Results Of Free Vibration Tests Research Articles

Full-Scale Static Loading and Free-Vibration Tests of a Real Bridge with Friction Pendulum Bearing System (FPS) and Design Parameter Estimation

ABSTRACT Although the friction pendulum bearing system (FPS) has been widely used in the construction of buildings, bridges, and other structures to enhance their seismic performance, the FPS was adopted in bridge construction in Japan for the first time in 2020 on the Tokai-Hokuriku expressway. To validate the design hypotheses of the real bridge with FPS, a series of static and dynamic tests were performed, and the parameters of the FPSs were estimated. The girder of the bridge supported by four FPSs was pushed to various specified displacements considered in a maximum considered earthquake event in a quasi-static manner by hydraulic jacks equipped with large-caliber valves to enable quick pressure release. The load of the jacks was then suddenly released so that the bridge entered free vibration. The bearings in the site static tests are found to undergo a noncontinuous sliding motion (stick – slip) because a small loading rate of jacks is applied. This stick – slip phenomenon is confirmed in both the site and laboratory tests in cases where the sliding velocity either approached 0 or departed from 0, i.e. at every velocity reversal. The friction coefficient is estimated from the force and displacement data at the sliding points, and good agreement is observed between the site static tests and the laboratory tests. The friction coefficient model for dynamic analysis, accounting for the stick – slip effect and several dependencies, is calibrated by these test results. The free-vibration test results, including measured displacements and accelerations, show agreement with the simulation results based on a simplified model of the bridge with the FPSs. Finally, the unscented Kalman filter and its adaptive variant are applied to estimate the design parameters of the FPSs.

Reliability analysis of damping estimation by random decrement technique for high‐rise buildings

AbstractDamping ratio is one of the most important parameters in the dynamic analysis of high‐rise buildings for structural design. As a practical damping estimation method, the random decrement technique (RDT) has been widely applied to civil structures in the past decades. The inherent limitation of the RDT is that it can only provide a single optimal damping result, while the reliability of its estimated result is unavailable. By combining the RDT with the bootstrap method, this paper presents a reliability analysis scheme for damping estimation. Based on the bootstrap‐based scheme, the reliability of damping results estimated by the RDT using four commonly used triggering conditions is systemically investigated through numerical simulations. The results show that the reliability of damping estimates can be effectively evaluated by providing the confidence interval. Moreover, based on free vibration test results and field measurements of acceleration responses of a 600‐m high skyscraper during a typhoon, the bootstrap‐based scheme is further verified to be applicable and effective for evaluating the reliability of damping results of high‐rise buildings in field measurements. This paper aims to provide useful implications for evaluating the reliability of damping estimates and thereby improve the accuracy of damping evaluation of high‐rise buildings.

Buckypaper/DYAD/Buckypaper and Buckypaper/DYAD/ (polyaniline/multiwalled carbon nanotube) composite sensors: Preparation and damping properties characterization

Buckypaper/DYAD/Buckypaper and Buckypaper/DYAD/(polyaniline/multiwalled carbon nanotube) composites films were made by frit compression method and layer-by-layer attachment method. Transmission electron microscopy and scanning electron microscopy were used to study the morphology properties of polyaniline/multiwalled carbon nanotube and the results showed that carbon nanotubes were well dispersed in the polymer matrix. Free vibration test results showed that the double-sided attachment of the sensor had higher damping ratio values than single-sided attachment. Also, damping ratios were higher when the composite sensor was placed at the beam's clamped end. Furthermore, the Buckypaper/DYAD/(polyaniline/multiwalled carbon nanotube) combination exhibited higher damping ratios than the other cases tested. Thus, these samples have the potential of being simultaneously strain sensors and dampers.

Using dynamic analysis of site vibration to select the suitable vibration limit

This study was done to select the appropriate vibration standard to be the adopted vibration standard for a construction project that was located in adjacent to many buildings. These buildings may be affected by project activities induced vibrations.Three common international vibration standards were revised and based on the dynamic analysis results, site conditions, the nature of the existing buildings and the expected construction activities, and the most appropriate vibration limit criteria were selected and tested then validated. The selection process was based on experimental study carried out using physical models and real vibration data recorded on the construction site. The validation process was based on monitoring the vibration effects on existing buildings.Experimental program was carried out using three physical models with dynamic characteristics within the range of the common existing buildings. These models were subjected to free vibration modal testing to ensure their dynamic characteristics. Three numerical finite element models for the three physical ones were built and calibrated using the free vibration test results. The numerical models were subjected to time history dynamic analysis using the vibration waveforms that represent the most effective construction vibrations. These waveforms were recorded onsite during different activities of the pilot project which was implemented to test all the project activities.The validation process was carried out by applying monitoring program on two of the project neighboring buildings. These buildings were selected to represent the most likely to be affected buildings by the construction activities. The monitoring program includes vibration and strain measurements for the building vibration responses.The results of validation process showed that the effectiveness of the selected vibration standard in estimating the level of damage on buildings subjected to the construction induced vibration.

Linear and Nonlinear Dynamic Response of Piles in Soft Marine Clay

AbstractThis paper presents the results of free vibration tests carried out at different load levels on a system of three near-shore steel pipe piles vibro-driven into soft marine clay. Piles are arranged in an L-shaped horizontal layout and are free at the head. The instrumentation consists of strain gauges placed at selected levels along the shaft of the loaded pile and accelerometers at the head of the receiver piles. The aim of this experiment is to analyze the dynamic soil–water–pile and pile-to-pile interaction and to investigate the development of nonlinearities at increasing load levels. The results of experimental modal analyses, in terms of natural frequencies and damping ratios of the system, are presented and the complex dynamic behavior of the vibrating soil–water–pile system and the pile-to-pile interaction are discussed. For a better reliability assessment of the system response in the range of linear behavior, the results of free vibration tests at the lowest level of the applied loads are...

