CE 72.52 - Lecture 8a - Retrofitting of RC Members

Dr. Pramin Norachan
Manager, Structural Engineering Unit
AIT Consulting
Advanced Concrete Structures

2. Global Retrofitting
Presentation Outline
1. Introduction
3. Local Retrofitting
4. Example 1: Five Story RC Frame-
Infill Building

Advanced Concrete Structures, Dr. Pramin Norachan 4

(Global Retrofitting)
(Local Retrofitting)

The design philosophy for strengthening can be divided into two approaches.
The first approach is the system strengthening (global strengthening) which new
elements are added to a building to enhance its global stiffness.
With an increase in the stiffness, the natural period of vibration of the building is to
decrease. This will result in a decrease in the amount of horizontal displacement that
must be achieved by the building to resist earthquakes.
Moreover, addition of new members to the building shall mostly increase the
horizontal load capacity of the building as well. Therefore, the increased capacity will
require greater ground motions to allow the building to develop a yielding behavior.
Thus, it can be said that the system strengthening does not only prevent collapsing
but also delays structural damages.

The second approach is element strengthening (local strengthening) which is a method
based on the insufficient capacities of members due to the sustained damages without
undertaking major changes in the load-deformation relationship of the building.
It should also be noted that there will be no significant changes in the displacement
demand after the member strengthening.

Among the global strengthening methods,
addition of RC shear walls is the most
popular one.
The installation of RC shear walls greatly
improves lateral load capacity and
stiffness of the structure.
In the strengthening method with shear
walls, the existing partition walls in the
building are removed and high strength
reinforced concrete shear walls are built
instead.

Shear walls have to be constructed from the foundation level and there may not
need to strengthen other components.
The shear walls can resist majority of the earthquake loads and limits the
displacement behavior of the building.

Under enormous cyclic forces
during a seismic effect, Buckling
Restrained Brace (BRB) which is
system based strengthening
techniques (global strengthening)
devices can be used to increase
the resistance of frame structures
by providing energy dissipation
and introducing nonlinear
behavior.

Testing and evaluating are
required for designing and
ensuring quality control.
The structures are susceptible to
collapse or large lateral
displacements due to earthquake
ground motions and require
special attention to limit the
displacement.
This displacement can be brought
into limit by providing BRB in the
structure.

Passive control systems reduce structural vibration and associated
forces through energy dissipation devices that do not require external power. These
devices utilize the motion of the structure to develop counteracting control forces
and absorb a portion of the input seismic energy.
Active control systems, however, enhance structural response through control forces
developed by force delivery devices that rely on external power to operate. The
actuator forces are controlled by real time controllers that process the information
obtained from sensors within the structure.
Semi-active control systems combine passive and active control devices and are
sometimes used to optimize the structural performance with minimal external
power requirements.

The seismic base isolation technology involves placing flexible isolation systems.
between the foundation and the superstructure.
By means of their flexibility and energy absorption capability, the isolation systems
reflect and absorb part of the earthquake input energy before this energy is fully
transmitted to the superstructure, reducing the energy dissipation demand on the
superstructure.

Base isolation causes the natural period of the structure to increase and results in
increased displacements across the isolation level and reduced accelerations and
displacements in the superstructure during an earthquake.
Base isolation is fundamentally concerned to reduce the horizontal seismic forces.

A typical base isolation system is evolved by the use of rubber bearing located at the
base of the building.
Rubber bearing consist of laminated layers of rubber and steel plates.
The main advantage are good protection against earthquake due to decrease shear.
Superstructure will need no reinforcement.

There are several options for the jacketing of concrete members which are element
based strengthening techniques (local strengthening). Usually, the exiting member is
wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or
with weld wire fabric. Based on this method, axial strength, bending strength, and
stiffness of the original column are increased. Reinforcement concrete jacketing can be
used as a repair of strengthening scheme. If there is damage in some of the existing
members, they should be repaired before jacketing.
There are several options for the
jacketing of concrete members which
are element based strengthening
techniques (local strengthening).
Usually, the exiting member is
wrapped with a jacket of concrete
reinforced with longitudinal steel
bars and ties, or with weld wire
fabric.
Based on this method, axial strength,
bending strength, and stiffness of the
original column are increased.

