Definition
In Civil engineering Retrofitting referred the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes.
OR
Due to passage of time the Structure got deteriorated due to which it lose its strength and also aesthetic view.
Intruduction
Civil engineering structures may be damaged due to various causes such as earthquakes, cyclones, blasting, etc. This kind of loading collapses the structure prematurely or causes extensive damage to them. When the damage is minor, it is possible to retrofit the structure.
Problems in RC Structures
The following are the major types of problems observed during earthquakes in RCC frame buildings:
• absence of ties in beam column joints
• inadequate confinement near beam column joint
• inadequate lap length and anchorage and splice at inappropriate position
• low concrete strength
• improperly anchored ties (90o hooks)
• inadequate lateral stiffness
• inadequate lateral strength
• irregularities in plan and elevation
• irregular distribution of loads and structural elements
other common structural deficiencies such as soft storey effect, short column effect, strong beam-weak column connections etc.Deteriorated Structure
RC Structures Resistance to Earthquake Enhanced by
1) Increasing seismic capacity of the building
It increase the lateral force resistance of the building structure by increasing stiffness, strength and ductility and reducing irregularities. This can be done by two ways
i) Strengthening of original structural members
ii) Introduction of New structural elements
2) Reducing seismic response of the building
Increasing damping in the building by means of energy dissipation devices, reducing mass, or isolating the building from the ground enhance the seismic structural response. A more recent approach includes the use of base isolation and supplemental damping devices in the building.
RETROFIT STRATEGIES
The following retrofit strategies for RC buildings are widely used after recent earthquakes in several places:
1) Jacketing
Jacketing of existing structural members may be of reinforced concrete, steel case or carbon fiber reinforced polymer (CFRP).
Reinforced Concrete Jacketing
This method involves addition of a layer of concrete, longitudinal bars and closely spaced ties on existing structural elements. The jacket increases both the flexural strength and shear strength of the column and beam.
Columns: The jacketing not only increases the flexural strength and shear strength of the column but also increases its ductility. The thickness of the jacket also gives additional stiffness to the concrete column.
Beams:Beams are retrofitted to increase their positive flexural strength, shear strength and the deformation capacity near the beam-column joints. The lack of adequate bottom bars and their anchorage at the joints needs to be addressed.
Steel Profile Jacketing
Steel profile jacketing refers to encasing frame elements with steel plates and filling the gap with non-shrink grout. This is generally used for improving ductility and shear strength and it provides confinement to structural element.
Columns:Steel profile jacketing of column consists of four longitudinal angles profiles placed one at each corner of the existing reinforced concrete column and connected together in a skeleton with transverse steel straps.
Beams: Steel plate reinforcement is a new technique which can be used for beams subject primarily to static loading to improve their shear strength or mid-span flexural strength.
CFRP Jacketing (Fiber Reinforced Polymer)
Seismic resistance of frame buildings can be improved significantly by using Fiber Reinforced Polymer overlays on RC elements of the building. FRP is light weight, high tensile strength material and has a major advantage of fast implementation.
2) Addition of RC Shear Wall
Adding shear walls is one of the most popular and economical methods to achieve seismic protection. Their purpose is to give additional strength and stiffness to the building and could be added to existing and new buildings.
3) Bracing
In this method diagonal braces are provided in the bays of the building. Diagonals stretch across the bay to form triangulated vertical frame and as triangles are able to handle stresses better than a rectangular frame the structure is also supposed to perform better. Braces can be configured as diagonals, X or even V shaped. Braces are of two types, concentric and eccentric.
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