El Centro Record Research Articles

Influence of the Duration Compression Ratio of the Input Motion on the Seismic Response of a Soil–Pile–Bridge Structure System in Shaking Table Tests

In the shaking table test of a soil–pile–bridge structure system, it is difficult to keep the similarity relations of the model structure and that of the model soil consistent. Due to the difference of geometry and material similarity ratios for the model structure and model soil, the determination of the duration compression ratio of input motions is a key problem. The spectrum characteristics of input motions will be varied by the duration compression ratio so that the seismic responses of structure and soil system will be affected. There are three commonly used approaches to determine the duration compression ratio of input motions in shaking table tests: the time similarity ratio of model structure; the time similarity ratio of model soil; and uncompressed. To study the influence of the duration compression ratio on the seismic response of a soil–structure system in shaking table tests, the El Centro record and the Wolong record were chosen as the input motions, and the durations were compressed by the three commonly used approaches in this paper. The influence of the duration compression ratios of the input motions on the acceleration response of a soil–pile–bridge structure system was compared and analyzed through a series of shaking table tests. The results showed that the duration compression ratio affected the acceleration response of the model soil and the model structure, and the effect was more obvious when the peak ground acceleration (PGA) was small. If the research is focused on the seismic response of the soil, it is recommended to use the time similarity ratio of the model soil to compress the input motions. If the research is focused on the seismic response of the structure, it is recommended to use the time similarity ratio of the model structure to compress the input motions. This study could provide a reference for the design of the shaking table test of a soil–pile–bridge structure system.

Shaking table test on seismic responses of a wind turbine tower subjected to pulse-type near-field ground motions

Near-field records with long-period velocity pulses can be considered as a most important factor in causing structural damage. In order to understand the effects of near-field records on the seismic behavior of a 2.0 MW land-based wind turbine, a 1/20-scaled model is designed and investigated by shaking table test. Two pulse-type near-field records (Westmorland record, Chi-Chi record) and two far-field records (Taft record, El-Centro record) are chosen as the input ground motions. The dynamic characteristic, acceleration, displacement, strain and shear force of this structure are measured and compared. The experimental results show that the wind turbine remains in elastic stage during this test. The pulse-type near-field records generally has an amplification effect on the acceleration response at the bottom and top of the tower. The effects of Chi-Chi records on the acceleration and displacement response of the tower top, nacelle and hub are particularly obvious. The vertical displacements at the tower top under Chi-Chi excitation are almost 5.6 times that under El-Centro record excitation. The maximum circumferential and vertical strains appear in the middle and lower part of this tower under the horizontal earthquake excitation, which occur at 1/2–3/4 height of the tower under the vertical earthquake. This study emphasizes the importance of incorporating the pulse-type near-field effects into seismic design of wind turbines, especially for those located in near active fault.

The variable resonance magnetorheological pendulum tuned mass damper: Mathematical modelling and seismic experimental studies

Tuned mass damper technologies are progressively advancing through innovative application of smart materials, facilitating more versatile infrastructure protection. During seismic events, primarily encountered surrounding fault lines, high-rise buildings and other civil structures can suffer catastrophic failures if not adequately protected. Where traditional passive structural protection may mitigate such damage, adaptive systems which provide controllable vibration attenuation across a wide range of excitation frequencies have seen growth in use, overcoming the challenges resulting from unpredictable seismic spectrums. As a robust solution to this problem, this article presents and analyses a variable resonance magnetorheological-fluid-based pendulum tuned mass damper which employs a rotary magnetorheological damper in a controllable differential transmission to add stiffness to a swinging pendulum mass. The device is mathematically modelled based on magnetic field analysis, the Bingham plastic shear-stress model for magnetorheological fluids, and planetary gearbox kinematic and torque relationships, with the model then being validated against experimental data. The passive and semi-active-controlled performance of the device in seismic vibration suppression is then experimentally investigated using a scale five-storey building. In tests conducted with the 1985 Mexico City record, the semi-active device outperformed the (optimal) passive-on tuning, at best reducing peak displacement by 15.47% and acceleration by 28.28%, with similar improvement seen against the passive-off case for the 1940 El Centro record.

