Seismic activities affect the constructed buildings in many ways and the awareness about the development of these seismic retrofitting techniques to make them more earthquake-resistant is available now. The damage due to seismic activities is caused by uncontrolled ground motion or soil failure after a massive earthquake.
Over time, scientists have gained more knowledge of the damages caused by earthquakes in different types of buildings. These have been used for designing the right types of strategies and techniques for protecting the buildings from the subsoil activities caused by seismic events.
In the present day, we can see many types of seismic resistant activities that are being carried over different types of buildings. This process of making the building resist the damages caused due to earthquake and related activities is called seismic retrofitting and this has emerged as a separate branch of the construction industry.
The seismic retrofitting is done for those buildings that have been constructed without adequate protection against seismic damages.
As there is imminent danger of the buildings from being affected by earthquakes seismic assessment is carried out frequently and suitable seismic retrofitting techniques are carried out to make them resist the damages caused by earthquakes.
The best advantage of the steps carried out to the buildings as seismic retrofits are capable of giving protection to the buildings against most of the natural calamities like tropical cyclones, tornadoes, and severe winds from thunderstorms.
The currently available seismic retrofitting is concerned with structural enhancements of the buildings and also offers protection to the non-structural elements so that full-fledged protection is available to the buildings. However, more attention is given to the modern buildings to make them earthquake-proof even at the stage of the initial structural design rather than making subsequent modifications made to the buildings after their construction.
In the present times, the new seismic retrofitting techniques are introduced and these are used with the introduction of new materials like fiber-reinforced polymers, fiber reinforced concrete and high strength steel, etc.
- Enhancing the level of global capacity or the inherent strength of the building by providing cross braces or new structural walls at the right places in the building.
- Drastically reducing the seismic demand of the building by using the techniques of supplementary damping and base isolation systems
- Providing more strength and capacity of individual structural elements in the building by selectively upgrading the local capacity through deformation, increasing the ductility, strength or stiffness of individual structural components
- Undertaking selective weakening retrofit by adopting a suitable counter-intuitive strategy for changing the characteristics of the inelastic mechanism of the structure with due recognition of the built-inn strength and elasticity parameters of the building.
- Providing sliding connections like passageway bridges to allow additional movement between seismically independent structures in the building
- Add more numbers of seismic friction dampers to provide the building with required amounts of damping for additional stiffness in the building.
These seismic retrofitting techniques are used to bring in more to the safety of the public with the help of the available engineering solutions that are in tune with the economic and political and other considerations that take up the responsibility of protecting against the damaging effects of the earthquakes.
As many new performance-based earthquake engineering solutions are designed, now they should be used to bring in the positive effects on the performance objectives sought to be achieved irrespective of the building types or the types of soil they are built on. These performance levels must be tuned towards providing
- Public Safety: the seismic retrofitting, in this case, must be done in such a way that there is minimum loss of life or injuries to the occupants of the building or those who are passing by during severe seismic activities. These retrofits are done in such a way that people inside the building have sufficient time to exit the building, and in case if there is a complete collapse of the building it must be able to be completely torn down and removed easily.
- Survivability of the Building: when this type of retrofit is applied to a dwelling unit or a commercial building it must remain safe to exit for sufficient time and it must be able to be repaired without making a complete replacement. The retrofits to these buildings can keep the building in a safe condition and it remains good for occupation.
- Retaining of Structural Functionalities: in this type of retrofit the base structure of the building is made to remain undamaged and the building continues to be in a state fit for human occupation and the repairs required after seismic activity are only cosmetic like plastering of minor cracks, setting right drywalls or stucco, etc
- Unaffected Structure: in this type of performance objective the retrofits are done in such a way that the building remains unscathed even after a severe seismic activity and the building remains in a fit condition as it was before the seismic event.
Some Of The Most Used Seismic Retrofitting Techniques:
- External Post-Tensioning: This type of seismic retrofitting is done in new structures using high strength steel tendons for providing an a-moment resisting system that can recenter itself in case of a severe seismic event. This is can be used for strengthening of bridges and non- ductile reinforced concrete frames. With the use of the pre-stressing technique, more strength is added to beams, columns, and beam-column joints.
- Base Isolators: Are the structural elements that are added to a building for providing it with necessary insulation from the shaking ground in case of an earthquake. By doing this, it is possible to protect the building and its integrity thereby boosting its seismic performance. This is usually done by using the earthquake engineering technology like seismic vibration control and it is now being modified and standardized so that it can be used both for new buildings as wells as the ones that are existing. This is a revolutionary technique in the seismic retrofitting in which the building is separated from the foundations using the steel or concrete reinforced beams placed underneath the foundation. Isolating pads or base isolators are used to completely restrict the transmission of the ground motion during an earthquake and help the building to be positioned properly onto the foundation thereby drastically increasing its stability.
- Supplementary Dampers: Help in absorbing the energy of motion and is provided with means to convert it to heat energy. This can reduce the resonant effects in the building structure which are rigidly attached to the ground. In addition to this supplementary damping is provided to reduce the displacement and acceleration demand that arises within the building structures. As these supplementary dampers act similarly to Shock absorbers in vehicles these are good to withstand the effects of the initial shock as well as the periodic resonant motion of the structure that repeated ground motion induces by way of aftershocks.
- Tuned Mass Dampers: Are used for reducing the swaying motion caused by the earthquake by using movable weights and a combination of springs and these are usually employed in structures that have more than ten stories.
- Slosh Tank: This is yet another of the seismic retrofitting techniques that use large tanks of water placed at strategic locations in the building to arrest the lateral swaying motions in building structures. The slosh tanks are designed in such a way that the water in the tank moves back and forth in a controlled motion countering the effects of the swaying and resonance inducing motions caused by earthquakes. This technique uses the principles of counteracting mass movement, energy dissipation, and vibration damping by converting the kinetic energy of the water in the tank to heat by the baffles
- Rigid Connections Between Buildings & Their Additions: Usually the additional structures built alongside the main structures will not be rigidly connected to the main structure. The relative motion between these two in case of a severe earth movement results in collision between the two causing very severe structural damage. While doing the seismic retrofitting the additional structure is rigidly fixed to the main structure so that there is no relative motion between the two or suitable dampers that are employed to dissipate the energy generated by the relative motion thus keeping the building wholly protected against damage.
Thus, now various techniques are being used to bring the desired performance levels in guarding different types of buildings from the devastating effects of the earthquakes and safeguard the life and properties during severe seismic events.