Mariacristina Spizzuoco

Experimental analysis of magnetorheological dampers for structural control

The possibility of reducing structural response under strong external excitations such as earthquakes and wind storms via control systems is attracting the interest of a large number of researchers. In the field of civil structures, control systems based on semi-active devices seem to be close to feasible implementation. Semi-active devices are typically passive elements capable of self-adjusting their own mechanical properties according to the instantaneous response of the hosting structure and, therefore, they can be considered as smart devices. Even though dampers based on magnetorheological fluids are considered very effective in practical implementations, the literature examining their properties from the structural control point of view is still quite limited. This paper aims to show the potential of such devices and to describe their properties from this special perspective. These properties include manufacturing issues, powering, range of variability of the mechanical parameters, their dependence on the feed current and overall response time.

Understanding and modelling the physical behaviour of magnetorheological dampers for seismic structural control

Semi-active magnetorheological (SA MR) dampers seem to represent the easiest way to materialize the concept of smart devices for semi-active structural control. SA MR dampers can be utilized as reactive force generators, when the control algorithms adopted to drive the devices are derived in the framework of control theory, or as smart dampers, when the real-time change of their mechanical properties is aimed at providing, at any time, the optimal amount of damping in a structure. The two approaches have different requirements in modelling the SA MR devices. Based on an experimental campaign on two prototype devices manufactured in Europe, the present paper compares the effectiveness of numerical models presented in the literature and analyses the response time and the dissipative capabilities of such devices.

Experimental issues in testing a semiactive technique to control earthquake induced vibration

This work focuses on the issues to deal with when approaching experimental testing of structures equipped with semiactive control (SA) systems. It starts from practical experience authors gained in a recent wide campaign on a large scale steel frame structure provided with a control system based on magnetorheological dampers. The latter are special devices able to achieve a wide range of physical behaviours using low-power electrical currents. Experimental activities involving the use of controllable devices require special attention in solving specific aspects that characterize each of the three phases of the SA control loop: acquisition, processing, and command. Most of them are uncommon to any other type of structural testing. This paper emphasizes the importance of the experimental assessment of SA systems and shows how many problematic issues likely to happen in real applications are also present when testing these systems experimentally. This paper highlights several problematic aspects and illustrates how they can be addressed in order to achieve amore realistic evaluation of the effectiveness of SA control solutions. Undesired and unavoidable effects like delays and control malfunction are also remarked. A discussion on the way to reduce their incidence is also offered.

Application of structural isolation and health monitoringThe ‘Our Lady of Tears Shrine’ in Syracuse (Italy)

The paper describes the seismic retrofit (carried out in the period February to March 2006) of the ‘Our Lady of Tears Shrine’ in Syracuse (Italy), made bysubstituting the bearings supporting its impressive dome with sliding seismic isolators equipped with elasto-plastic dissipators. After an accurate description of the construction and of its new antiseismic devices, installed between the dome and its 22 supporting columns, the dome raising and lowering phases, together with the instrumentation and data acquired during the operations, are reported in detail. The improvement of the structural behaviour after the seismic retrofit is demonstrated through the results of a numerical investigation under seismic excitation. Finally, the rehabilitation and reactivation of the pre-existing continuous monitoring system of the temple is examined. The structure has been recently included among those of the Italian network of buildings and bridges permanently monitored by the Italian Department of Civil Protection.

Skyhook-based monitoring and control of a steel building under seismic action

Sky-hook damping is one of the most promising techniques for feedback control of structural vibrations. It is based on the idea of connecting the structure to an ideal fixed point of the space through passive dissipative devices. Herein the benefit of semi-active (SA) sky hook (SH) damping is investigated for seismic protection of a two-story steel frame via shaking table tests. This kind of SA control is achieved implementing a continuous monitoring of selected structural response parameters and using variable dampers. The damping properties of the latter are changed in real-time so as to make the force provided by the damper match the desired SH damping force as closely as possible. To this aim, two prototype magnetorheological dampers have been installed at the first level of the frame and remotely driven by a SH controller. The high effectiveness of the control strategy is proved comparing the floor accelerations and interstory drift in both uncontrolled and controlled configurations. Practical issues about the implementation of the measurement system needed for the monitoring activity are finally discussed.

