Low-intensity Earthquakes Research Articles

Polarization sensing of network health and seismic activity over a live terrestrial fiber-optic cable

Wide-scale sensing of natural and human-made events is critical for protecting against environmental disasters and reducing the monetary losses associated with telecommunication service downtime. However, achieving dense sensing coverage is difficult, given the high deployment overhead of modern sensor networks. Here we offer an in-depth exploration of state-of-polarization sensing over fiber-optic networks using unmodified optical transceivers to establish a strong correlation with ground truth distributed acoustic sensing. To validate our sensing methodology, we collect 85 days of polarization and distributed acoustic sensing measurements along two colocated, 50 km fiber-optic cables in Southern California. We then examine how polarization sensing can improve network reliability by accurately modeling overall network health and preemptively detecting traffic loss. Finally, we explore the feasibility of wide-scale seismic monitoring with polarization sensing, showcasing the polarization perturbations following low-intensity earthquakes and the potential to more than double seismic monitoring coverage in Southern California alone.

Empirical Fragility Analysis of Haitian Reinforced Concrete and Masonry Buildings

This study develops empirical fragility curves for concrete and masonry buildings in Haiti, utilizing data from the 2021 earthquake. A dataset of 3527 buildings from the StEER database, encompassing a diverse range of building types, is used. These buildings types include reinforced concrete structures with masonry infills, confined masonry buildings, reinforced masonry bearing walls, and unreinforced masonry bearing walls. Shakemaps from the USGS are utilized to assess the earthquake’s intensity at each building, with the peak ground acceleration (PGA) as the intensity measure. Damage is classified into five distinct states: no damage, minor, moderate, severe, and partial or total collapse. For each of these states, the corresponding probabilities of exceedance are calculated, and log-normal cumulative distribution functions were fitted to those data to produce empirical fragility curves. The results show a notable similarity in performance among the four types, each having high probability of failure even under low-intensity earthquakes. Total fragility curves (including all four building types) are developed subsequently and they are convolved to the probabilistic seismic hazard map of Haiti to assess the seismic risk. This includes estimating the annual probability of partial/total collapse and the probability of partial/total collapse in the event of 475-year and 2475-year earthquakes. The results indicate a significant risk, with up to 64% probability of collapse in certain areas for the 2475-year earthquake and a probability of collapse of 15% for a 475-year earthquake. These findings underscore the critical vulnerability of Haiti’s buildings to seismic events and the urgent need for their retrofit.

Experimental study on the coordinated behavior of the Combined Viscous-Steel Damping System (CVSDS) for multi-level seismic mitigation

Passive energy dissipation devices, such as the viscous fluid damper (VFD) and the steel yielding damper (SYD), are widely adopted to alleviate structural seismic responses. The Combined Viscous-Steel Damping System (CVSDS) is proposed as a hybrid passive damping device for multi-level seismic mitigation. As two components in the CVSDS, VFD, and SYD are adopted to resist frequent and rare earthquakes, respectively. This study aims to investigate and compare the seismic behavior of CVSDS, VFD, and SYD by shake table tests. Seismic excitations with different intensities are applied to a single-degree-of-freedom system. The comparison of displacement and hysteresis behavior is analyzed by time history analysis and Dynamic Time Warping (DTW) method in the time domain. The time-frequency characteristics are explored by the Wavelet Transform (WT) in the frequency domain. The results indicate that the locking function of the CVSDS remains inactive during low-intensity earthquakes, but is triggered during high-intensity ones. Locking depends on the preset locking force and external loads. Both the analyses in the time domain and frequency domain demonstrate that prior to locking, CVSDS and standalone VFD display similar characteristics, but locked CVSDS resembles standalone SYD. CVSDS allows for VFD with smaller tonnage and stronger vibration control capacity. Increased vibration frequency will be observed in the locked CVSDS during strong earthquakes due to the supplementary stiffness of the SYD.

