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CHAPTER 1

INTRODUCTION

1.1 GENERAL

One of the most frightening and destructive phenomena of nature is

an earthquake and its terrible after-effects. Every year several million

earthquakes occur on the earth. Many of these are of small intensity and do not

cause any damage. However, earthquakes of large intensity in the vicinity of

populated areas cause considerable damage and loss of life. Much of the

existing buildings in India and worldwide consists of structures designed

without the benefit of current seismic design procedures and, therefore, are

vulnerable to damage during a seismic event.

Reinforced concrete (RC) frames infilled with masonry form the

structural system of many of these vulnerable buildings. Recent earthquakes

across the world revealed major seismic deficiencies in RC buildings, some of

which led to catastrophic collapses causing a death toll measured in thousands.

The RC frames typically have been designed for gravity loads only, and

common design practice considers the infill a non-structural component. By

neglecting the masonry infill during design of the frame, one is assuming that

the final infilled structure will have the same reliability as the frame alone.

Such a belief is vastly misleading. Historically, such structures have been

plagued with poor performance during seismic events (Paulay and Priestley

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1992). Clearly the “non-structural” masonry infill can drastically alter the

seismic behaviour of infilled frames. The complex interaction between frame

and infill makes lateral strength and seismic behaviour difficult to quantify.

One of the observed common RC structural failures during recent

earthquakes is column shear failure. The types of columns that are susceptible

to shear failure are columns with low shear span/depth ratio. These columns

may have been originally designed as long columns and then partial

supporting walls were later constructed, which created a captive column.

Earthquake damage reports, with few exceptions worldwide, present numerous

cases of captive-column effect. Although the problem shows itself as damage

to the column, the cause usually rest with non-structural elements imposing a

pattern of response to the earthquake motions different from the expected

behaviour of the column by itself without the non-structural elements.

1.2 CAPTIVE-COLUMN EFFECT

The captive-column effect is caused by a non-intended

modification to the original structural configuration of the column that restricts

the ability of the column to deform laterally by partially confining it with

building components (Guevara and Garcia 2005). The column is kept captive

by these components as shown in Figure 1.1 and only a fraction of its height

can deform laterally, corresponding to the free portion; thus the term captive

column. In general, the captive-column effect is often represented as a short-

column effect. Although the terms captive column and short column have been

used interchangeably in the literature, the reasons that cause them are

completely different. In the former case, the column is affected by the

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presence of adjoining non-structural elements. In a short-column effect, the

column is made shorter than neighbouring columns by horizontal structural

elements, such as beams, girders, and staircase landing slabs that frame at

mid-height of the column.

Figure 1.1 The Captive-Column Behaviour

Architectural decisions based on functional or aesthetic aspects are

the most common reasons for the creation of captive columns. The need for

incorporating openings to the walls of a building in order to provide natural

lighting and ventilation leads to partial lateral confinement along the height of

the column by rigid elements, such as internal partitions and facades. The

column ends up having adjoining walls in all its height, except in the upper

part where the opening is located. The length of the column that would be free

to deform laterally is reduced from the vertical floor to ceiling distance to just

barely the height of the opening as shown in Figure 1.1. This type of

configuration is often found in school classrooms, store rooms, rest rooms,

doctors’ consulting rooms, and so on where there is an apparent need to

provide lighting and ventilation while restricting visibility from one space to

the other. In these cases, the non-structural walls are higher than the height

generally allowed for normal window sills, and in order to comply with

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ventilation and lighting regulations, the high windows extend from column to

column. Figures 1.2 and 1.3 show some typical captive-column conditions in

RC buildings with partial masonry infill.

Figure 1.2 Captive-Column Conditions in a Warehouse

Figure 1.3 Partial-Infill in the Outer Walls of a Factory Unit

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1.3 BEHAVIOUR OF PARTIALLY INFILLED FRAMES

A masonry infilled frame typically comprises of RC frame with

masonry infilling, in which the restraint against lateral load is provided by the

composite action of the infill and the frame. In case of partially infilled

frames, for functional or aesthetic reasons, the infill wall will be constructed

only for a certain height of the frame thereby partially confining the columns

with building components. When the column is restrained by adjoined non-

structural walls, column and walls interact, restricting the lateral deformation

of the column (Figure 1.1). The design level of shear force D

V in a column

(Paulay and Priestley 1992) will be

c

BT

D

l

MMV

+=

(1.1)

where c

l is the clear storey height, and T

M and B

M are moments at

the top and bottom of the column. However, in reality, due to the presence of a

restraining element external to it, the clear height is significantly reduced,

increasing the shear force in inverse proportion. This increased shear force for

which the column is not designed for, can cause a shear failure of the column

resulting in the collapse of the building.

Dogangun (2004) has reported that many columns in RC frames

have failed due to short-column (captive-column) effect created by the

openings provided in the infill wall between columns during the Bingol

earthquake in Turkey on May 1, 2003. Cagatay (2005) has reported that all

columns on the outer side of an industrial building failed due to short-column

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(captive-column) effect induced by the partial infill during the Adana-Ceyhan

earthquake in Turkey on June 27, 1998 (Figure 1.4). Karakostas et al (2005)

have reported the shear failure of columns in RC frames due to openings in the

brick infill walls during the Lefkada, Greece earthquake on August 14, 2003

(Figure 1.5). They identified that the collapse took place in an area where

damage to other RC buildings was limited, and soil conditions were better

than in other town districts. The collapse can thus be attributed to the overall

poor seismic resistance of the building than the intensity of the seismic

motion. Aliaari and Memari (2005) have reported the formation of short-

column (captive-column) effect due to partial tight fit infill walls in an RC

building during the Peru earthquake in June 2001 (Figure 1.6). Moretti and

Tassios (2006) have reported a damaged column in a partially infilled RC

frame at Athens during an earthquake in 1999 with the upper part behaved as a

short column owing to restraints in displacements by adjacent masonry infills.

