• Earthquakes and Structures
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• 제18권1호
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• pp.45-56
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• 2020

The application of critical excitation method with displacement-based objective function for multi degree of freedom (MDOF) systems is investigated. To this end, a new critical excitation method is developed to find the critical input motion of a MDOF system as a synthetic accelerogram. The upper bound of earthquake input energy per unit mass is considered as a new constraint for the problem, and its advantages are discussed. Considering this constraint, the critical excitation method is then used to generate synthetic accelerograms for MDOF models corresponding to three shear buildings of 10, 16, and 22 stories. In order to demonstrate the reliability of generated accelerograms to estimate dynamic response of the structures, three target ground motions with considerable level of energy contents are selected to represent "real critical excitation" of each model, and the method is used to re-generate these ground motions. Afterwards, linear dynamic analyses are conducted using these accelerograms along with the generated critical excitations, to investigate the key parameters of response including maximum displacement, maximum interstory drift, and maximum absolute acceleration of stories. The results show that the generated critical excitations can make an acceptable estimate of the structural behavior compared to the target ground motions. Therefore, the method can be reliably implemented to generate critical excitation of the structure when real one is not available.

• Earthquakes and Structures
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• 제1권1호
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• pp.43-67
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• 2010

A gradient-based evolutionary optimization methodology is presented for finding the optimal design of both the added dampers and their supporting members to minimize an objective function of a linear multi-storey structure subjected to the critical ground acceleration. The objective function is taken as the sum of the stochastic interstorey drifts. A frequency-dependent viscoelastic damper and the supporting member are treated as a vibration control device. Due to the added stiffness by the supplemental viscoelastic damper, the variable critical excitation needs to be updated simultaneously within the evolutionary phase of the optimal damper placement. Two different models of the entire damper unit are investigated. The first model is a detailed model referred to as "the 3N model" where the relative displacement in each component (i.e., the spring and the dashpot) of the damper unit is defined. The second model is a simpler model referred to as "the N model" where the entire damper unit is converted into an equivalent frequency-dependent Kelvin-Voigt model. Numerical analyses for 3 and 10-storey building models are conducted to investigate the characters of the optimal design using these models and to examine the validity of the proposed technique.

• Earthquakes and Structures
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• 제9권5호
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• pp.1115-1132
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• 2015

The study considers earthquake shake table testing of bending-torsion coupled structures under multi-component stationary random earthquake excitations. An experimental procedure to arrive at the optimal excitation cross-power spectral density (psd) functions which maximize/minimize the steady state variance of a chosen response variable is proposed. These optimal functions are shown to be derivable in terms of a set of system frequency response functions which could be measured experimentally without necessitating an idealized mathematical model to be postulated for the structure under study. The relationship between these optimized cross-psd functions to the most favourable/least favourable angle of incidence of seismic waves on the structure is noted. The optimal functions are also shown to be system dependent, mathematically the sharpest, and correspond to neither fully correlated motions nor independent motions. The proposed experimental procedure is demonstrated through shake table studies on two laboratory scale building frame models.

• Structural Engineering and Mechanics
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• 제68권5호
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• pp.575-589
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• 2018

It is well known that the incidence angle of seismic excitation has an influence on the structural response of buildings, and this effect can be more significant in the case of near-fault signals. However, current seismic codes do not include detailed requirements regarding the direction of application of the seismic action and they have only recently introduced specific provisions about near-fault earthquakes. Thus, engineers have the task of evaluating all the relevant directions or the most critical conditions case by case, in order to avoid underestimating structural demand. To facilitate the identification of the most critical incidence angle, this paper presents a procedure which makes use of a two-degree of freedom model for representing a building. The proposed procedure makes it possible to avoid the extensive computational effort of multiple dynamic analyses with varying angles of incidence of ground motion excitation, which is required if a spatial multi-degree of freedom model is used for representing a building. The procedure is validated through the analysis of two case studies consisting of an eight- and a six-storey reinforced concrete frame building, selected as representative of existing structures located in Italy. A set of 124 near-fault ground motion records oriented along 8 incidence angles, varying from 0 to 180 degrees, with increments of 22.5 degrees, is used to excite the structures. Comparisons between the results obtained with detailed models of the two structures and the proposed procedure are used to show the accuracy of the latter in the prediction of the most critical angle of seismic incidence.

• Earthquakes and Structures
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• 제1권2호
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• pp.147-162
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• 2010

The ground acceleration measured at a point on the earth's surface is composed of several waves that have different phase velocities, arrival times, amplitudes, and frequency contents. For instance, body waves contain primary and secondary waves that have high frequency content and reach the site first. Surface waves are composed of Rayleigh and Love waves that have lower phase velocity, lower frequency content and reach the site next. Some of these waves could be of more damage to the structure depending on their frequency content and associated amplitude. This paper models critical earthquake loads for single-degree-of-freedom (SDOF) inelastic structures considering evolution of the seismic waves in time and frequency. The ground acceleration is represented as combination of seismic waves with different characteristics. Each seismic wave represents the energy of the ground motion in certain frequency band and time interval. The amplitudes and phase angles of these waves are optimized to produce the highest damage in the structure subject to explicit constraints on the energy and the peak ground acceleration and implicit constraints on the frequency content and the arrival time of the seismic waves. The material nonlinearity is modeled using bilinear inelastic law. The study explores also the influence of the properties of the seismic waves on the energy demand and damage state of the structure. Numerical illustrations on modeling critical earthquake excitations for one-storey inelastic frame structures are provided.

