CHAPTER 1 – INTRODUCTION
WHAT IS SEISMICALLY-ISOLATED BUILDING?
In India, Zone4 & Zone 5 is one of the highest seismic-risk areas and has experienced many earthquake disasters from the last five decades. Seismic isolation technology evolved from various early ideas and types into what is often referred to as “Base isolation” as shown in figure 1.1. A base-isolated building has an “isolation level” between the foundations of the building and the upper structure (commonly referred to as the superstructure). Isolators and dampers are installed at the isolation level to decouple the building from earthquake ground motion.
Figure 1.1 Seismically-isolated building
1.1.1Laminated rubber bearings (LRB) LRB are most often use as isolators. Rubber bearings have very low horizontal stiffness, which allows horizontal movement of the superstructure and reduction of forces that are induced in the structure by earthquakes. Seismic isolation systems reduce the swaying of buildings, by increasing the amount of horizontal movement at the base. Figure 1.1.1 demonstrates this characteristic by comparing a seismically-isolated building to an ordinary fixed building and to a high-rise building without seismic isolation. This vertical stiffness of most rubber bearing is similar to that of the foundation of conventional building. This means that the vertical characteristic and movements of seismically isolated building are essentially the same as those of conventional buildings. Dampers are also included in the isolation system: to absorb earthquake energy, thus and reduce horizontal displacement.
Figure 1.1.1 Earthquake loads and periods of buildings during earthquakes
The gentle movement of a seismically-isolated building offers protection to the inhabitants and to the structure itself. It also minimizes damage to the non-structural components, such as equipment’s, curtain walls and partition walls, and the building contents. These characteristic distinguish seismically-isolated buildings from ordinary fixed-based buildings, thus allowing the building to remain fully operational during and after a strong earthquake. For this reason, seismic isolation technology should be considered for any building that severs essential functions.
The seismic isolation system has become more widespread in recent years, including as retrofits of exiting building, and in wide variety of different application for new building. The Table 1 show the seismically isolated building start built by year by different countries.
1 New Zealand 1974
2 US 1984
3 Japan 1988
4 US (Bridge) 1982
5 India 2003
Table 1 Seismically-isolated building starting being builtup in the world
How do seismically-isolated buildings behave in earthquake?
Despite the images implied by the word “isolate”, seismically-isolated building move horizontally at the isolation level, which is between the superstructure and the foundation: by horizontally-flexible isolation device. Most seismically-isolated buildings move slowly in horizontal direction, but with long horizontal displacement, while the ground moves rapidly and violently below (Fig 1-3). The relatively slow movement of the superstructure results in lower superstructure-deformation, thus building occupants do not feel the violent jolts of acceleration of the ground. The dampers in the isolation system absorb seismic energy, which maintains the stability of the superstructure. Along with the dampers and rubber bearings, the acceleration in the superstructure is much less than the acceleration of the ground below.
In contrast, ordinary fixed-base structures absorb seismic energy by plastic deformation of their structural members, such as beams columns, or walls , and the acceleration increases throughout height of the building (Fig 1-4)
Many seismically-isolated building are equipped with strong-motion accelerometer to measure building response in earthquakes, and many of the record obtained from these accelerometers are published. Table 2 shows the maximum earthquake occurs observed in India.
Date Place Magnitude
June 16, 1819 Kutch, Gujarat 8
Jan 10, 1869 Near Cachar, Assam 7.5
May 30, 1885 Sopore, Jammu and Kashmir 7
June 12, 1897 Shillong Plateau, Meghalaya 8.7
April 4, 1905 Kangra, Himachal Pradesh 8
July 8, 1918 Srimangal, Assam 7.6
July 2, 1930 Dhubri, Assam 7.1
Jan 15, 1934 Bihar-Nepal Border 8.3
June 26, 1941 Andaman Islands 8.1
Oct 23, 1943 Assam 7.2
Aug 15, 1950 Arunachal Pradesh-China Border 8.5
July 21, 1956 Anjar, Gujarat 7
Dec 10, 1967 Koyna, Maharashtra 6.5
Jan 19, 1975 Kinnaur, Himachal Pradesh 6.2
Aug 06, 1988 Manipur-Myanmar Border 6.6
Aug 21, 1988 Bihar-Nepal Border 6.4
Oct 20, 1991 Uttarkashi, up hills 6.6
Sept 30, 1993 Latur-Osmanabad, Maharashtra 6.3
May 20, 1997 Jabalpur, Madhya Pradesh 6
Mar 29, 1999 Chamoli District, Uttar Pradesh 6.8
Jan 26, 2001 Bhuj, Gujarat 7.7
Sept13, 2002 Andaman Islands, India region 6.5
Dec 26, 2004 Nicobar Islands, India region 7.1
Dec 14, 2005 Uttaranchal, India 5.1
Feb 14, 2006 Sikkim, India 5.3
Nov 6, 2007 Gujarat, India 5.0
Feb 6, 2008 West Bengal, India 4.3
Sept16, 2008 Maharashtra, India 5.0
Aug 10, 2009 Andaman Islands, India region, Andaman Islands earthquake7.5
June 12, 2010 Nicobar Islands, India region 7.5
Feb 4, 2011 Myanmar–India border region 6.2
Sept 18, 2011 India–Nepal border region, Sikkim earthquake 6.9
Oct 29, 2011 Sikkim, India 3.5
May 1, 2013 Eastern Kashmir, India 5.4
Jan 3, 2016 Manipur, India region, Imphal earthquake 6.7
Jan 3, 2017 India–Bangladesh border region, Tripura earthquake 5.5
Figure 1-5 Earthquake occurs in india (1985-2001)
What are the design advantage of Seimically-isolated building?
