Far-field Earthquakes Research Articles

Fuzzy Friction Controllers For Semi-active Seismic Isolation Systems

Some studies have shown that a conventional seismic isolation system may suffer from an excessive isolator displacement when subjected to a near-fault earthquake that usually has a long-period velocity pulse waveform. In order to alleviate this problem, a semi-active isolation system (SAIS) with a variable friction damper (VFD) controlled by proposed fuzzy controllers is investigated in this study. By varying the clamping force in the VFD damper, the slip force of the damper applied on the isolation system can be regulated on-line. Moreover, in order to determine the clamping force, four types of simple fuzzy controllers are developed based on the concept of antilock braking systems used in automobiles, and their isolation performances are simulated and compared. In addition, in order to assure a fair comparison, four types of earthquakes, namely far-field, weak near-fault, strong near-fault, extreme near-fault earthquakes, representing a wide variety of ground motions are considered as the ground excitations in the simulation. The numerical result shows that among the four fuzzy controllers proposed, the one that takes the ground velocity as an input variable has the best overall performance. As compared to the uncontrolled passive isolation system, the SAIS system with this controller greatly reduces the structural acceleration and base displacement responses simultaneously in a strong or extreme near-fault earthquake, whereas the controller results in a roughly equal level of acceleration response and a much less base displacement in a far-field or weak near-fault earthquake.

Mid-Column Pounding of Multi-Story Reinforced Concrete Buildings considering Soil Effects

In this paper an analysis of seismic pounding of reinforced concrete buildings with non-equal story heights including soil-structure interaction is presented. Two building configurations, configuration A: 10-story and 9-story buildings and configuration B: two 5-story buildings, are considered. The discrete model is used to incorporate foundation-soil interaction and pounding between buildings is incorporated through impact elements that consist of a gap element and a Kelvin-Voigt element. The responses of the buildings are compared in terms of impact forces, interstory displacements and normalized story shear for two near-field and two far-field earthquakes. The buildings under consideration experience the maximum impact forces and interstory displacements due to the near-field earthquakes for both fixed foundation and flexible (soil) foundation cases. Significant reduction in impact forces are observed when the underlying soil effect is considered. The maximum interstory displacements occurred when there is no pounding. In most of the cases, the consideration of soil results in lower normalized story shear.

Dynamic behavior of nonlinear rolling isolation system

The linear isolator has fixed vibration frequency. When structures with these linear isolators are located near a fault, it may cause resonance and large displacement response. Hence, nonlinear isolation may avoid this situation. In this study, an eccentric nonlinear rolling isolator with a parameter, which is the eccentricity of the pin connection of the mass block (facility) to the circular isolator, is investigated. If the eccentricity is not equal to zero, the dynamic response is nonlinear rolling behavior. The equation of motion of the isolation system is derived. The frequency of the isolator increases with the eccentricity under the same initial angle. The influence of the eccentricity to the effect of isolation is scrutinized. Finally, the feasibility of the proposed isolation device is verified numerically. If the proposed isolator is designed properly, it is effective for far-field earthquake (El Centro earthquake). Even though the linearized frequency of the proposed isolator falls into the dominant frequency range of near-fault earthquake (Chi-Chi earthquake), resonance can be avoided due to nonlinearity. Copyright © 2008 John Wiley & Sons, Ltd.

Seismic Performance of Guideway Sliding Isolator with Gap Springs for Precision Machinery

The performance of gap springs in a guideway sliding isolator (GSI) system developed to protect precision machinery against seismic motion has been studied. A spring is initially distanced from the system by a gap, causing the isolation system to exhibit nonlinear performance once the gap is closed, reducing the chance of resonance. A full-scale shaking table test of a 22-ton specimen and a numerical model simulation in SAP2000 have been performed. The study shows that springs possessing the appropriate gaps are more effective in controlling relative displacements than is a pure friction system. The optimal gap for a system subjected to far-field earthquakes was found to be 5mm. In addition, supplemental viscous damping of less than 15% of the critical damping had no significant effect on the GSI system far-field seismic response, but it did reduce the relative displacements of the system for near-fault seismic excitations.