• 한국해양공학회지
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• 제37권4호
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• pp.164-171
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• 2023

Vibration and noise must be considered to maximize the efficiency of a yaw system and reduce the fatigue load acting on a wind turbine. This study investigated a method for analyzing yaw-system vibration based on the change in the load-duration distribution (LDD). A substructure synthesis method was combined with a planetary gear train rotational vibration model and finite element models of the housing and carriers. For the vibration excitation sources, the mass imbalance, gear mesh frequency, and bearing defect frequency were considered, and a critical speed analysis was performed. The analysis results showed that the critical speed did not occur within the operating speed range, but a defect occurred in the bearing of the first-stage planetary gear system. It was found that the bearing stiffness and first natural frequency increased with the LDD load. In addition, no vibration occurred in the operating speed range under any of the LDD loads. Because the rolling bearing stiffness changed with the LDD, it was necessary to consider the LDD when analyzing the wind turbine vibration.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2004년도 학술대회지
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• pp.780-783
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• 2004

The two-dimensional modeling of the non-stationary thermal state and voltage responsivity of the sensitive elements usually used in solid-state pyroelectric focal plane arrays are presented. Temperature distributions under periodical thermal excitation and the response of the thermal imaging device, which is composed of the pyroelectric sensitive elements mounted on a single silicon substrate, are numerically calculated. The sensitive element consists of a covering metal layer, infrared polymer absorber, front metal contact, sensitive pyroelectric element, the interconnecting column and the bulk silicon readout. The results of the numerical modeling show that the thermal crosstalk between sensitive elements to be critical especially at low frequency (f < 10Hz) of periodically modulated light. It is also shown that the use of our models gives the possibility to improve the design, operating regimes and sensitivity of the device.

• Smart Structures and Systems
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• 제6권5_6호
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• pp.525-543
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• 2010

Testing and validation processes are critical tasks in developing a new hardware platform based on a new technology. This paper describes a series of experiments to evaluate the performance of a newly developed MEMS-based wireless sensor node as part of a wireless sensor network (WSN). The sensor node consists of a sensor board with four accelerometers, a thermometer and filtering and digitization units, and a MICAz mote for control, local computation and communication. The experiments include calibration and linearity tests for all sensor channels on the sensor boards, dynamic range tests to evaluate their performance when subjected to varying excitation, noise characteristic tests to quantify the noise floor of the sensor board, and temperature tests to study the behavior of the sensors under changing temperature profiles. The paper also describes a large-scale deployment of the WSN on a long-span suspension bridge, which lasted over three months and continuously collected ambient vibration and temperature data on the bridge. Statistical modal properties of a bridge tower are presented and compared with similar estimates from a previous deployment of sensors on the bridge and finite element models.

• Journal of Advanced Marine Engineering and Technology
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• 제38권3호
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• pp.253-261
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• 2014

본 연구에서는 스퍼 유성기어시스템 2단, 헬리컬기어 시스템 1단으로 구성되어 있고, 메인 축과 증속기가 플랜지 결합되어 있는 동력분기식 2.5MW 풍력발전기에 대해 진동해석을 수행하였다. 유성기어시스템, 헬리컬기어시스템, 메인 축 등은 MASTA 상용프로그램으로 모델링하고, 기어박스 케이스, 토크암, 유성캐리어, 플랜지 등은 유한요소법으로 모델링하였으며, 부분구조합성법으로 조합하여 진동해석을 수행하였다. 2.5MW 동력분기식 풍력발전기 기어트레인의 고유진동 특성 해석을 수행하였고, 블레이드 풍하중에 의한 가진, 질량불평형, 축 어긋남 등에 발생하는 가진원에 대해 위험속도 분석을 하였다.

• 한국전산구조공학회논문집
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• 제37권1호
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• pp.9-16
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• 2024

일반적으로 적합직교분해(proper orthogonal decomposition, POD) 기반의 침습적(intrusive) 차수축소모델(reduced order model, ROM)을 활용하면 구조 시스템의 전체 자유도를 크게 줄이고 외연적 시간 적분법에서 해의 안정성을 만족하는 임계 시간 간격을 증가시킬 수 있다. 따라서 본 연구에서는 POD-ROM을 활용하여 Voronoi-cell 격자 요소로 이산화된 구조 시스템의 축소와 이에 따른 외연적 시간 적분법의 임계 시간 간격 및 해석 정확도 변화를 살펴보았다. 또한 지진하중과 같은 불규칙한 하중 이력을 받는 구조물 응답 해석에 POD-ROM을 적용하였다. 해석 결과 ROM을 통해 해의 정확도를 충분히 확보하면서 연산 시간을 크게 단축할 수 있음을 확인하였다. 또한 POD-ROM과 VCLM의 연계 방안의 적절성을 확인하였다. 향후 해당 연구는 고정밀 대용량 동적 구조해석의 실용성을 높이고, 설계 변수에 따른 구조물 동적 거동의 실시간 예측을 위한 기반 연구로 활용될 수 있다.