Building Technology and Materials-VI

Earth Quake resisting Structures

Class notes 2018-19 BKPS CoA
Earthquake-Resistant Systems

Structural Systems Defined:

The Indian Standard Code for earthquake Resistant building provisions recognizes these building
structural systems:

1- Bearing Wall Systems

2- Building Frame Systems
3- Moment Resisting Frame Systems
4- Dual Systems

1- Bearing wall systems consist of vertical load carrying walls located along exterior wall lines and at
interior locations as necessary. Many of these bearing walls are also used to resist lateral forces and are
then called shear walls. Bearing wall systems do not contain complete vertical load carrying space
frames but may use some columns to support floor and roof vertical loads.

2- Building frame systems use a complete three dimensional space frame to support vertical loads, but
use either shear walls or braced frames to resist lateral forces. A building frame system with shear walls
is shown in Figure
3- Moment-resisting frame systems, shown in Figure. provide a complete space frame throughout the
building to carry vertical loads, and they use some of those same frame elements to resist lateral forces.

4. A dual system is a structural system in which an essentially complete frame provides support for
gravity loads, and resistance to lateral loads is provided by a specially detailed moment-resisting frame
and shear walls or braced frames. The moment-resisting frame must be capable of resisting at least 25
percent of the base shear, and the two systems must be designed to resist the total lateral load in
proportion to their relative rigidities. This system, which provides good redundancy, is suitable for
medium-to-high rise buildings where perimeter frames are used in conjunction with central shear wall
core. Concrete intermediate frames cannot be used in seismic zones 3 or 4.
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3-2Lateral-Force-Resisting Elements
Lateral-force-resisting elements must be provided in every structure to brace it against wind and seismic
forces. The three principal types of resisting elements are shear walls, braced frames, and moment-
resisting frames.

3-2-1 Shear Walls:

A shear wall is a vertical structural element that resists lateral forces in the plane of the wall through
shear and bending. Such a wall acts as a beam cantilevered out of the foundation, and, just as with a
beam, part of its strength derives from its depth. Fig. (3.3) shows two examples of a shear wall, one in a
simple one-story building and another in a multistory building.

the shear walls are oriented in one direction, so only lateral forces in this direction can be resisted. The
roof serves as the horizontal diaphragm and must also be designed to resist the lateral loads and transfer
them to the shear walls. Fig. also shows an important aspect of shear walls in particular and vertical
elements in general. This is the aspect of symmetry that has a bearing on whether torsional effects will
be produced. The shear walls in Fig. show the shear walls symmetrical in the plane of loading.
Fig. illustrates a common use of shear walls at the interior of a multistory building. Because walls
enclosing stairways, elevator shafts, and mechanical shafts are mostly solid and run the entire height of
the building, they are often used for shear walls. Although not as efficient from a strictly structural point
of view, interior shear walls do leave the exterior of the building open for windows. Notice that in Fig.
(3.3.b) there are shear walls in both directions, which is a more realistic situation because both wind and
earthquake forces need to be resisted in both directions. In this diagram, the two shear walls are
symmetrical in one direction, but the single shear wall produces a nonsymmetrical condition in the other
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since it is off center. Shear walls do not need to be symmetrical in a building, but symmetry is preferred
to avoid torsional effects. Shear walls, when used a lone, are suitable for medium rise buildings up to 20
stories high. Shear walls may have openings in them, but the calculations are more difficult and their
ability to resist lateral loads is reduced depending on the percentage of open area.

What is a Shear Wall Building?

Reinforced concrete buildings often have vertical plate-like RC walls called Shear Walls in addition to
slabs, beams and columns. These walls generally start at foundation level and are continuous throughout
the building height. Their thickness can be as low as 150mm, or as high as 400mm in high rise
buildings. Shear walls are usually provided along both length and width of buildings. Shear walls are
like vertically-oriented wide beams that carry earthquake loads downwards to the foundation.