SEISMIC SHAKING IN SINGAPORE DUE TO PAST SUMATRAN EARTHQUAKES

In 1996, the Meteorological Service of Singapore (MSS) installed a network of seven seismic stations. Nanyang Technological University (NTU) has also installed two additional seismic stations. Together, the nine stations form a network called the Singapore Array for Earthquake Response (SAFER). One of the stations installed by NTU consists of two sets of four accelerometers installed in a 66-storey commercial building for the study of building response to far-field earthquakes.This paper summarizes the research work that has been developed from the network of sensors. During the operation of the SAFER array, far-field earthquake ground motions have been recorded for many Sumatra earthquake events. From this, local site characteristics have been studied and hazard maps showing the amplified peak ground acceleration of the earthquake has been developed for the local sites. A case study for the hazard map due to the Bengkulu earthquake (Mw= 7.7) of June 4, 2000, is shown. Based on numerical studies of typical building structures in Singapore, an additional response map showing spatial variation of approximate base shear of buildings has been developed for Singapore. A case study of the response map due to the Bengkulu earthquake (Mw= 7.7) of June 4, 2000, is also shown.For future seismic hazard assessments of Singapore, a set of attenuation relationships that can reasonably predict the ground-motion intensity in Singapore generated by potential seismic sources have to be established. These attenuation relationships have to be developed using synthetic seismograms because the ground motion data that have been recorded within the last 10 years is not sufficient to develop them empirically. However, the available ground motions play a critical role in validating the synthetic attenuation relationships.The ultimate objective of this continuing research work is to incorporate a real-time monitoring system with the ground motion prediction models into hazard and response maps for scenario earthquakes. Such an integrated system when developed may assist in the planning of emergency responses to various earthquake scenarios.

Response of Nonstructural Components in Structures with Damping Systems

An analytical study of buildings without and with seismic damping systems was conducted to investigate the dynamic response of secondary systems. The buildings were represented by steel frames designed by the procedures described in the 2000 NEHRP Recommended Provisions. Nonlinear response analyses were performed for the undamped frames and the damped frames equipped with different energy dissipation systems and for far-field, near-field, and soft-soil earthquake histories. The study shows that significant benefits may be provided by viscous damping systems in terms of reduced floor spectral accelerations and floor absolute velocities.

Resistance of a Portal Frame Bridge Pier Under Earthquake Waves from Arbitrary Directions

Steel portal frames are used in Japan as basic structural frames for viaducts in metropolitan areas. These structures may be subject to both near-field and far-field earthquake motions from all directions. Although for isolated single piers the earthquake-resistant design appears to be well established, the earthquake response and the resistance are not fully clarified for portal frames. A study on elastoplastic finite displacement analysis by a finite element model for earthquake resistance of steel portal frame piers of an elevated highway is presented. The incidental earthquake waves are assumed to come from arbitrary directions; for simplicity, the analysis considers five different directions covering the in-plane of the frame (0 deg from the transverse direction of the bridge axis) and the out-of-plane directions (30 deg, 60 deg, 75 deg and 90 deg). Some results were compared with those from experiments on portal rigid-frame bridge piers under in-plane loading conducted at Kyoto University. It was found that it is important to take the correlation of in-plane and out-of-plane responses into consideration for the rational earthquake-resistant design of a portal frame bridge pier.

Piezoelectric friction dampers for earthquake mitigation of buildings: design, fabrication, and characterization

In this paper, the design, fabrication and characterization of a piezoelectric friction damper are presented. It was sized with the proposed practical procedure to minimize the story drift and floor acceleration of an existing 1/4-scale, three-story frame structure under both near-fault and far-field earthquakes. The design operation friction force in kip was numerically determined to range from 2.2 to 3.3 times the value of the peak ground acceleration in g (gravitational acceleration). Experimental results indicated that the load-displacement loop of the damper is nearly rectangular in shape and independent of the excitation frequency. The coefficient of friction of the damper is approximately 0.85 when the clamping force on the damper is above 400 lbs. It was found that the friction force variation of the damper generated by piezoelectric actuators with 1000 Volts is approximately 90% of the expected value. The properties of the damper are insensitive to its ambient temperature and remain almost the same after being tested for more than 12,000 cycles.

Novel Semiactive Friction Controller for Linear Structures against Earthquakes

Semiactive friction dampers (SAFD) have been shown to be more effective than passive friction dampers in reducing structural response due to earthquakes. The effectiveness of SAFD, however, depends on the control strategy used. Because of the nonlinear nature of friction dampers, the establishment of an effective control strategy is a challenging effort. The motion of friction dampers, either passive or semiactive, involves sticking and slipping phases. The idea for increasing the performance of SAFD is to maintain its motion in the slipping phase as much as possible, as energy is dissipated during the slipping phase rather than the sticking phase. In this paper, the authors propose control strategies which are capable of: 1) maintaining the motion of SAFD in the slipping phase as much as possible; and 2) eliminating the undesirable acceleration spikes by introducing an appropriate boundary layer in the control strategy. Two structures, a 6-story base-isolated structure and a 3-story fixed base building model, were used to demonstrate the performance of the proposed control strategies using different far-field and near-field earthquakes. Further, the performances of various combinations of passive and semiactive energy dissipation devices have been evaluated and compared. Based on numerical simulation results, it is demonstrated that the proposed semiactive friction control strategies are very effective.