Estimation of effective rigidity in reinforced concrete structures through modal identification tests and sclerometry

It is currently known that the structural elements of a building can crack due to structural failures under service loads, due to frequent low-intensity earthquakes or during the first moments of a severe earthquake, which could result in cracked sections when the action of the earthquake occurs. of maximum intensity, for such considerations, the Peruvian norm of seismic analysis does not contemplate cracked sections for seismic analysis. The purpose of this study was to estimate the effective stiffness factors of 4 reinforced concrete buildings. The research has a quantitative approach of non-experimental design, the sample is made up of 4 reinforced concrete buildings in the city of Juliaca, to which sclerometry and modal identification tests were carried out with X16 - 1D sensors to obtain compressive strength. and natural periods of field vibration respectively. Numerical models were obtained with their real concrete resistance considering the existing partitions, to which stiffness reduction factors will be applied using international codes and iterations to approximate their natural periods to the natural periods obtained by modal identification. The results show that there is a difference in natural periods of vibration between the numerical model with real resistances considering the effects of the partitioning and those obtained with modal identification tests in a range of 5 % to 30 %, which shows that the fundamental period field vibration is greater than projected.

Study on the performance of a novel rotation-amplified brace and its vibration control for a frame structure

A novel rotation-amplified brace (RAB) with higher energy dissipation capacity is proposed based on the bridge amplification working mechanism to solve the problem that traditional displacement-dependent braces are unable to fully perform under low-intensity earthquakes due to their small deformation. First, the basic construction and working mechanism are introduced. Then, a theoretical model is derived that can precisely predict the hysteretic characteristics of the brace and verified by numerical simulation. Additionally, the mechanical properties of the brace under different initial amplifying angles are presented. Finally, its control effect on a rotation-amplified braced frame (RABF) is studied. The numerical simulation results show that the yield-bearing capacity, maximum bearing capacity, initial stiffness and energy dissipation capacity of RAB significantly increase with decreasing initial amplification angle. Meanwhile, a smaller initial amplification angle can bring out a greater plastic deformation area of the energy-dissipation steel tube to improve the energy dissipation capacity. However, it is necessary to ensure that the ratio of the maximum tensile-compressive capacity meets the limit stipulated by the American Steel Structure Association at 1.3. It is especially noteworthy that the maximum reduction ratio of maximum interstory drift is as high as 61% for the RABF structure with braces in an inverted V-shape arrangement, which further verifies the effectiveness of RAB in improving seismic performance.

Seismic Behaviour of Multistorey Steel Framed Tall Buildings Using Intentionally Eccentric Braces

Braces with intentional eccentricity (BIE) are recently proposed to improve the seismic behaviour of conventional buckling braces (CBBs) by inserting intentional eccentricity along the brace length. Due to this eccentricity and the resultant bending moment, the BIE bends uniformly from small storey drifts and moves smoothly into the postbuckling behaviour under compression and sustains trilinear behaviour under tension. This behaviour delays the appearance of midlength local buckling which causes unstable energy dissipation. BIEs have a desirable postyielding stiffness which results in stable energy dissipation during cyclic loading and are capable of dissipating energy during low-intensity earthquakes. The seismic behaviour of structures with BIEs for use in buildings has not yet been investigated, specifically in tall buildings. Therefore, this study concentrates on investigating the seismic behaviour of tall buildings equipped with BIEs that uses a 3-dimensional (3D) finite element model in ETABS. In the first step, a 20-storey structure is designed using both eccentric brace frame (EBF) and BIE system and their seismic performance under the TABAS earthquake record is compared. In the second step, the seismic performance of a 25-storey irregular structure is assessed to evaluate the efficiency of the BIE system in irregular structures. Results show the desirable performance and energy dissipation capacity of the BIE system but it also shows large out-of-plane deformation in some cases.

