Earthquake disasters have consequences and characteristics abundant completely different from different disasters. The main characteristic of this disaster is its uncertainty. An earthquake event is uncertain with reference to its location, magnitude, intensity, time of incidence, duration, etc. The earthquake happens without any warning and therefore the whole damaging effects occur within a seconds. The earthquake problem is further sophisticated by the consequences of different structure with different properties and varying degree of design and construction.
Designing structures to resist earthquakes may be a difficult task, for satisfying the conflicting parameters of safety and, economy. The task is additional intrigued, by the uncertainty of the planning event; but, it’s necessary to adopt a very best approach for earthquake resistant design based on the design requirements and economic concerns.
Conventional approach adopted to resist earthquake generated force and hence to minimize of to prolong the damage of the structure is to provide correct strength, stiffness and inelastic deformation capacity i.e. Ductility (IS 13920:1993). In conventional earthquake resistant design philosophy the structure is predicted according to perform without major damage under minor and moderate earthquake and under major earthquake the damage may be vital without collapse. The target thought-about during this design philosophy is that collapse of structure is delay in order that humans will safely leave the building. The earthquake energy is absorbed by structure itself leading to damage.
Under gravity load (static load) condition the tensile stresses develop at a specific location within the component of structure, like as bottom of beam, slab, etc. below such loading no tensile stresses develop within the columns as they’re subjected to axial loading only. Therefore once the structure is meant as a standard moment resisting frame for static load, tensile reinforcement is provided at these locations only.
But below earthquake excitation force (dynamic forces) the tensile forces could develop at any location of the elements of structure such as either top or bottom of beam and additionally within the columns. Since concrete cannot carry this tension, reinforcement is required at each faces of beams to resist reversal of bending moment. Otherwise failure can occur at the locations wherever tensile reinforcement isn’t provided.
To avoid this failure it becomes necessary to enhance the ductility of the structure in order that it will deform without collapse throughout earthquake shaking. Under this condition structures have to be compelled to design with failure-mode-control approach. Specially appropriated ductile elements are designed to withstand many several cycles well on the far side yield underneath reversed loading.
The yield levels being chosen in order that the forces transmitted to different element of the structure are limited to their elastic or low ductility range. The yielding lengthens the basic amount of the structure, detuning the response aloof from the energetic period of most of the earthquake ground motion. The hysteretic behavior of the ductile element provides energy dissipation to damp the response motions.
The ductile behavior of the chosen elements ensures spare deformation capability, over a number of cycles of motion, for the structure as whole to ride out the earthquake attack. In well-designed typical structures the yielding action is meant to occur at intervals the structural members at specially designed locations (plastic hinge zones). Yielding of structural members is an inherently damaging mechanism, even supposing applicable choice of the hinge locations and careful description will ensure structural integrity.
Massive deformations at intervals the structure itself are required to withstand strong earthquake motions. These deformations cause issues, for the design of components not supposed to supply seismic resistance, as a result of it’s difficult to confirm that unintended loads don’t seem to be transmitted to them once structure is distorted significantly from its rest position. Additional drawback happens within the detailing of items such as windows and partitions, and for the seismic design of building services.
Passive control systems don’t need associate external power supply for their functioning. Passive control devices import the forces that are developed in response to the motion of the structure. The energy in passive controlled structural system, as well as the passive devices, accumulated inflated by the passive control devices. Passive control devices generally need little maintenance. The main two types of passive control systems are, Base isolation and Passive seismic dampers.
The Seismic Base Isolation techniques to reduce the earthquake effect on the structure up to negligible level. The base isolation seismic technique, consist of a bearing allowing the horizontal movement, a damper controlling the displacement and component providing fixed under lateral load. In base isolation system foundation and super structure are isolated with help of isolation system.
If it is possible at one and the same time to hold up the building and let the ground move underneath then the large displacements, story drift and hence damage to the super structure will be greatly reduced. Base isolators reduce the structural response by filtering the seismic excitation and by dissipating energy, thereby reducing the energy that need to be dissipated by structure. During earthquake the ground accelerations induce large displacements at the isolator level and minimize the acceleration and story drift of super structure. The isolation system does not absorb the earthquake energy, but rather deflect it through the dynamics of the system.
The basic idea of base isolation is to prolong the period of time of the structure by incorporating flexible elements between the super structure and foundation. Because of flexibility of the isolator layer, the time period of motion of the isolator is comparatively long; as a result the utilization of isolator shifts the elemental period of the structure off from the predominant periods of ground excitation.
Standard structures have relatively lower time period and that they match with predominant period of most of the earthquakes. As a result that they experience large accelerations and hence large forces. In contrast the base isolated structure includes a longer time-period leading to lower acceleration. Consequently the standard structures experience lower displacements and base isolated structure high displacement at isolator level. Considering large displacement is at isolator level, the super structure displaces as a rigid body.
Active control system is requires a large power source for operation of electro-hydraulic or electro-mechanical actuators that provide management forces to the structures. Control forces are developed based on feedback from sensors that measure response of the structures. Once earthquake excitation force reach to a particular level computer activate the active seismic dampers, which absorb or dissipate, the energy or add a counter, balancing force.
Hybrid control system implies the combined use of active and passive control system. In this system-base isolators used as a passive device and active seismic dampers used as an active, device. The base isolator primarily; performs an open loop control role by. Acting as a superstructure input signal filter while active dampers (TMD) perform a closed loop control role. The hybrid system involves a reduction of overall system response even in presence of nonlinear behavior of isolator.