An experimental study of nominally symmetric and asymmetric structures isolated with the FPS

AbstractThis experimental investigation deals with the earthquake behaviour of a nominally symmetric and a mass‐asymmetric three‐storey structural model isolated with the frictional pendulum system (FPS). Both accidental and natural torsion are evaluated in the structure by using recorded accelerations in all building floors and measured deformations at the isolation level. A 3D‐shaking table was used to subject the model to five different ground motions, including impulsive as well as far‐field subduction‐zone type earthquakes. Results show that the analytical predictions of the earthquake behaviour of the isolated structure, as obtained from a physical model of the FPS, are in close agreement with the true complex inelastic measured behaviour of the FPS. Besides, experimental results also validate previous observations about the importance of accounting for the variability of the normal loads in modelling the earthquake behaviour of FPS isolators. Measured torsional deformation amplifications at the base of the building vary, in the mean, from 2.5% to 6% for the symmetric and asymmetric structural configurations, respectively. In relation to the fixed base structure, the reduction factors for the base shear of the isolated structure are, in the mean, about 3.9 for both configurations. Finally, it is concluded that the FPS is capable of controlling the lateral–torsional motions of mass‐asymmetric structures quite effectively by aligning the centre of mass of the superstructure with the centre of pendular and frictional resistance of the isolation system. Copyright © 2003 John Wiley & Sons, Ltd.

Response spectrum predictions for potential near-field and far-field earthquakes affecting Hong Kong: soil sites

This paper considers the modelling of design-level earthquake response spectra for soil sites in Hong Kong. It forms part of a series of connected studies aimed at quantifying the detailed characteristics of seismic activity and design ground shaking levels in Hong Kong and its surrounding region. This series of papers should appeal to a broad international readership interested in adopting a methodology to determine seismic demand for infrastructure in megacities located within a low or moderate seismic region. The peak velocity and displacement demands (soil RSV max and soil RSD max, respectively) have been obtained from soil response analyses for numerous representative soil column models. The comparison of the soil RSV max between individual soil columns displays interesting similarities for a given earthquake scenario. A unique upper bound RSV max value has been identified for each probability of exceedence (PE). The predicted soil RSV max was found to be relatively insensitive to the lengthening of return period. Another significant trend is the excellent linear correlation between the peak displacement demand (soil RSD max) and the site natural period ( T s). A simple relationship has accordingly been developed to determine the soil RSD max from the given soil RSV max and T s. Maximum soil amplification factors (spectral ratios) in the order of 5–6 have been observed in the present study, for Hong Kong soil sites resonating under the action of bedrock shaking induced by distant, large magnitude earthquake events associated with a range of PE. Previous studies have grossly underestimated this factor, due to inaccurate representation of the bedrock motions generated by far-field earthquakes and due to underestimation of the soil-rock impedance contrast. Furthermore, codified estimates of site amplification may significantly underestimate the effect of resonance, due to distortion introduced by various forms of idealisation and normalisation when dealing with the soil and bedrock response spectra.

Experimental Verification of Seismic Response of Building Frame with Adaptive Sliding Base-Isolation System

The paper presented herein investigates the ability of an adaptive seismic isolation system to protect structures subjected to disparate earthquake ground motions. The isolation system consists of sliding isolation bearings in combination with an adaptive hydraulic damper. The damping capacity of the hydraulic damper can be modified in real time to respond to the effects that the earthquake ground motion has on the structure. An experimental laboratory implementation of the adaptive isolation system within a scale-model building structure is described. Analytical models of the isolation system components and the test structure are developed and calibrated through experimental system identification tests. Results from experimental shaking table tests are then used to validate the results from numerical simulations which utilized the analytical models. Although the adaptive base-isolation system results in a complex nonlinear dynamic system, the analytical predictions agreed reasonably well with the experimental test data. The experimental and analytical results demonstrate that, for both near-field (pulse-type) and far-field earthquake ground motions, an adaptive sliding base isolation system is capable of reducing the interstory drift response of structures while simultaneously limiting the displacement response of the isolation system.