Magnetotellurics (MT) and Gravity Study of a Possible Active Fault in Southern Garut, West Java, Indonesia

In recent years, dozen low-intensity earthquakes occurred in southern Garut, West Java Indonesia; two of them were reported destructive. However, those shallow earthquake clusters are hardly associated with well-known active faults in the area. Hence, we conducted 3D gravity combined with 2D magnetotellurics (MT) inversions to study the subsurface. Gravity and MT modeling confirm a basin with around 5 km depth consisting of two subbasins separated by a NE-SW trending local-high ridge. The local high coincides with the magmatic intrusion in geothermal fields and aligns with a series of volcanic bodies’ lineament observed on the surface. We interpret this structural high as a preexisting fault that serves as a magma pathway in the tectonomagmatic interaction. Shallow low-magnitude seismicity in the southern Garut area tends to occur in the resistive bodies. We interpret that heat from the cooling magmatic intrusion may decrease the effective fault-normal stress of the rocks, leading to a decrease in fault failure resistance and may initiate rupture. The resistivity structure around the initial rupture may affect whether or not the nucleation will end up as a large-magnitude earthquake. Furthermore, the unconsolidated young volcanic cover in this area could amplify ground shaking when earthquake occurs that might lead to more extensive damage.

Damage indices of steel moment-resisting frames equipped with fluid viscous dampers

ABSTRACT Deformations, accelerations, shear forces, and energy demands are normally utilized to evaluate the performance of steel frames equipped with fluid viscous dampers (FVDs). This study aims to quantify damage indices (DI) and their distributions along the structural height for steel frames (3, 6, 9, and 20 stories) equipped with FVDs. The damage model incorporating both deformation and hysteretic plastic energy is adopted to quantify damage developed in structures. A set of 20 pairs of ground motions is used to perform nonlinear time history analyses. Influences of structural property and FVD features (supplementary damping ratio and velocity power ) on DI are investigated. The results show that FVDs with lower α would reduce DI. However, nonlinear FVDs lead to higher DI compared to linear ones when >30%, particularly for 9- and 20-story structures. Nonlinear FVDs () are more effective than linear ones in reducing DI during low-intensity earthquakes. The results further demonstrate that the distribution of DI along the structural height is mostly determined by structural properties, FVD characteristics, ground motion intensity, and the type of damage model used.

Simplified framework for economic convenience of base isolation as seismic retrofit solution for existing RC buildings

Modern seismic retrofit strategies aim at increasing the building safety under high intensity earthquakes and minimizing the possible damage and, in turn the expected economic losses (EAL) under frequent and low intensity earthquakes. Classic techniques based on tradition or innovative material such as steel jacketing or FRP wrapping aims at increasing the strength/ductility of reinforced concrete (RC) members, but they do not significantly act in reducing the engineering demand parameters EDPs such as acceleration and drift. On the other side innovative techniques such as base isolation are capable of combining the increase of seismic safety with the significant reduction of acceleration and drift demand on the superstructure, thus, resulting in a significant reduction of the EALs. Although the number of applications is significantly increasing in Italy, its wide use is discouraged by the high cost of installation. Considering the annual savings related to the reduction of the EALs, the payback time (PBT) of the economic investment in a retrofitting by using base isolation can be comparable to that of other techniques. However simple tools to easily assess the PBT knowing major building characteristics are currently not available.This paper proposes a novel framework to estimate the EALs and the PBT of RC buildings retrofitted by means of FRP or Base Isolation based on the main building characteristics (i.e. in-plane geometry, number of floors, seismic). It is implemented in a MATLAB code to perform NLTHs on simplified structural models and calculate the EALs by using the full PBEE framework and considering the uncertainties in the hazard, damage and loss analysis. The framework is validated against the results of a case study building analyzed by means of a refined 3D FEM model and calculating the EAL by means of the FEMA P-58 approach. A parametric study is carried out varying the concrete compressive strength and the reinforcement details to check the sensitivity of the framework to these variables. The preliminary results are presented and discussed.