Figures 1.7 to 1.9 show few of the shear failure in columns due to captive-

column effect during earthquakes in various parts of the world.

Figure 1.4 Captive-Column Failure in an Industrial Building at Adana

(Turkey), 1998 Earthquake (Cagatay 2005)

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Figure 1.5 Captive-Column Failure in an RC Frame at Lefkada (Greece),

2003 Earthquake (Karakostas et al 2005)

Figure 1.6 Captive-Column Effects due to Partial Tight-Fit Infill Walls in

an RC Building at Peru, 2001 Earthquake (Aliaari and Memari 2005)

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Figure 1.7 Captive-Column Failure in a High School Building at Sichuan

(China), 2008 Earthquake

Figure 1.8 X-Shaped Cracks in a Captive Column at Athens (Greece),

1999 Earthquake

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Figure 1.9 Captive-Column Failures in an RC Building at Bhuj (India),

2001 Earthquake

1.4 STATEMENT OF THE PROBLEM

The severity of the damage caused by earthquakes to buildings due

to the presence of captive-column/partial-infill requires a better understanding

of the failure mechanism of partially infilled frames under lateral loading.

When it is not possible to avoid captive columns, this effect must be addressed

in structural design. The columns of such partially infilled buildings must be

designed for the higher shear force considering the height of the opening to

avoid shear failure (Paulay and Priestley 1992). The Indian standard

IS-13920 (1993) for ductile detailing of RC structures requires special

confining reinforcement (closely-spaced closed ties) to be provided over the

full height of columns that are likely to sustain captive-column effect. The

special confining reinforcement must extend beyond the particular column

into the columns vertically above and below by a certain distance equal to the

development length of the largest longitudinal bar in the column. Since earlier

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versions of the local building code did not contain the provisions for special

confining reinforcement, many existing buildings with captive-column

conditions would not survive an earthquake that reaches a certain level of

intensity. The condition will be more critical in seismic regions, where new

standards are more comprehensive than the old ones.

For buildings in India not meeting the current Indian seismic code

requirements (IS-13920 1993), a structural upgrading or retrofit scheme

should be considered. To date, it has been reported that many of the owners

have not taken remedial actions, which may be attributed to high retrofitting

costs. Traditional retrofitting techniques that use steel and cementitious

materials do not always offer the most appropriate solutions. Most of these

methods have proven to be impractical, labour intensive, add considerable

mass, and cause significant impact on the occupant, all resulting in very high

costs. This may lead to a ‘‘do nothing’’ choice, in which the owner decides

that the risk of economic loss and occupant injury does not justify the

significant cost of strengthening.

Therefore, some efforts must be made to develop effective and

economic retrofit techniques for upgrading the vulnerable columns in existing

partially infilled RC structures to increase their safety levels and to improve

the expected behaviour during seismic activity in the future. Retrofitting with

fibre-reinforced polymers may provide a more economical and technically

superior alternative to the traditional techniques in many situations.

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The evaluation of the seismic resistance of existing structures and

their deficiencies is essential before an appropriate repair or upgrade system

can be designed. Understanding the behaviour of partially infilled frames and

having a satisfactory method of analysis will help us to have more realistic and

economical solutions. The behaviour of such frames under seismic loading is

very complex and complicated. Since the behaviour is nonlinear and closely

related to the complex interaction between the frame and the partial infill, it is

very difficult to predict it by analytical methods unless the analytical models

are supported and revised by using the experimental data. Due to the complex

behaviour, experimental research is of great importance to determine the

strength, stiffness and dynamic characteristics of partially infilled frames at

each stage of loading.

1.5 FIBRE-REINFORCED POLYMERS

Fibre-reinforced polymer (FRP) is a composite material composed

of matrix of polymeric material reinforced by uni-directional or multi-

directional fibres, usually 3 to 5 microns in diameter, placed in a resin matrix,

polymer, and hence stems the name. The resin matrix binds the fibres together,

allows load transfer between fibres and it also protects the fibres from

environment. The FRPs are mechanically different from steel in a sense that it

is anisotropic, linearly elastic and it is usually of higher strength with a lower

modulus of elasticity than steel. The FRPs have desirable physical properties

over steel, like corrosion resistance, high strength-to-weight ratio, high fatigue

resistance, and dimensional stability. The FRP also has the disadvantages of

the susceptibility to moisture and chemicals, the loss of properties at high

temperatures, as in the case of fire, and the damage from ultra-violet light.

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The production of FRP started since the 1940’s, where it was used

in variety of industries, such as aerospace, automotive, shipbuilding, chemical

processing, etc., for many years. Their application in civil engineering,

however, has been very limited. Their high strength-to-weight ratio and

excellent resistance to corrosion make them attractive material for structural

applications. Presently, several types of FRP materials have been considered

for repair and retrofit of concrete and masonry structures, among them are the

glass fibre-reinforced polymers “GFRP”, carbon fibre-reinforced polymers

“CFRP”, and aramid fibre-reinforced polymers “AFRP”. Glass has been the

predominant fibre for many civil engineering applications because of the

economical balance of cost and strength properties.

Although the FRP can be used as a structural stand-alone material

like the structural steel shapes or reinforcement bars for new reinforced

concrete structures, yet its most extensive use to-date is to retrofit existing

structures in the form of bonded laminates. The laminates are made by

stacking a number of thin layers of fibres and matrix and consolidate them into

the desired thickness. Fibre orientation in each layer as well as the stacking

sequence of the various layers can be controlled to generate a range of

physical and mechanical properties. Laminates are used either in the form of

dry plates or wet lay-up of a single lamina or multiple laminates. The plates

are to be bonded to the surface using the appropriate adhesive, whereas the

wet lay-up involves wetting (impregnating) the fabric at the time of

installation in-situ with the appropriate polymer, in this case the polymer

serves both as a binding matrix as well as bonding the FRP to the surface of

the structure. Figure 1.10 shows a typical example of FRP laminates available

in the form of a mat.

