Antonio Occhiuzzi

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.

Use of seismic early warning information to calibrate variable dampers for structural control of a highway bridge: evaluation of the system robustness

The seismic events occurred in recent years highlighted the extreme vulnerability of large part of the existing constructed facilities and the need to adopt innovative solutions to improve their seismic performance. With this purpose, the possible exploitation of a seismic early warning system (SEWS) in the framework of semi-active structural control using magnetorheological (MR) dampers is herein investigated. The main idea consists in the use of these time-varying properties devices to control an hosting structure by changing their behaviour according to an anticipate estimate, provided by the SEWS, of the peak ground acceleration (PGA) of the incoming earthquake. In this way, the dampers are able to adapt their mechanical characteristics to the specific earthquake obtaining the optimal seismic response. The present paper describes the application of this protection technique to a case-study problem, a highway bridge located in Southern California. The seismic response of the benchmark bridge is investigated by nonlinear time-history analyses by adopting 16 real earthquake ground excitations. These accelerograms cover a wide variety of magnitudes, distances to fault and soil types. Possible errors on estimation of PGA provided by SEWS and their effects on the proposed control system are also considered. The results obtained confirm that unavoidable errors in the PGA estimates provided by the SEWS do not propagate to the seismic response. Conversely, the proposed strategy turns out to damp these errors, resulting in a robust seismic behaviour of the protected structure.

A Semi-active Control System for Wind Turbines

A semi-active (SA) control system based on the use of smart magnetorheological (MR) dampers to control the structural response of a wind turbine is proposed herein. The innovative approach is based on the implementation and use of a variable-properties base restraint. This is able to modify in real time its mechanical properties according to the instantaneous decision of a given control logic, the latter addressed to control one or more structural response parameters. The smart base restraint is thought to be a combination of a smooth hinge, elastic springs, large-scale adjustable MR dampers, and a control algorithm that instantaneously commands the latter during the motion, making them to modulate the reactive force as needed to achieve the performance goals. The design and operation of such a system are shown with reference to a case study consisting of an almost 100 m tall wind turbine, realized in a 1/20 scale model at the Denmark Technical University (DTU). Shaking table tests have been performed under the action of two different types of wind loads and by using two purposely written control logics, highlighting the high effectiveness of the proposed SA control technique and encouraging to further investigate in such direction.

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.

A Semi-active Oleodynamic Damper for Earthquake Control. Part 1: Design, Manufacturing and Experimental Analysis of the Device

Within an experimental research project financed by the European Union, a prototype semi-active oleodynamic damper has been conceived and manufactured in Italy starting from technology today currently adopted for passive energy dissipation devices. The new device was obtained by just adding two identical electrovalves and an external oleodynamic circuit to a commercially available silicon oil damper. Static and dynamic tests performed on the damper allowed to determine its stiffness and energy dissipation characteristics when passively operating (both electrovalves are always closed or open) as well as to measure the release and insertion times of the electrovalves during opening and closing tests. The experiments indicated a non-linear quadratic viscous constitutive law for the damper as well as operating times of electrovalves in good agreement with specifications given by the manufacturer. A semi-active assembly composed by the prototype damper and external flexible steel plates was then mounted and subjected to further tests in order to definitively characterize through a simple mathematical model the entire system and to completely identify its operating delays and their physical sources.

Effects of Environmental Conditioning on the Bond Behavior of FRP and FRCM Systems Applied to Concrete Elements

AbstractThis paper presents the results of an extensive experimental campaign of bond tests aimed to assess and compare the influence of several environmental conditioning factors (humidity and tem...

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.

Smart passive control of seismic structural vibrations exploiting early warning information

The combined use of two emerging technologies in the field of seismic engineering, apparently far each other, is focused herein. The first, semi-active (SA) control aims at smartly reduce the seismic effects induced by the earthquake on the structures; the second, seismic early warning (SEW) is instead addressed to make anticipate estimate of the intensity measures of an incoming earthquake some seconds before it strikes a given site. Current research on SEW include the anticipate estimate of the peak ground acceleration (PGA). The paper proposes the exploitation of this earthquake parameter in the framework of SA control strategies. The effectiveness of the proposed strategy is assessed and compared with other more consolidated control strategies. Possible errors on estimation of PGA provided by the SEW system and their influence on the effectiveness of the proposed control strategy are also 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.