Reinforced Concrete Shear Wall

Advantages and Disadvantages of Shear Walls in

Reinforced Concrete Buildings:
Properly designed and detailed buildings with shear walls have shown very good performance in past
earthquakes. Shear walls in high seismic regions require special detailing. However, in past earthquakes,
even buildings with sufficient amount of walls that were not specially detailed for seismic performance
(but had enough well distributed reinforcement) were saved from collapse. Shear wall buildings are a
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popular choice in many earthquake prone countries, like Chile, New Zealand and USA. Shear walls are
easy to construct, because reinforcement detailing of walls is relatively straightforward and therefore
easily implemented at site. Shear walls are efficient, both in terms of construction cost and effectiveness
in minimizing earthquake damage in structural and nonstructural elements(like glass windows and
building contents). On the other hand, shear walls present barriers, which may interfere with
architectural and services requirement. Added to this, lateral load resistance in shear wall buildings is
usually concentrated on a few walls rather than on large number of columns.

Architectural Aspects of Shear Walls:

Most RC buildings with shear walls also have columns; these columns primarily carry gravity loads (i.e.
those due to self-weight and contents of building). Shear walls provide large strength and stiffness to
buildings in the direction of their orientation, which significantly reduces lateral sway of the building
and thereby reduces damage to structure and its contents. Since shear walls carry large horizontal
earthquake forces, the overturning effects on them are large. Thus, design of their foundations requires
special attention. Shear walls should be provided along preferably both length and width. However, if
they are provided along only one direction, a proper grid of beams and columns in the vertical
plane(called a moment-resistant frame) must be provided along the other direction to resist strong
earthquake effects. Door or window openings can be provided in shear walls, but their size must be
small to ensure least interruption to force flow through walls. Moreover, openings should be
symmetrically located. Special design checks are required to ensure that the net cross-sectional area of a
wall at an opening is sufficient to carry the horizontal earthquake force.

Shear walls in buildings must be symmetrically located in plan to reduce ill effects of twist in buildings.
They could be placed symmetrically along one or both directions in plan. Shear walls are more effective
when located along exterior perimeter of the building – such a layout increases resistance of the building
to twisting
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Class notes 2018-19 BKPS CoA
Ductile Design of ShearWalls:
Just like reinforced concrete beams and columns, reinforced concrete shear walls also perform much
better if designed to be ductile. Overall geometric proportions of the wall, types and amount of
reinforcement, and connection with remaining elements in the building help in improving the ductility of
walls.
Overall Geometry of Walls:
Shear walls are rectangular in cross-section, i.e., one dimension of the cross-section is much larger than
the other. While rectangular cross-section is common, L- and U-shaped sections are also used (Fig. 3.6).
Thin-walled hollow reinforced concrete shafts around the elevator core of buildings also act as shear
walls, and should be taken advantage of to resist earthquake forces.

Shear Wall Geometry

Braced Frames:
A braced frame is a truss system of the concentric or eccentric type in which the lateral forces are
resisted through axial stresses in the members. Just as with a truss, the braced frame depends on
diagonal members to provide a load path for lateral forces from each building element to the foundation.
Fig. shows a simple one-story braced frame. At one end of the building two bays are braced, and at the
other end only one bay is braced. As with Fig., this building is only braced in one direction and uses
compression braces because the diagonal member may be either in tension or compression, depending
on which way the force is applied. Fig. (3.7.b) shows two methods of bracing a multistory building. A
single diagonal compression member in one bay can be used to brace against lateral loads coming from
either direction. Alternately, tension diagonals can be used to accomplish the same result, but they must
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be run both ways to account for the load coming from either direction. Braced framing can be placed on
the exterior or interior of a building, and may be placed in one structural bay or several. Obviously, a
braced frame can present design problems for windows and doorways, but it is a very efficient and rigid
lateral force resisting system.