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Figure 1.10 FRP Laminates in the Form of Mat

The most important characteristics of a strengthening work are the

predominance of labour and shutdown costs, time, site constraints and long-

term durability. In addition to their outstanding mechanical properties, the

advantages of FRP composites versus conventional materials for strengthening

of structural and non-structural elements include lower installation costs,

improved corrosion resistance, onsite flexibility of use, and minimum changes

in the member size after repair. From the architectural point of view, this

constitutes a huge advantage for the FRPs against traditional strengthening

techniques, because the use of conventional methods may violate the

aesthetics of building facades and they may intrude on usable space adjacent

to the strengthened components. More importantly, from the structural point of

view, the dynamic properties of the structure remain unchanged because there

is little addition of weight and stiffness. Any alteration to the aforementioned

properties would typically result in an increase in seismic forces. Additionally,

the ease with which FRP composites can be installed on the structural

elements of RC frames makes this form of strengthening attractive to the

owner, considering both reduced installation cost and down-time.

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1.6 LITERATURE REVIEW

The complex interaction of RC frame and infill has been the subject

of numerous investigations conducted by the researchers throughout the world.

Different approaches had been adopted starting from simple strength of

materials approach, passing through trials to match experimental results using

simple models. Methods based on the theory of elasticity, plastic analysis, and

finally finite element analysis were also used.

In order to assess and to critically evaluate the research work done

on infilled RC frames and to identify a suitable retrofit strategy for captive-

column defects, a detailed review of literature has been undertaken. The

literature survey in this study is divided into two sections. The first section

highlights various experimental and theoretical studies conducted to date in

the area of masonry infilled RC frames with emphasis on the conclusions

reached. The second section shows some of the retrofitting techniques with

FRP composites adopted for strengthening RC structural elements and frames.

1.6.1 Previous Research on Masonry-Infilled RC Frames

Several dozen experimental and analytical investigations of infilled

structures have been conducted over the past 50 years, and important advances

have been made for RC frames infilled with masonry. Much of this work has

focused on the concept of stiffness. An early contribution to understanding the

complex nature of masonry infill frames was introduced by Smith (1966). He

examined the behaviour of infilled frames by the finite-difference method and

adopted a simplified equivalent single strut model to replace the wall. Since

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then the equivalent strut model has been widely used by engineers and

researchers. For design and analysis, Holmes (1961), Stafford Smith and

Carter (1969), and Saneinejad and Hobbs (1995) have proposed the idea of

equivalent diagonal strut and derived systematic methods to calculate the

mechanical properties of such struts. Madan et al. (1997) further extended the

work of Saneinejad and Hobbs (1995) by including a smooth hysteretic model

for the equivalent diagonal strut. The hysteresis model uses degrading control

parameters for stiffness and strength degradation and slip (pinching).

Paulay and Priestley (1992) have suggested treating the infill walls

as diagonal bracing members connected by pins to the frame members. They

have also suggested to calculate the stiffness of the structure and hence its

natural period based on considering the effective strut width to be one quarter

of the wall diagonal.

Cyclic testing of masonry infilled specimens began during the

1970’s with research addressing seismic performance for both evaluation and

retrofit. Researchers such as Klingner and Bertero (1978), Kahn and Hanson

(1979), Liauw and Kwan (1985) have conducted experimental investigations

on the lateral stiffness and strengths of concrete frames infilled with reinforced

and unreinforced masonry panels.

Bertero and Brokken (1983) have conducted a series of quasi-static

cyclic and monotonic load tests on one-third scale models of 11 storey-three

bay RC frame infilled in the outer bays. Different panel material and

reinforcement combinations were tested. In this study, the effective

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inter-storey lateral stiffness of infilled frames was 5.3-11.7 times the lateral

stiffness of the bare frame depending on the type of infill. The maximum

lateral resistance of infilled frames was 4.8-5.8 times of that obtained for the

bare frame. The implications of experimentally obtained results were analysed

by investigating how the infills affect the dynamic response of RC moment

resisting frame buildings.

Achintya and Jain (1991) have presented the behaviour of brick

infilled RC frames subjected to lateral load, through an experimental

approach. The strength of mortar is found to have considerable influence on

lateral stiffness and strength of the infilled frames. Frames tested with

reinforced brick panel have shown insignificant improvement in failure

strength. The stiffness of the infilled frame has decreased very rapidly after the

initiation of cracks. Lack of fit between the infill and the frame due to

shrinkage of infilled material is also noted.

Mehrabi et al (1996) have tested 12 half scale, single-storey, single-

bay, RC frames infilled with concrete block masonry that were designed in

accordance with code provisions. The objectives were to evaluate the

influence of the relative strength and stiffness of infill panels with respect to

those of the bounding frame, the lateral load history, the panel aspect ratio, the

magnitude and the distribution of vertical loads, and the adjacent infilled bays

on the performance of these frames. The experimental results indicated that

infill panels can significantly improve the performance of RC frames. They

concluded that the lateral loads developed by the infilled frame specimens

were always higher than that of the bare frame.

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Negro and Verzeletti (1996) have conducted a series of pseudo-

dynamic tests on a full-scale 4-storey RC building designed according to

Eurocodes. The tests were conducted on the bare frame, as well as on the

frame with two different configurations of non-structural masonry infills. The

experimental results indicated that the presence of light non-structural

masonry infills can change the response of the structure to a large extent and

the presence of regular pattern of infills to a large extent prevents energy

dissipation from taking place in the frame.

