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Wednesday, January 11, 2017

BASE ISOLATION AND SEISMIC CONSIDERATION IN EARTHQUAKE RESISTANT STRUCTURES - 3

Page 3                                                                                                                      
CHAPTER 4
FUTURE TRENDS IN BASE ISOLATION 

Many of the base isolation techniques described above involve the materials, which are susceptible to deterioration with time. Regular inspection and maintenance of the system is required. Special measures need to be taken for fire protection. As such it is desirable to develop such an isolator, which has a life span equal to the life of a structure, free from effects of environment and fire. Also it should be free from maintenance. Hence it will be an ideal case if researchers develop an isolator using materials which are unaffected by environment or affected by it to very low extent like natural earth of specific qualities having inherent properties of spring action and friction. 
The equivalent spring constants and damping co-efficient for foundations resting on soil can be worked out using equations given in the table (4.1). Equations for damping accounts for material as well as radiation effects.
From the equations which are valid for low strains, it can be seen that the spring stiffness is more for large size foundations and for greater value of shear modulus G. Also it is dependent on Poisson’s ratio of soil. Thus the spring values of the soil can be altered by varying the above parameters by choosing soil of appropriate properties. In similar way the damping properties of the soil medium can be altered. Also the effect of damping and isolation can be obtained by allowing the structure to slide on a soil medium to required extent. However, in the case of using soft geological materials such as soil, sand, pulverised granite etc, the material will see large strains. 

Table 4.1.Spring constant and damping coefficients
for foundation on homogeneous half space

(Source: S. J. Patil, G. R. Reddy Website: www.ijetae.com)

Where - 
Ρ = mass density of soil 
Vs = Shear wave velocity of soil medium 
G = ρ Vs2 
ν = Poisson’s ratio of soil medium 
R = equivalent radius for rectangular foundation (R= √BL/π for translation and R= 4√4BL3 / 3π for rocking). 
B = width of the foundation perpendicular to the direction of horizontal excitation 
L = length of the foundation in the direction of horizontal excitation 
I0 = total mass moment of inertia of structure and foundation about the rocking axis at the base 
IT = polar mass moment of inertia of structure and foundation 
βx, βψ, and βv are constants depending on ratio L/B 
C1= 0.5 ; C2 =0.30/(1+ βψ); C3 = 0.8



CHAPTER 5
ADVANTAGES & LIMITATIONS

5.1. Advantages
Structural  Damage  is  restricted  when  the  structure  is  built  on  a  suitable  seismic isolating system.  
Damage  to  indoor  services  and  facilities  would  be  of  little  concern  which  would normally  affect  gas,  water  or  swage  leakage  for  unfortified  structures.  The  base Isolation  will  protect  the  structure  by  preventing  plastic  deformation  of  structural elements,  because,  the  super-structure  demonstrates  elastic  behaviour  during  initial and following excitation of the base.  
Secondary damage and injury as a result of falling furniture would be restricted. In the other words,  the  level  of  safety  is  increased  significantly when  using  base  isolation system rather than conventional systems.  
The function of buildings can be ensured during an excitation or even after a major earthquake as super-structure is designed to remain elastic.  Therefore,  plastic deformation  of  structural  elements  can  be  prevented  and  the  building  is  still  a  safe place to remain and life can continue as normal.  
Evacuation routes and corridors are normally secured in a base-isolated building after an  earthquake  so,  horror  of  earthquake  can  be  eased  and  psychological  burden  is alleviated.  
Reduction  in  earthquake  input  forces,  coul lead to slender structural elements  and consequently the considerable reduction in the whole weight of structure, which givesthe noteworthy reduction in construction materials and construction costs. 
Considerable safety improvements would reduce disaster management protocol for such buildings during an earthquake and reduction of repair costs after an earthquake, seismic isolation can reduce life cycle cost.

5.2. Limitations

Base isolation enables the reduction in earthquake-induced forces by lengthening the period of vibration of the structure. However, Base isolation is not suitable for all buildings. Most suitable candidates for base-isolation are low to medium-rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation.. Period of vibration in building increases with increasing height. Taller buildings reach a limit at which the natural period is long enough to attract low earthquake forces without isolation. Therefore, seismic isolation is most applicable to low and medium rise buildings and becomes less effective for tall ones. The cut off mainly depends on structural systems or type of framing system. Cost involved in constructing a new building is higher than the cost of conventional earthquake resistant structural system. Seismic isolation bearings are expensive. Due to these economic considerations, even in developed countries these devices have so far been used for important buildings only. To enable its use for common buildings, some low cost devices have to be developed.

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