Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. Every structure must have two load resisting systems, a vertical load resisting system for transferring the vertical load to the ground and a horizontal load resisting system for transferring the horizontal load to the vertical load system. The seismic forces must be properly collected by the horizontal framing system and properly transferred into vertical lateral resisting systems. Any discontinuity/irregularity in this load path or load transfer may cause one of the major contributions to structural damage during strong earthquakes.
Why do we need seismic retrofitting?
- For ensuring safety and security of a building
- To reduce hazards and losses from non-structural elements
- Structural improvement to reduce seismic hazard
- To be aligned with upgraded codes
- To address the lateral strength or stiffness and lateral displacement or ductility of the structures
- To sustain these lateral deformations
- To modify discontinuous load path
Seismic retrofit techniques for existing RC Buildings
The selection of seismic retrofitting depends on the horizontal and vertical load resisting system of the structure and the type of materials used for construction. The understanding of the mode of failure, structural behaviour and weak and strong design aspects as derived from the earthquake damage surveys. Seismic retrofitting is achieved by the inclusion of structural improvements that may prevent the building and people from damage by seismic waves. Different techniques are discussed below;
External post-tensioning
Post-tensioning tendons with strands and wires can be stressed if required. Typical applications for external tendons with strands and wires include bridge construction projects, where they are used as longitudinal tendons between bulkheads, and in the prestressing of concrete and hybrid wind towers, where they are used as vertical tendons. Pre-stressing can increase the capacity of structural elements such as beam, column and beam-column joints. External pre-stressing is also used for structural upgrade for gravity/live loading.
Seismic Base Isolation
Base isolation technology can make medium-rise masonry (stone or brick) or reinforced concrete structures capable of withstanding earthquakes, protecting them and their occupants from major damage or injury. It is not suitable for all types of structures such as taller buildings, as base isolators have a limited ability to cope with tension, meaning a taller building could overturn or topple during an earthquake. Base isolation is a collection of structural elements of a building that should substantially decouple the building’s structure from the shaking ground thus protecting the building’s integrity and enhancing its seismic performance. This earthquake engineering technology, which is a kind of seismic vibration control, can be applied both to a newly designed building and to seismic upgrading of existing structures.
Concrete Shear Walls
Shear walls are structural member used to resist lateral forces. For slender walls where the flexural deformation is more, shear wall resists the loads due to cantilever action. These are vertical elements of the horizontal force resisting system. It is designed in such a way that the lateral seismic load and gravity load is carried by consistent shear walls. Shear wall system is one of the most common and effective lateral load resisting systems that is widely used in medium- to high-rise buildings.
Steel Braced Frames
They are widely used as lateral-load resisting systems in buildings. The standard bridge or roof truss is loaded vertically by gravity and spans horizontally, while the braced frame is loaded primarily horizontally by seismic inertia loads and acts as a vertical cantilever. The source of truss stability is based on its basic unit of triangle which is a structural unit that resists structural loads via development of axial forces in its members.
Supplementary Damping
Supplementary dampers absorb the energy of motion and convert it to heat, thus damping resonant effects in structures that are rigidly attached to the ground. In addition to adding energy dissipation capacity to the structure, supplementary damping can reduce the displacement and acceleration demand within the structures. The device simply responds to the structure’s movement according to basic physical principles and helps reduce it. Thus the moving mass supplements the damping already inherent in the structure.
Moment-Resisting Steel Frames
Moment-resisting frame is a rectilinear assemblage of beams and columns, with the beams rigidly connected to the columns. Resistance to lateral forces is provided primarily by rigid frame action – that is, by the development of bending moment and shear force in the frame members and joints.
Jacketing
Jacketing is a method of structural retrofitting and strengthening It is used to increase bearing load capacity following a modification of the structural design or to restore structural design integrity due to a failure in the structural member. This technique is used on vertical surfaces such as walls, columns and other combinations such as beam sides and bottoms. It consists of added concrete with longitudinal and transverse reinforcement around the existing column. Jacketing is the process whereby a section of an existing structural member is restored to original dimensions or increased in size by encasement using suitable materials. A steel reinforcement cage or composite material wrap can be constructed around the damaged section onto which shotcrete or cast-in-place concrete is placed.
Concrete Diaphragm walls
The diaphragms allow the walls or frames to work as a group in resisting lateral forces. The vertical structural elements that resist lateral forces would be disorganized trying to function alone if they weren’t linked together. Due to out-of-plane resistance capacities are very small for walls, moment-resisting frames, and braced frames, it is only through the diaphragms that the vertical elements can transmit their stabilizing effect to other elements.
FRP composites
In recent years it has been found that among various retrofitting methods, seismic retrofit with FRP materials has gained notable acceptance. Retrofitting with FRP materials is now extensively being used as a seismic retrofitting method and it is a technically sound and cost effective repairing technology. FRP Composites FRP Systems are used to enhances structural capacity of members in shear, flexure, compression, to aid in blast mitigation, upgrades for seismic loads and to controls propagation of existing cracks.
Conclusion
Seismic retrofit aims on the modification of existing structures to enhance their capability to resist earthquakes. It is achieved by the inclusion of structural improvements that may prevent the building from damage by seismic waves. In seismic zones, retrofitting may be essential for the bridges, overpasses, tunnels, and buildings, while the new construction would require compliance to updated seismic standards.