1,720,986 research outputs found
Insights into inter-story isolation design through the analysis of two case studies
No full textInter-story isolation system (IIS, also appointed as mid-story isolation), is currently spreading and gaining significant popularity, mainly in Japan. However, while more than sixty applications have been realized in nearly twenty years, the conceptual framework for dealing with the design problem of IIS is not well established, since the IIS combines isolation and mass damping control strategies. In this paper two case studies of real inter-story isolated buildings are in depth examined for interpreting the latest design practice in the light of approaches and indications coming from the world of research. The two buildings, very different from each other, cover the wide applicability of IIS. One of them is a paradigmatic example of ideal IIS, with dynamic characteristics commonly adopted in the scientific community, i.e. a very rigid superstructure, and frequencies of the two structural parts well separated from the isolation frequency. The other building is a non-typical case of IIS, with the upper structures less rigid than the lower one, and both structural portions quite flexible. Modal and frequency response analyses are carried out on simplified two- or three- degree-of-freedom models, representing the reduced-order models utilized in the inherent scientific literature for grasping the influence of the main design parameters governing the dynamic problem. Modal and non-linear time history analyses are then carried out on multi degree-of-freedom models, for the seismic assessment of the buildings. The main results are reported and design implications are discussed
Vibration characteristics and higher mode coupling in intermediate isolation systems (IIS): a parametric analysis
No full textIntermediate isolation system (IIS) is currently spreading and gaining significant popularity, mainly in Japan. However, its potentials are not so well-known in European countries, and in USA only one application to building retrofit is registered. The dynamic behaviour of intermediate isolation systems, more complex than the two-degree-of-freedom behaviour of base isolation systems, gives rise to a twofold control mode, which combines isolation and mass damping strategies. However, the research contributions provided in the scientific literature usually concentrate on one single control mode, either isolation or mass damping, and the relevant design methods and criteria. This paper addresses the IIS design problem from a wider perspective and presents an explorative study on the vibration characteristics and dynamic behaviour of IIS, in order to identify the range of different behavioural modes and to propose relevant design guidelines. For these aims, a parametric analysis is carried out, varying the main design parameters, namely: isolation period and ratio, location of isolation layer and mass ratio, distributions of stiffness and mass in the upper and lower structures. A classical modal approach is initially assumed for assessing the contributions of each vibration mode on the global dynamic behaviour of IIS, with a particular focus on the effect of coupling of higher modes. However, since IIS is a non-proportionally damped system, a state space formulation is subsequently adopted for establishing the cases for which the simplified classical approach, only considering two damping values for the isolation and structural parts, can be adopted in a preliminary design stage. Finally, frequency response analysis is carried out for identifying the ranges of predominant isolation and mass damping behaviour and the effect of mode coupling both in terms of local and global response of the isolated models. Design implications are finally derived from the analysis results
Seismic Retrofit of Existing Masonry Buildings through Inter-story Isolation System: A Case Study and General Design Criteria
No full textThis paper investigates the potentials of inter-story isolation systems (IIS) for retrofitting irregular masonry buildings by means of an isolated vertical addition realised on the roof of the existing structure. By assuming a case study, a wide parametric analysis is firstly carried out on lumped-masses model for identifying the dynamic properties of the isolated superstructure that minimizes the global seismic response. Hence, a 3D FE model is adopted for the detailed design and analysis of the IIS structure, with different solutions, in order to regularize the global seismic response of the IIS complex. Finally, some general design criteria are suggested
Intermediate Isolation System for Adding Floors in Existing Buildings: Chart-Based Design
No full textImproving the Seismic Response of Tall Buildings: From Diagrid to Megastructures and Mega-Subcontrol Systems
Full text linkBackground:
Diagrid structures, widely used for the tall buildings of the third millennium, are characterized by a very effective behaviour in the elastic field due
to the grid triangulation. In particular, under horizontal actions, axial forces and deformations mainly arise in the structural members of the diagrid,
thus resulting in the reduction of the shear lag effect and racking deformations. The response to incremental horizontal actions beyond the plastic
threshold, however, shows a poor plastic redistribution capacity, with consequent low values of global ductility, in spite of a significant design
overstrength.
