Showing papers by "National Center for Research on Earthquake Engineering published in 2013"

Optimal design formulas for viscous tuned mass dampers in wind‐excited structures

Abstract: SUMMARY Optimal design for tuned mass dampers (TMDs) with linear or nonlinear viscous damping is formulated in order for design practitioners to directly compute the optimal parameters of a TMD in a damped structure subjected to wind excitations. The optimal TMD tuning frequency ratio and damping coefficient for a viscous TMD system installed in a damped structure under 10 white noise excitations are determined by using the time-domain optimization procedure, which minimizes the structural response. By applying a sequence of curve-fitting schemes to the obtained optimal values, design formulas for optimal TMDs are then derived. These are expressed as a function of the mass ratio and damping power-law exponent of the TMD as well as the damping ratio of the structure. The feasibility of the proposed optimal design formulas is verified in terms of formulary accuracy and of comparisons with existing formulas from previous research works. In addition, one numerical example of the Taipei 101 building with a nonlinear TMD, which is redesigned according to the proposed optimal formulas, is illustrated in effort to describe the use of the formulas in the TMD design procedure and to investigate the effectiveness of the optimal TMD. The results indicate that the proposed optimal design formulas provide a convenient and effective approach for designing a viscous TMD installed in a wind-excited damped structure. Copyright © 2011 John Wiley & Sons, Ltd.

Optimal design of friction pendulum tuned mass damper with varying friction coefficient

Abstract: SUMMARY Tuned mass dampers with viscous damping and isolation systems with friction pendulums have been proposed and widely applied over the past several decades. By combining these two ideas, a friction pendulum tuned mass damper (FPTMD) is proposed in this study. Because the restoring and friction forces are provided by the spherical surface of the FPTMD, springs and dampers are not needed. Moreover, suspension is not necessary and the installation space is greatly reduced. The optimal design of the FPTMD with varying friction coefficients for wind-excited high-rise structures is investigated. The optimization procedures are demonstrated by the FPTMD implemented on Taipei 101 under white-noise wind force, and the optimization results are validated by three-dimensional graphs. From the results of the sensitivity study, the effectiveness of the FPTMD with two different patterns of friction coefficients is sensitive to the tuning frequency ratio but not very sensitive to the friction parameters. Moreover, an FPTMD with a friction coefficient that linearly varies with displacement is even less sensitive to the friction parameters and the amplitude of excitation. The feasibility of the FPTMD with two different patterns of friction coefficient is illustrated by Taipei 101 subjected to the design wind force with a return period of 50 years. Following design optimization and numerical verification, the effect of vibration reduction for Taipei 101 is demonstrated. Copyright © 2012 John Wiley & Sons, Ltd.

Dual compensation strategy for real‐time hybrid testing

Abstract: SUMMARY Real-time hybrid testing is an experimental technique for evaluating the dynamic responses of structural systems under seismic loading. Servo-hydraulic actuators, by nature, induce inevitable time delay between the command and the achieved displacements. This delay would lead to incorrect test results and even cause instability of the system; therefore, delay compensation is critical for stability and accuracy of hybrid simulations of structural dynamic response. In this paper, a dual delay compensation strategy is proposed by a combination of a phase lead compensator and a restoring force compensator. An outer-loop feed-forward phase lead compensator is derived by introducing the inverse model in the z domain. The adaptive law based on the gradient algorithm is used to estimate the system delay in the format of parametric model during the test. It is shown mathematically that the parameter in the delay estimator is guaranteed to converge. The restoring force compensator is adopted to improve the accuracy of experimental results especially when the structure is subjected to high frequency excitations. Finally, analytical simulations of an inelastic SDOF structure are conducted to investigate the feasibility of the proposed strategy. The accuracy of the dual compensation strategy is demonstrated through several shaking table tests. Copyright © 2012 John Wiley & Sons, Ltd.

Anomalous frequency characteristics of groundwater level before major earthquakes in Taiwan

Abstract: . Unusual decreases of water levels were consistently observed in 78% (= 42/54) of wells in the Choshuichi Alluvial Fan of central Taiwan about 250 days before the Chi-Chi earthquake (M = 7.6 on 20 September 1999) while possible factors of barometric pressure, earth tides, precipitation as well as artificial pumping were removed. Variations in groundwater levels measured on anomalous wells from 1 August 1997 to 19 September 1999, which covers the 250 day unusual decreases, were transferred into the frequency domain to unveil frequency characteristics associated with the Chi-Chi earthquake. Analytical results show that amplitudes at the frequency band between 0.02 day−1 and 0.04 day−1 generally maintained at the low stage and were apparently enhanced a few weeks before the Chi-Chi earthquake. Variations of amplitude at this particular frequency band were further examined along with other Taiwan earthquakes (M > 6) from 1 August 1997 to 31 December 2009. Features of the enhanced amplitudes at the frequency band are consistently observed prior to the other two earthquakes (the Rei-Li and Ming-Jian earthquakes) during the 12.5 yr study period. In addition, surface displacements recorded from GPS, which provides insights into understanding stress status in subsurface during the Chi-Chi earthquake, are also inspected. The result confirms that abnormal rise and fall changes in groundwater level yield an agreement with forward and backward surface displacements around the epicentre prior to the Chi-Chi earthquake.

