Effects of False Floors on Vibration Serviceability of Building Floors. II: Response to Pedestrian Excitation

This is the second of two papers that present the results of a comprehensive and systematic study into the effects of false flooring on the vibration serviceability of long-span concrete floors. In this paper, controlled pedestrian response measurements were utilized to determine the effects of the installation of false flooring on the vibration serviceability of long-span concrete floors. It was found that, in most cases, the installation of false flooring tended to reduce the measured vibration response of the floor under controlled pedestrian excitation. This was more significant for false floors with a relatively high finished floor height (FFH) than for floors with a lower finished floor height. It is tentatively proposed that the effects of false flooring be incorporated into existing design procedures by multiplying calculated responses by a vibration response reduction factor. This factor would be 0.9 for false flooring with FFH less than 500 mm or 0.8 for false flooring with FFH of 500 mm or greater.     

Vibration Serviceability of a Building Floor Structure. I: Dynamic Testing and Computer Modeling

Buildings with large column-free floors or long-cantilevered structures can be susceptible to annoying vibrations due to everyday occupants’ activities such as walking. Computer modeling and analytical representation of building structural properties to predict the floor response subjected to excitations due to human activities are important issues that require further studies. Vibration testing and analysis of built structures can assist in more accurate estimation of structure dynamic properties. This paper presents the results of the modal testing conducted on an office building floor and analysis of the collected vibration measurements. It compares these results with the structural response using computer analyses. It also presents a sensitivity study to assess the importance of various structural parameters on the floor dynamic response. From the results presented, it is concluded that for the structure used in this study the raised flooring and nonstructural elements acted mainly as added mass and did not contribute to the floor damping. Conclusions are also made on the importance of various structural parameters on floor response and the analysis of the modal test results.     

Probabilistic Modeling of Walking Excitation for Building Floors

Slender floor structures are becoming increasingly prone to excessive vibration due to human-induced walking excitation. To prevent discomfort of floor occupants and/or malfunctioning of sensitive equipment, it is necessary to have a reliable means of estimating floor vibration in the design phase. For accurate estimation of the floor vibration, both reliable excitation and structural models are required. This paper concentrates on the former by evaluating the performance of the existing force models and suggesting their improvement. For this a force model adopted in the United Kingdom by the Concrete Society was applied to four nominally identical floors using their experimentally identified modal properties. After comparison with experimental data the drawbacks of the force model were identified after which an improved model of the walking-induced dynamic force, based on the combination of two existing methodologies used separately for low- and high-frequency floors, is proposed. The improved model accounts for the intersubject variability in the walking force with respect to the pacing frequency, step length, and forcing magnitude. Moreover, it includes all relevant frequency components of the walking force into analysis, removing the need for classification of floors as low or high frequency. The proposed approach should help designers and building owners to make more informed decisions when evaluating vibration serviceability of floor structures.     

Ambient Vibration of Long-Span Cable-Stayed Bridge

The investigation of dynamic response for long-span cable-stayed bridges largely depends on a detailed understanding of their dynamic characteristics, such as the natural frequencies, mode shapes, and modal damping ratios. In this paper, the dynamic characteristics of a fairly long cable-stayed bridge in Hong Kong are studied using finite-element analysis and ambient vibration measurements. A three-dimensional finite-element model is first established for the bridge based on design drawings. The dynamic characteristics are then analyzed from the statically deformed configuration. Ambient vibration measurements are also conducted to obtain the dynamic characteristics of the bridge. Comparison between these two results shows that, for the most part, a total of 31 modes can be correlated with a reasonable agreement. However, the frequency differences of the higher modes can range between 15 and 30%. This implies that, if the measurement is more reliable, a finite-element model updating is necessary in order to achieve better correlation between these two results.     

Aerodynamic Coupling Effects on Flutter and Buffeting of Bridges

The effects of aerodynamic coupling among modes of vibration on the flutter and buffeting response of long-span bridges are investigated. By introducing the unsteady, self-excited aerodynamic forces in terms of rational function approximations, the equations of motion in generalized modal coordinates are transformed into a frequency-independent state-space format. The frequencies, damping ratios, and complex mode shapes at a prescribed wind velocity, and the critical flutter conditions, are identified by solving a complex eigenvalue problem. A significant feature of this approach is that an iterative solution for determining the flutter conditions is not necessary, because the equations of motion are independent of frequency. The energy increase in each flutter motion cycle is examined using the work done by the generalized aerodynamic forces or by the self-excited forces along the bridge axis. Accordingly, their contribution to the aerodynamic damping can be clearly identified. The multimode flutter generation mechanism and the roles of flutter derivatives are investigated. Finally, the coupling effects on the buffeting response due to self-excited forces are also discussed.     