Application of EPS Geofoam to a Soil–Steel Bridge to Reduce Seismic Excitations

There have only been a limited number of analyses of soil–steel bridges under seismic and anthropogenic (rockburst) excitations. Rockbursts are phenomena similar to low-intensity natural earthquakes. They can be observed in Poland (Upper and Lower Silesia) as well as in many parts of the world where coal and gas are mined. The influence of rockbursts and natural earthquakes on soil–steel bridges should be investigated because the ground motions caused by these two kinds of excitations differ. In the present paper, a non-linear analysis of a soil–steel bridge was carried out. Expanded polystyrene (EPS) geofoam blocks were used in a numerical model of the soil–steel bridge to buffer the seismic wave induced by a rockburst (coming from a coal mine) as well as a natural earthquake (El Centro record). The analyzed soil–steel bridge had two closed pipe arches in its cross-section. The span of the shells was 4.40 m and the height of the shells was 2.80 m. The numerical analysis was conducted using the DIANA program based on the finite element method (FEM). The paper presents the FEM results of a 3D numerical study of a soil–steel bridge both with and without the application of the EPS geofoam under seismic excitations. The obtained results can be interesting to bridge engineers and scientists dealing with the design and analysis of bridges situated in seismic and mining areas.

ExactH2optimal solutions to inerter‐based isolation systems for building structures

This paper presents two inerter-based isolation systems, namely, an inerter-damper (ID)–based isolation with an inerter in parallel with a viscous damper and a tuned ID (TID)–based isolation composed of an inerter in series with a spring-damper pair. A base-isolated building with two degrees of freedom (DOF) is considered, where the superstructure above the base is simplified as a single DOF. The H2 norm performances of the ID- and TID-based isolation systems are introduced in comparison with the traditional isolation system, with the aim to minimize structure damage under random excitation from ground acceleration. The closed-form solutions are obtained, including the damping ratio of the traditional isolation, the damping ratio and the inertance-to-mass ratio of the ID-based isolation, and the damping ratio and inerter frequency tuning ratio of the TID-based isolation. It is shown that the TID-based isolation is superior to both the traditional and ID-based isolation systems for vibration control. Specifically, the optimal H2 norm of the transmission from the ground acceleration to the building relative displacement in the TID-based isolation can be reduced by 7.5%. The influence of the primary damping of the building structures is also studied numerically, in both the frequency and time domains, and compared with the exact optimal solution of the undamped structure. The simulations using recorded earthquake spectra show that the ID- and TID-based isolations can further reduce the story drift, the base displacement as much as 48.5% and 66.3% (root mean square) and 37.8% and 71.9% (peak) for the El Centro record compared with the traditional isolation system.

Shaking table tests and numerical simulations of a small radius curved bridge considering SSI effect

Focusing on a small radius curved bridge, a serious of shaking table tests and corresponding numerical finite element analyses were conducted for a 1/16 scaled curved bridge model considering SSI (Soil-Structure Interaction) effect. In the testing and simulating, the strong motion records with different frequency characteristics and durations, El Centro record from Ms6.7 Imperial Valley earthquake and Wolong record from Ms8.0 Wenchuan earthquake, were as the input motions. The dynamic property, vibration response, damage rule, and failure mode of the curved bridge were obtained preliminarily. The experimental results showed that the transverse stiffness of this soil-pile-bridge structure system was larger than the longitudinal stiffness; the obvious torsion effect of the model structure was observed in the tests under the input motion in uni-direction, and the larger was the input motion PGA, more obvious was the torsion effect; the seismic responses were more sensitive to input motions with relative low-frequency components; the damage degree of the pier bottom increased from short to high pier, and the rubber bearings at side piers appeared two different failure modes induced by the longitudinal slope. Compared with straight bridges, the curvature radius made curved bridges more prone to causing rotation and displacement. Furthermore, finite element analyses were conducted by using the plastic-damage model for concrete, and equivalent soil springs method for pile-soil interaction, respectively. The simulations were in agreement with the test results satisfactorily, moreover the differences between these results were discussed in detail and the errors were in an acceptable range. These studies would be used to provide insights for the further research and theoretical design of curves bridges.

