Determining minimum analysis conditions of scale ratio change to evaluate modal damping ratio in long-span bridge

Seungtaek Oh,Hoyeop Lee,Sung-Soon Yhim,Hak-Eun Lee,Nakhyun Chun 국제구조공학회 2018 Smart Structures and Systems, An International Jou Vol.22 No.1

Damping ratio and frequency have influence on dynamic serviceability or instability such as vortex-induced vibration and displacement amplification due to earthquake and critical flutter velocity, and it is thus important to make determination of damping ratio and frequency accurate. As bridges are getting longer, small scale model test considering similitude law must be conducted to evaluate damping ratio and frequency. Analysis conditions modified by similitude law are applied to experimental test considering different scale ratios. Generally, Nyquist frequency condition based on natural frequency modified by similitude law has been used to determine sampling rate for different scale ratios, and total time length has been determined by users arbitrarily or by considering similitude law with respect to time for different scale ratios. However, Nyquist frequency condition is not suitable for multimode system with noisy signals. In addition, there is no specified criteria for determination of total time length. Those analysis conditions severely affect accuracy of damping ratio. The focus of this study is made on the determination of minimum analysis conditions for different scale ratios. Influence of signal to noise ratio is studied according to the level of noise level. Free initial value problem is proposed to resolve the condition that is difficult to know original initial value for free vibration. Ambient and free vibration tests were used to analyze the dynamic properties of a system using data collected from tests with a two degree-of-freedom section model and performed on full bridge 3D models of cable stayed bridges. The free decay is estimated with the stochastic subspace identification method that uses displacement data to measure damping ratios under noisy conditions, and the iterative least squares method that adopts low pass filtering and fourth order central differencing. Reasonable results were yielded in numerical and experimental tests.

Determining minimum analysis conditions of scale ratio change to evaluate modal damping ratio in long-span bridge

Oh, Seungtaek,Lee, Hoyeop,Yhim, Sung-Soon,Lee, Hak-Eun,Chun, Nakhyun Techno-Press 2018 Smart Structures and Systems, An International Jou Vol.22 No.1

Damping ratio and frequency have influence on dynamic serviceability or instability such as vortex-induced vibration and displacement amplification due to earthquake and critical flutter velocity, and it is thus important to make determination of damping ratio and frequency accurate. As bridges are getting longer, small scale model test considering similitude law must be conducted to evaluate damping ratio and frequency. Analysis conditions modified by similitude law are applied to experimental test considering different scale ratios. Generally, Nyquist frequency condition based on natural frequency modified by similitude law has been used to determine sampling rate for different scale ratios, and total time length has been determined by users arbitrarily or by considering similitude law with respect to time for different scale ratios. However, Nyquist frequency condition is not suitable for multimode system with noisy signals. In addition, there is no specified criteria for determination of total time length. Those analysis conditions severely affect accuracy of damping ratio. The focus of this study is made on the determination of minimum analysis conditions for different scale ratios. Influence of signal to noise ratio is studied according to the level of noise level. Free initial value problem is proposed to resolve the condition that is difficult to know original initial value for free vibration. Ambient and free vibration tests were used to analyze the dynamic properties of a system using data collected from tests with a two degree-of-freedom section model and performed on full bridge 3D models of cable stayed bridges. The free decay is estimated with the stochastic subspace identification method that uses displacement data to measure damping ratios under noisy conditions, and the iterative least squares method that adopts low pass filtering and fourth order central differencing. Reasonable results were yielded in numerical and experimental tests.