3-2-3 Moment-Resisting Frames:

Moment-resisting frames carry lateral loads primarily by flexure in the members and joints. Joints are
designed and constructed so they are theoretically completely rigid, and therefore any lateral deflection
of the frame occurs from the bending of columns and beams. They are used in low-to medium rise
buildings. The UBC differentiates between three types of moment resisting frames. The first type is the
special moment-resisting frame that must be specifically detailed to provide ductile behavior and
comply with the provisions of the UBC. The second type is the intermediate moment-resisting frame,
which is a concrete frame with less restrictive requirements than special moment-resisting frames.
However, intermediate frames cannot be used in seismic zones 3 or 4. The third type is the ordinary
moment-resisting frame. This concrete moment-resisting frame does not meet the special detailing
requirements for ductile behavior. Ordinary concrete frames cannot be used in zones 3 or 4. Moment-
resisting frames are more flexible than shear wall structures or braced frames; the horizontal deflection,
or drift, is greater, and thus non-structural elements become more problematic. Adjacent buildings
cannot be located too close to each other, and special attention must be paid to the eccentricity
developed in columns, which increases the column bending stresses. Two types of moment-resisting
frames are shown in Fig.
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Class notes 2018-19 BKPS CoA

Moment Resisting Frames

Advantages:

- Provide a potentially high-ductile system with a good degree of redundancy, which can allow freedom
in architectural planning of internal spaces and external cladding.

- Their flexibility and associated long period may serve to detune the structure from the forcing motions
on stiff soil or rock sites.

Disadvantages:
- Poorly designed, moment resisting frames have been observed to fail catastrophically in earthquakes,
mainly by formation of weak stories and failures around beam-column joints.

- Beam column joints represent an area of high stress concentration, which needs considerable skill to
design successfully.

- Requires good fixing skills and concreting.

3-2-4 Horizontal Elements (Diaphragms):

In all lateral force-resisting systems, there must be a way to transmit lateral forces to the vertical
resisting elements. This is done with several types of structures, but the most common way used is the
diaphragm.
A diaphragm acts as a horizontal beam resisting forces with shear and bending action. There are two
types of diaphragms: flexible and rigid. Although no horizontal element is completely flexible or rigid,
distinction is made between the two types because the type affects the way in which lateral forces are
distributed. A flexible diaphragm is one that has a maximum lateral deformation more than two times the
average story drift of that story. This deformation can be determined by comparing the midpoint in-plane
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deflection of the diaphragm with the story drift of the adjoining vertical resisting elements under
equivalent tributary load. The lateral load is distributed according to tributary areas as shown in Fig.
With a rigid diaphragm, the shear forces transmitted from the diaphragm to the vertical elements will be
in proportion to the relative stiffness of the vertical elements (assuming there is no torsion), as shown in
Fig, (3.9.b). If the end walls in the diagram are twice as stiff as the interior walls, then one-third of the
load is distributed to each end wall and one-third to the two interior walls, which is equally divided
between these two. The illustration shows symmetrically placed shear walls, so the distribution is equal.
However, if the vertical resisting elements are asymmetric, the shearing forces are unequal. Concrete
floors are considered rigid diaphragms, as are steel and concrete composite deck construction. Steel
decks may be either flexible or rigid, depending on the details of their construction. Wood decks are
considered flexible diaphragms.

Diaphragm Load Distribution

Load Path:
The structure shall contain one complete load path for Life Safety for seismic force effects from any
horizontal direction that serves to transfer the inertial forces from the mass to the foundation. There must
be a complete lateral-force-resisting system that forms a continuous load path between the foundation,
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all diaphragm levels, and all portions of the building for proper seismic performance The general load
path is as follows: seismic forces originating throughout the building are delivered through structural
connections to horizontal diaphragms; the diaphragms distribute these forces to vertical lateral-force
resisting elements such as shear walls and frames; the vertical elements transfer the forces into the
foundation; and the foundation transfers the forces into the supporting soil. If there is a discontinuity in
the load path, the building is unable to resist seismic forces regardless of the strength of the existing
elements. Mitigation with elements or connections needed to complete the load path is necessary to
achieve the selected performance level.
The design professional should be watchful for gaps in the load path. Examples would include a shear
wall that does not extend to the foundation, a missing shear transfer connection between a diaphragm
and vertical element, a discontinuous chord at a diaphragm notch, or a missing collector. In cases where
there is a structural discontinuity, a load path may exist but it may be a very undesirable one. At a
discontinuous shear walls, for example, the diaphragm may transfer the forces to frames not intended to
be part of the lateral-force-resisting system. While not ideal, it may be possible to show that the load
path is acceptable.

