOF systems with such devices under the excitation of ground motion. The influences of mass ratio, damping ratio and structural frequency on control efficiency of PDs and TLDs are studied. The results from the parametric analyses demonstrate that all the aforementioned factors have significant impacts on the vibration control performance of both dampers. Most experimental results indicate that the PDs have better vibration control performance than the counterpart TLDs.

A Study of Ground Motion Excitation Based on the Earthquake of September 8, 2017: Evidence that Normal Faults Influence the Stress Parameter

We performed a study of ground motion modeling based on multiple linear regression and random vibration theory using the earthquake sequence of September 8, 2017 (Mw = 8.2). Our results show that there is high attenuation in the region of Oaxaca–Chiapas and central Mexico; however, the peak amplitude excitations of normal fault earthquakes are higher than those of thrust fault earthquakes. The earthquake sequence involved a large number of events, but not all the seismic events were aftershocks because many earthquakes had different rupture styles. The thrust events follow a simple scaling relation with a constant stress parameter, but the normal fault events are difficult to represent with a constant self-similar model. Our best result consists of a $$Q_{0} = 240 \pm 032$$ with a frequency-dependent factor of $$0.63 \pm 0.05$$ . The excitation term is well determined with a stress parameter of 80 bars for the thrust faults, but for the normal faults, the stress parameter is at least 180 bars with a non-self-similar model. Our results show that it is necessary to include new ground motion prediction equations in seismic hazard analysis for “meganormal” faults, which have not previously been considered in subduction zones.

Seismic performance evaluation of self-centering K-type and D-type eccentrically braced frame systems

This paper presents a numerical study of steel self-centering eccentrically braced frame (SCEBF) buildings subjected to seismic excitation. This study aims to investigate the nonlinear seismic system behavior of two types of SCEBF systems - K-type and D-type SCEBFs under design basis earthquake (DBE) level. Using two prototype framed structures – a 3-story D-type and a 4-story K-type SCEBF building designed for Los Angeles, California, it is demonstrated that SCEBF systems can be designed to provide the strength and stiffness comparable to conventional steel EBFs while maintaining plumbness under DBE level seismic ground motion excitation. The nonlinear time history analysis results show that residual drifts of the SCEBFs are negligibly small and damages in the structure are largely confined to fuse devices under DBE. Additionally, a parametric study of both K-type and D-type SCEBFs with three different levels of post-tensioning (PT) cables’ initial stress has been done to demonstrate the approach to tuning the self-centering EBF’s seismic behavior. The parametric study results suggest that key structural parameters such as the equivalent yield strength and the post-gap opening stiffness of SCEBFs can be adjusted through properly selecting PT cable’s length and initial stress level.

Nonlinear random vibrations of moored floating structures under seismic and sea wave excitations

A numerical model is formulated for the nonlinear random vibrations of the offshore floating structures moored by cables under seismic and sea wave excitations. The upper end of each mooring cable is connected to the floating structure and the other end of each cable is fixed to the seabed. Nonlinear equations of motions of the mooring cables are formulated by using the nonlinear cable elements that are formulated based on the extended Hamilton principle. The floating platform is modeled as a rigid body with three degrees of freedom. The connection conditions which represent the relationships between the displacements of the floating platform and cables are given to derive the equations of motions of the whole system. The effects of nonlinear hydrodynamic drag forces and added-mass on both the floating platform and cables are taken into consideration. The random vibrations of the moored floating structure under both horizontal seismic ground motion and sea wave excitation are analyzed by using the Monte Carlo simulation method. The probability density functions of the displacements of the moored floating structure and the maximum tensile force in the cables are analyzed. The influences of different sag-to-span ratios, inclination angles and diameters of the mooring cables on the mean value and standard deviation of the displacements of the floating structure and the maximum tensile force in the cables are studied.

Shaking Table Test Study on Seismic Performance of Hollow Rectangular Piers

To study the seismic performance of hollow reinforced concrete piers under dynamic loads, nine hollow pier specimens with different stirrup ratios, reinforcement ratios, and axial compression ratios are designed and manufactured. The El Centro wave, Taft wave, and artificial Lanzhou wave are selected as seismic excitation for the shaking table test. The effects of the reinforcement ratio, stirrup ratio, and axial compression ratio on the failure mode, period, damping, acceleration and displacement response, dynamic magnification factor, ductility, and energy dissipation of specimens under different working conditions are studied. The results show that all the nine reinforced concrete piers have good seismic performance. Subjected to ground motion excitation, horizontal through cracks appeared on the pier surface. With the increase of ground motion excitation, the period of piers increases but the maximum period does not exceed 0.62 s, and the damping ratio increases as well and ranges from 0.02 to 0.064. With the increase of the ground motion excitation, the acceleration response of pier specimens increases, the dynamic magnification factor decreases, the displacement ductility coefficient decreases, and the energy dissipation of the specimens increases. The reinforcement ratio, stirrup ratio, and axial compression ratio have different effects on the above parameters. The test results can provide reference for seismic design of hollow rectangular piers and have certain engineering significance and value.