가상 질량 동조기에 의한 구조물의 감쇠비 추정

황재승 한국풍공학회 2016 한국풍공학회지 Vol.20 No.1

본 연구는 가상 질량 동조기(VMT)를 이용하여 구조물의 응답으로부터 구조물의 감쇠비를 구하는 기법을 다룬다. 백색잡음뿐만아니라, 특정 진동수 가진, 충격하중 가진 등 다양한 하중종류 및 특성에 의하여 발생한 구조물의 계측응답에 대하여 VMT를 적용하였을 때 구조물의 감쇠비를 추정하는 기법에 대해서 다루고, VMT의 동적특성이 구조물 감쇠비 추정에 미치는 영향을 수치적으로 분석하였다. 수치해석결과에 의하면, VMT에 의하여 구조물 감쇠비 추정이 충분한 신뢰성을 가지고 예측되는 것을 확인 할 수 있었다. 충격하중을 받는 응답을 이용하는 경우 VMT에 의하여 가장 안정적으로 구조물 감쇠비 추정이 가능한 것을 알 수 있었으며, 백색잡음, 협소대역 하중에 대해서도 VMT의 지정 감쇠비를 너무 작게 산정하지 않으면 감쇠비 추정이 가능한 것으로 나타났다. 정현파의 경우에는 가진 진동수의 VMT 비율을 이용하여 감쇠비 추정이 가능한데 고유진동수와 인접할수록 정밀도가 높아지는 것을 알 수 있었다. This study outlines a technique to obtain a structural damping ratio from measured responses of the structure using the virtual mass tuner(VMT). For the structural responses induced by the different load types such as white noise, narrow band load, impact load and sine wave load, numerical analyses are performed to verify the applicability of VMT in the estimation of the damping ratio, and to consider the effect of the characteristics of loads and VMT on the accuracy of the damping ratio. From the numerical results, it is found that VMT is able to precisely estimate the damping ratio with a sufficient reliability regardless of the load type. When using a response by the impact load, the damping ratio is much more stably estimated. In addition, if the damping ratio of VMT is designated not too small, the damping ratio can be accurately estimated for the response induced by the white noise and narrow band load. In the case of the sine wave, it is possible to estimate the structural damping ratio using a variance ratio of the VMT at the different excitation frequencies.

Amplitude dependency of damping of tall structures by the random decrement technique

An Xu,Zhuangning Xie,Ming Gu,Jiurong Wu 한국풍공학회 2015 Wind and Structures, An International Journal (WAS Vol.21 No.2

This study focuses on the amplitude dependency of damping of tall structures by the random decrement technique (RDT). Many researchers have adopted RDT to establish the amplitude dependency of damping ratios in super-tall buildings under strong wind loads. In this study, a series of simulated examples were analyzed to examine the reliability of this method. Results show that damping ratios increase as vibration amplitudes increase in several cases; however, the damping ratios in the simulated signals were preset as constants. This finding reveals that this method and the derived amplitude-dependent damping ratio characteristics are unreliable. Moreover, this method would obviously yield misleading results if the simulated signals contain Gaussian white noise. Full-scale measurements on a super-tall building were conducted during four typhoons, and the recorded data were analyzed to observe the amplitude dependency of damping ratio. Relatively wide scatter is observed in the resulting damping ratios, and the damping ratios do not appear to have an obvious nonlinear relationship with vibration amplitude. Numerical simulation and field measurement results indicate that the widely-used method for establishing the amplitude-dependent damping characteristics of super-tall buildings and the conclusions derived from it might be questionable at the least. More field-measured data must be collected under strong wind loads, and the damping characteristics of super-tall buildings should be investigated further.

Amplitude dependency of damping of tall structures by the random decrement technique

Xu, An,Xie, Zhuangning,Gu, Ming,Wu, Jiurong Techno-Press 2015 Wind and Structures, An International Journal (WAS Vol.21 No.2

This study focuses on the amplitude dependency of damping of tall structures by the random decrement technique (RDT). Many researchers have adopted RDT to establish the amplitude dependency of damping ratios in super-tall buildings under strong wind loads. In this study, a series of simulated examples were analyzed to examine the reliability of this method. Results show that damping ratios increase as vibration amplitudes increase in several cases; however, the damping ratios in the simulated signals were preset as constants. This finding reveals that this method and the derived amplitude-dependent damping ratio characteristics are unreliable. Moreover, this method would obviously yield misleading results if the simulated signals contain Gaussian white noise. Full-scale measurements on a super-tall building were conducted during four typhoons, and the recorded data were analyzed to observe the amplitude dependency of damping ratio. Relatively wide scatter is observed in the resulting damping ratios, and the damping ratios do not appear to have an obvious nonlinear relationship with vibration amplitude. Numerical simulation and field measurement results indicate that the widely-used method for establishing the amplitude-dependent damping characteristics of super-tall buildings and the conclusions derived from it might be questionable at the least. More field-measured data must be collected under strong wind loads, and the damping characteristics of super-tall buildings should be investigated further.