Primary Load-Path Elements:

Within every building, there are multiple elements that are used to transmit and resist lateral forces.
These transmitting and resisting elements define the building’s lateral-load path. This path extends from
the uppermost roof or parapet, through each element and connection, to the foundation. An appreciation
of the critical importance of a complete load path is essential for everyone involved in the design,
construction, and inspection of buildings that must resist earthquakes
There are two orientations of primary elements in the load path: those that are vertical, such as shear
walls, braced frames, and moment frames, and those that are essentially horizontal, such as the roof,
floors, and foundation.
The roof and floor elements are known as diaphragms. Diaphragms serve primarily as force-transmitting
or force distributing elements that take horizontal forces from the stories at and above their level and
deliver them to walls or frames in the story immediately below. Diaphragms are classified as either
flexible or rigid, and the method of distributing earthquake forces from the diaphragm to the resisting
elements depends on that classification. Concrete diaphragms are considered rigid. Shear walls and
frames are primarily lateral force- resisting elements but can also perform force-transmitting functions.
For example and while not necessarily desirable, an upper story interior shear wall may not continue to
the base of the building and therefore must transmit its forces to a floor diaphragm. Also, at the base of a
frame or a shear wall, forces are transmitted into a foundation element.
The primary structural elements that participate in the earthquake load path are shown in Fig

Primary Structural Load Path Elements

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Foundations form the final link in the load path by collecting the base shear and transmitting it to the
ground. Foundations resist lateral forces through a combination of frictional resistance along their lower
surface and lateral bearing against the depth of soil in which they are embedded. Foundations must also
support additional vertical loads caused by the overturning forces from shear walls and frame columns.
/SEAOC Joint Venture Training Cu
Secondary Load-Path Elements:
Within the primary load-path elements, there are individual secondary elements needed to resist specific
forces or to provide specific pathways along which lateral forces are transmitted. Particular attention
must be given to transmitting forces between horizontal seismic elements (diaphragms) and vertical
seismic elements. Two important secondary elements are chords and collectors. A chord is a structural
member along the boundary of a diaphragm that resists tension and compression forces. A collector is a
structural member that transmits diaphragm forces into shear walls or frames. Fig. depicts the overall
function of chords and collectors.

In the case of floors and roofs, the perimeter edges or boundaries are critical locations because they form
the interface between the diaphragms and the perimeter walls. The perimeter is typically the location for
vertical seismic elements, although many buildings also have shear walls or frames at interior locations.
An interior line of resistance also creates a diaphragm boundary. Boundary elements in diaphragms
usually serve as both chords and collectors, depending on the axis along which lateral loads are
considered to be applied. As shown in, the forces acting perpendicular to the boundary elements tend to
bend the diaphragm, and the chord member must resist the associated tension and compression. Similar
to a uniformly loaded beam, a diaphragm experiences the greatest bending stress and largest deflection
at or near the center of its span between vertical resisting seismic elements. The chord on the side of the
diaphragm along which the forces are being applied is in compression, and the chord on the opposite
side is in tension. These tension and compression forces reverse when the earthquake forces reverse.
Therefore, each chord must diaphragm level to resist the out-of-plane bending in the be designed for
both tension and compression. In concrete walls, reinforcing steel is placed at the wall. Collectors are
needed when an individual shear wall or frame in the story immediately below the diaphragm is not
continuous along the diaphragm boundary. This is a very common situation because shear walls are
often interrupted by openings for windows and doors, and because resisting frames are normally located
in only a few of the frame bays along a diaphragm boundary. A path must be provided to collect the
lateral forces from portions of a diaphragm located between vertical resisting seismic elements and to
deliver those forces to each individual shear wall or frame.
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Use of Collector Element at Interior Shear Wall