Vibration Control of Stay Cables of the Shandong Binzhou Yellow River Highway Bridge Using Magnetorheological Fluid Dampers

The Shandong Binzhou Yellow River Highway Bridge is a three-tower, cable-stayed bridge in Shandong Province, China. Because the stay cables are prone to vibration, 40 magnetorheological (MR) fluid dampers were attached to the 20 longest cables of this bridge to suppress possible vibration. An innovative control algorithm for active and semiactive control of mass-distributed dynamic systems, e.g., stay cables, was proposed. The frequencies and modal damping ratios of the unimpeded tested cable were identified through an ambient vibration test and free vibration tests, respectively. Subsequently, a series of field tests were carried out to investigate the control efficacy of the free cable vibrations achieved by semiactive MR dampers, “Passive-off” MR dampers and “Passive-on” MR dampers. The first three modal damping ratios of the cable incorporated with the MR dampers were also identified from the in situ experiments. The field experiment results indicated that the semiactive MR dampers can provide significantly greater supplemental damping for the cable than either the Passive-off or the Passive-on MR dampers because of the pseudonegative stiffness generated by the semiactive MR dampers.     

Identification of Natural Frequencies and Dampings of In Situ Tall Buildings Using Ambient Wind Vibration Data

An accurate prediction for the response of tall buildings subject to strong wind gusts or earthquakes requires the information of in situ dynamic properties of the building, including natural frequencies and damping ratios. This paper presents a method of identifying natural frequencies and damping ratios of in situ tall buildings using ambient wind vibration data. Our approach is based on the empirical mode decomposition (EMD) method, the random decrement technique (RDT), and the Hilbert–Huang transform. Our method requires only one acceleration sensor. The noisy measurement of the building acceleration is first processed through the EMD method to determine the response of each mode. Then, RDT is used to obtain the free vibration modal response. Finally, the Hilbert transform is applied to each free vibration modal response to identify natural frequencies and damping ratios of in situ tall buildings. The application of the proposed methodology is demonstrated in detail using simulated response data of a 76-story benchmark building polluted by noise. Both the along-wind and across-wind vibration measurements have been illustrated. Simulation results demonstrate that the accuracy of the proposed method in identifying natural frequencies and damping ratios is remarkable. The methodology proposed herein provides a new and effective tool for the parametric identification of in situ tall buildings.     

Analytical Study on Bending Effects in a Stay Cable with a Damper

The effects of bending on the modal properties of a stay cable with a transverse damper are analytically studied. Considering that the value of the flexural rigidity in the stay cable is small in practice, an explicit asymptotic formula for the modal damping of a cable with a general type of damper is derived. For a viscous damper, the asymptotic formula obtained is compact, accurate, and thus is very suitable for practical design. Furthermore, for the first few vibration modes of interest, the asymptotic solution is independent of the modal index. It is shown that flexure in the cable reduces the maximum attainable modal damping, possibly up to 20%, while it significantly increases the optimal damping coefficient of the damper.     

Dynamic Field Test, System Identification, and Modal Validation of an RC Minaret: Preprocessing and Postprocessing the Wind-Induced Ambient Vibration Data

The investigation of dynamic response for civil engineering structures largely depends on a detailed understanding of their dynamic characteristics, such as the natural frequencies, mode shapes, and modal damping ratios. Dynamic characteristics of structures may be obtained numerically and experimentally. The finite-element method is widely used to model structural systems numerically. However, there are some uncertainties in numerical models. Material properties and boundary conditions may not be modeled correctly. There may be some microcracks in the structures, and these cracks may directly affect the modeling parameters. Modal testing gives correct uncertain modeling parameters that lead to better predictions of the dynamic behavior of a target structure. Therefore, dynamic behavior of special structures, such as minarets, should be determined with ambient vibration tests. The vibration test results may be used to update numerical models and to detect microcracks distributed along the structure. The operational modal analysis procedure consists of several phases. First, vibration tests are carried out, spectral functions are produced from raw measured acceleration records, dynamic characteristics are determined by analyzing processed spectral functions, and finally analytical models are calibrated or updated depending on experimental analysis results. In this study, an ambient vibration test is conducted on the minaret under natural excitations, such as wind effects and human movement. The dynamic response of the minaret is measured through an array of four trixial force-balanced accelerometers deployed along the whole length of the minaret. The raw measured data obtained from ambient vibration testing are analyzed with the SignalCAD program, which was developed in MATLAB. The employed system identification procedures are based on output-only measurements because the forcing functions are not available during ambient vibration tests. The ModalCAD program developed in MATLAB is used for dynamic characteristic identification. A three-dimensional model of the minaret is constructed, and its modal analysis is performed to obtain analytical frequencies and mode shapes by using the ANSYS finite-element program. The obtained system identification results have very good agreement, thus providing a reliable set of identified modal properties (natural frequencies, damping ratios, and mode shapes) of the structure, which can be used to calibrate finite-element models and as a baseline in health monitoring studies.     