Objective:
In this paper, it is proposed to exploit the high elastic efficiency of the diagrid type and use a vibration control system, based on mass damping
mechanism with large mass ratios, to reduce a priori the inelastic demands due to seismic actions.
Methods:
Starting from the analysis of the seismic behavior of archetype diagrid buildings, a case study is selected to assess the effectiveness of the proposed
motion-based design approach. For this purpose, the diagrid is first transformed into a megastructure (MS) configuration by densifying the
diagonal elements at the most stressed corner areas and transfer floors, suitably chosen. Then, the exterior mega-frame is detached from interior
sub-structures, thus allowing for a relative motion between the two structural portions according to a “mega-sub-structure control system” (MSCS),
which activates the mass damping mechanism.
Results:
Time-history analyses carried out on simplified lumped-mass models confirm the effectiveness of the proposed strategy in reducing the seismic
response.
Conclusion:
Finally, the practical feasibility of the MSCS and engineering solutions for the relevant structural organization are discussed
Steel Vertical Extension of Existing Buildings with Isolation System: Diagram-Based Design Approach and Validation Through a Case Study
No full textInnovative mass-damping-based approaches for seismic design of tall buildings
Full text linkMass damping is a well known principle for the reduction of structural vibrations and applied in tall building design in a variety of configurations. With mass usually small (around 1% of building mass), the properly “tuned” mass damper (TMD) shows great effectiveness in reducing wind vibrations, but minor advantages under earthquake excitations. The above limitation can be surpassed by utilizing relatively large mass TMD. For this purpose, two different solutions are here proposed. In both cases, the idea is to separate the building into two or more parts, thus allowing for a relative motion between them, and activating the mass damping mechanism. In the first solution, the building is subdivided along elevation into an upper and a lower structure, separated by means of an intermediate isolation system (IIS). In the second solution, by revisiting the classical mega-frame typology, the exterior full-height structure provides the global strength and stiffness, and secondary structures, extending between two transfer levels, are physically detached from the main structure at each floor and isolated at transfer level. Simplified lumped-mass models are developed for illustrating the dynamic behaviour of the two solutions and carrying out parametric analyses. Procedures for deriving optimum values of design parameters are also proposed and compared to the parametric study
A diagram-based design procedure for Intermediate Isolation System in existing buildings with inelastic behavior
No full textVertical extensions of existing buildings can be realized through Intermediate Isolation System (IIS): the extension, equipped with a base isolation system on the rooftop of the existing building, can work as a mass damper, thus reducing the seismic demand on the old structure. The idea proposed in this paper is to predict the elastic or inelastic response of the existing structure in the IIS configuration by means of the results of simple linear analyses. Parametric response spectrum analyses are performed on simplified two degree-of-freedom models by varying the mass ratio and the periods of both the existing building and the new isolated vertical extension. So-called IIS design spectra are derived, and the results are provided as design charts. Given the period of the existing building and the mass ratio, the period of the new isolated vertical extension is selected to obtain the required/desired response of the existing building. For existing buildings working in the elastic field, the designer can derive the response of the existing building by utilizing the IIS design spectra as design charts. For existing building working in the inelastic field, the design charts can still be adopted, though within a more complex procedure, which accounts for two limit behaviors that the extended building exhibits in the inelastic field. The outlined design procedure is applied to some case studies and validated through the comparison with the results of nonlinear time history analyses
Coupling of Structural Additions for the Mitigation of Seismic Response in Existing Buildings
No full textIn the present paper, additions in structural steelwork are utilized for giving new life to old buildings in regions characterized by medium/high seismic hazard. Two models are here proposed, i.e.: vertical addition for masonry buildings and lateral addition for r.c. buildings. For the model of vertical addition, the connection between the masonry and steel structures is realized by means of an Intermediate Isolation System (IIS). For the model of lateral addition, an exoskeleton (EXO) is connected to the existing building by means of rigid or flexible and dissipative link. Two buildings, representative of the heterogeneous Italian building stock, are selected as case studies. Parametric analyses are firstly performed on lumped mass models to explore the feasibility and effectiveness of the IIS and EXO systems in reducing the seismic response of the case studies; then, once selected the design configurations of the new additions, more refined 3D FE models are adopted for the detailed analysis of the two solutions
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