Experimental Assessment of a Skyhook Semiactive Strategy for Seismic Vibration Control of a Steel Structure

Sky-hook damping is one of the most promising techniques for feedback control of structural vibrations. It is based on the idea of connecting the structure to an ideal fixed point of the space through passive dissipative devices. Herein the benefit of semiactive (SA) sky-hook (SH) damping is investigated for seismic protection of a two-storey steel frame via shaking table tests. This kind of SA control is achieved implementing a continuous monitoring of selected structural response parameters and using variable dampers. The damping properties of the latter are changed in real-time so as to make the force provided by the damper match the desired SH damping force as closely as possible. To this aim, two prototype magnetorheological dampers have been installed at the first level of the frame and remotely driven by a SH controller. The effectiveness of the control strategy is measured as response to reduction in terms of floor accelerations and interstory drift in respect to the uncontrolled configuration. Two different calibrations of the SH controller have been tested. The experimental results are deeply discussed in order to identify the optimal one and understand the motivations of its better performance.

Monitoring and Identification of the Seismically Isolated “Our Lady of Tears” Shrine in Syracuse

This paper describes the installation and management of the monitoring system of the “Our Lady of Tears Shrine” in Syracuse, whose dome is an imposing r.c. and prestressed r.c. structure of about 22,000 ton that was seismically isolated by flat sliding devices with hysteretic dampers. The monitoring system, representing an upgrading and improvement of an old system never made working, has some innovative features, because it allows to manage with the same dedicated hardware and software both the slow (thermal variations, relative humidity, wind direction and velocity) and the fast acquisitions (dynamic vibrations by wind and earthquake). The monitoring system was inserted among those structures maintained and controlled by the Seismic Observatory of Structures of the National Department of Civil Protection. Some records of low magnitude earthquakes allowed to validate the correct behaviour of the whole structure, as well as to make a dynamic identification of the complex construction and to calibrate a detailed finite element model of the Sanctuary, thus predicting isolators’ behaviour under design earthquake.

Design and Retrofit of Multistory Frames with Elastic-Deformable Viscous Damping Braces

ABSTRACT This paper provides an exhaustive treatment for the analysis of non-proportional damping structures equipped with elastically deformable viscous damping braces. A frequency domain approach is adopted for solving the equation of motion as a function of nondimensional parameters. Accounting for the brace flexibility, an optimal damping system is suggested in order to minimize the resonance peak of the top-story displacement response on the first vibration mode. The effect of variation of the dissipative bracing system is demonstrated not only in terms of provided equivalent viscous damping and maximum response, but also in terms of dominant frequency and mode shapes.

Real-time monitoring and structural control of a wind turbine using a rocking system

The design of a semi-active (SA) control system to mitigate wind induced structural demand to high wind turbine towers is discussed herein. A variable restraint at the base, able to modify in real time its mechanical properties according to the instantaneous response of the tower, is proposed. A special control algorithm has been properly designed to drive MR dampers. It requires the tower is equipped with sensors for measurement of displacements and stresses. The real-time monitoring of the tower response is needed in order to make the SA system works in the sense of mitigating the structural demand against wind gust. A finite element model of a wind turbine model has been adopted to perform several numerical simulations. On the basis of these results, the optimal calibration of the controller has been found as the one allows to achieve different and conflicting, structural goals.

A Semi-active Rocking System to Enhance the Seismic Dissipative Capability of Precast RC Columns

This work is inspired by the idea of dissipating seismic energy at the base of prefabricated RC columns via semi-active (SA) variable dampers exploiting the base rocking. It was performed a wide numerical campaign to investigate the seismic behavior of a precast RC column with a variable base restraint. The latter is based on the combined use of a hinge, elastic springs, and magnetorheological (MR) dampers remotely controlled according to the instantaneous response of the structural component. The MR devices are driven by a SA control algorithm purposely written to modulate the dissipative capability so as to reduce base bending moment without causing excessive displacement at the top. The proposed strategy results to be really promising, since the base restraint relaxation, that favours the base moment demand reduction, is accompanied by an high enhancement of the dissipated energy due to rocking that can be even able to reduce top displacement in respect to the “fixed base rotation” conditions.