磁気ばねを用いた動吸振器による低振動数構造物の制振に関する研究

The present study investigates the vibration suppression for a structure with low natural frequency using a dynamic absorber which is connected to a main system with two magnetic springs. It is important for the assurance of safety and the improvement of reliability to suppress vibration of flexible structures such as skyscrapers and space structures whose natural frequencies are very low. Though some countermeasures against a low frequency vibration such as a pendulum dynamic absorber have been proposed, the passive control technique for a very low frequency vibration can stand further improvement. In the present experimental apparatus, the main system and the dynamic absorber are composed of two rigid body pendulums in order to have a very low natural frequency. Two magnetic springs connecting between the main system and the dynamic absorber are composed of three permanent magnets whose same magnetic poles are faced each other and can adjust the natural frequency of the dynamic absorber by changing the distance between the magnets. It was confirmed by conducting the time history response analysis by the direct numerical integration that it is effective for the transient response by tuning to the optimal parameters of the dynamic absorber derived according to the fixed point theory. Furthermore, free vibration test results using the experimental apparatus revealed the effect of the dynamic absorber with magnetic spring on a very low frequency vibration and the experimental results agreed qualitatively with the numerical analysis results.

Predictive Capability of a 2D FNPF Fluid-Structure Interaction Model

The predictive capability of two-dimensional (2D) fully-nonlinear-potential-flow (FNPF) models of an experimental submerged moored sphere system subjected to waves is examined in this study. The experimental system considered includes both single-degree-of-freedom (SDOF) surge-only and two-degree-of-freedom (2DOF) surge-heave coupled motions, with main sources of nonlinearity from free surface boundary, large geometry, and coupled fluid-structure interaction. The FNPF models that track the nonlinear free-surface boundary exactly hence can accurately model highly nonlinear (nonbreaking) waves. To examine the predictive capability of the approximate 2D models and keep the computational effort manageable, the structural sphere is converted to an equivalent 2D cylinder. Fluid-structure interaction is coupled through an implicit boundary condition enforcing the instantaneous dynamic equilibrium between the fluid and the structure. The numerical models are first calibrated using free-vibration test results and then employed to investigate the wave-excited experimental responses via comparisons of time history and frequency response diagrams. Under monochromatic wave excitations, both SDOF and 2DOF models exhibit complex nonlinear experimental responses including coexistence, harmonics, subharmonics, and superharmonics. It is found that the numerical models can predict the general qualitative nonlinear behavior, harmonic and subharmonic responses as well as bifurcation structure. However, the predictive capability of the models deteriorates for superharmonic resonance possibly due to three-dimensional (3D) effects including diffraction and reflection. To accurately predict the nonlinear behavior of moored sphere motions in the highly sensitive response region, it is recommended that the more computationally intensive 3D numerical models be employed.

A Study on Countermeasures against Wind-Induced Cable Vibrations in Cable-Stayed Bridge by Changing Vibration Modes. 4th Report. Experimental Application to an Actual Cable.

In order to suppress vibrations of stay cables in cable-stayed bridges, the authors developed a new vibration reduction called Changing vibration modes method using a permanent magnet. With this method, growth of the unstable fundamental vibration mode is prevented by dissipating energy of higher stable modes. The authors have confirmed in former reports that the proposed effectively reduces the self-induced cable vibrations by both theoretical analyses and wind tunnel tests and design of the device was proposed. In this report, first, the design process of a prototype of a control device is described. Second, higher mode vibrations generated by the action of the device were confirmed from the results of free vibration tests. Finally, efficiency of the proposed countermeasure against rain-wind-induced vibrations of an actual cable is clarified.

METHODS FOR MEASURING VERTICAL TIRE STIFFNESS

Vertical stiffness was measured for a 260/80R20 radial ply agricultural drive tire using five methods; load-deflection,non-rolling vertical free vibration, non-rolling equilibrium load-deflection, rolling vertical free vibration, androlling equilibrium load-deflection. Tests were conducted at three inflation pressures (41, 83, and 124 kPa). Non-rollingfree vibration resulted in the highest stiffness for all inflation pressures. Load-deflection and non-rolling equilibrium load-deflectionresults were similar at all inflation pressures. Rolling vertical free vibration and rolling equilibrium load-deflectionresults were similar at inflation pressures of 83 and 124 kPa. Non-rolling free vibration is not an adequatemethod for determining vertical tire stiffness when the tire is represented by a spring and viscous damper in parallel. Thediscrepancy between load-deflection and free vibration test results was attributed to hysteresis in the tire. Tire propertiesshould be measured at the desired forward velocity when the tire is modeled as a spring and viscous damper in parallel.

Cracking and Shear Effects on Structural Walls

A mathematical model taking into account flexural and shear deformation, rotary moment of inertia, and axial loads is developed to calculate the natural frequencies of isolated walls. Results of free vibration tests of nine 1/3‐scale reinforced concrete isolated walls are evaluated using the mathematical model. The measured “uncracked” fundamental frequency varied from 90%‐100% of the calculated value. Results of the free vibration tests are also compared with calculated frequencies using simplified formulas available in standard textbooks. It is shown that damping increases with the amplitude of the free vibration and with structural damage. It is concluded that shear deformations have a significant effect in the calculation of natural frequencies, particularly for walls with large boundary elements, and linear analyses based on “uncracked” properties may give unreasonably conservative values for earthquake design because of rapid reduction in natural frequencies caused by cracking.