Seismic Retrofitting For Post Earthquake Rehabilitation
- Author Anjum Anwer
- Published May 11, 2019
- Word count 1,155
There are several methods by which the buildings can be restored to previous strength after an earthquake. The retrofitting technique improves the building condition after a mild or serious earthquake outbreak. Retrofitting is employed to strengthen the building to counter expected future earthquake. This should be done with extreme care, as the technique varies with type of building, number of floors, loading condition, material quality, workmanship and type of damage. The region affected should be studied, as the earthquake is area-specific and it differentiates with seismic zones. The first step is analyzing the hazard and the estimation of destruction caused.
Analysis of Hazard and estimation
In the hazard analysis, the seismic environment is inspected to examine the magnitude and frequency recurrence rate. Estimation of hazard is done by counting the buildings affected partially or fully by earthquake waves. The hazard is analyzed with ground motions related with the exceedence probability in 50 years. The seismic excitation and design of structure should be structurally analyzed. After getting a comprehensive data base about the damage, the structural systems are designed accordingly. The data analyzed should cover the details relating the earthquake severity with structural stability. Vulnerability of damage is estimated to identify the expected loss expressed a scale from 0 to 1
Reasons for failure
The common reason for failure includes poorly constructed buildings, collapsing either totally or partially, walls collapsing within narrow sheets, untied roofs and cantilevers, free standing high boundary walls, parapets and balconies falling, gable walls falling over after cracking under lateral thrust of the roof etc.
Damage types and its effect
Slight Nonstructural damage- results in cracks in plaster of walls or falling of plaster in some regions. No serious issues on roof & floor. Seen small cracks or disturbed tiles only.
Slight Structural Damage – Small cracks appeared to form on walls. Nonstructural elements like parapets are damaged. Small cracks in slabs/ A.C. sheets
Moderate Structural Damage – formation of deep cracks in walls & columns. The structure loses its load carrying capacity. Large cracks in slabs & 25% tiles are fallen or disturbed
Severe Structural Damage- Collapse of one or two walls. Approximately fifty percent of the main structural elements fail. Floors are badly cracked and tiles badly affected & fallen.
Collapse – A large part of the building collapses.
Repair & Restoration on structures
After analyzing the structure and developing strategy to rebuild it, proper measure of repairing & restoration is done prior to retrofitting eventhough no structural damage is encountered. 'Repair' will leave the building permanently weak. After repairing, the structure becomes architecturally viable. Restoration will bring the strength of the building to pre-earth-quake level. In buildings with historical value, restoration is done to protect its heritage value. Restoration accurately reveals, recover or represent the state of a historic building, as it appeared at a particular period in its history. Whereas seismic strengthening or Retrofitting makes the building safe under the occurrence of probable future earthquake loads. Appropriate methods of repair and restoration are taken into account after evaluating the building type and damage type.
Repairing of building includes:
i) Patching up of defects as cracks and fall of plaster and re-plastering if needed.
ii) Repairing doors, windows and replacement of glass panes.
iii) Checking and repairing electrical connections, gas connections, plumbing, heating, ventilation
iv) Rebuilding non-structural walls, chimneys, boundary walls.
v) Relaying cracked flooring at ground level and roofing sheets or tiles.
Vi) Redecoration work
Restoration of building includes:
i) Removal of portions of cracked masonry walls and piers, and rebuilding them in richer mortar.
ii) Addition of reinforcing mesh on both faces of the cracked wall, holding it to the wall through spikes or bolts and then covering it suitably with micro-concrete or 1:3 cement -coarse sand plaster.
iii) Injecting neat cement slurry or epoxy like material, which is strong in tension, into the cracks in walls, columns, beams etc.
Seismic Strengthening (Retrofitting)
By seismic strengthening the seismic resistance of an existing building is updated. It includes Structural restoration and cosmetic repairs. The works included in retrofitting are:
i) Modification of roofs
ii) Substitution or strengthening of floors
iii) Modification in the building plan
iv) Strengthening of walls including provision of horizontal and vertical bands or belts, introduction of 'through' or header stones in thick stone walls, and injection grouting etc.
v) Adding to the sections of beams and columns by casing or jacketing etc.
vi) Adding shear walls or diagonal bracings,
vii) Strengthening of foundations if found necessary
EARTHQUAKE RESISTANT RETROFITTING OF BUILDINGS
The following retrofitting actions are recommended against collapse in a future severe earthquake
i) Check length, height and thickness of walls and modify to conform to the Code. The maximum length of wall for RR masonry is limited to 5 m and storey height is restricted to 2.7 m.
ii) Check the positions and sizes of openings in walls and modify as required, or provide reinforcement. The distance of jamb from internal corner should not be less than 450 mm. The distance between consecutive openings is restricted to 600 mm
iii) If there are no 'through' stones in thick stone walls, then provide RC headers, by making 'through' hole by removing the stones in opposite Wythes, inserting an iron link and filling the hole with concrete.
iv) Provide seismic belt below roof and above door/window lintel level. For this use weld mesh reinforcement.
Through bond element
Usually a Through bond stone is introduced in RR masonry walls to run through its intermediate height. The points are selected first where the ‘through’ stones to be installed. After that the plaster is removed from surface to expose stones. a gentle push is given to loosen the stone. The inner material is gradually removed to form a 75 mm hole in the wall. Locate position of the opposite stone on the other face of the wall by gentle tapping in the hole. Remove the identified stone slowly by same gentle process.
Horizontal seismic belts
Seismic belt is given to the building to take additional load during earthquake. These belts are provided above openings and below roof at eave level, on gable wall, door, window and for rafter with collar tie.
Method of fixing seismic belts
• Remove plaster in the height of the belt
• Rake out Mortar joints to 12-15 mm depth
• Clean the surface and wet it with water
• Apply neat Cement slurry and apply first coat of 12 mm thickness.
• Roughen its Surface after initial set.
• Fix the mesh with 150 mm long nails at about 300 mm apart while plaster is still green.
• Apply second coat of plaster of 16 mm thickness.
The retrofitting of building is also done by providing vertical reinforcement at corners, junctions of walls, stiffening flat wooden floor and roof etc. The retrofitting is done to initiate energy conservation and to reduce energy consumption. The aim is to create a high performing building with an effective integrated design process. From studies it is clear that the Retrofitting of an existing building shall be more cost effective than building a new facility.
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