Combined Experimental-Operational Modal Testing of Footbridges

In combined vibration testing, an artificial, measured force is used in operational conditions. This requires the identification of a system model that takes both the measured and the operational excitation into account. Advantages with respect to the classical operational modal analysis approach are the possibility of obtaining mass-normalized mode shapes and the increase of the excitation level and its frequency content. An advantage with respect to the classical experimental modal analysis approach, where the ambient excitation is not modeled, but considered as disturbing noise, is the possibility of using excitation levels that are of the same amplitude, or even smaller, than the ambient excitation levels. In this paper, combined modal testing of footbridges is explored using two case studies: a steel arch footbridge with spans of 75.2 m and 30.3 m and a concrete stress-ribbon footbridge with spans of 30 m and 28 m. The comparison of the modal parameters (eigenfrequencies, damping ratios, mode shapes, and modal scaling factors) obtained from a combined vibration test with the ones obtained from other modal tests and from a finite-element model, demonstrates the feasibility of using small and practical excitation devices for the modal testing of footbridges.     

Challenges in the Application of Stochastic Modal Identification Methods to a Cable-Stayed Bridge

This paper presents an analysis of the data collected in the ambient vibration test of the International Guadiana cable-stayed Bridge, which links Portugal and Spain, based on different output-only identification techniques: peak-picking, frequency domain decomposition, covariance-driven stochastic subspace identification, and data-driven stochastic subspace identification. The purpose of the analysis is to compare the performance of the four techniques and evaluate their efficiency in dealing with specific challenges involved in the modal identification of the tested cable-stayed bridge, namely the existence of closely spaced modes, the perturbation produced by the local vibration of stay-cables, and the variation of modal damping coefficients with wind velocity. The identified natural frequencies and mode shapes are compared with the corresponding modal parameters provided by a previously developed numerical model. Additionally, the variability of some modal damping coefficients is related with the variation of the wind characteristics and associated with a component of aerodynamic damping.     

浅谈现浇混凝土楼板裂缝在施工过程中的控制

现浇混凝土楼板经常出现裂缝，造成裂缝原因很多，裂缝形成多种多样，本文介绍了裂缝的存在的形式，产生原因，以及在设计施工，原材料控制中采取减少或避免楼板产生裂缝的措施。     

Vibration Studies of a Cantilevered Structure Subjected to Human Activities Using a Remote Monitoring System

Long cantilevered balconies used as seating areas in auditoriums, theaters, churches, and stadiums are often susceptible to excessive vibrations because of crowd movements. Measurement and analysis of the responses of such structures when subjected to human movements can provide a reasonable estimate of their dynamic properties. However, it is generally very difficult to artificially excite such massive structures with a measured input force at the same level as that exerted by a crowd. In addition, it is not yet well understood how human occupants’ presence may change the dynamic properties of these structures. This paper presents details of a remote vibration monitoring system (RVMS) installed on a large cantilevered balcony structure to collect the vibration records generated by rhythmic crowd activities. The results of the studies conducted indicate that the presence of human occupants resulted in a consistent reduction in the natural frequencies of the structure and an increase in the damping ratios for higher modes. Conclusions were also drawn regarding the applicability of the damping ratios recommended by a number of standards and design guides for the structure used in this study.     

Frequency Spectrum Analysis of Impact-Echo Waveforms for T-Beams

Numerical and experimental studies were performed to assess the transient impact response of 11 T-beams with various dimensions and aspect ratios. Numerical modeling was performed using a three stage finite-element modeling procedure which included modal analysis, resonant analysis, and three-dimensional transient dynamic analysis. The response at impact locations on both the top centerline of the flange and the bottom centerline of the web was investigated. Physical models of three of the beams were constructed in the laboratory to determine the physical response of the beams when subjected to a transient impact and to verify the numerical results. Relationships between the fundamental frequencies and the frequencies of higher cross-sectional modes of vibration were established for the various aspect ratios. Shape factors were derived from the numerical and experimental results. The practical significance of the results is demonstrated for a project where impact-echo testing was used to nondestructively assess the condition of a decommissioned concrete T-shaped girder.     

