International Research Journal of Engineering and Technology (IRJET) Volume: 04 Issue: 02 | Feb -2017
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e-ISSN: 2395 -0056 p-ISSN: 2395-0072
Seismic Base Isolators under Individual and Combined Use in Multi Storied Buildings – A Review Nithin A V1, Asha Varma P2 1PG
Student, Department of Civil Engineering, NSS College of Engineering, Palakkad, Kerala, India Department of Civil Engineering, NSS College of Engineering, Palakkad, Kerala, India
2Professer,
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Abstract - Base isolation refers to the principle which
of the input energy to the isolators is lost in the hysteresis loop. Thus the low lateral stiffness and hysteretic behaviour makes them effective in passive seismic control of seismic response of buildings. These also re-centres the structure when ground motion due to earthquake ceases and should be designed in such a way that necessary rigidity is provided under low service loads. This technique is also used for the seismic retrofit of historic structures, buildings containing motion sensitive equipment, high risk buildings (eg, nuclear power plants), buildings of special importance after earthquakes (hospitals, disaster management centres) etc.
introduces flexibility to the supports of the building in the horizontal plane and ensures the period of the buildings outside that of the earthquakes acting on it. These isolators are either installed as a single type or using different isolators on the base of same building. The concept of combined and multiple isolation techniques is not new. The combined isolation is often confused with hybrid isolation where active or semi active isolation systems are used in combination so that effective reduction of seismic response can be carried out in case one of the system fails during the event. The use of one or more passive isolation in combination or the same with a friction slider mechanism is referred as combined base isolation. To avoid the complexity of design and analysis the studies were mostly limited to implementing these systems at the base of the building. The multiple isolation structure has additional isolation devices in stories by inserting isolators in the middle story in addition to the base. In this paper, the suitability of type of base isolators under individual and under combined base isolation and their effectives to reduce interstory drifts and accelerations of the buildings is studied. A brief idea about base isolation and its different types is needed beforehand.
A typical base isolation must satisfy the three basic criteria listed below: 1. Decoupling action between foundation superstructure with or without flexible mounting.
Base isolation decouples the structure from ground during a seismic event and prevent damages that may occur due to the seismic event. For the past decades, its proven that base isolation is one the effective and popular methods to reduce the seismic response on these types of structures. It is a passive control device which is installed between the foundation and base of the building. The basic principle is either deflection or absorbing the seismic energy. First is achieved by making the building flexible at the base in lateral directions, this increase the fundamental time period of the structure. Also the buildings having longer time periods attract less seismic forces. The nonlinear response of isolators helps in seismic energy absorption. The (internal) force–displacement curve of isolators under sinusoidal excitation exhibits hysteretic behaviour and, therefore, much
Impact Factor value: 5.181
3.
Enough rigidity under low in-service loads
Generally, types of the base isolators can be grouped into (i) Elastomeric bearings, and (ii) Friction sliding bearings and (iii) Roller bearings. The Elastomeric bearings include, laminated lead rubber bearings (LRB), High Damping laminated Rubber Bearing (HDRB) and Low damping laminated rubber bearings. In the friction type, Flat sliding bearings, friction pendulum systems (FPS), Spherical sliding bearing are used. Each isolator differs in their response to an earthquake. Each isolator has its own stiffness and damping characteristics and also each earthquake response varies based on the method of analysis performed. Thus an effective stiffness of base isolation acts in case of combined isolation technique. Most commonly used bearings are Lead Rubber and Friction pendulum bearings. Lead Rubber bearings consist of alternate layers of rubber and steel shims with a central lead core. Fig.1. The elastomeric material provides the horizontal flexibility to the isolator, the lead core provides energy dissipation, while the internal steel shims enhance the vertical load capacity and minimizing lateral buckling. The steel shims also contribute to plastic deformation of the central lead core. The rubber layers deform laterally during an earthquake event of the structure and allows the building
1.INTRODUCTION
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Energy dissipation action
2. BASE ISOLATION SYSTEMS
Key Words: Base isolation, Lead Rubber Bearings, Friction Pendulum Bearings, Non linear dynamic analysis, Combined base isolation.
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