Amar and Cherifati (1999) have reported vibration measurements

performed on two adjacent, three-storey reinforced concrete frame buildings

with hollow clay brick infill panels. The first building was a bare frame and

the second one was a similar frame infilled with brick panels. The

fundamental period for the infilled frame building was much smaller than that

of the bare frame building. Using shear beam lumped-mass models and the

vibration data, the actual lateral stiffness of both buildings was identified. The

lateral stiffness of the infilled frame building was found to be seven times

greater than that of the bare frame building.

Singh et al (2001) reported the tests conducted on infilled frames

with and without shear connectors and found that the stiffness, strength,

ductility and energy absorption capacity is clearly superior when the interface

connection is used.

Lee and Woo (2002) have investigated the effect of masonry infills

on the seismic performance of low-rise RC frames with non-seismic detailing.

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For this purpose, a 2-bay 3-storey masonry-infilled RC frame was selected and

a 1:5 scale model was constructed with non-seismic detailing. A series of

earthquake simulation tests and a pushover test were performed on this model.

When the results of these tests are compared with those in the case of the bare

frame, it can be recognised that the masonry infills contribute to the large

increase in the stiffness and strength of the global structure whereas they also

accompany the increase of earthquake shear forces. The failure mode of the

masonry-infilled frame was that of shear failure due to the bed-joint sliding of

the masonry infills while that of the bare frame appeared to be the soft-storey

plastic mechanism at the first storey.

Al-Chaar et al (2002) have presented their research on the

behaviour of a type of building popular in high seismic zones with a lateral-

load-resisting system consisting of masonry-infilled RC frames. Older

buildings of this type typically were designed for gravity loads in combination

with insufficient or no lateral loads; therefore they do not meet current seismic

code requirements. Also, the participation of infill panels in the lateral load

resistance of RC frames was not recognized in the original design, often

resulting in an overly conservative design. In an attempt to determine the

seismic vulnerability of this type of structure, an experimental program was

carried out to evaluate the behaviour of five half-scale, single-storey

laboratory models with different numbers of bays. The results indicated that

infilled RC frames exhibit significantly higher ultimate strength, residual

strength, and initial stiffness than bare frames without compromising any

ductility in the load–deflection response.

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Ghosh and Amde (2002) have verified the design of infilled frames

to resist lateral loads on buildings in terms of their failure modes, failure loads,

and initial stiffnesses using procedures proposed by previous authors. This

verification was made by comparing the results of the analytical procedures of

the previous authors with those of a new finite element model for infilled

frames, which are verified using experimental results. To model the interface

between the frame and the infill and the mortar joints surrounding the blocks

of masonry, a non-associated interface model was formulated using the

available test data on masonry joints. The failure criteria for masonry include

the Von Mises criterion for the plane stress condition for uncracked masonry

(assembly) and a smeared crack model. Additionally, the finite element model

has provided more insights into the failure mechanisms of the infilled frames.

A comparative study on the nonlinear behaviour of reinforced

concrete multi-storey structures was carried out by Lu (2002) on the basis of

measured response of four six-storey, three-bay framed structures, namely a

regular bare frame, a discontinuous-column frame, a partially masonry-infilled

frame, and a wall-frame system. The structures were designed for similar

seismic requirements in accordance with Eurocode, and their 1:5.5 scaled

models were subjected to similar earthquake simulation tests. Experimental

observations and numerical analyses showed that the distribution of the storey

shear over-strength is a rather stable indicator of the general inelastic

behaviour of frames, and hence, can be employed as a characteristic parameter

to quantify the frame irregularity for design purposes. He commented that

abrupt discontinuity of the geometry or arrangement of structurally effective

elements, where unavoidable, may be compensated by strength enhancement

targeting a smoothed over-strength profile to allow for distributed inelastic

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deformation, and this principle applies as well to non-uniformly masonry

infilled frames.

Henderson et al (2003) have reported a five-year, large- and small-

scale, static and dynamic experimental research program, in which more than

700 tests were conducted. The program has demonstrated that unreinforced

masonry infills are more ductile and resist lateral loads more effectively than

anticipated by conventional code procedures. The tests were conducted both in

the laboratory and on existing structures. The experimental data indicated that

the combination of a frame and infill material efficiently resists lateral loads-

the infilling provides significant lateral stiffness while the surrounding frame

adds ductility and confinement to the overall system. The results from

approximately 25 moderate- and full-scale tests on infills have showed that

with simulated seismic loads the frames confined the masonry, and the load-

carrying capacity of the infill was considerably above the load that caused

initial cracking.

Mohamed et al (2003) have reported the seismic performance of

concrete-backed stone masonry walls subject to cyclic load based on

experimental tests. Six, one-third scale, single-storey, single-bay wall samples

were tested. Three of these samples were constructed using old construction

methods and the other three were constructed using a new construction

method. The influence of the type of construction, applied vertical loads,

ductility, energy dissipation, stiffness, and failure mechanisms were

investigated. The experimental results indicated that an increase in the applied

vertical load resulted in a substantial increase in both lateral strength and

stiffness of the tested samples.

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Pujol and Fick (2010) have tested a full-scale three-storey RC

structure with and without infill walls made out of solid clay bricks to study

the effect of infill wall in RC frames under strong ground motion. During the

test without the walls, the structure experienced a punching shear failure at a

slab–column connection. After this first test, infill walls were built with solid

bricks. The walls filled completely full bays and ran continuously from the

foundation to the roof. It was observed that the walls increased the stiffness

and the strength of the structure. The drift capacity of the structure with walls

was observed to be 1.5%. Up to this level of deformation, masonry infill walls

in structures similar to the one described here can be expected to help control

inter-storey drift provided that measures are taken to prevent their out-of-plane

failure.