The collector member provides that path. Collectors are commonly called drag struts or ties. Collectors
are also needed when an interior shear wall or frame is provided. In this case, the collector is placed in
the diaphragm, aligned with the wall or frame, and extends to the diaphragm edges beyond each end of
the wall or frame. Collectors can occur in spandrel beams, of concrete, that link sections of shear walls
together. The following statements contained in the 1997 UBC clearly require that a complete load path
be provided throughout a building to resist lateral forces. “All parts of a structure shall be interconnected
and connections shall be capable of transmitting the seismic force induced by the parts being connected.
“Any system or method of construction shall be based on a rational analysis... Such analysis shall result
in a system that provides a complete load path capable of transferring all loads and forces from their
point of origin to the load resisting elements.” To fulfill these requirements, connections must be
provided between every element in the load path. When a building is shaken by an earthquake, every
connection in the lateral force load path is tested. If one or more connections fail because they were not
properly designed or constructed, those remaining in parallel paths receive additional force, which may
cause them to become overstressed and to fail. If this progression of individual connection failures
continues, it can result in the failure of a complete resisting seismic element and, potentially, the entire
lateral-force resisting system. Consequently, connections are essential for providing adequate resistance
to earthquakes and must be given special attention by both designers and inspectors. Connections are
details of construction that perform the work of force transfer between the individual primary and
secondary structural elements discussed above. They include a vast array of materials, products, and
methods of Construction. In concrete construction, diaphragm-reinforcing steel resists forces in the
diaphragm and chord tension stresses, and reinforcing dowels are generally used to transfer forces from
the diaphragm boundaries to concrete walls or frames.
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Base Isolation:
Traditionally, while designing structures to withstand vibrations due to an earthquake or wind, the basic
consideration was to make the structure resistant to vibrations by improving its strength, ductility, and
stiffness. On the other hand devices that prevent propagation of vibrations to the structures or that absorb
the energy of vibration were proposed as substitutes for the traditional design practices. It is only
recently, however, that the study in this direction has progressed and the findings have been used in
building construction. The aim of these techniques is to improve the safety of structures by damping
their response.
Response-control structures are generally made by attaching special elements to normal structural
members. In the case of base isolation technique, which is the most popular response-control structure
technique, a device having some damping properties and sufficient bearing strength is used in the
structure. In addition, especially in the case of tower-like structures, an added-mass mechanism is used.
A small mass is added to the main structure thereby converting the vibration energy of the main structure
into vibration energy of the added mass.

A damper is an important element for structures since it absorbs vibration energy developed during
earthquakes, thereby reducing vibration response. In the case of base isolation structures, which have
long fundamental periods of oscillation, dampers are generally employed to restrict the excess
deformation of base isolation devices. Even in the case of towers or similar structures such as high-rise
buildings, dampers are used to suppress the response during strong winds or small to medium
earthquakes. Dampers can be roughly classified into the following two types:
1. Viscous or viscoelastic dampers: This is a damper where the damping power is proportional to the
velocity (for example: oil damper).
2. Hysteresis-type dampers: In dampers such as steel damper, lead damper, friction damper, etc., the
vibration energy is dissipated as the hysteretic energy in the force deformation relation of damper
materials.
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In either case the vibration energy of the structure is converted into thermal energy. In a mass-effect
mechanism, mass is added to the structure such that vibration energy of the structure is converted into
the vibration energy of the added mass. This is also referred to as damper or dynamic damper but will
not be discussed here since it is not a base isolation method.

Most of the dampers were developed for structures based on base isolation and are available in various
types. All are used for controlling the relative displacement of the structure in the horizontal direction
and are designed to move freely in the forward and backward directions. The limiting relative
displacement during the operation is about 20 to 40 cm.
1. Viscous and viscoelastic dampers

a. Oil Dampers
Oil dampers used for base isolation structures are basically the same as those used for an automotive
vehicle. They use the resistance encountered at the orifice when a piston moves through the cylinder
filled with oil. The damping power of oil dampers is broadly proportional to the velocity of
vibrations. Since it has almost no stiffness the base isolation effect is observed not only during large but
also during small or medium earthquakes.

b. Visco elastic Dampers

Dampers using visco elastic material are of two types: those used for base isolation and those used for
multi-storied buildings. In either case, visco elastic material of high-polymer origin is placed between
two points where relative displacement takes place during an earthquake or strong winds. The vibrations
are damped using the viscous resistance in this region. In the case of base isolation structures, a viscous
material is filled between two flat places so that viscous resistance is offered during the relative
displacement of these two plates. In the case of multi-storied buildings, two methods may be used—in
one a device is inserted between the braces and in the other method viscous material is inserted inside
the wall. These dampers, similar to oil dampers, are effective even in the case of small deformation
(displacement). However, some of these devices show considerable dependence on temperature and
vibration frequency.