Reinforcement concrete jacketing can
be used as a repair of strengthening
scheme.
If there is damage in some of the
existing members, they should be
repaired before jacketing.

Jacketing of beams is recommended for several
purposes as it gives continuity to the columns
and increases the strength and stiffness of the
structure.
While jacketing a beam, its flexural resistance
must be carefully computed to avoid the
creation of a strong beam-weak column system.

Jacketing of beam may be carried out under different ways, the most common are one-
sided jackets or 3- and 4-sided jackets.
The beam should be jacketed through its whole length.
The reinforcement has also been added to increase beam flexural capacity moderately
and to produce high joint shear stresses.

The steel jacket retrofit has been used as a method to
enhance the shear strength and ductility of square
reinforced concrete (RC) columns in existing buildings
Local strengthening of columns has been frequently
accomplished by jacketing with steel plates.

FRP composite materials have experienced a continuous increase of use in structural
strengthening and repair applications around the world in the last fifteen years.
In general, applications that allow complete wrapping of the member with FRP have
proven to be effective.

Wrapping of columns to increase their load and deformation capacity is the most
effective and most commonly used method of retrofitting with composites.
However, certain performance and failure mode issues regarding different wrapping
configuration and fiber orientations.

Influence of shear strengthening and anchorage on FRP strengthened beam behavior
under cyclic loading by using FRP plates in various configurations.
It can be seen from this figure that flexural strengthening of beams without proper
attention to brittle shear and debonding failure modes not only renders the
strengthening application ineffective, but also harms the member by decreasing its
ductility.

Seismic performance review of an existing 5-story RC frame-Infill wall school
building and comparison of various retrofit options is presented here.
Rastriya Higher Secondary
School Building, Nepal

Stage I: Collecting As-built Building Information
The architectural and structural drawings of the building are provided by the client.
However, it is understood that the drawings for the extension part of the building are
not available. On-site measurements and investigation are carried out to collect the as-
built information of extension part.
Stage II: Performance Based Evaluation for the Existing Building
Performance based evaluation is carried out to check the seismic performance of the
existing building using the as-built information from the previous stage.
Stage III: Performance Based Evaluation for the Strengthened Buildings
Performance based evaluation is carried out to check the seismic performance of the
strengthened buildings based on common strengthening techniques used by practical
engineers.

MCE level response spectrum is estimated by increasing the spectra values of DBE level
response spectrum by 2.0 times.
Modal pushover analysis (MPA) is conducted to determine the inelastic response of the
building.
Response spectrum for seismic zone V (Z = 0.36) based on type III of subsoil (soft soil),
specified in IS 1893:2002, approximately equivalent to the response spectrum with
1.1 of seismic zone factor, mentioned in NBC 105:1994.
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
SpectralAcceleration,Sa(g)
Natural Period(s)
ResponseSpectra
DBE
MCE
T1T2T3T4T5T6
MCE level response
spectrum is estimated by
increasing the spectra
values of DBE level
response spectrum by 2.0
times.
Modal pushover analysis
(MPA) is conducted to
determine the inelastic
response of the building.

Finite Element Model (SAP2000)
Nonlinear Components
Columns :
- Fiber hinge
- PMM hinge
Girders :
- Moment hinge
Shear Walls:
- Layered Shell

Modal analysis is performed in order to determine the vibration modes of a building.
The first fundamental mode is coupled both X and Y direction (torsion) due to
unsymmetrical configuration of the building as expected.
It is found that more than 90% of total mass is participating to 6 modes.
Mode Natural Period (s) UX UY RZ
1 0.90 2.0% 63.8% 20.9%
2 0.86 9.4% 19.5% 45.4%
3 0.67 76.6% 0.1% 14.7%
4 0.32 0.4% 4.7% 0.5%
5 0.30 0.3% 6.6% 9.3%
6 0.23 9.2% 0.0% 0.9%
Total 97.8% 94.7% 91.7%

In terms of lateral stiffness, the building is flexible is Y-direction since the building
dimension in Y-direction is smaller than that in X-direction.