The mechanism of hindering occupants’ evacuation from seismic responses of building

Pedestrian evacuation from buildings during an earthquake needs to consider human behavior and building shaking. This study sets up an indoor evacuation model based on the social force and dynamic mechanics. First, social forces were formulated in the Eulerian coordinate system, seismic force that excites on pedestrians in a multi-story building is derived from structural acceleration, and an evacuation criterion is given based on above forces. Second, a simulation was performed through VB programming, which accounts for the situation that people evacuate from a walkway. Parameters of the social force model are modified in order to estimate pedestrians’ acceleration in concerned situation. Third, structural dynamic responses under a series of ground motion excitations with varying peak values are acquired through finite element analysis to determine pedestrians’ seismic forces. Then, pedestrians’ ability to escape safely is evaluated according to evacuation criterion. Results show that seismic force would increase when pedestrian located on higher floor or ground excitation is of more dramatic level. Additionally, the possibility of survival is likely minimized as long as seismic force is larger than social force. This proposed model is capable of describing the effects of environment on human behavior during earthquake evacuation.

Hysteresis models for response history analyses of recycled rubber–fiber reinforced bearings (RR-FRBs) base isolated buildings

The aim of this study is to assess the applicability of two well-known hysteresis models for the prediction of the response of a Recycled Rubber – Fiber Reinforced Bearings (RR-FRBs) base isolated building under ground motion excitation. Results of experimental tests on low-cost rubber bearings are used to determine their response under cyclic loading, and to derive the parameters of the Bilinear and the Bouc–Wen hysteresis models using consolidated procedures. Response History Analyses (RHAs) are then performed to assess the behavior of a prototype building when base isolated with the low-cost devices. The outputs of RHAs are then compared to shaking table test results to validate the capability of the hysteresis models of predicting the peak structural responses under ground motion excitations. Findings of this research confirm the advantages of using low-cost rubber devices for earthquake hazard mitigation. The potentials and limitations of adopting simple models of hysteresis when assessing the seismic response of a RR-FRBs base isolated structure are discussed in this study.

Dynamic response of post-tensioned rocking wall-moment frames under near-fault ground excitation

The dynamic responses of a rocking wall-moment frame (RWMF) with a post-tensioned cable are investigated. The nonlinear equations of motions are developed, which can be categorized as a single-degree-of-freedom (SDOF) model. The model is validated through comparison of the rocking response of the rigid rocking wall (RRW) and displacement of the moment frame (MF) against that obtained from Finite Element analysis when subjected ground motion excitation. A comprehensive parametric analysis is carried out to determine the seismic performance factors of the RWMF systems under near-fault trigonometric pulse excitation. The horizontal displacement of the RWMF system is compared with that of MF structures without RRW, revealing the damping effect of the RRW. Frame displacement spectra excited by trigonometric pulses and recorded earthquake ground motions are constructed. The effects of pulse type, mass ratio, frame stiffness, and wall slenderness variations on the displacement spectra are presented. The paper shows that the coupling with a RRW has mixed results on suppressing the maximum displacement response of the frame.

Evaluation of Seismic Behavior of Steel Moment Resisting Frames Considering Nonlinear Soil-structure Interaction

In structural analysis, the base of structures is usually assumed to be completely rigid. However, the combination of foundation and the subsurface soil, makes in fact a flexible-base for the soil-structure system. It is well-known that the structural responses can be significantly affected by incorporating the Soil-structure Interaction (SSI) effects. The aim of the present study is to provide more accurate structural responses analysis by considering the influence of SSI. It is noteworthy that the input ground motion records imposed to the combination of the soil, foundation and structure were selected in a such way that their characteristics were completely matched with the subsurface soil of structures. For this purpose, 3, 6, 9, 12, 15, 18 and 20-storey structures resting on a shallow foundation were selected and the concept of Beam on Nonlinear Winkler Foundation (BNWF) model is employed. The seismic responses of these structures were calculated based on the five different types of soil and the outcomes were compared with those from fixed-base structures. A set of 35 ground motion excitations recorded on different soil types, is selected which categorized to 5 sets consist of 7 records. Non-Linear Response History Analysis (NL-RHA) was performed and radiation damping considered for all of the structures and soil types. The results clearly showed that the inter-storey drift ratio was reduced in lower stories considering SSI effects. These effects are strongly increased, especially with increasing the slenderness ratio of the structures and softening the subsurface soil. Finally, the period lengthening ratio of studied structures, for various soil types was investigated.

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.

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 cable-stayed 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 non-isolated 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.

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.