Francesco Ricciardelli

Passive and active mass damper control of the response of tall buildings to wind gustiness

Abstract The performance of passive (TMD), active (AMD) and hybrid (ATMD) mass dampers for the reduction of the buffeting response of tall buildings is investigated. First a simple 1+1 DoF system is considered to model the main structure provided with a mass damper, and the wind buffeting force is simplified into a white noise excitation. A linear quadratic regulator (LQR) control law is used, and in the case of the ATMD, the results are compared to those obtained with a closed form design procedure from the literature. Second a 64-storey building is considered, and modelled accounting for its first four longitudinal modes. In the latter case a more realistic buffeting excitation is considered, accounting for the frequency distribution of the atmospheric turbulence, and for its vertical correlation. It is pointed out how the performance of each device is strongly related to the response parameter to be mitigated, and how simplified 1+1 DoF models can inaccurately estimate the system response, and therefore the control performance.

Semi-active Tuned Mass Damper control strategy for wind-excited structures

Abstract Uncertainties in the main structure dynamic properties as well as those in the characteristics of the excitation may cause a deterioration of the performance of Tuned Mass Dampers. This may happen because the choice of tuning and damping ratio of the auxiliary system, based on the expected values of the dynamic properties of the main structure and on a simple excitation pattern, may prove away from the optimum. An empirical algorithm is presented in this paper which allows the performance of the TMD to be optimised, based on the measured response. The algorithm relies on two assumptions: the smoothness of the spectrum of excitation and the availability of an estimate of the dynamic properties of the main structure. As an example the algorithm is applied to a 64-story building subjected to turbulence buffeting.

Pressure distribution, aerodynamic forces and dynamic response of box bridge sections

Static and dynamic wind tunnel tests were carried out in smooth flow on a section model of the Sunshine Skyway Bridge. Deck surface pressures, support forces, structural response, as well as wake flow velocities were measured in the tests. In this paper, a first step is made to investigate the characteristics of the aeroelastic behaviour of a bridge deck, by relating the mean and fluctuating aerodynamic forces, deck pressure distributions and dynamic response at different wind speeds, and by comparing their values with those measured on the stationary deck. Three different response regimes of forced motion, torsional vortex shedding lock-in and torsional flutter were observed, associated with different ranges of the oncoming wind speed. In the forced vibration regime the aerodynamic forces and the pressure distributions compare very well to those measured in the static tests, indicating a quasi-steady behaviour. In the lock-in and flutter regimes, however, these quantities take values quite different from those measured on the stationary model, showing the nature of the aeroelastic interaction.

On the amount of tuned mass to be added for the reduction of the shedding-induced response of chimneys

Abstract Tuned mass dampers are commonly used to reduce the shedding-induced response of chimneys. In this paper, criteria for the choice of the amount of tuned mass to be added are presented. Collapse, displacement and fatigue limit states are considered. As a first step, the minimum additional mass required to prevent vortex shedding lock-in is evaluated. In a second stage, the additional mass required to limit the stable motion to a prescribed RMS displacement is computed, including the effect of errors in the tuning of the device. Finally, expressions to evaluate the fatigue life of the chimney are presented. An application to a particular chimney is used to illustrate the procedures and to compare the requirements, in terms of additional mass, related to the three different limit states.

Lateral Pedestrian-Induced Vibrations of Footbridges: Characteristics of Walking Forces

The forces a walker exerts to an oscillating floor differ significantly from those exerted to a fixed floor. This is because of a natural and unconscious gait modification, deriving from the necessity of minimizing energy consumption while maintaining balance. Among other factors, these forces depend on the frequency and amplitude of oscillation of the floor, and this dependency is claimed to be responsible for most of the large amplitude vibrations observed on flexible footbridges. This paper reports the results of an extensive experimental campaign carried out using an instrumented treadmill, and aimed at evaluating the characteristics of the lateral force exerted by pedestrians when walking on a floor driven into a harmonic lateral motion with varying frequency and amplitude. The total measured force is separated into motion-dependent or self-excited components proportional to the floor velocity and acceleration, respectively, and an external excitation, which is independent of the floor motion. The variation of the acceleration- and velocity-proportional forces together with the characteristics of the spectra of the external excitation is discussed.

Prediction of the response of suspension and cable-stayed bridge towers to wind loading

Abstract A semi-empirical method for the analysis of the response of some high-rise bridge towers to wind loading on the basis of simple wind tunnel tests on fixed sectional models is shown. The procedure operates in the framework of the linearized quasi-steady theory, and allows to predict the stable linearized mean and peak structural response. The towers considered are those made of two vertical or nearly vertical legs connected through horizontal members. If the connecting beams are stiff enough to make the whole tower behave as a beam, the well-kown theory for line-like structures can be applied. The input data, i.e. the mean values and power spectral density functions of the aerodynamic coefficients, can be drawn from wind tunnel pressure measurements on fixed sectional models. The calculation of the response needs to be done numerically. In some cases it is useful to apply a procedure based on the use of influence lines for the direct calculation of the response. The procedure is applied to the calculation of the response of a 107.50 m high suspension bridge tower, and the results are compared with those obtained by means of wind tunnel tests on an aeroelastic replica model.

Assessment of the Peak Response of a 5MW HAWT Under Combined Wind and Seismic Induced Loads

The rapid growth of the wind energy industry has brought the construction of large-scale wind turbines with the aim of increasing their performance and profits to areas characterized by high seismic hazard. Previous research demonstrated the seismic vulnerability of large-scale wind turbines when seismic and wind actions are considered simultaneously in the demand model. In this study, the response of the supporting structure of a land-based horizontal axis wind turbine under the combined effects induced by wind and earthquake is presented.

Deterministic and Probabilistic Serviceability Assessment of Footbridge Vibrations due to a Single Walker Crossing

This paper presents a numerical study on the deterministic and probabilistic serviceability assessment of footbridge vibrations due to a single walker crossing. The dynamic response of the footbridge is analyzed by means of modal analysis, considering only the first lateral and vertical modes. Single span footbridges with uniform mass distribution are considered, with different values of the span length, natural frequencies, mass, and structural damping and with different support conditions. The load induced by a single walker crossing the footbridge is modeled as a moving sinusoidal force either in the lateral or in the vertical direction. The variability of the characteristics of the load induced by walkers is modeled using probability distributions taken from the literature defining a Standard Population of walkers. Deterministic and probabilistic approaches were adopted to assess the peak response. Based on the results of the simulations, deterministic and probabilistic vibration serviceability assessment methods are proposed, not requiring numerical analyses. Finally, an example of the application of the proposed method to a truss steel footbridge is presented. The results highlight the advantages of the probabilistic procedure in terms of reliability quantification.

Pedestrian-induced lateral forces on footbridges

This paper investigates the phenomenon of excessive pedestrian-induced lateral vibrations as observed on several high-profile footbridges. The vibrations are a consequence of human-structure interaction, in which the forces generated by the pedestrians depend strongly on the vibration of the underlying pavement. An extensive experimental analysis has been carried out to determine the lateral forces generated by pedestrians when walking on a laterally moving treadmill. Two different conditions are investigated; initially the treadmill is fixed and then it is laterally driven in a sinusoidal motion at varying combinations of frequencies (0.33–1.07 Hz) and amplitudes (4.5–48 mm). The component of the pedestrian-induced force which is caused by the laterally moving surface is herewith quantified through equivalent velocity and acceleration proportional coefficients. It is shown that large amplitude lateral vibrations are the results of correlated pedestrian forces in the form of negative damping, with amplitudes that depend on the relationship between the step frequency and the frequency of the lateral movement.