Shake table tests on a reinforced concrete waffle‐flat plate structure with new hybrid energy dissipation devices

AbstractThis paper presents a new hybrid energy dissipation device and investigates experimentally its capability to improve the seismic response of an inherently very flexible structural system: a reinforced concrete waffle‐flat plate structure. The new device combines in parallel within a single device a low‐cost viscoelastic (VE) component and a metallic yielding (MY) component. The device has a gap that prevents deformations on the MY component in the range of displacements caused by wind or low intensity earthquakes, to avoid high‐cycle fatigue damage. The MY component is expected to activate under the design or the maximum credible earthquake, providing the main structure with lateral stiffness, and a reliable and large source of energy dissipation capacity. Six new hybrid energy dissipation devices (energy dissipation system) were installed in a scaled two‐story portion of a prototype RC waffle‐flat plate structure (main structure) designed only for gravity loads, without considering special ductility detailing or capacity design rules for the columns. The VE component of the hybrid energy dissipation devices reduced the translational periods with largest effective modal mass along the horizontal directions X and Y to 60%, and increased the fraction of damping to about 12%. The main structure with the energy dissipation system was subjected to bidirectional shake table tests that represented frequent, design and maximum credible earthquakes. In the tests, the main structure remained basically undamaged under the frequent and the design earthquakes, whereas it suffered minor (reparable) damage under the maximum credible earthquake. The MY component remained undeformed under low intensity earthquakes.

Seismic response of large offshore wind turbines on monopile foundations including dynamic soil–structure interaction

This paper presents the results of a study on the dynamic and seismic response of the support structures of three reference Offshore Wind Turbines (OWT) of increasing rated power, founded to the seabed through monopile foundations. Thus, the structural behaviour of the NREL 5 MW, IEA Wind 10 MW and IEA Wind 15 MW reference OWTs under seismic input is analysed. To do so, a model based on the aero-hydro-servo-elastic OpenFAST open-software code, modified to include dynamic Soil–Structure Interaction (SSI) and input ground motion, is employed. Dynamic SSI phenomena are incorporated through lumped parameter models fitted to the impedances computed using an advanced boundary elements–finite elements model of the soil–foundation system in which the monopile is discretized as a steel pipe buried in the unbounded seabed. The fore–aft and side-to-side responses of the systems are computed under power production, parked and emergency shutdown operating conditions considering different earthquakes and arrival times. It is found that even low and moderate intensity earthquakes can produce significant increases in the structural demands of large OWTs. There exists a clear tendency for SSI to be beneficial when the size of the OWT increases.

Theoretical and experimental investigation on the seismic performance of a novel variable-damping viscous fluid damper

This paper presents a novel three-stage variable-damping-coefficient viscous fluid damper (VCVFD) and investigates its effectiveness in mitigating the seismic structural response. The concept and design of the displacement-dependent VCVFD were based on the equations for one-dimensional viscous flow in the damper orifice. A series of performance tests on VCVFDs were conducted to investigate the mechanical behaviour of the dampers with different design parameters under various loading conditions. The damping force increases more significantly than in the conventional viscous fluid damper (VFD) when the operating displacement is larger, and the expected effect of variable damping is achieved. A smaller gap height is more beneficial to energy dissipation in the VCVFD. Moreover, the hysteresis loop of the VCVFD was consistent with the theoretical results. To validate the effectiveness of the VCVFD in structural control, shaking table tests of two-storey steel frames equipped with VCVFDs under different seismic intensities were conducted. VCVFD can slightly increase the natural frequencies of the steel frame and significantly increase the structural damping ratio. The effectiveness of the VCVFD in mitigating the structural acceleration under low earthquake intensities was not prominent, but as the earthquake intensity increased, the acceleration amplification factor decreased. The added damping contributed by VCVFDs can significantly reduce the displacement response. It is concluded that with reasonable damping structural design, the VCVFD can achieve an almost similar mitigation effect as VFD in low-intensity earthquakes, whereas the VCVFD can provide higher damping under high-intensity earthquakes.