Seismic Analysis of Steel Solid Web Girder-RC Tubular Column Hybrid Structure

This paper aims to investigate the seismic performance of a novel type of steel–concrete hybrid supporting structure consisting of reinforced concrete (RC) tubular columns, steel solid web girder platform, and A-shaped steel frames. It is typically used to house air-cooled condensers (ACC) in thermal power plants (TPPs). First, the finite-element (FE) model was implemented in ABAQUS and the simulation approaches were validated by pseudo-dynamic test results of a scaled steel-concrete hybrid supporting structure. Then, the elasto-plastic time-history analysis of the steel solid web girder-RC tubular column hybrid structure was conducted. The El Centro (NS) record was scaled to peak ground acceleration (PGA) of 0.07, 0.20, 0.40 and 0.62 g to reflect the frequent, basic, rare, and very rare earthquakes. The dynamic characteristics, base shear force, lateral deformation performance, stiffness deterioration, and damage evolution characteristics were analyzed. The numerical results showed that the first vibration mode of this hybrid structure is torsion, due to its small torsional stiffness and the nonuniform distribution characteristics of stiffness and mass in the vertical direction; the lateral deformation shape is shear mode; and the damage mainly occurred on the RC tubular columns, while the steel components did not yield under severe earthquakes. In general, the overall seismic performance of the steel solid web girder-RC tubular column hybrid structural system could meet the seismic design requirements with respect to the high-intensity earthquakes.

Seismic analysis of pile in liquefiable soil and plastic hinge

Liquefaction is one of the leading seismic actions that cause extensive damage to buildings and infrastructure during earthquakes. In many historic cases, plastic hinge formations in piles were observed at inexplicable locations. This project investigated the behaviour of piled foundations within soils susceptible to liquefaction by using numerical analysis carried out in Abaqus software in terms of plastic hinge development. Three different soil profiles were considered in this project that varied in the thickness of both the liquefiable and non-liquefiable layers, pile length and free- and fixed-head pile conditions. Modelling a single pile as a beam–column element carrying both axial and El Centro record earthquake loading produced results of the seismic behaviour of piles that could be assessed by force-based seismic design approaches. The displacements and deformations induced by dynamic loads were analysed for piles affected by liquefaction, and the results were used to demonstrate the pile capacity and discuss the damage patterns and location of plastic hinges. Parametric studies generally demonstrate that plastic hinge formation occurs at the boundaries of the liquefiable and non-liquefiable layers; however, the location can be affected by a variety of factors such as material properties, pile length and thickness of the liquefied soil layer.

Impact of artificially seismic loading on the response of building structure in various site classifications

The lack of local ground motion records has led to a direct adoption of El Centro accelerogram in time history technique as the most reliable method to observe structural responses. Program based simulations with respect to the provision of Indonesian standard were engaged to obtain artificial seismic accelerations for each site classification. Time history technique is utilized to analyze and compare the response of a dual system structure against seismic loadings in terms of maximum story displacement, base reaction, pier moment, story acceleration and story shear.Spectral matching process using Etabs yields better average spectral curves than using Seismomatch. This, however, relies upon the scaling method and number of iterations. Structural analysis results show that the artificial records of Lacc North, Friuli, Petrolia and Trinidad create extreme story displacement and story acceleration for site class B, C, D and E in that order. Artificial load of Friuli, Lucerne and Sylmarf yield the largest base reactions whereas maximum story shear is caused by the artificial ground motion of Chichi, Laccnorth, Petrolia and Trinidad for the ordered site classes. The average displacement at the top story of matched accelerogram or site B is 50% below the displacement by the original El Centro record while for site C the displacement reduces 10% and remains stabled in site D but increases 7% in site E. The base reaction falls about 20%–30% in site B, C and D and rises 14% in site E. Pier moment due to matched records decreases up to 6% as compared to the influence of reference record in all sites while story acceleration experienced 17% increase in site B. The artificial time history records adversely affect on the story shear response up to 51% higher than El Centro record. The result of F.TEST shows 77% difference between both techniques. The selection of correct, appropriate and sufficient ground motion records may produce ideal artificial accelerations and it is, therefore, profound to select such records since the possible difference may affect the final design of the building structure using linear time history analysis.

