New trends in the architectural design of footbridges feature an unprecedented slenderness, especially when these are located in the urban environment. For this reason, static analyses and a design towards the ultimate limit state have proven inadequate in many circumstances, and the main objective in the structural design is becoming that of assessing the serviceability limit state through dynamic analyses. On the other hand, the key issue of dynamic analyses is the availability of reliable models for the structure and for loads, and in the particular case of pedestrian action the lack of commonly accepted models for walking, running and jumping has become the weak link in the whole structural design process. In a first stage of the present work, vibration measurements were taken on a recently built cable-stayed footbridge, whose second vibration mode was excited by runners. As a second step, a dynamic loading model for the vertical component of the running-induced force was developed, which was used for the finite element analyses of the footbridge. Finally, tuned mass dampers (TMDs) represent a quite mature technology for reducing the resonant response of flexible structures, but their effectiveness is heavily dependent on the tuning ratio. In the case of footbridges, pedestrians can act as a significant part of the vibrating mass; thus, varying the vibration frequency, which makes it difficult to properly tune the damper frequency. Semi-active TMDs can be looked at as passive devices able to adjust their dynamic parameters according to a given control logic. A physical description of a control algorithm is given in the paper, and its performance is discussed.

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.

This 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 temperature) on the bond behavior of two different types of composites systems glued to concrete elements: a fiber-reinforced polymer (FRP) system made of a carbon sheet applied with epoxy resin and a polybenzoxozole (PBO) grid applied with a cement-based mortar, i.e., a fiber-reinforced cementitious matrix (FRCM) system. Several environmental conditions have been considered (partial immersion in water at 23, 30, and 40°C for short and long periods with and without further drying processes, exposure in air at 30 and 40°C) before testing the specimens according to two well-known setups for bond tests: a single push-pull shear test and a beam test. The experimental results were mainly analyzed in terms of failure modes and loads, showing a clearly negative effect of the conditioning factors for the specimens with the carbon fibre reinforced polymer (CFRP) sheet as the conditioning time increases because of the plasticization phenomena of the epoxy adhesive. Conversely, for the specimens with the PBO grid, the failure loads were slightly lower or even greater than the ones relieved for the reference specimens as the exposure periods increase, whereas in the case of short exposure, the bond strength reduced and the scattering of the experimental resulted increased

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.

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The experimental analyses of response time and dissipative capability of two prototype magnetorheological semi-active dampers are presented herein. These activities have been conducted during an Italian research project on devices manufactured in Germany. A detailed report of the response time analysis based on experimental data is presented and commented. It is shown how the control delays are strongly dependent on the effectiveness of the electric part of the control hardware, generally being less than 10 ms if special care is paid in designing the whole control chain. The dissipative capacity of the devices is further analyzed under the action of different imposed displacement laws, investigating a large range of displacement amplitudes, frequencies, and feeding currents. Interesting comparisons in terms of energy are finally drawn between magnetorheological damper used in a passive (constant current) and in a semi-active mode (variable current commanded by an energy-based control logic)

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