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)
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
ADVANTAGES & LIMITATIONS
• 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.
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.