The use of finite element models has been greatly advanced by

Liauw and Kwan (1983) during the 1980’s with companion experiments, as a

tool for understanding the behaviour of infilled frames. By introducing

plasticity models for the infill and interface elements between frame and infill,

they have identified new frame failure modes. They reported that crushing or

softening of infill regions of high compressive stress, at the ends of a main

diagonal strut, may significantly reduce lateral support to the column provided

by the infill, resulting in a “short-column effect”.

May and Nazi (1991) have developed a nonlinear analytical model

for infilled frames under monotonic and cyclic loading. The model has

represented the panel elements as eight noded isoparametric elements, the

frame members as three noded frame elements with shear locking effects

considered. The material of the infill was concrete and was represented by an

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elasto-plastic work hardening model. In tension, fixed crack models with

strain softening were used. The beam and column have bi-linear moment

curvature relationship and Newton-Raphson method had been used for the

solution.

Haddad (1991) have conducted finite element analysis of infilled

frames considering cracking and separation. The effect of frame infill

separation was included in the model. From the analysis results, it was

concluded that as the infill-frame relative stiffness factor increases, bending

moments and deflections at the frame joints were decreased.

Harpal Singh et al (1998) have reported an inelastic finite element

model to simulate the behaviour of RC frames infilled with masonry panels

and subjected to static load and earthquake excitation. Under the loads, the

mortar may crack causing sliding and separation at the interface between the

frame and the infill. Further, the infill may get cracked and/or crushed which

changes its structural behaviour and may render the infill ineffective, leaving

the bare frame to take all the loads which may lead to the failure of the

framing system itself. In this study, a mathematical model to incorporate this

behaviour has been developed.

Dawe et al (2001) have developed a computer model for the

structural analysis of masonry infilled frames. Structural interaction of the

panel and its peripheral frame was considered. Various failure criteria were

incorporated into the model and special elements were developed to account

for masonry failure by cracking and crushing as well as to account for the

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complex interaction of frame and panel. Comparison of analytical findings

with the results of 31 laboratory tests on steel and RC infilled frames showed

favourable correlation and verification of the computer model.

Das and Murty (2004) have reported the nonlinear pushover

analysis performed on five RC framed buildings with brick masonry infills for

the same seismic hazard as per Eurocode, Nepal building code, Indian code,

and the equivalent braced frame method given in the literature. Infills are

considered in the modelling. They have concluded that infills reduce the

overall structural ductility, but increase the total strength. In addition, the

buildings designed by the equivalent braced frame method showed better

overall performance.

Stavridis and Shing (2010) have addressed pertinent issues on the

development and calibration of nonlinear finite element models for assessing

the seismic performance of masonry-infilled RC structures. The modelling

scheme considered by them has combined the smeared and discrete crack

approaches to capture the different failure modes of infilled frames, including

the mixed-mode fracture of mortar joints and the shear failure of RC members.

A systematic approach was presented to calibrate the material parameters, and

the accuracy of the nonlinear finite-element models has been evaluated with

experimental data. The comparison of the numerical and experimental results

indicated that the models can successfully capture the highly nonlinear

behaviour of the physical specimens and accurately predict their strength and

failure mechanisms. Araki et al (2011) have developed finite element models

and reviewed the out-of-plane response of masonry walls retrofitted by

inserting inclined stainless steel bars.

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The studies of several researchers are generally based on the

investigations conducted on fully infilled RC frames. Very few studies have

been reported on RC frames with partial infill or with openings in the infill.

Achyutha et al (1986) have proposed an iterative finite element method of

analysis to simulate the elastic behaviour of infilled frames with and without

opening. The results of the effect of size of opening are reported. Some of the

theoretical results were compared with those of experiments carried out on

half-scale models.

Negro and Colombo (1997) have reported the effects of the non-

structural masonry infills on the global seismic behaviour of frame structures,

based on the results of full-scale pseudo-dynamic tests and the post test

calculations. The tests were performed on a four-storey framed structure

designed according to Eurocode, with different infill configurations. The

results show that an irregular distribution of the panels yields unacceptably

large damage in the frame. They have also stated that the effects of non-

structural masonry infills can modify the seismic behaviour of framed

buildings to a large extent. These effects are generally positive. On the other

hand, potentially negative effects such as soft-storey effects induced by

irregularities in elevation, short-column effects due to openings should also be

considered. Hence neglecting the effects of non-structural infills does not, in

general, result in a safe design even though this is the practice suggested by

most design codes.

Buonopane and White (1999) have performed pseudo-dynamic

testing of a half-scale specimen for seismic evaluation of a two-storey, two-

bay RC frame infilled with masonry. The second-storey infill included

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window openings. The specimen was subjected to four tests of increasing

magnitude based on the Taft ground motion. The final sequence of tests

produced diagonal cracking in the upper storey, but primarily bed joint shear

cracking in the lower storey. Relations between the types of observed cracking

and storey drift-storey shear response were explored. Estimates of storey

stiffness from several simple strut models were found to bind the

experimentally measured values for both the first- and second-storey walls

prior to significant damage. It is concluded that the available methods for

estimating shear strength that neglect infill-frame interaction were found to

largely underestimate the measured shear strength.

Chiou et al (1999) have studied the structural behaviour of a one-

bay, one-storey framed masonry wall subjected to in-plane monotonic loading

by a full-scale test on using the method of discontinuous deformation analysis.