Seismic Intensity Estimation Using Multilayer Perceptron for Onsite Earthquake Early Warning

Automated early preventive measures are necessary to reduce secondary hazards in case of high-intensity earthquakes, as these have the potential to damage civil structures over a larger distance from the epicentre. The warning in case of a low-intensity earthquake can, however, disrupt human life and cause unnecessary losses. Hence, in this article, a Multilayer Perceptron-classifier is modeled to provide ensuing severity-based warning by predicting the possibility of the onsite intensity exceeding a pre-trained Peak Ground Acceleration threshold related to damaging intensities in the MMI scale. The supervised learning of the classifier utilized seismic features extracted from the strong-motion signal after the onset of the p-wave. A stratified differential feature-window resampling is applied, which enhanced the F-score of intensity class prediction from 79% to 91%. The trained model is realized on a Faster Than Real-Time (FTRT) simulation environment and tested with sliding windows for estimation of earthquake intensity classes for early warning, predicting with 89% accuracy. A seismic dataset consisting of around eight thousand non-earthquake seismic events from the National Capital Region, India, is generated using a fleet of developed seismic sensing nodes (SSNs) over a period of four years. The cross-dataset validation on this generated dataset showed 95.98% accuracy. A novel Factor of Early Warning (FoEW) metric is introduced to measure the effectiveness of warning by normalizing the achieved lead time with the corresponding maximum warning time. Mean FoEW exceeded 85% for the earthquakes with moderate to heavy damage potentials, i.e., MMI ≥ VI, suitable for onsite earthquake early warning.

Seismic fragility and post-earthquake reparability of concrete frame with low-bond high-strength reinforced concrete column

To further study the global seismic behaviour and post-earthquake reparability of RC building frames with the proposed self-centring columns with low-bond high-strength reinforcements (LBHSRs), incremental dynamic analysis (IDA) of five-floor and ten-floor frame archetypes under excitation by twenty ground motions (GMs) was performed. First, the pushover results indicated that the use of LBHSR could substantially improve the yield and ultimate lateral drift of both the archetypes, although the archetype had a smaller longitudinal reinforcement ratio (LR) of the LBHSR and similar seismic resistance. The dynamic response results indicated that the archetype with LBHSRs exhibited a smaller residual story lateral drift although the effectiveness of the use of LBHSR to reduce seismic response was not apparent for the archetype subjected to a low-intensity earthquake. The seismic fragility results showed that LBHSR was more effective for preventing seismic collapse than for attaining the immediate occupancy (IO), life safety (LS), and collapse prevention (CP) limit states. Furthermore, the higher the LR, the lower was the likelihood of seismic collapse. The fragility curves of the residual story lateral drifts indicate that the use of LBHSR can significantly mitigate the residual deformation in the DS1, DS2, and DS3 damage states. Moreover, the effectiveness increases with the increase in the LR and earthquake intensity. Comparisons of residual story lateral drifts between the predicted results and IDA results indicated that the present calculation models are not suitable for predicting residual deformation. The model needs to be studied further.

A shaking table-based experimental study of seismic response of shield-enlarge-dig type's underground subway station in liquefiable ground

In instances where earthquakes with large magnitude occur, liquefaction-induced lateral deformation of ground has caused thorough destruction to the underground structure of urban rail transit. A sequence of shaking table experiments was conducted on shield-enlarge-dig type's subway station structure in liquefiable ground exposed to the near field earthquake and the far field earthquake. Outcomes of these experiments are discussed in items of foundation deformation, pore water pressure, acceleration response, dynamic soil pressure, and macroscopic failure of model structure. The measured data infers that, when subjected to a high intensity earthquake, the model structure generates an upward movement relative to the foundation. The difference between the soil pressure above and below the structure is the internal cause of the liquefaction-induced uplift of the structure. The shear deformation to the soil layer of the foundation happened, and the displacement peak values of the left and right pendula were asymmetric. The acceleration response and its amplification coefficient in the model foundation gradually increases towards the surface from the bottom as a result of a low intensity earthquake, but for medium and high intensity earthquakes, the acceleration response decreases first and then increases. A notable phenomenon of high frequency filtering and low frequency amplification ensued during the seismic wave propagation from the bottom to the surface of soil. For the same depth of measuring points in foundation soil, the time when the pore-pressure ratio reaches the peak lags behind the time when the acceleration reaches the peak, and the hysteresis is more intense with a rise in magnitude of ground movement. The response spectrum of a low intensity earthquake is characterized by low period accumulation and amplification, while the response spectrum of medium and high intensity is characterized by moves from short period to long period and multi-peaks. The pore pressure accumulation of the model foundation experienced a change process of “rapid growth and slow dissipation,” and the time period of its sharp increase was consistent with that of the sharp increase of the acceleration arias strength. The existence of the underground structure notably inhibits an increase in pore water pressure. The earthquake damage mechanism of the underground subway station of shield-enlarge-dig type in liquefiable ground occurred in three phases: shear failure occurred on the column and damaged the opening position and spandrel of the tunnel; connection parts of the side wall and roof exhibited tensile damage; and the underground structure collapsed.