Evaluation of seismo-electric anomalies using magnetic data in Taiwan

Abstract: . The Parkinson vectors derived from 3-component geomagnetic data via the magnetic transfer function are discussed with respect to epicentre locations and hypocentre depths of 16 earthquakes (M ≥ 5.5) in Taiwan during a period of 2002–2005. To find out whether electric conductivity changes would happen particularly in the seismoactive depth ranges, i.e. in the vicinity of the earthquake foci, the frequency dependent penetration depth of the electromagnetic waves (skin effect) is taken into account. The background distributions involving the general conductivity structure and the coast effect at 20 particular depths are constructed using the Parkinson vectors during the entire study period. The background distributions are subtracted from the time-varying monitor distributions, which are computed using the Parkinson vectors within the 15-day moving window, to remove responses of the coast effect and underlying conductivity structure. Anomalous depth sections are identified by deviating distributions and agree with the hypocentre depths of 15 thrust and/or strike-slip earthquakes with only one exception of a normal fault event.

Analytical and experimental studies on midstory isolated buildings with modal coupling effect

Abstract: SUMMARY In a midstory isolated building, the isolation system is incorporated into the midstory rather than the base of the building. Because of the flexibility of the substructure below the isolation system in a midstory isolated building, the contribution of higher modes to the seismic responses of the midstory isolated structure may not be negligible, especially when the coupling of higher modes exists. To investigate this modal coupling effect, a simplified three-lumped-mass structural model of the midstory isolated building is assumed in this study. Through the equivalent linear analysis and shaking table tests, it is found that the coupling of higher modes may lead to enlarged acceleration responses at the super-floor and superstructure above the isolation layer. Accordingly, a simple method to prevent the midstory isolation design from the coupling of higher modes attributed to the improper design of the substructure and superstructure is proposed in this paper. Copyright © 2012 John Wiley & Sons, Ltd.

Use of centrifuge experiments and discrete element analysis to model the reverse fault slip

Abstract: This study presents a series of physical model tests and numerical simulations using PFC 2D (both with a dip slip angle=60° and a soil bed thickness of 0.2 m in model scale)at the acceleration conditions of 1g, 40g, and 80 g to model reverse faulting. The soil deposits in prototype scale have thicknesses of 0.2 m, 8 m, and 16 m, respectively. This study also investigates the evolution of a surface deformation profile and the propagation of subsurface rupture traces through overlying sand. This study proposes a methodology for calibrating the micromechanical material parameters used in the numerical simulation based on the measured surface settlements of the tested sand bed in the self-weight consolidation stage. The test results show that steeper surface slope on the surface deformation profile, a wider shear band on the major faulting-induced distortion zone, and more faulting appeared in the shallower depths in the 1-g reverse faulting model test than in the tests involving higher-g levels. The surface deformation profile measured from the higher-g physical modeling and that calculated from numerical modeling show good agreement. The width of the shear band obtained from the numerical simulation was slightly wider than that from the physical modeling at the same g-levels and the position of the shear band moved an offset of 15 mm in model scale to the footwall compared with the results of physical modeling.

Behavior of oblong and rectangular bridge columns with conventional tie and multi-spiral transverse reinforcement under combined axial and flexural loads

Abstract: This study investigates the behavior of transversely reinforced oblong and rectangular bridge columns under combined axial and flexural loading, including eccentric compression and lateral cyclic loading under constant axial load. The transverse reinforcement schemes include conventional tie and multi-spiral reinforcement. The multi-spiral reinforcement for the oblong column comprises two interlocking spirals and, for rectangular columns, comprises two interlocking large central spirals interlocked with four small spirals at the corners. The amount of transverse reinforcement for all of the columns conforms to the current seismic bridge design specifications. Test results indicate that all of the columns exhibit ductile behavior with ductility capacities significantly higher than the ductility capacity required by the design code. The oblong spiral column with an amount of transverse reinforcement 43% that of the corresponding tied column shows strength, ductility, energy dissipation, and over-str...