The numerical solutions were compared with the experimental results and a

satisfactory agreement was obtained. The filled masonry wall had affected

dominantly the behaviour of the framed masonry structure. The partially

infilled masonry wall induced a short column effect and leads to a severe

failure of the column. On the other hand, the completely filled masonry wall

increased the stiffness of the structure and the adjacent column failed in the

configuration of nearly uniform cracks.

By means of a new finite element technique, Asteris (2003) has

investigated the influence of the masonry infill panel opening in the reduction

of the infilled frames stiffness. A parametric study has been carried out using

as parameters the position and the percentage of the masonry infill panel

opening for the case of one-storey one-bay infilled frame. The investigation

26

has been extended to the case of multi-storey, fully or partially infilled frames.

In particular, the redistribution of action effects of infilled frames under lateral

loads has been studied. It is shown that the redistribution of shear force is

critically influenced by the presence and continuity of infill panels. The

presence of infills leads, in general, to decreased shear forces on the frame

columns. However, in the case of an infilled frame with a soft ground storey,

the shear forces acting on columns were considerably higher than those

obtained from the analysis of the bare frame.

Dogan (2011) have recently reported that different column heights

cause failure during earthquakes. Based on the results obtained from

numerical solutions and earthquake damage examinations, it was concluded

that short columns (captive columns) must not be constructed. If such columns

are to be used, certain precautions must be taken during design and

construction. He has also added that in existing buildings with walls of partial

height, the simplest solution is to close the openings by building a wall of full

height – this will eliminate the short-column (captive-column) effect. If that is

not possible, the columns need to be strengthened using one of the well

established retrofit techniques.

1.6.2 Retrofit Techniques Using FRP Composites

In recent years, external application of FRP wraps are being used in

RC structural elements such as beams, columns, and beam-column joints to

increase their performance. Mirmiran and Shahawy (1997) have provided a

framework for better understanding of the behaviour of fibre-wrapped or FRP-

encased concrete columns. Results from a series of uniaxial compression tests

27

on concrete-filled FRP tubes were compared with the available confinement

models in the literature. They have also reported that external confinement of

concrete by means of high-strength fibre composites can significantly enhance

its strength and ductility as well as result in large energy absorption capacity.

Sheikh (2002) has reported that retrofitting with FRP to strengthen

and repair damaged structures is a relatively new technique. In an extensive

research program conducted at the University of Toronto, application of FRP

in concrete structures was investigated for its effectiveness in enhancing

structural performance both in terms of strength and ductility. The structural

components tested include slabs, beams, columns, and bridge culverts. Results

so far indicated that retrofitting with FRP offers an attractive alternative to the

traditional techniques. He also added that FRP can provide the most

economical and superior solution for a structural rehabilitation program.

Shehata et al (2002) have conducted tests on 54 short column

specimens to investigate the gain in strength and ductility of concrete columns

externally confined by CFRP wrapping. The variables studied were the

column cross section shape and the amount of confinement expressed in the

number of CFRP sheet layers applied to the models. On the basis of the

obtained results, equations were proposed to calculate the confined concrete

strength and the ultimate concrete strain as a function of the confining lateral

stress for each of the cross section geometry used, circular, square, and

rectangular.

28

Ghobarah and Said (2002) have constructed and tested several RC

beam-column joints to develop effective selective rehabilitation schemes using

advanced composite materials. The joints were designed to simulate non-

ductile detailing characteristics of pre-seismic code construction. The control

specimens showed joint shear failure when subjected to cyclic loading at the

beam tip. Different fibre-wrap rehabilitation schemes were applied to the joint

panel with the objective of upgrading the shear strength of the joint.

In the studies conducted by Ye et al (2003), eight specimens,

including two strengthened after being loaded to yield level to imitate

strengthening with some damage and one strengthened under a sustained axial

load to imitate strengthening under service condition, were tested under

constant axial load and lateral cyclic load to investigate the seismic

performance of RC columns strengthened with CFRP sheets. Based on the

experimental results, a confinement factor of CFRP and an equivalent

transversal reinforcement index were suggested.

Harajli and Rteil (2004) have presented the results of an

experimental investigation undertaken to evaluate the seismic performance of

RC columns designed for gravity load and confined externally with CFRP

flexible sheets. Other types of confinement including the use of steel fibre

reinforcement, or conventional transverse reinforcement were also evaluated

for comparison. The specimens consisted of 150 x 300 x 1000 mm-long

columns projecting outside a stiff column stub. The main parameters included

the reinforcement ratio in the columns, the area of CFRP sheets, and the

volume fraction of steel fibres. Confining the concrete with a relatively small

area of CFRP sheets reduced the bond deterioration, considerably increased

29

the energy absorption and dissipation capabilities of the columns, and resulted

in a significant improvement in seismic performance. In addition, they have

also reported that while confinement with ordinary transverse steel enhanced

the seismic behaviour and increased the energy absorption capacities of the

columns, it was not as effective as CFRP or steel fibre reinforcement.

Balsamo et al (2005) have assessed the use of CFRP composites for

the seismic repair of RC structures on a full-scale dual system subjected to

pseudo-dynamic tests. The aim of the CFRP repair was to recover the

structural properties that the frame had before the seismic actions by providing

both columns and joints with more deformation capacity. Comparisons

between original and repaired structures were discussed in terms of global and

local performance. In addition to the validation of the proposed technique, the

experimental results will represent a reference database for the development of

design criteria for the seismic repair of RC frames using composite materials.

Pan et al (2007) have conducted axial compression tests on six

elliptical modified rectangular slender RC columns wrapped with FRP, with a

slenderness ratio ranging between 4.5 and 17.5. The test result showed that the

effect of the slenderness ratio on the load carrying capacity of FRP-wrapped

concrete columns is more significant than that of ordinary RC columns. The

strengthening effect decreases with increase of slenderness ratio.