Seismic behavior of concrete filled steel tubular arch structures

Shaking table tests of a 1:10 scale arch model performed to investigate the seismic behavior and resistance of concrete filled steel tubular (CFT) arch structures are described in this paper. The El-Centro record and Shanghai artificial wave were adopted as the input excitation. The entire test process can be divided into three stages depending on the lateral brace configurations, i.e., fully (five) braced, two braces removed, and all braces removed. A total of 46 tests, starting from the elastic state to failure condition, have been conducted. The natural vibration frequencies, responses of acceleration, displacement and strain were measured. From the test results, it is demonstrated that the CFT arch structures are capable of resisting severe ground motions and that CFT arches offer a credible alternative to reinforced concrete arches, especially in regions of high seismic intensity.

Response of seismically isolated buildings with buffers subjected to near-source ground motions and possible alternative isolation systems

The response of a seismically isolated building with lead rubber bearings (LRB) to near source ground motions from large earthquakes was investigated. The building was assumed to have a buffer to limit the maximum bearing displacement in a rare event of large magnitude and the buffer gap was assumed to be only 150mm (the level of maximum isolator displacement used in the 1980s). The structure was assumed to be designed for 1.5 times the NS component of the 1940 El Centro record. The 15% damped (an amount of damping which is close to the equivalent damping ratio for an seismically isolated building at its isolator design displacement) displacement spectrum of the design motion is only 40% that of the Sylmar County Hospital Parking Lot record from the 1994 Northridge earthquake (Mw= 6.7) in the period range around the first modal periods of both isolated and un-isolated structure used in the present study. Among the near-source records that are available, the near-source Sylmar record from the 1994 Northridge earthquake was found to have a very large displacement demand in a period range of 2.0 - 3.0s and this record is thought to be a better representation of the expected near-source motions than the 1.5 times the 1940 El Centro record. Structure-buffer impact was found to impose very large inter-storey drifts and produce very large storey accelerations, when the building was subjected to the excitation of the Sylmar record. The structure-buffer impact was found to be detrimental to the structural response if the structure was not designed to provide inelastic deformation capacity, and the structural response did not improve when the gap was increased to 200-250 mm, the expected maximum displacement capacity of the LRBs used in the building. An alternative isolation system of LRBs and hysteretic dampers was investigated and found to be adequate for resisting near-source motions. A large initial damper stiffness and relatively small buffer stiffness (compared with the total initial stiffness of LRBs) were found to be effective in reducing inter-storey drifts and storey accelerations at floors except for the base and roof of the structure. A disadvantage of such a system is the relatively large base and roof accelerations. The system has relatively large inter-storey drifts and storey accelerations compared with an isolated structure using LRBs only when the structure was subjected to either the 1940 El Centro type ground motions or the Joshua Tree type ground motions with backward directivity effect. Such an isolation system would still enable the structure to respond essentially elastically under the excitation of the Sylmar record even though the isolated structure was designed for a much lower level of ground shaking. As the upper structure of a seismically isolated building is usually designed to respond essentially elastically, the detailing used in the design of a reinforced concrete structure to provide inelastic deformation capacity was generally uncommon and was not fully accounted for in the present study.