콘크리트 댐 내진성능평가 시 감쇠비 적용 방안 고찰

오정근,정영석,권민호 한국구조물진단유지관리공학회 2023 한국구조물진단유지관리공학회 논문집 Vol.27 No.1

The purpose of this paper is to review the appropriateness of the application method for the value of the damping ratio suggested in thecurrent design standards and evaluation guidelines when evaluating the seismic performance of concrete dams and to suggest improvements. As a resultof the study, for the magnitude of the damping ratio in the dynamic elastic analysis, it is necessary to refer to the case of a similar dam in which themagnitude of the earthquake load is similar and the reproducibility of the damping ratio has been verified. Considering this, it is necessary to applya low damping ratio and consider adding hysteresis damping in case of nonlinear behavior. In addition, since the concrete dam body located on therock has insignificant radiation attenuation effect, it is not reasonable to increase the damping ratio of the concrete dam body to reflect the radiationdamping. Therefore, in order to evaluate the realistic seismic performance of concrete dams, it is necessary to revise the damping ratio-related contentscontained in the current dam design standards and evaluation guidelines. 이 논문의 목적은 콘크리트 댐의 내진성능평가 시 현행 설계기준과 평가요령에서 제시하는 감쇠비 값 크기에 대한 적용방안의 적정성을 검토하고 개선안을 제시하는 것이다. 연구결과 동적탄성해석 시 감쇠비의 크기는 지진하중의 크기가 유사하고 감쇠비에 대한 재현성이 검증된 유사댐의 사례를 참조할 필요가 있으며, 동적소성해석 시에는 지진하중의 크기와 상관없이 초기 탄성거동을 고려하여 감쇠비를 낮은값으로 적용하고 비선형 거동 시 이력감쇠가 추가되도록 고려할 필요가 있다. 또한 암반상에 위치한 콘크리트 댐체는 방사감쇠 효과가 미미하므로 방사감쇠를 반영하기 위해 콘크리트 댐체의 감쇠비를 증가시키는 것은 합리적이지 않다. 따라서 콘크리트 댐의 현실적인 내진성능평가를 위해 현행 댐 설계기준과 평가요령에 수록된 감쇠비 관련내용에 대해 개정이 필요하다.

Wind-induced conductor response considering the nonproportionality of generalized aerodynamic damping

Wenjuan Lou,Dengguo Wu,Haiwei Xu 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.7

Aerodynamic damping is a key factor that influences the wind-induced responses of transmission conductors. The inaccurate estimation of aerodynamic damping leads to misunderstanding of conductor response to wind actions. For dynamic response analysis in the frequency domain, the generalized aerodynamic damping matrix used to solve conductor motion is diagonal. This study reanalyzed the conductor generalized aerodynamic damping matrix by considering its horizontal and vertical motions and the coupling effects of different modes. The derived generalized aerodynamic damping was a nondiagonal matrix, and we defined it as nonproportional generalized aerodynamic damping. Variations in the aerodynamic damping ratio and the nonproportionality of generalized aerodynamic damping with wind speed were investigated through numerical studies of single- and triple-span conductors. Triple-span conductors with different span lengths, hanging heights, and initial pretension forces were adopted to discuss the effects of the nonproportionality of generalized aerodynamic damping on estimating the dynamic responses of conductors. Results showed that neglecting the vertical motion of a conductor leads to an overestimation of the aerodynamic damping ratio, especially under high wind speed conditions. The nonproportionality of generalized aerodynamic damping is highest when the incoming wind speed at 10 m is 25 m/s. Ignoring such nonproportionality leads to an overestimation of the resonant responses of conductors but exerts minimal effects on total fluctuating responses mainly contributed by background components. Therefore, for a transmission conductor with a significant resonant response, the nonproportionality effect of generalized aerodynamic damping must be considered. Increasing span length and conductor height or reducing initial pretension force seems to enhance nonproportionality effects on conductor responses.

