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
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.
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
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.