Sadeghian et al (2009) have presented the results of experimental

studies about axial stress-strain behaviour of retrofitted slender concrete

columns with CFRP composites. In this study, 30 unreinforced concrete

30

cylinders 100 mm diameter with variable height were prepared and retrofitted.

In each group, a plain specimen (unwrapped) and five wrapped specimens

with different fibre orientations were tested under compressive axial force

upto failure. The results have shown that the CFRP composites are most

effective in increasing the strength and ductility in slender columns.

Though the externally bonded FRP has become increasingly

popular for civil infrastructure applications, the use of GFRP composites has

rapidly increased during the last decade because of its economical balance of

cost and strength properties. Wang and Restrepo (2001) have proposed an

analytical method for evaluating the short-term axial load deformation

behaviour of rectangular and square RC compression members with GFRP

jackets and steel hoops. Three square and three rectangular columns were

tested under axial tension/compression upto failure. The results have clearly

showed the efficiency of the jackets in enhancing the ultimate strain and

strength of columns. The jackets were also very effective in preventing

longitudinal bar buckling from occurring. The analytical model was calibrated

using data from the tests and closed-form equations were proposed for

evaluating the short-term load-deformation behaviour of columns confined

with GFRP jackets.

El-Amoury and Ghobarah (2002) have proposed techniques for

upgrading the shear strength of beam-column joints using GFRP sheets. The

sheets were wrapped around the joint to prevent the joint shear failure. Three

beam-column joints were tested; namely a control specimen and two

rehabilitated specimens. The specimens were tested under quasi-static load to

failure. From the test results, they have found that the control specimen

31

showed combined brittle joint shear and bond failure modes while the

rehabilitated specimens showed a more ductile failure.

Li et al (2003) have conducted uni-axial compression tests on three

different strengths of concrete cylinders confined with different number of

layers of CFRP to develop a constitutive model for confined concrete in the

use of retrofitting and strengthening RC structures. The peak strength of this

constitutive model (named L-L model) was derived from the Mohr-Coulomb

failure envelope theory and can be explicitly expressed as a function of the

unconfined concrete strength, the lateral confining stress, and the angle of

internal friction of concrete. A second-order polynomial equation was used to

present the stress-strain curve of the L-L model. Test results of 108 concrete

cylinders confined by CFRP material were recorded to show the accuracy and

effectiveness of the L-L model. For concrete confined by steel reinforcement

and CFRP material, a modified L-L model was proposed. They have also

reported that this model can be applied to other confining materials such as

GFRP.

Galal et al (2005) have evaluated the performance enhancement of

short RC columns with high and low transverse steel content when retrofitted

using FRP composites. Seven RC short columns were designed and tested

under lateral cyclic loading and constant axial load. GFRP composites were

used to strengthen the short columns. It was found that short columns suffered

brittle shear failures. From the investigation, it was demonstrated

experimentally that it is possible to strengthen the shear resistance of short

columns with GFRP composites such that a flexural ductile failure occurs by

developing plastic hinges at both ends of the column.

32

Hodhod et al (2005) have conducted an experimental investigation

into the behaviour of high strength concrete square short columns subjected to

biaxial bending moments and strengthened by GFRP laminates. The

parameters considered in this study were: number of FRP layers and

arrangement of wraps. Test results of the full scale concrete columns were

presented and discussed. The study has shown that GFRP wraps can be used

successfully to enhance the ductility of high strength concrete columns

subjected to biaxial bending by 300%.

Balsamo et al (2005 a) have conducted full-scale pseudo-dynamic

tests on torsionally unbalanced three-storey RC frame structures with an aim

to pursue a better understanding of the potential of seismic rehabilitation

methods. The strategy of retrofitting with GFRP wraps was explained and the

performance of specimens with and without GFRP wraps during the pseudo-

dynamic tests was described. Through the experimental data, the effectiveness

of the retrofitting strategy was assessed.

Prota et al (2006) have assessed the effectiveness of FRP

confinement on rectangular RC columns with high aspect ratio (wall-like).

Their study aimed at providing more experimental evidence about the

behaviour of such members confined with GFRP laminates. Test results on

nine axially loaded columns were presented. The analysis of test results has

highlighted that GFRP confinement could determine significant strength and

ductility increases and the discussion of failure modes pointed out that the

failure of GFRP confined wall-like columns is controlled by the shape of the

cross-section. Theoretical-experimental comparisons were also performed

using some available models for strength prediction of such members.

33

Balasubramanian et al (2007) have conducted experimental

investigations on RC structural elements to assess the efficiency of CFRP and

GFRP wraps used for the retrofitting purposes. Twelve numbers of beam

specimens and thirteen numbers of column specimens were cast and tested to

check the efficiency of CFRP/GFRP wrapping. For the RC columns retrofitted

with single layer of CFRP/GFRP wrap, peak load, maximum strains as well as

ductility index were higher than the control RC column. In addition, the

performance of beams was found to be improved after retrofitting using FRP

wrapping. The performance of the retrofitted RC beams using a single layer of

both CFRP and GFRP were almost similar. Hence from economical

considerations, GFRP wrapping may be preferred.

Kumutha et al (2007) have examined the several aspects related to

the use of GFRP fabrics for strengthening rectangular columns subjected to

axial compression. To cover a wide range of cross-sectional dimension ratios,

three aspect ratios were studied and specimens with different layers of GFRP

wrap were investigated. Totally nine specimens were subjected to axial

compression which includes three control specimens. All the test specimens

were loaded to failure in axial compression and the behaviour of the

specimens in the axial and transverse directions was investigated. They

concluded that better confinement was achieved with GFRP composite sheets

resulting in enhanced load carrying capacity of the column, in addition to the

improvement of ductility.

