What is Seismic Retrofitting?
Seismic Retrofitting refers to increasing the strength of a building to ensure that it is more durable and resistant to the effects of seismic event or earthquakes. It might be needed because a building might not have been designed to be earthquake resistant. Also, some buildings after a long period of time may get structurally weak and might have to be reinforced.
A few steps can be taken that can help to fix a building’s weakness towards earthquake damage.
Seismic Base Isolation
Base Isolation is extensively used around the world to reinforce structures against seismic activity. Base isolators connect the building to the foundation and act as a suspension. These isolators make use of materials and techniques that absorb a large part of vibrations from the shaking ground. These may include roller balls and bearings, sliders or elastomeric bearings. In case of an earthquake, the vibrations transferred from the ground to the building are considerably reduced when compared to structures that are built directly on foundations without employing isolators. This reduces the oscillation or movement of building and lowers chances of it getting damaged.
Addition of Concrete Columns to Exterior
To increase the strength of a building and to prevent it from shaking, columns made from reinforced concrete maybe constructed on the exterior of the building. Addition of these columns to an already existing building encases the building in a protective frame and ensures higher resistance to damage from seismic activity.
External Post Tensioning
There are multiple points in a building that are under constant pressure from the load they are supporting. Some of these may be overstressed and might have weakened overtime. Their symptoms include bending, distortions or cracks in the structure. Buildings with these are already under undue pressure and any earthquake activity will have a greater impact on it. To make the establishment more durable, these parts are retro-fitted with post-tensioning systems that help to share the load. Generally steel bars, cables and strands and may be installed externally or inside the original structure to provide post tensioning. They may also exert a force in opposite direction to normalize the distorted shape. These also prevent spreading of cracks and increase the load carrying capacity.
Braced Frames and Shear Walls
Shear walls and braced frames provide support to structure and counter and stabilize any motion or vibration in the building resulting from shaking of ground during an earth quake. These act as a brace to the walls of a building. While braced frames are constructed out of steel, shear walls are made from wood or concrete. These braces are highly ductile and stretch whenever they are under pressure due to lateral movement in the building.
In old concrete buildings, with the passing of time, the walls might develop cracks or the plaster might deteriorate and its binding strength will decrease. It may come loose in places and this will hamper the structural integrity of the building. To counter this, Gunite may be applied. It is a combination of cement and sand supplemented with water resulting in concentrated concrete that is sprayed at high pressure over the surface to be repaired. It bonds strongly to the existing structure and increases its structural integrity making it more resilient to earthquakes.