2. Hysteresis-type dampers
a. Steel Dampers
A number of versions are available in steel dampers such as dampers using straight rods, steel rods
clubbed together, steel soils, steel plates shaped like an arch with a gap in-between, or steel pipes. Most
of these are used in base isolation type structures. All of these dampers use the bending deformation
properties of steel and its restoring force characteristics are mostly bi-linear. Accordingly, the base
isolation effect is minimal during small to medium earthquakes since the stiffness in this region is high.

b. Lead Dampers
In these dampers, lead is formed into a cylinder which shows uniform energy absorption properties
irrespective of the amplitude of deformation.
c. Friction Dampers
Friction dampers are also classified into two types—base-isolation-type structures and multi-storied
structures. In the former, a friction plate isinserted between two stainless steel plates and held together
by bolts. In the case of multi-storied buildings, this is in the form of a pump having an outer cylinder and
a rod. There is a friction plate between the outer cylinder and the rod.
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Building Technology and Materials-VI
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 Building Technology and Materials-VI
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RETRO-FITTING:

Retrofitting is the process of structural upgrading of an existing building to meet seismic design
standards close to or equivalent to standards expected of new buildings. Structural assessment is needed
to identify the structural problems exactly. Both structural and non-structural elements require analysis.
For example, the primary horizontal load resisting system may be very weak and prone to collapse, and
infill walls may be likely to collapse under out-of-plane-loads. During this detailed analysis, the
structural engineer will consider what elements require complete replacement or merely upgrading.
Perhaps a new structural system, like a shear wall, needs to be inserted into the existing building. In
the worst cases, new structural systems will be required to resist horizontal loads in both the X and Y-
directions. The three most common structural systems used for retrofitting are R.C Shear wall, Steel
cross bracing and R.C or Steel moment resisting frames.

Ideally, the chosen system will possess greater (or at least similar) horizontal stiffness to that of the
existing building. If the new system is too flexible, then the existing elements will be subject to large
horizontal deflections which they may not be able to sustain without collapse. For example, an
inadequately reinforced column can take only so much horizontal deflection before it loses its capacity
to withstand gravity loads.

The best retrofitting option is to use new RC shear walls. They have, by far, the best seismic
performance record of the three systems. They are therefore the most reliable. New shear walls almost
always require new foundations to prevent overturning.
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The new systems must be configured in such a way as to not only resist plan orthogonal loads, but also
to resist torsion. If the black coloured walls are new shear walls, then the left-hand layout is unsuitable
because no two walls form anti-torsion couples. The middle scheme is suitable because of the couple
formed by the Y-direction walls, but the right-hand scheme is the best due to its largest possible lever-
arms between both sets of new walls contributing increased torsional resistance.

If possible it is constructionally easier if the walls can be constructed on the outside of the building.
New foundations will be much more straight forward to construct. In this situation of exterior walls,
careful attention must be paid to their architectural impacts on the existing building.

the diaphragms play crucial role of transferring horizontal loads. Their strength should be checked and
upgraded if necessary so as they can resist and transfer all the horizontal inertia loads into the new
vertical structural systems.

Sometimes it is possible to upgrade existing structural elements like columns. For example, a weak
column - strong beam frame can be improved by jacketing its columns with confining steel and adding
additional vertical reinforcing. But the problem of under-reinforced beam column joints and other
detailing defects are much more difficult to solve. Since the seismic performance of frames is so
dependant upon high quality detailing and construction, this approach has a far lower chance of being
effective than the introduction of new RC walls.

Ideally, retrofitting schemes should be designed by experienced structural engineers only.