Various
methodologies are available for analysis and retrofitting of masonry building
structures. The proposed retrofitting schemes are based on predicted behavior
of this class of buildings which is based on observed behavior in the past
earthquakes. However, these buildings could be brought to seismic safety level
recommended by various Building Codes within economic limits. The economically
viable option with less intervention would be more desirable though various
other intervention options are available worldwide.
Major types of
problems and basic damage patterns during earthquakes in Masonry buildings:
•
Non-integrity of wall,
floor and roof structures and their components
•
Out-of-plane collapse due
to lack of anchoring elements on upper parts of the wall of the flexible roof
buildings
•
Separate orthogonal walls
at junctions due to developing cracks
•
Collapse of gable wall
since it behaves as a free cantilever
•
Reduce wall stiffness or
storey stiffness due to large opening
•
Out-of plane failure of
walls due to lack of cross walls
•
Collapse of the building
due to rapid cracking and disintegrating of various parts due to brittle nature
Concept of
Retrofitting:
The concept of
retrofitting masonry buildings starts from enhancing integrity to the structure
by providing proper connections between its resisting elements in such a way
that inertia forces generated by the vibration of the building can be
transmitted to the members that have ability to resist.
Typical important
aspects are the connection
a) between
components of floors and roof;
b) between roof or
floors and walls;
c) between
intersecting walls; and
d) walls and
foundation.
Commonly used
improvement methods include eliminating features that are
a) sources of
weakness or which produce concentrations of stresses in some members,
b) abrupt change of stiffness from floor to
floor,
c) concentration
of large masses, and
d) large openings
in walls without proper peripheral reinforcement.
Increasing the
lateral strength in one or both directions, by reinforcing or by increasing
wall plan areas or the number of walls may be required in some cases.
Avoiding the
possibility of brittle mode of failure by providing proper reinforcement and
connection of load resisting members is the overall objective.
Jacketing
This method is
adopted on buildings constructed with a material that is of heavy in weight,
weak in strength, and brittle. It helps
to basket the wall, hence improve its shear strength and ductility. This method
also improves integrity and deformability.
Main improvements in different
structural elements of the building by this method are as follows:
Walls: To improve strength,
deformability and to reduce risk of disintegration, delamination of walls resulting in total collapse of the building,
thin reinforcement concrete jacketing of all the walls is done. In this
alternative two steel meshes should be placed on either two sides or one side
of the wall and both the meshes should be connected by some steel bars
connectors passing through the wall. The thickness of the added concrete should
be about 40 to 50 mm thick. The concrete used ought to be a micro-concrete i.e.
concrete with small aggregates. Selection of one side jacketing or two side
jacketing depends on the analysis result.
Floors: If the floor is
flexible, bracing of the floor elements with steel or timber sections and tie
up of the floor elements with walls should be done to improve stiffness of the
floor system and integrity between walls and floor.
Roof: If the roof is flexible,
similar to floor, bracing of the roof elements with steel or timber sections
and tie up of the roof elements with walls should be done to improve stiffness
of the roof system and integrity between roof and walls.
False Ceiling: Ceiling may need
replacement with a light ceiling system and better anchorage system.
The Splint and
Bandage system is considered as an economic version of jacketing where
reinforcing bars are provided at most critical locations, wherever stress
concentrations can develop. Splints are vertical elements provided at corners,
wall junctions and jambs of openings in the external faces of the building. The
objective is to provide integrity in vertical direction. 
The bandages are
horizontal elements running around all the walls and building to integrate
various walls together thereby preventing potential out of plane collapse of
walls. In addition, openings are also surrounded by splints and bandages to
prevent initiation and widening of cracks from their corners. Splints are
provided in the external face only. The bandages could be provided on both the
faces of the walls just above the lintel level and eaves level. This method is
inferior to jacketing but better than bolting as discussed below in terms of
safety enhancement.
In splint and bandage
system, the strengthening and stiffening of the floor and roof is made in the
same way as discussed above under Section Jacketing.
A horizontal compression
state induced by horizontal tendons is used to improve the shear strength of
in-plane walls. This also considerably improves the connections between
orthogonal walls. The easiest way of affecting the pre-stressing is to place
two steel rods on the two sides of the wall and strengthening them by
turnbuckles (Figure below). These are done at two levels each storey viz. a)
lintel level and b) just below the floor and roof structure. This method
improves the earthquake resistance of the building and will delay the collapse,
but it is still much inferior to the jacketing or split and bandage in terms of
increasing safety. This method is cheaper and will be effective for small and
simple buildings.
Confinement with
Reinforced Concrete Elements
Confinement with
reinforced concrete elements (beam and columns) make the existing masonry act
as “confined masonry” in the sense that reinforced concrete elements are
inserted surrounding the wall panel or middle of the long wall, allowing the
entire wall, or its portion, to act as a truss element, where the struts are
inclined strip of unreinforced masonry. In this way, brittle and non-ductile
wall becomes more ductile and its load carrying capacity increased several
times with added confinement of the reinforced concrete elements. It is more
suitable for building up to one to three storey height with monolithic
reinforced concrete slab and horizontal bands over the load bearing walls at
the lintel level. However, implementation of this method of retrofitting is
more complex and needs special improvements for foundation also.
Wall Bracing
Wall bracing increases
the lateral load resisting capacity of the load bearing walls improving lateral
stability. Bracing materials may be of reinforced concrete, steel section,
Carbon fiber reinforced polymer (CFRP) or wood. Relevant standards have to be
used to determine number, location and tie down of bracing units. Special
attention is to be given at joints of bracing and connection with existing
wall.
Summary
Summary
Retrofitting Options
|
||||||
Jacketing
|
Splint and Bandage
|
Bolting/Pre-stressing
|
Confinement with reinforced concrete
elements
|
Base
Isolation
|
Strengthening with FRPs
|
|
Maximum
Nos. of
|
Suitable up to four
|
Suitable up to three
|
Suitable
up to two
|
Suitable
up to three storey
|
Suitable
for low to
|
Suitable
for low rise
|
Storey
|
storey
|
storey, preferable for two
storey
|
storey
|
medium
rise buildings
with
time period up to
0.5sec
|
buildings
up to 2 Stories
|
|
Architectural
Changes
|
Extensive
|
Moderate
|
Less
|
Significant
|
Insignificant
|
Less
|
Intervention
time
|
Long
|
Moderate
|
Short
|
Long
|
Long
|
Less
|
Cost
|
High
|
Moderate
|
Low
|
High
|
Extensive
|
High
|
Safety
achieved
up to MMI
IX
|
Life safety
-
Immediate
Occupancy
|
Life
safety
|
Brittle
collapse prevention
|
Life
safety
|
Immediate
Occupancy
|
Life
safety
|
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