Earthquake response analyses of a full‐scale five‐story steel frame equipped with two types of dampers

Abstract: SUMMARY The seismic performance tests of a full-scale five-story passively controlled steel building were conducted on the E-Defense shaking table in Japan in March 2009. Before the tests, a blind prediction contest was held to allow researchers and practitioners from all over the world to construct analytical models and predict the dynamic responses of the steel frame specimen equipped with buckling-restrained braces (BRBs) or viscous dampers (VDs). This paper presents the details of two refined prediction models made and results obtained before the tests. When the proposed analytical modeling techniques are adopted as in the two refined prediction models, the overall prediction accuracy is about 90%. Sensitivity studies conducted after the tests are also presented in this paper. The effects of varying each modeling feature on the response simulation accuracy have been investigated. The analytical results suggest that considering concrete full-composite actions for beam members could improve prediction accuracy by about 20% against using the simplified bare steel beam model. Adopting refined BRB stiffness computed from incorporating finite-element gusset stiffness only improves the overall prediction accuracy by 0.9%. Considering the BRB dynamic loading test results for analytical BRB strength reduces the error by 1.9%. For the VD frame, incorporating the brace and VD stiffness could improve the overall prediction accuracy by about 15%. Copyright © 2012 John Wiley & Sons, Ltd.

A novel lifetime cost-benefit analysis method for seismic retrofitting of low-rise reinforced concrete buildings

Abstract: This article presents a novel simplified method for assessing seismic damage to low-rise reinforced concrete (RC) buildings by using the hazard curve of response spectral acceleration. Moreover, the occurrence of an earthquake is assumed to follow a Poisson process when analysing the occurrence probability of a specified damage state in the remaining service life and expected losses induced by seismic damage. Then, a novel procedure for estimating lifetime costs and benefits of seismic retrofitting is proposed. In the case study, 16 practical design projects for seismic retrofitting of RC school buildings in Taipei are subjected to lifetime cost-benefit analysis using the proposed method. It can be found that not only lifetime cost-benefit ratios but also the financial return period for each dollar invested seismic retrofitting can be identified conveniently. Additionally, they are useful information for making decisions about whether to retrofit a building or not.

A frequency response function change method for damage localization and quantification in a shear building under ground excitation

Abstract: A frequency response function change (FRFC) method to detect damage location and extent based on the change in the frequency response functions of a shear building under the effects of ground excitation was proposed in this paper. The damage identification equation was derived from the motion equations of the system before and after the occurrence of the damage. Efforts to make the FRFC method less model-dependent were made. Intact system matrices, which could be estimated using the measured data without the need for an analytical model, and the frequency response functions were required for the FRFC method. The effects of measurement noise and model parameter error in the FRFC method were studied numerically. The proposed FRFC method was validated by experimental studies of a six-story steel building structure with single and multiple damage cases. Copyright © 2012 John Wiley & Sons, Ltd.

Soil gas radon–thoron monitoring in Dharamsala area of north-west Himalayas, India using solid state nuclear track detectors

Abstract: The study described here is based on the measurements of soil gas radon–thoron concentrations performed at Dharamsala region of north-west (NW) Himalayas, India. The study area is tectonically and environmentally significant and shows the features of ductile shear zone due to the presence of distinct thrust planes. Solid state nuclear track detectors (LR-115 films) have been used for the soil gas radon–thoron monitoring. Twenty five radon–thoron discriminators with LR-115 films were installed in the borehole of about 50 cm in the study areas. The recorded radon concentration varies from 1593 to 13570 Bq/m3 with an average value of 5292 Bq/m3. The recorded thoron concentration varies from 223 to 2920 Bq/m3 with an average value of 901 Bq/m3. The anomalous value of radon–thoron has been observed near to the faults like main boundary thrust (MBT and MBT2) as well as neotectonic lineaments in the region.

Structural Damage Assessment Using Output-Only Measurement: Localization and Quantification

Abstract: One of the important issues to conduct the damage detection of a structure using vibration-based damage detection (VBDD) is not only to detect the damage but also to locate and quantify the damage. In this paper a systematic way of damage assessment, including identification of damage location and damage quantification, is proposed by using output-only measurement. Four level of damage identification algorithms are proposed. First, to identify the damage occurrence, null-space and subspace damage index are used. The eigenvalue difference ratio is also discussed for detecting the damage. Second, to locate the damage, the change of mode shape slope ratio and the prediction error from response using singular spectrum analysis are used. Finally, to quantify the damage the RSSI-COV algorithm is used to identify the change of dynamic characteristics together with the model updating technique, the loss of stiffness can be identified. Experimental data collected from the bridge foundation scouring in hydraulic lab was used to demonstrate the applicability of the proposed methods. The computation efficiency of each method is also discussed so as to accommodate the online damage detection.Copyright © 2013 by ASME

Vibration-based damage identification of reinforced concrete member using optical sensor array data:

Abstract: To detect and locate the structural damage from direct measurements can be done only when the sensors are very closely located to the damage initiating point, which is generally impossible to predict, particularly for the reinforced concrete structures. With the availability of high-resolution distributed sensing, using optical tracker on light targets, the damage location as well as the level of damage can be identified. The objective of this article is to conduct structural system identification of a reinforced concrete frame by using the proposed structural integrity index (identify the element curvature and null-space damage index) and the estimation of finite element strain. Finally, a discussion on the identified time-varying system natural frequency and stiffness/strength degradation of the reinforced concrete structure from global measurement in relation to the calculated structural integrity index using optical sensing array data and element strain on the identification of damage location and dam...