이선형 단자유도 감쇠시스템의 비탄성변위비

한상환,배문수,조종 한국지진공학회 2007 한국지진공학회논문집 Vol.11 No.6

NEHRP 지반조건 B,C,D에서 이선형 단자유도 감쇠시스템의 지반조건, 후탄성기울기, 감쇠비, 항복강도 감소계수, 고유 주기 등의 변화가 비탄성변위비에 미치는 영향을 조사하였다. 기존의 제안식은 변위일정 법칙을 따라 일정주기 이상에서 비탄성 변위비를 과대평가하게 된다. 또한 기존식은 5%이상의 감쇠비에 대하여만 제안되었다. 본 연구는 후탄성기울기, 감쇠비 20% 이하의 이선형 시스템의 비탄성 변위비의 평균과 편차를 제안하였고 범용적으로 사용할 수 있음을 보였다. 제안식을 사용하여 비탄성 변위비의 확률적 분포를 계산하여 구조물의 성능기반설계에 이용할 수 있다. This study investigates the effect of site class, post-yield stiffness ratio, damping ratio, yield-strength reduction factor, and natural period on inelastic displacement ratio of bilinear SDF systems located at the sites classified as NEHRP site class B,C,D. The previous studies developed inelastic displacement ratio using equal displacement rule in the intermediate and long period range. But, this approximation overestimates the inelastic displacement ratio. Furthermore, inelastic displacement ratio has not been developed for the systems having a damping ratio less than 5%. This study conducts nonlinear regression analysis for proposing equations for calculating median and deviation of the inelastic displacement ratio of the bilinear SDOF system having damping ratios ranging from 0 to 20%. Using median and deviation of the inelastic displacement ratio, probabilistic inelastic displacement ratio is estimated, which can be used for performance-based seismic evaluation.

Effects of damping ratio on dynamic increase factor in progressive collapse

Hamed Saffari,Javad Mashhadi 국제구조공학회 2016 Steel and Composite Structures, An International J Vol.22 No.3

In this paper, the effect of damping ratio on nonlinear dynamic analysis response and dynamic increase factor (DIF) in nonlinear static analysis of structures against column removal are investigated and a modified empirical DIF is presented. To this end, series of low and mid-rise moment frame structures with different span lengths and number of storeys are designed and the effect of damping ratio in DIF is investigated, performing several nonlinear static and dynamic analyses. For each damping ratio, a nonlinear dynamic analysis and a step by step nonlinear static analysis are carried out and the modified empirical DIF formulas are derived. The results of the analysis reveal that DIF is decreased with increasing damping ratio. Finally, an empirical formula is recommended that relates to damping ratio. Therefore, the new modified DIF can be used with nonlinear static analysis instead of nonlinear dynamic analysis to assess the progressive collapse potential of moment frame buildings with different damping ratios.

한국 표준설계스펙트럼의 감쇠보정계수에 대한 연구

허태민,김재관,김정한,이진호 한국지진공학회 2018 한국지진공학회논문집 Vol.22 No.1

In this study, we develop and propose damping correction factors for the Korean standard design spectra. The newly proposed Korean standard design spectra has been given only for 5% damping ratio. But in practice, engineers need design spectra for damping values other than 5%. To obtain design spectra for various damping values from the standard spectra, damping correction factors are derived. These factors modify the shape of design spectra in accordance with the damping ratio. Response spectra for various damping values are calculated from the earthquake records that had been used to calculate standard design spectra. They consist of 55 records from 18 earthquakes occurred in overseas intraplate regions and Korea. The regressed spectra for the damping values ranging from 0.5% to 50% are compared with standard spectra at three regions acceleration, velocity and displacement sensitive regions. The regression analysis of these data rendered formula for damping correction factors. Finally, a single formula for damping correction factors is recommended that is valid for both horizontal and vertical design spectra and that is applicable to the entire range of periods. One thing to note that recommended damping correction factors is valid for the design spectrum of the rock grounds because the design spectra was developed based on the earthquake records of the rock ground.