Robust gain-scheduled control for vibration suppression

Limited control authority is a key issue in the field of structural control and is a major research area since most of the practical control problems are dominated by constraints on the control signal. The paper presents a simple and practical gain-scheduled controller design procedure for active vibration suppression of a three-storey flexible structure. First, system identification experiments are performed and the plant’s uncertainty is derived. Next, robust controller design with constraint on the control signal is presented. For a better trade-off between control performance and control constraint a gain-scheduling approach is investigated. Stability analysis of the gain-scheduled controller is analysed using a parameter-independent Lyapunov function (quadratic stability) as well as a parameter-dependent Lyapunov function (biquadratic stability). Finally, the gain-scheduled controller is tested experimentally when the flexible structure is excited with a scaled historical earthquake record (1940 El Centro record). Successful experimental results show that the proposed robust gain-scheduled control approach offers good performance in the case of control authority limitation.

Robust active vibration suppression control with constraint on the control signal: application to flexible structures

AbstractA unified mathematical framework, sustained by experimental results, is presented for robust controller design taking into account the constraint on the control signal. The design procedure is exemplified for an active vibration suppression control problem with applications to flexible structures. The considered experimental set‐up is a three‐storey flexible structure with an active mass driver placed on the last storey. First, the considered flexible structure is identified and the model's parametric uncertainties are deduced. Next, control constraints are presented for the robust control design problem, taking into account the restriction imposed on the control signal. Finally, the effectiveness of the control system is tested through experiments, when the input disturbance is assumed to be a sinusoidal one as well as a historical earthquake record (1940 El Centro record). Copyright © 2003 John Wiley & Sons, Ltd.

Fault Tolerance of Semiactive Seismic Isolation

A 6-story seismically isolated structure fitted with semiactive hydraulic devices is analyzed in order to study effects of time lag in the devices and mass eccentricity in the superstructure on the lateral-torsional behavior. The computer program 3DBASIS, which allows the nonlinear dynamic analysis of 3-D structures, is used in this work. Appropriate modifications were made to the program to incorporate behavior of semiactive hydraulic devices. Three different types of base isolation systems were considered: 1) lead rubber bearings (LRBs); 2) LRBs with supplemental viscous damping; and 3) LRBs with semiactive viscous damping. A comparison of these 3 base-isolation systems, considering both effects of eccentricity in the structure and differential time lags in semiactive hydraulic devices are studied. The peak isolator shear, isolation drift, rotation, and torsional moment, are reported. Three major earthquake motion records, namely, the El Centro record of the 1940 Imperial Valley earthquake, the Meloland record of the 1979 Imperial Valley earthquake, and the Sylmar free field record of the 1994 Northridge earthquake were used as inputs in the analyses.

Robust-Controller design with hard constraint on the control signal - application for active vibration suppression of flexible structure

The paper deals with robust-controller design, as an application for active vibration suppression of flexible structures, in the case of a hard constraint on the control signal. The experimental set-up is a three-story flexible structure with an active mass driver placed on the last story. First, the flexible structure is identified, and the model's parametric uncertainties are deduced. Next, control constraints are presented for the robust-control design problem, taking into account the restriction imposed on the control signal. Finally, the effectiveness of the control system is tested through experiments where the input disturbance is assumed to be a sinusoidal one or a scaled simulation of a historical earthquake (1940 El Centro record).

Gray box identification of flexible structures: application to robust active vibration suppression control

AbstractThe paper deals with gray box identification of flexible structures and active vibration suppression from a robust control perspective. First, the linearized mathematical model of an N‐storey flexible structure is presented. Next, the generalized mathematical model is particularized for the investigated three‐storey flexible structure. The considered flexible structure is identified based on black box and gray box identification methods and the model's parametric uncertainties are deduced. Furthermore, control constraints are presented for the design problem, in case of velocity as well as acceleration feedback, from a robust control perspective. Finally, the effectiveness of the control system is tested through experiments, when the input disturbance is assumed to be a sinusoidal one as well as a historical earthquake record (1940 El Centro record). Copyright © 2001 John Wiley & Sons, Ltd.