A Hamiltonian state space approach to anisotropic elasticity and piezoelasticity

Abstract: We present a Hamiltonian state space approach for problems of anisotropic elasticity and piezoelasticity. By means of Legendre’s transformation, the basic equations of piezoelasticity are formulated into a state equation and an output equation in terms of the state vector that comprises the generalized displacement vector and the conjugate generalized traction vector as the dual variables. The Hamiltonian features and symplectic orthogonality of the system, which are essential for the solution approach using eigenfunction expansion, are delineated at length. We show that the solution to 3D problems of a prismatic body hinges upon a 2D Hamiltonian eigensystem and the eigensolution associated with the zero eigenvalue leads to the solution to the generalized plane problem naturally. Based on the formalism, the solution to a problem of piezoelasticity is no more difficult than its elastic counterpart.

Full-scale testing of self-centering steel plate shear walls

Abstract: This paper presents the results of a self-centering steel plate shear wall (SC-SPSW) experimental program conducted at the National Center for Research on Earthquake Engineering (NCREE) as part of a collaborative research endeavor. Two full-scale two-story SC-SPSW specimens were tested under pseudo-dynamic loading. The specimens investigated two different post-tensioned (PT) beam-to-column connection configurations—one using a PT connection detail where a gap forms in a connection as the beam rocks about its flanges, and one using a PT connection (called the NewZBREAKSS connection) where the beam in a connection always rocks about its top flanges, thus eliminating the problem of frame expansion. The test specimens also incorporated a post-tensioned column base connection that allowed the column to rock about its flanges, relying on vertical post-tensioned rods anchored along the column height. The PT column base provides additional recentering capabilities, as well as eliminates the damage and residual plastic deformations that occur in the moment resisting base connections of SC-SPSWs. The results from this project will be used to validate numerical models and inform construction and design recommendations.

Dynamics, control and real-time issues related to substructuring techniques: application to the testing of isolated structure systems:

Abstract: This article presents the designs of output-based adaptive and numerical-substructure-based controllers for the testing of a base-isolated substructured system. The linear numerical-substructure-based controllers, which are developed using state-space and transfer-function methods and a higher-order actuator model, are introduced in order to address new substructured eigenvalue techniques in relation to testing stability and accuracy. An output-based framework for the synthesis of new adaptive substructuring controllers and for the associated stability proof is discussed, based upon an ad hoc reference model concept. Implementation studies favourably verify the proposed control and analysis strategies, showing that the adaptive controller effectively compensates for time-varying and unwanted parameters within the actuator systems, and using the substructured eigenvalue can explicitly predict the testing performance in advance. In addition, the results also indicate that a feedforward controller incorporat...

Geological significance of radon gas in soil and underground water: A case study of Nurpur area, District Kangra, Himachal Pradesh, India

Abstract: Remote sensing satellite data have been used to recognize structures having tectonic significance. Based on satellite data, lineament map of Nurpur and its adjoining area of District Kangra, Himachal Pradesh, has been generated. LR-115 solid state nuclear track detectors have been used for the measurement of soil gas radon at 71 different locations of the study area. Radon monitoring in underground water at 26 different locations of the study area has been carried by scintillometry. The results indicate zones of lineament density and tectonically induced radon in soil and underground water. The results are co-relatable with regional geology of the area.

Water velocimeter and turbidity-meter using visible light communication modules

Abstract: Traditionally, water turbidity can be measured based on the absorption or scattering effects. Water flow velocity can be measured based on Doppler frequency shift (DFS) estimation. In this work, we aim to provide a simple, robust, low-cost and accurate method for both measurements by using visible light communication (VLC) modules. Signal processing methods to extract the water turbidity and water flow speed based on the VLC transceiver modules are designed and implemented.

Application of Supplemental Damping Characteristics to Response Spectrum Analyses of Nonproportionally Damped Multistory Asymmetric-Plan Buildings

Abstract: For practicing engineers, knowledge of the characteristics of supplemental damping in buildings is essential to understand the resultant effects of added damping. Characterizing the overall system ...