Softening Effects on Bearing Capacity of Mine Floors

A major cause for massive collapse of coal mines in the Illinois Basin is the softening of mine floors. Certain materials present in mine floors soften after coal extraction resulting in bearing capacity failure of coal pillars. The softening mechanisms considered in this study were (1) slaking∕swelling due to moisture exposure; and (2) creep or strain softening due to sustained loads. The effect of floor slaking or swelling on floor stability can be fairly dramatic. In some cases, shortly after pooling of water on the mine floor, collapse occurs resulting in surface subsidence. In other cases, the time factor is greater. The effect of softening on the ultimate bearing capacity of the mine floor was assessed by modeling representative conditions using FEM and elastoplastic elements. In the models, the properties of softened mine floors were determined from results from subsurface exploration work in floor areas that were stable and others that had failed. The zone of softening was evaluated by stress-field analysis.     

Damping of Vibration by Actively Controlled Initial Distortions

A concept for the artificial damping of free vibration by means of actively controlled initial distortions imposed on the structure is presented. Two formulations for active control are presented: The first simulates the natural damping properties of structures, while the second uses a more sophisticated modal strategy of control (but with a faster damping process). The general idea of damping by actively forced distortions is explained and followed by a simple example for a one‐degree‐of‐freedom system. Then, the simulation of natural damping (which is a particular case of active control) and the possibility of accelerating the damping process by the modal optimal strategy are discussed and demonstrated with some examples for a two‐degree‐of‐freedom system. Finally, the vibration control of a four‐degree‐of‐freedom system is presented to demonstrate the efficiency of the proposed method. The method of active damping is described for truss structures, but it can be easily generalized to include frame structures as well.     

Free Vibrations of Taut Cable with Attached Damper. I: Linear Viscous Damper

Free vibrations of a taut cable with an attached linear viscous damper are investigated in detail using an analytical formulation of the complex eigenvalue problem. This problem is of considerable practical interest in the context of stay-cable vibration suppression in bridges. An expression for the eigenvalues is derived that is independent of the damper coefficient, giving the range of attainable modal damping ratios and corresponding oscillation frequencies in every mode for a given damper location without approximation. This formulation reveals the importance of damper-induced frequency shifts in characterizing the response of the system. New regimes of behavior are observed when these frequency shifts are large, as is the case in higher modes and for damper locations further from the end of the cable. For a damper located sufficiently near the antinode in a given mode, a regime of solutions is identified for which the damping approaches critical as the damper coefficient approaches a critical value. A regime diagram is developed to indicate the type of behavior in each mode for any given damper location.     

Equivalent Modal Damping of Short-Span Bridges Subjected to Strong Motion

In this paper four different methods are investigated for estimating the equivalent modal damping ratios of a short-span bridge under strong ground motion by considering the energy dissipation at the boundary. The Painter Street Overcrossing (PSO) is investigated because of seismic data availability. Computed responses using the response-spectrum method with the equivalent damping ratios estimates are compared with the recorded responses. The results show that the four methods provide reasonable estimation of equivalent modal damping ratios and that neglecting off-diagonal elements in the damping matrix is the most efficient and practical method. The equivalent damping ratio of the PSO was nearly 25% under an earthquake with peak ground acceleration of 0.55g, which is much higher than the conventional assumption of 5%.     

Damping of Cables by a Transverse Force

A general asymptotic format is presented for the effect on the modal vibrations of a transverse damper close to the end of a cable. Complete locking of the damper leads to an increase of the natural frequencies, and it is demonstrated that the maximum attainable damping is a certain fraction of the relative frequency increase, depending on the type of damping device. The asymptotic format only includes a real and a complex nondimensional parameter, and it is demonstrated how these parameters can be determined from the frequency increase by locking and from an energy balance on the undamped natural vibration modes. It is shown how the asymptotic format can incorporate sag of the cable, and specific results are presented for viscous damping, the effect of stiffness and mass, fractional viscous damping, and a nonlinear viscous damper. The relation of the stiffness component to active and semiactive damping is discussed.