“Smart” Base Isolation Strategies Employing Magnetorheological Dampers

One of the most successful means of protecting structures against severe seismic events is base isolation. However, optimal design of base isolation systems depends on the magnitude of the design level earthquake that is considered. The features of an isolation system designed for an El Centro-type earthquake typically will not be optimal for a Northridge-type earthquake and vice versa. To be effective during a wide range of seismic events, an isolation system must be adaptable. To demonstrate the efficacy of recently proposed ''smart'' base isolation paradigms, this paper presents the results of an experimental study of a particular adaptable, or smart, base isolation system that employs magnetorheological ~MR! dampers. The experimental structure, constructed and tested at the Structural Dynamics and Control/Earthquake Engineering Laboratory at the Univ. of Notre Dame, is a base-isolated two-degree-of-freedom building model subjected to simulated ground motion. A sponge-type MR damper is installed between the base and the ground to provide controllable damping for the system. The effectiveness of the proposed smart base isolation system is demonstrated for both far-field and near-field earthquake excitations.

Response spectrum predictions for potential near-field and far-field earthquakes affecting Hong Kong: rock sites

To realistically assess the seismic risk relating to built infrastructures in Hong Kong and in the neighbouring coastal cities of southern Guangdong province, it is necessary to predict ground shaking induced by different earthquake scenarios with good accuracy. A companion paper has described the modelling of the spatial and temporal distribution of the diffused seismic activities in the region, based on the newly-developed ‘Expanding Circular Disc’ (ECD) method. Representative Magnitude–Distance (M–R) combinations for both near-field and far-field earthquakes (in relation to Hong Kong) have been derived using the ECD method. The present paper describes the modelling of the response spectrum on rock sites associated with the predicted M–R combinations, using the Component Attenuation Model (CAM) that was also developed recently by the authors, based on stochastic simulations of the seismological model. The significant effects of soil resonance on the response spectrum are described in a separate publication. The accuracy of CAM in modelling ground motion properties on rock sites has been tested here by comparisons with (i) strong motions recorded in Taiwan and South China from the 1999 ‘Chi-Chi’ earthquake in Taiwan ( M=7.6), (ii) motions recorded in South China from another earthquake occurring in the southern Taiwan Strait in the same year ( M=5.1), and (iii) historical seismic intensity data obtained within South China. The overall capability of CAM in modelling both near-field and far-field attenuation has been shown to be unmatched by existing empirical models. Results of the comparison studies confirm the accuracy of CAM, particularly within an epicentral distance of 300–400 km. This study shows that the developed serviceability response spectra (i.e. at short return periods) are controlled mainly by the earthquake recurrence behaviour of major distant seismic sources. In contrast, the ultimate response spectra (i.e. at long return periods) relate to events with magnitudes close to the maximum credible earthquake (MCE) limit, the effect of which may also be represented by the Characteristic Response Spectrum (CRS). Both types of earthquake scenario can be significantly affected by the regional crustal properties. The proposed response spectrum envelopes have been compared with previously developed recommendations, and a critical review has been conducted. The intrinsic advantages of the ECD–CAM modelling approach have been highlighted, emphasising its directness and transparency when compared with the more complex process required to implement traditional Probabilistic Seismic Hazard Assessment (PSHA).

Scenario predictions for potential near-field and far-field earthquakes affecting Hong Kong

Historical earthquakes noted in the written records of the South China region, including Hong Kong, are not well delineated along identified prominent fault sources. Despite the lack of any definitive, localised trend in the spatial distribution of seismic activity in the region, there does appear to be some major disparity in the seismic activity rates (especially for large magnitude earthquakes) between the near-field and the far-field regions of Hong Kong. Despite this observation, previous studies of the regional seismic activity and seismic ground motion hazard (the latter using a probabilistic seismic hazard assessment, PSHA) have considered very broad source zone regions, in which uniform levels of seismic activity have been assumed. The present paper further scrutinises this broad source zone (BSZ) approach by adopting a novel expanding circular disc (ECD) method to determine the rates of earthquake recurrence. Such a method is intended to counter-check previously developed models by determining earthquake scenario events in terms of magnitude–distance (M–R) pairs or combinations, having defined values of average return period. Unlike the BSZ approach, the ECD method specifically accounts for the supposed variations in the seismic activity rates between events in the near-field and the far-field of Hong Kong. The form of the developed method is particularly suited to the determination of design-level earthquake ground motions for bedrock sites, since it assumes a directionally-independent attenuation model as described in the companion paper. It is found that, whilst the BSZ approach may indicate the overall average levels of hazard that are representative of the South China region as a whole, it does not capture the large disparity in seismic activity rates between near-field and far-field events. This important feature is expected to have a significant impact upon engineering assessments of the seismic safety of structures in Hong Kong and elsewhere in the South China region. For example, it is found that for events with M≥6, the seismic activity rate (normalised by time and area) in the very far-field is around 3.5 times larger than in the near-field and medium-field of Hong Kong. The resulting design M-R combinations, covering a range of return periods from 70 to 2500 years, are limited, for very long return periods and for distant events, by the maximum credible earthquake (MCE) magnitude. Intensive research to determine this seismic hazard parameter is recommended, in order to refine further the results of the ECD analysis, which presently conservatively assumes the MCE to range between M=6 in the near-field of Hong Kong to M=8 in the very far-field, at distances greater than 280 km from Hong Kong.