The Guadiaro-Baños contourite drifts (SW Mediterranean). A geotechnical approach to stability analysis

Two Quaternary plastered contourite drifts, with terraced and low-mounded morphologies, make up the continental slope and base-of-slope in the northwestern Alboran Sea, respectively, between the Guadiaro and Baños turbidite systems, close to the Strait of Gibraltar.Considering their significant lateral extent, the link between the contourite drift deposits and landslides may be particularly important for hazard assessment. The physical properties, composition and geometry of contourite drifts have been proposed as key factors in slope stability, although this relationship still needs to be better constrained. In this work, new in-situ geotechnical data (cone penetration tests; CPTu) has been combined with morphostratigraphic, sedimentological, and (laboratory) geotechnical properties to determine the stability of the Guadiaro-Baños drifts.For the depositional domains of both drifts, the resulting sedimentary and geotechnical model describes low-plasticity granular and silty sands on the erosive terraced domain that evolve seawards to silty and silty-clay deposits with a higher plasticity and uniform geomechanical properties. For the shallower coarse-grained contourite sediments, the cohesion (c') and internal friction angle (ϕ') values are 0–9 kPa and 46–30°, respectively, whereas for the distal fine contourites the undrained shear strength gradient (∇Su) is 2 kPa/m. These properties allow us to establish high factors of safety for all the scenarios considered, including seismic loading. Slope failure may be triggered in the unlikely event that there is seismic acceleration of PGA > 0.19, although no potential glide planes have been observed within the first 20 m below the seafloor.This suggests that the contourite drifts studied tend to resist failure better than others with similar sedimentary characteristics. The interplay of several processes is proposed to explain the enhanced undrained shear strength: 1) the geometry of the drifts, defined by an upper contouritic terrace and lower low-mounded shapes; 2) recurrent low-intensity earthquakes with insufficient energy to trigger landslides, favouring increased strength due to dynamic compaction; and 3) cyclic loading induced by solitons/internal waves acting on the sediment.

An adaptive control algorithm for smart base isolation systems based on seismic early warning system

The common control strategy of smart base isolation composed of a low-damping rubber bearing and a magneto-rheological (MR) damper is to use MR damper in semi-active form. One drawback of this strategy is increasing the acceleration of isolated structure due to adding MR damper, especially under low-intensity earthquakes. This paper proposes a new adaptive-passive control approach for smart base isolation system. In this approach, MR damper performs in passive form while its behaviour is adjusted proportionally to intensity of the incoming earthquake which can be provided by a seismic early warning system (SEWS). For comparison purposes, hybrid control system has been also designed using the common control strategy in semi-active form. Because of the lack of high accuracy of SEWS in predicting the earthquake intensity, the performance of proposed control approach has been evaluated for an error of 20% in the estimated earthquake intensity. Evaluation of the performance of these control strategies under testing earthquake records shows that adaptive-passive control algorithm works better than semi-active control algorithm in controlling the seismic response of structure and the proposed control algorithm has effective performance under a wide range of earthquake intensities.