In seimically-isolated buildings, the seismic deformation and energy absorption are primarliy transmitted to the device that comprise the isolatioon system. Compared wth conventional fixed-base buildings, design planning for seismically-isolated building is not as restrictive regarding the placement of seismic elements, such as shear walls and braces. Additionally, it is easer to position partition walls, allowng more flexible layouts for many types of buildings such as commerical builldings, hospitals and museums.
More-complicated configuration of seismic elements are often necessary in order to reduce eccentricity, which is turn control stiffness. Such is the case of core-type building with reinforced-concrete walls, that have their elevators and stairways, or mechnaical rooms loacted offset in plan, or buildings that are setback or staggered in plan.
In seismicaly-isolated builldings, superstructure eccentrcity can be controlled by adjusting the loaction of the centre of the mass of the superstructure relative to the center of the stiffness of the isolation system.
What is a seismic isolation devive?
Seismic isolation device element that are installed between the foundation and upper structure of a building (commonly referred to as the superstructure) to reduce the effects of earthquake ground motion on the building. Seismic isolation device have four function.
Decouple the structure from the ground, and minimize the transmission of earthquake motion to the building (isolation function).
Suppport the weight of the superstructure in a stable manner at all times (load carrying function).
3. Reduce the amplitude shaking caused by earthquake (damping function).
4. Ensure that the superstructure returns to its original position after an earthquake (re-centering function)
Depending on required functions, seismic isolation devices are classified as either an “isolator “or a “damper”: the former satisfies the requirement of (1) and (2) (some types of (3) and (4) and the latter satisfies (3).
Isolators are designed to provide low horizontal stiffness with constant performance, such as stable load carrying capability to support the building weight. To reduce the level of movement to as practical range, damping is also required in the isolation system. Dampers absorb earthquake energy that results from the velocity of motion, and assist in the reduction of displacement of the isolation system.
In India, there are many types of isolators, such as Laminated Rubber Bearing LRB, Flat type Slider, Sphiercal slider and roller bearing. Some type of isolators have damping function, such as LRB , High-damping rubber bearing and slider. Dampers are typically metallic devices of steel or lead , fluid dampers, using soft, sticky fluid such as oil and friciton dapers.
The primary objective of this current work is to study of the earthquake response of multi-storey
Buildings using performance based seismic isolators. The effect of seismic response on G+5 storey building with the help of ETAB software 16.2.1 version, for various types of LRB isolators at different location has been investigated.
The main objectives of the present thesis are as follows;
To design a G+5 storey, reinforced concrete frame building using ETAB
To confirm the effectiveness of LRB under extreme field ground motion in comparison with fixed based structure.
To perform earthquake response study with different isolation system
To compare earthquake response on base isolated building and a regular building, determining their effectiveness.
To examine the effectiveness of LRB under low frequency ground motion
To explore possibility of modification in LRB for increasing its effectiveness under all types of ground motion.
According to the previous works done, first of all the thesis focuses on the general considerations of seismic isolated buildings which are world-wide applications of seismic isolated buildings, different type of isolators, the installation process of them, their mechanical characteristics, cost of isolated buildings and location. Finally, the design methods for different types of isolators are discussed to present simple, concise, and practical information and principles required by practitioners in seismic isolated buildings. Therefore G+ 5 storeys building in zone 5 are analyzed by LRB isolators to come up with the optimum case according to the seismic demand. Microsoft Excel spreadsheet is utilized in order to design Lead Rubber Bearing isolators. Finally, a G+5 storey building fixed-base and optimum isolated structures are analyzed in order to clarify differences between the performances of these two types of structures. Moreover to what have already been done, these two buildings are designed and their materials are compared to reveal the differences.
The scope of present study aims at evaluation of RC building (Designing according to IS code 456:200 ; IS 13920:2016 First revision) using response spectrum analysis. The performance based earthquake engineering technique known has Nonlinear response procedure has been effectively used in this respect. The Response spectrum has been carried out using ETAB 2016, a product of CSI.
The outcomes of analysis are compared in terms of Base shear, Spectral Displacement in x direction, Spectral Displacement in y direction, Storey drift ratio etc. Determine the best possible combination of reinforcement that would be economical, effective and must be limited in order to enable immediate occupancy.