Thesis title: Movable Façades for vibration reduction of buildings
The growing international demand for energy efficiency has paved the way for new attempts in the architectural field, moving towards significant improvements in sustainable building design. The desire to reduce the energy consumption is closely related to the spread of Nearly Zero-Energy Buildings (NZEB) capable of coupling the aesthetic and functional features with energetic aspects to minimize building consumptions. Among the latest generation strategies that have made it possible to achieve this appealing target, façade systems play a fundamental role for the entire building system, not only from an aesthetic point of view but above all in terms of energy saving. Since the nineteenth century, the façade industry has evolved rapidly, leading to the transition from the load-bearing masonry wall to the external self-supporting envelope (the so-called curtain wall) and, subsequently, to the shift of climateenergy functions from inside to outside, from which the Double Skin Façade (DSF) system was born. Recently, the concept to let DSF be movable with respect to the structure in order to improve the dynamical response of buildings has been proposed by Moon. While, in the past, the idea to let the cladding surfaces to undergo potentially large displacements, was hindered by the difficulty to realize suitable connections, due to the renewed interaction between technology and architecture, the possibility to realize movable and adaptive façades could nowadays be considered technically feasible. All the above mentioned facts provide the main motivation for the work described in this thesis, which aims to investigate advantages and disadvantages of the use of Movable Façade (MF) for vibration reduction of buildings. After a critical analysis of the state of the art, in the first part of the work, the performances of buildings equipped with MF are compared to those of analogous structures equipped with standard Tuned Mass Damper (TMD). As compared to classic TMDs, MFs offer an important advantage stemming from the fact that they use mass already available in the building, without wasting useful indoor space. The analyses show that, despite the strong similarities in the resulting equation of motions, the two systems may exhibit quite different responses and a critical analysis of the differences between the two systems is provided. The comparison shows that, depending on the mass and stiffness ratios, MFs can be potentially very efficient in reducing vibrations of the main structure, reaching levels of efficiency even larger than those obtainable by TMDs. However, the same study also shows that high efficiency of the MF can be obtained only at the price of displacements of the façade so large to significantly exceed functionally admissible levels. The main outcome of the first part of the work is that the limitation of the façade displacements is the paramount issue to be solved to let any application of MF feasible. Accordingly, the rest of the thesis is devoted to propose solutions aimed to solve this problem. In the second part of the thesis, a new connection device for MF is proposed. The device combines two main functional principles: a friction slider, inspired by the Variable Friction Cladding Connection (VFCC) device proposed by Laflamme and coworkers, and a system of dissipative bumper dampers, inspired by solutions adopted in the field of seismic pounding. The conception of the device and its nonlinear modeling are addressed in order to identify the main design parameters that influence the performances of the connection. In the third part of the thesis, the performances of MF connected to the building by simple friction sliders without bumpers are subject to a preliminary evaluation, based on a simplified Two-Degrees-of-Freedom (2DOF) model of a mid-rise RC frame building under harmonic excitation. This study shows that, although the presence of large friction dissipation can be, in some conditions, beneficial, large displacements of the façade remain an issue, hence strongly confirming the need of a complete devices equipped with bumpers. The performances of the complete device are then studied by means of parametric nonlinear dynamic analyses aimed to investigate the influence of the main design parameters. The main outcome of this part is that bumpers may be very efficient in reducing the façade displacements while keeping dynamical efficiency. However, it also turns out that the balance between the vibration reduction efficiency and façade displacement control may be delicate, as quite different results can be obtained depending on the frequency of the excitation. This result pointed out the need to re-evaluate the performances of MF under the more realistic situation of a building subject to wind actions, where the combination of several frequencies may give different results with respect to single-harmonic forcing. In the fourth part of the thesis, a Multi-Degrees-of-Freedom (MDOF) model of the same building considered in the third one, under the action of wind excitation is developed and used to investigate the performances of the MF connected to the structure by the complete device proposed in the second part. The analyses show that the use of connection devices with larger gaps with respect to bumpers provide better performances in terms of structure displacement although in some conditions acceleration performances may be reduced by impacts with bumpers. In the last part of the thesis, the case of the Isozaki tower, a 51-floors, 220-meters tall building recently realized in Milan (Italy) is studied. To mitigate wind-induced vibrations, Isozaki tower has been equipped with large viscoelastic dampers installed on the top of inclined trusses anchored to the façade and to the ground, externally to the building. Since such trusses have a strong impact in the visual appearance of the building, this case is a prominent example of interaction between structural and architectural aspects. The idea has been to redesign the façade of the tower in such a way to let eight cladding blocks to rigidly move, relatively to the structure. A MDOF nonlinear model of the Isozaki tower with a multiblock MF has been realized and the performances of the building under the action of wind has been evaluated. The analyses show that, after a proper calibration of the various design parameters, the MF could achieve the desired serviceability performance levels both in terms of accelerations and displacements, without impacting in a significant way the architecture of the building. In conclusion, although real applications of MF are still to come, this thesis addressed some of the main conceptual issues to be solved. Of course, several other problems remain to be solved, especially at the technological level, but the preliminary results obtained here, seem to confirm that the idea initially advanced by Moon could become applicable.