Investigating the seismic isolation effect of the cushioned pile raft foundation in soft clay through dynamic centrifuge tests

Cushioned pile-raft foundation (Cushioned-PR) is a new type of foundation for sea-crossing bridges in deep water (more than 50 m in depth). Cushioned-PR is able to tolerate a certain amount of slide that occurs between the raft and the piles under strong earthquakes. This characteristic could reduce the seismic motion transmitted to the superstructure to maintain the integrity of the bridge. Thus, Cushioned-PR has become popular for bridges under the threat of strong earthquakes. However, the dynamic responses of the Cushioned-PR under various earthquake intensities in soft clay have not been thoroughly understood. It is essential to better evaluate the isolation effect of Cushioned-PR to improve the design. In this study, the non-linear behavior of Cushioned-PR and the isolation effect of its interposed layer are evaluated using centrifuge tests. A series of dynamic centrifuge tests are performed with a 3 × 3 pile group foundation embedded in soft clay under different earthquake motions. Both the cushioned pile raft foundation system (Cushioned-PR) and the connected pile raft foundation system (Connected-PR) are tested in the centrifuge tests. Acceleration and residual displacement of the super-structure, and bending moment of columns and piles, are monitored during the experiments. The results show that the interposed layer in Cushioned-PR has a significant impact on the bending moment and residual displacement of the pile-raft system. Based on the observation from this study, the maximum bending moment of the column in the Cushioned-PR is around 28%–54% of the values from the Connected-PR. On the other side, the maximum horizontal displacement of Cushioned-PR is about 2–10 times larger than that of the Connected-PR. The results show that Cushioned-PR can effectively reduce the seismic excitation transmitted to the super-structure if the seismic intensity is high. However, the benefit of Cushioned-PR is marginal under low-intensity earthquakes.

Ground motion duration effect on responses of hydraulic shallow-buried tunnel under SV-waves excitations

Although intensive research of the influence of ground motion duration on structural cumulative damage has been carried out, the influence of dynamic responses in underground tunnels remains a heated debate. This study attempts to highlight the importance of the ground motion duration effect on hydraulic tunnels subjected to deep-focus earthquakes. In the study, a set of 18 recorded accelerograms with a wide-range of durations were employed. A spectrally equivalent method serves to distinguish the effect of duration from other ground motion features, and then the seismic input model was simulated using SV-wave excitation based on a viscous-spring boundary, which was verified by the time-domain waves analysis method. The nonlinear analysis results demonstrate that the risk of collapse of the hydraulic tunnel is higher under long-duration ground motion than that of short-duration ground motion of the same seismic intensity. In a low intensity earthquake, the ground motion duration has little effect on the damage energy consumption of a hydraulic tunnel lining, but in a high intensity earthquake, dissipation of the damage energy and damage index of concrete shows a nonlinear growth trend accompanied by the increase of ground motion duration, which has a great influence on the deformation and stress of hydraulic tunnels, and correlation analysis shows that the correlation coefficient is greater than 0.8. Therefore, the duration of ground motion should be taken into consideration except for its intensity and frequency content in the design of hydraulic tunnel, and evaluation of seismic risk.

Enhancing Friction Pendulum Isolation Systems Using Passive and Semi-Active Dampers

Friction pendulum systems (FPSs) are a common solution for isolating civil engineering structures under ground movements. The result is a base-isolated structure in which the base exhibits low shear stiffness in such a way that the input energy of the earthquake is concentrated and dissipated into it, leaving the superstructure free of damage. As a consequence, large displacements of the FPS may be demanded depending on the earthquake intensity and the fundamental period of the FPS. To accommodate these displacements, large-size isolators with high friction coefficients are usually required. However, the FPS will then exhibit poor re-centering capacity and the risk of future shocks will increase due to previous residual displacements, especially for low-intensity earthquakes. An alternative solution is to include a semi-active damper to the FPS, keeping the friction coefficient low and achieving both, limited base displacement under high-intensity earthquakes and good re-centering capacity under low-intensity ones. Thus, this work presents a design methodology for base isolators formed by an FPS with a damper added. The design methodology is applied to an FPS with a passive damper and to an FPS with a semi-active damper. Two ON-OFF control strategies are studied: (i) a fairly simple phase control, and (ii), a mechanical energy-predictive based algorithm. The advantages of semi-active FPSs with low friction coefficients with respect to FPS with high friction coefficients are demonstrated. The results with the designed semi-active FPS are compared with the single FPS and the FPS with a passive damper. Finally, the use of semi-active FPS allows us to enhance the FPS performance as the isolator size can be reduced while keeping the capacity to cope with low and high-intensity earthquakes without residual displacements.