Response of low-rise buildings to moderate ground shaking, particularly the May 1990 Weber earthquake

In the Weber earthquake of 13 May 1990 the stronger component of the ground motions recorded in Dannevirke was similar in strength to the El Centro S00E record from the 1940 Imperial Valley earthquake which underlies the New Zealand loadings code, The Modified Mercalli intensity in Dannevirke however was only about MM7 1⁄2, whereas the intensity corresponding to the 1984 earthquake code is about MM8 1⁄2 for the Dannevirke area. This paper compares the strength of the Dannevirke record in terms of spectral accelerations with (i) the above El Centro record, (ii) the Matahina dam record of the 1987 Edgecumbe earthquake, and (iii) the loadings of the 1984 and 1992 New Zealand codes. Also described in the paper are time-history analyses of one- and two- storey buildings subjected to the above ground motions in an attempt to explain why the damage levels were lower than might be expected from the strength of the recorded accelerograms. Comparisons are made of the seismic performance of moment-resisting frames and walled structures. Comments are made on two of the provisions of the 1992 loadings code.

Sensitivity of seismic response to uncertainties in restoring force model: a Monte Carlo simulation case study

A rigid slab supported by two lateral load resisting elements with random yield strength is used to evaluate the sensitivity of structural response to the uncertainty in the restoring force of these elements. The input ground acceleration consists of scaled versions of the strong-motion protion of the 1940 El Centro record. The structure is symmetric in the elastic range and behaves as a single-degree-of-freedom system. However, it can experience translational and torsional vibrations following the first yielding because of the difference between the values of the element yield strengths, caused by uncertainty. The dissipated energy and the maximum displacement, ductility, and rotation are used to quantify the response sensitivity to the uncertainty in the restoring force model. The dissipated energy is the least sensitive response measure. On the other hand, the maximum rotation of the structure can be strongly affected by this uncertainty. Therefore, special design measures are needed to limit the uncertainties in this response measure.

Seismic response of unbraced steel frames

In the analysis and design of unbraced steel frames various models are employed to represent the behaviour of beam-to-column connections. In one such model, termed here as ‘Simple Construction’, pinned connections are assumed when resisting gravity loads, whereas the same connections are assumed to be moment-resistant rigid connections when resisting lateral loads due to an earthquake or wind. Such connections are designed for moments due to lateral loads only; thus, they are not only flexible but may yield when the gravity and lateral loads act concurrently. This paper establishes the seismic performance of two (one 5-storey and the other 10-storey) unbraced steel building frames designed based on the ‘Simple Construction’ technique and on limit state principles. The first part of the paper describes briefly the design of such frames and compares their static responses with the corresponding responses of frames designed based on the ‘Continuous Construction’ assumption. Using realistic moment-rotation behaviour for flexible beam-to-column connections and realistic member behaviour, the non-linear dynamic responses of such frames for the 1940 El Centro record and 2 times the 1952 Taft record have been established using step-by-step time-history analyses. Floor lateral displacement envelopes, storey shear envelopes and cumulative inelastic rotations of beams, columns and connections are presented. The results indicate that the ‘Simple Construction’ frames experience larger lateral deflections while attracting lesser storey shears. During a major earthquake, the columns and connections of the ‘Simple Construction’ frames experience yielding, whereas in ‘Continuous Construction’ frames the beams and columns experience yielding. The cyclic plastic rotations in the connections and in the columns associated with ‘Simple Construction’ frames are found to be considerably higher.

A STATIC AND DYNAMIC NONLINEAR BEHAVIOUR OF STEEL TOWERS OF LONG-SPAN SUSPENSION BRIDGES SUBJECT TO WIND AND EARTHQUAKE LOADINGS

The static ultimate strength analyses of the tower-cable system of Akashi-Kaikyo Bridge are performed for the investigations of the planar and spatial behaviour of the structures subject to a variety of the combinations of the vertical dead and live loads, and the horizontal wind loads both in the directions of perpendicular-to-bridge axis and bridge axis. The obtained ultimate loads based on three different loading paths are compared with the design loads specified in the current codes of huge towers. The nonlinear dynamic responses of the planar tower and tower-cable models are obtained to examine the safety of the superstructures of Akashi towers against horizontal ground motions of the 1940 El Centro record with variable amplitudes up to four times the design acceleration specified in the codes.