Colomb et al (2008) have reported that the GFRP material appears

to be a good solution during the elastic analysis. They have tested eight short

concrete columns, under a constant compression load combined with a

34

horizontal quasi-static cyclic load. Seven columns were reinforced by CFRP

or GFRP. They concluded that composite material reinforcement endowed the

short columns with ductile behaviour, although the columns did not contain

the necessary transversal reinforcement ratio.

Ludovico et al (2008) have reported on a series of bidirectional

pseudo-dynamic tests performed on a full-scale torsionally unbalanced three-

storey RC frame structure. The structure was designed only for gravity loads

and was subjected to two rounds of tests both on the “as-built” and GFRP

retrofitted configuration. The aim of the GFRP retrofit was to enhance the

global performance of the structure in terms of ductility and energy dissipation

by providing columns with more deformation capacity and preventing the

development of brittle failure modes. The driving principles for the design of

GFRP retrofit and the experimental results in terms of global and local

performance were presented and compared in order to validate the proposed

retrofit technique. It was concluded that the GFRP laminates have provided a

considerable increase in the structural global deformation capacity without

significantly affecting its strength. The shear retrofit of the exterior beam-

column joints have prevented brittle mechanisms and thus allowed to fully

exploit the improved energy dissipating capacity of the structure.

1.7 NEED FOR THE PRESENT PROBLEM

It is now widely recognised that masonry infill walls used for

cladding and/or partition in buildings, significantly alter their seismic

response, and their effect in changing the stiffness, the ultimate lateral load

capacity as well as the ductility of the building system should be accounted for

35

in analysis and design. From the extensive review of literature carried out and

presented in the earlier sections, it is seen that exhaustive theoretical and also

experimental studies have been done on lateral behaviour of masonry infilled

multi-storey RC frames. But it is found that the work done to study the effect

of openings in the infill of such frames is limited. The actual behaviour of RC

frames with partial masonry infill under lateral loads simulating earthquake

effects such as captive-column condition is not available.

Due to the complexity of the contact problem, the sophisticated

composite action of the frame and the partial masonry infill, and the

incomplete understanding of the infill role, as well as the numerous

uncertainties involved in modelling the effect of infills; design aids such as

manuals and software as well as related code provisions hardly include any

detailed guidance to take into account the effect of the partial infills. This

enforces the need to pursue research work on RC frames with partial masonry

infill under simulated earthquake loads to identify the seismic effects such as

formation of captive-column condition and column shear failure which had

been observed during several earthquakes and reported.

On the other hand, there is an urgent need to establish effective and

economic retrofitting techniques for seismic up-gradation of captive columns

in existing partially infilled RC structures. The beneficial effect of using

GFRP laminates in retrofitting RC structural elements is clearly established in

the literature. The behaviour of partially infilled RC frames with the adopted

retrofit techniques should also be investigated.

36

1.8 RESEARCH OBJECTIVES

The objective of this research study is to develop a retrofit solution

for strengthening existing RC structures with captive-column defects. In this

study, two different retrofit schemes using FRP laminates and masonry inserts

were proposed. One of the retrofit schemes with GFRP laminates was

proposed to provide a simple and technically superior alternative to the

traditional techniques for improving the shear strength of captive columns and

preventing their failure. Another retrofit scheme was a novel approach

proposed to avoid captive-column failures by simply defending the column

with the addition of non-structural masonry inserts, partly closing the gap that

causes the captive-column effect.

In the experimental study, three partially infilled RC frames were

tested under quasi-static cyclic loads simulating seismic action. In order to

investigate the seismic performance of the proposed retrofit schemes, two

specimens were retrofitted using GFRP laminates and masonry inserts

respectively. Another specimen was the control frame tested to identify the

captive-column behaviour. An analytical validation was performed using

IDARC2D, a software package for the inelastic damage analysis of buildings,

developed by Valles et al (1996) in the National Center for Earthquake

Engineering Research (NCEER) at Buffalo, New York.

1.9 ORGANISATION OF THE DISSERTATION

This dissertation is organised according to the stages followed for

the development of the investigation. Thus, Chapter 1 introduces a general

37

statement of the problem and the objectives of this research. The chapter also

reviews the available literature discussing various studies conducted on

masonry infilled RC frames and on some of the retrofitting techniques for

strengthening RC structural frames and elements using FRP composites.

Chapter 2 describes the details of the test specimens, material properties,

casting and erection of specimens, test setup, and the testing procedure

followed. This chapter also describes the two different retrofit schemes

adopted in this study for strengthening the partially infilled frames. Chapter 3

describes the first phase of the experimental investigation conducted with

quasi-static cyclic loads on a partially infilled RC frame to evaluate the effect

of partial infill and the formation of captive-column conditions.

Chapter 4 gives the details of second phase of the experimental

investigation conducted with quasi-static cyclic loads on a similar frame as

presented in Chapter 3 but retrofitted using GFRP laminates. Chapter 5

presents the third phase of the experimental investigation conducted with

quasi-static cyclic loads on a frame retrofitted using masonry inserts. In

Chapters 3, 4, and 5, various parameters like lateral deflection, strength,

stiffness, ductility, and energy dissipation capacity are considered to study the

behaviour of the frames and the mechanism of failures are identified.

Chapter 6 describes a simple, easy to use analytical approach for validation of

the experimental results carried out on partially infilled frames. The

assumptions and expressions used for the development of analytical models

are presented. The analytical values were confronted with the experimental

values. In Chapter 7, the test results of Chapters 3, 4, and 5 are interpreted and

effectiveness of the adopted retrofit schemes is evaluated. Finally, the

Chapter 7 provides conclusions, recommendations to the designers, and the

scope for further research in the area of partially infilled RC frames.