For repairing and strengthening of the existing building, three common strengthening
techniques used by practical engineers, column jacketing, adding steel braces and
adding new shear walls were used to improve the seismic performance of the existing
building.
Then, the efficiency of each strengthening method was investigated on the basis of
member strength and deformation.

based strengthening techniques (local strengthening).
Usually, the exiting member is wrapped with a jacket of concrete reinforced with
longitudinal steel bars and ties, or with weld wire fabric.
Based on this method, axial strength, bending strength, and stiffness of the original
column are increased.

Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If
there is damage in some of the existing members, they should be repaired before
jacketing. The details of concrete jacketing with longitudinal steel bars are illustrated
in the following figure.

Among the global strengthening methods, addition of RC shear walls is the most
popular one. Many researchers have focused on the addition of RC shear walls and
found that the installation of RC shear walls greatly improves lateral load capacity and
stiffness of the structure.
In the strengthening method with shear walls, the existing partition walls in the
building are removed and high strength reinforced concrete shear walls are built
instead.

In this method, shear walls have to be constructed the foundation level and there
may not need to strengthen other components. The shear walls bear majority of the
earthquake loads and limits the displacement behavior of the building.

Under enormous cyclic forces during a seismic effect, Buckling Restrained Brace (BRB)
which is system based strengthening techniques (global strengthening) devices can
be used to increase the resistance of frame structures by providing energy dissipation
and introducing nonlinear behavior.
Testing and evaluating are required for designing and ensuring quality control.

The structures are susceptible to collapse or large lateral displacements due to
earthquake ground motions and require special attention to limit the displacement.
This displacement can be brought into limit by providing BRB in the structure.

The existing building is increased stiffness by column jacketing, adding shear walls
and BRB which is the main reason in reducing time period when compared with that
of the existing building. Shear wall and BRB can contribute more stiffness than
column jacketing.
Mode Natural Period (s)
Original Jacketing SW BRB
1 0.90 0.76 0.43 0.52
2 0.85 0.72 0.39 0.50
3 0.66 0.54 0.30 0.41
4 0.32 0.25 0.17 0.18
5 0.30 0.24 0.15 0.18
6 0.22 0.18 0.15 0.16

The pushover curves for different buildings in Y direction (weak direction) which
represent the relationship between base shear and roof displacement are plotted in
the following figure.
0
1000
2000
3000
4000
5000
0.00 0.05 0.10 0.15 0.20 0.25
BaseShear(KN)
Roof Displacement (m)
Pushover Curves for Different Buildings
Existing
Column Jacketing
SW
BRB
The results show all strengthening
methods increased the building
base shear, while they reduced the
maximum roof displacement.
The shear walls were more
effective than other strengthening
methods in this purpose. In the
term of ductility, the results show
that column jacketing technique
caused the highest ductility, while
shear walls significantly reduced
the ductility.

Base shear resulting from modal pushover analysis (MPA) at MCE levels of different
strengthened buildings are summarized in the following figures.
28.4
15.0
45.0
26.5
48.5
40.340.9
34.4
0
10
20
30
40
50
60
X Y
BaseShear(%)
Along Direction
Base Shear Percentage in Total Weight of Building
MPA-Existing
MPA-Jacketing
MPA-SW
MPA-BRB
Results showed that base shear
of all strengthened buildings are
increased approximately 1.5
times in X-direction and 2 times
in Y-direction, respectively.
Column jacketing, adding shear
walls and BRB cause the increase
in the structural base shear
because they contribute more
stiffness to the existing building.