Earthquake responses of RC moment frames subjected to near‐fault ground motions

AbstractThere are three objectives in this paper. The first objective is to compare the dynamic behaviour of a reinforced concrete building structure subjected to near‐fault and far‐field ground motions. A twelve‐storey and a five‐storey reinforced concrete building with moment resisting frames were selected in this study. The Chi‐Chi earthquake was selected as a first set in this study to test near‐fault earthquake characteristics. Further, another earthquake record of an event at the same site was selected to test the far‐field earthquake characteristics for comparison. Through nonlinear time history analyses, the results show that the near‐fault earthquake results in much more damage than the far‐field earthquake. The second objective of this paper is to compare the predictions for ductility demand by the nonlinear time history analyses with those obtained by the pushover analysis procedure. The third objective is to explore the parameters that will more significantly affect the the building structure's dynamic response characteristics of base shear reduction and displacement amplification. Copyright © 2001 John Wiley & Sons, Ltd.

EVALUATION OF SEISMIC DEMANDS IN A CONTINUOUS BRIDGE

This paper examines the dynamic behavior of a highway RC-bridge subjected to both near-fault and far-field ground motions. The bridge consists of a hinge supported continuous girder with six concrete piers and the bridge is designed according to the Taiwan seismic design code. To investigate the hysteretic behavior of the bridge piers, cyclic loading tests were carried out at the National Center for Research on Earthquake Engineering (NCREE). The Chi-Chi earthquake ground motion record was adopted as the near-fault earthquake characteristics whereas another earthquake record was selected for the far-field earthquake characteristics. The ductility demands and base shear demands due to the near-fault and the far-field earthquake ground motions are compared and conclusions drawn from the study. The stipulation of code limitations and the present calculated demands are discussed.

Abnormal historic sea-surface fluctuations, SW England

Abstract This paper presents a series of documentary accounts of abnormal sea surface changes that have affected the coastline of SW England during historic times. An attempt is made to consider possible origins for individual events although it is recognised that certain episodes of coastal flooding appear to have no simple explanation. However, certain coastal floods appear to have been generated by tsunamis triggered by far-field offshore earthquakes.

Elastic-wave propagation and site amplification in the Salt Lake valley, Utah, from simulated normal faulting earthquakes

AbstractThe two-dimensional seismic response of the Salt Lake valley to near- and far-field earthquakes has been investigated from simulations of vertically incident plane waves and from normal-faulting earthquakes generated on the basin-bounding Wasatch fault. The response to normal faulting earthquakes was simulated using a two-dimensional finite-element method and the plane-wave response was calculated from two-dimensional finite-difference simulations. The plane-wave simulations were then compared with observed site amplifications in the Salt Lake valley, based on seismic recordings from nuclear explosions in southern Nevada, that show 10 times greater amplification within the basin than measured values on hard-rock sites. While previous studies attribute this increased site amplification to the near-surface unconsolidated/consolidated alluvial fill contact, our synthetic seismograms suggest that in the frequency band 0.3 to 1.5 Hz at least one-half the site amplification can be attributed to the impedance contrast between the basin sediments and higher velocity basement rocks. Synthetic seismograms from vertically incident plane-wave sources and buried double-couple sources predict large amplitude Rayleigh-wave propagation from the edges of the basin and, in general, uniform site amplification. In contrast, near-field simulations of basin-bounding, normal-faulting earthquakes predict large-amplitude Rayleigh waves propagating westward from the fault across the basin. Spectra of synthetic accelerograms computed from the normal-faulting earthquakes shows that spectral amplification within the basin is primarily due to source directivity with a maxima near the surface projection of the fault that decays rapidly away from the fault. Importantly, the synthetic modeling of near-field earthquake sources show that near-field directivity effects are important and should be considered in an earthquake hazard assessment of the Salt Lake valley and similar geologic settings along the Wasatch Front.