1
2
3
4
5
6
-6000 -4000 -2000 0 2000 4000 6000
StoryLevel
Shear Force (KN)
Story Shear in X-Direction
Existing
Jacketing
SW
BRB
1
2
3
4
5
6
-6000 -4000 -2000 0 2000 4000 6000
StoryLevel
Shear Force (KN)
Story Shear in Y-Direction
Existing
Jacketing
SW
BRB

0
1
2
3
4
5
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
StoryLevel
Displacement (m)
MPA - MCE - Story Displacement (X)
Original
Jacketi
ng
SW
BRB
Limit
(H/200)
0
1
2
3
4
5
-0.30 -0.20 -0.10 0.00 0.10 0.20 0.30
StoryLevel
Displacement (m)
MPA - MCE - Story Displacement (Y)
Original
Jacketing
SW
BRB
Limit
(H/200)
The story displacement and story drift for
the buildings with shear walls (SW) and
BRB are significantly decreased.

Based on the results, the shear capacity of many girders seems to be adequate to
resist the probable shear demand with approximately 90% of D/C smaller than one.
Even thought the seismic performance existing structure is improved by using column
jacketing, adding shear wall or BRB, it cannot avoid shear failure in girders because
girders are primary structure which are used to transfer loads to vertical members.
Thus, it is recommended that only some girders about 10% need to be strengthened
to resist shear demand. The strengthening method can perform by using concrete
girder jacketing or CFRP.

In terms of shear capacity, for the building with column jacketing is the most effective
method to resist shear demand under MCE earthquake level, while many columns of
the other strengthened buildings seem to be insufficient to resist the shear demand.
Therefore, it is recommended that the retrofit for shear capacity of columns have to
be provided for only few columns for the column jacketing building, while most
columns of the other strengthened buildings need to be strengthened.

The axial-flexural interaction capacity of buildings with column jacketing, shear wall
or BRB seem to be improved.
However, some columns need to be retrofitted to increase the capacity to resist these
biaxial demand forces.

Girders:
Flexural deformation of all girders generally acceptable for both DBE and MCE level
earthquakes, while few girders seem to be inadequate to resist the demand forces at
MCE level. However, the retrofitting for girder flexure is negligible. For girder shear
capacity, only 10% of girders need to be strengthened to resist shear demand.

Columns:
For shear capacity, most columns are insufficient to resist the shear demand for both
earthquake levels. Thus, it is recommended almost all columns need to be
strengthened to increase shear capacities.

Columns:
Moreover, in terms of axial-flexural interaction capacity, approximately 50% of
columns seem to be overstressed under earthquakes at MCE level. Therefore, these
columns need to be strengthened to resist the biaxial demand force.

For strengthening of the existing building, three common strengthening techniques
were used to improve the seismic performances of the existing building.
• Column jacketing (local strengthening approach)
• Adding new shear walls, SW (global strengthening approach)
• Adding bucking restrained braces, BRB (global strengthening approach)
With increase in the stiffness for all three strengthening methods, results show that
the natural period of vibration of the building is reduced, while the base shear is
increased.
The story displacement and story drift for the buildings with shear walls (SW) and
BRB are significantly decreased.

Shear and axial-flexural interaction capacities of columns seem to be reduced
because new shear walls and BRB can help to resist some shear demand. However,
local strengthening is still required for both girders and columns.
However, in the case of the column jacketing, shears capacities of the girders and
tension, compression, shear and axial-flexural interaction capacities of columns seem
to be adequate to resist demand forces under MCE earthquake level.
In conclusion, the results indicate that displacement at roof and story drift are within
the limitation. Therefore, adding new shear walls and BRB may not be necessary
because the local strengthening is still required for both girders and columns.
Moreover, in general, adding new shear walls are more expensive since additional
foundation should be provided in this method. Therefore, based on these results, it
might be concluded that the column jacketing is the most effective and the most
economic strengthening method for this building. However, there are still 16% of
girders that need to be strengthened to increase their shear capacity.

CE 72.52 - Lecture 8a - Retrofitting of RC Members

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