Report

Behaviour And Failure Mechanism Of Infill Walls

CHAPTER 1

INTRODUCTION

1.1 General

Masonry infilled walls are provided within the reinforced concrete structures

without being analyzed as a combination of concrete and brick elements, though in reality

they act as a single unit during earthquakes. The performance of such structures during

earthquakes has proved to be superior in comparison to the bare frames in terms of

stiffness, strength and energy dissipation. There are plenty of researches done so far for

infilled frames, however partially infilled frames are still the topic of interest. Though it

has been understood that the infills play significant role in enhancing the lateral stiffness

of complete structure, the past experience in various earthquakes have proved that the

partially infilled framed structures somehow are affected adversely. This report intends to

highlight the need of knowledge on partially infilled frames and the composite action. It

also summarizes the findings till date done by various researchers on the behavior of

partial infilled frames under lateral loads.

Reinforced concrete frames with Masonry infills are a popular form of

construction of high-rise buildings in urban and semi urban areas around the world. The

term infilled frame is used to denote a composite structure formed by the combination of a

moment resisting plane frame and infill walls. The masonry can be of brick, concrete

units, or stones. Usually the RC frame is filled with bricks as non structural wall for

partition of rooms .Social and functional needs for vehicle parking, shops, reception etc

are compelling to provide an open first storey in high rise building. Parking floor has

become an unavoidable feature for the most of urban multistoried buildings. Though

multistoried buildings with parking floor (soft storey) are vulnerable to collapse due to

earthquake loads, their construction is still widespread. These buildings are generally

designed as framed structures without regard to structural action of masonry infill walls.

They are considered as non structural elements. Due to this in seismic action, RC frames

purely acts as moment resisting frames leading to variation in expected structural

response. The effect of infill panels on the response of RC frames subjected to seismic

action is widely recognized and has been subject of numerous experimental and analytical

Dept. of Civil Engineering 1 BIT, Bangalore


investigations over last five decades. In the current practice of structural design in India

masonry infill panels are treated as nonstructural element and their strength and stiffness

contributions are neglected. In reality the presence of infill wall changes the behavior of

frame action into truss action thus changing the lateral load transfer mechanism.

Infill panel elements, as part of the building RC structures, play a very important

role on the seismic performance of the building structure. If rightfully accounted for its

mechanical properties in the analysis and design process, as well as in their

implementation in practice, they can markedly increase the total structural strength and

energy dissipation abilities of buildings.

Infill walls are usually provided for functional and architectural reasons and they

are normally considered as non-structural elements and their strength and stiffness

contributions are ignored in the analysis works despite significant advances in computer

technology and availability of modern computational resources. The reasons for ignoring

their presence may be due to the complication involved in analysis and also the

uncertainty about the non-integral action between infill and the frame. Thus, the analyses

of structures are being based on the frames.

When subjected to gravity loads only, the infill walls only add their self weight.

However, an infill wall tends to interact with the frame when subjected to seismic forces.

The performance of structures can be greatly improved by the increase in strength arising

from the non-structural components; on the contrary, this increase in strength also

accompanies an increase in initial stiffness of the structure, which may consequently

attract additional seismically induced lateral inertia forces. An infill wall also exhibits

energy dissipation characteristics under earthquake loading as the frame members

compress the infills at some locations. The infill walls when compressed carry a part of

the load by providing strut action to the frame. As such, the infill walls contribute as a

surplus benefit during the times of earthquakes.

Generally, all parts of the frame may not include infills as they are provided as per

the functional and architectural needs. It has been observed from past earthquakes that the

infills contribute in the enhancement of overall lateral stiffness of the structure. Strong

infills have often prevented collapse of relatively flexible and weak reinforced concrete

frames. Brick masonry, in cement mortar, exhibits highly non-homogeneous behavior due



to relatively weak shear strength of mortar and sometimes due to weak compressive

strength of bricks.

1.2 Types of Infill Provisions

Infills are provided fully or with openings as per the needs for provisions of

partitions or for doors and windows. The four different general types of frames are shown

in the figures below:

1. Bare frame (Fig.1)

2. Fully infilled frame (Fig.2)

3. Infilled frame with opening (Fig.3) and

4. Partial infilled frame (Fig.4).

Fig.1.1 Types of Infill Provisions



CHAPTER 2

INFLUENCE OF INFILL WALLS

2.1 General

Infill walls are interior or exterior walls that ‘fill in’ between structural elements of

a building. They are made of many materials.

It is a general practice in developing countries to provide brick masonry infill

walls within the columns and beam of Reinforced concrete frame structures. Such

composite structures formed by the combination of a moment resisting plane frame and

infill walls is termed as "infilled frames". Infill walls provide durable and economical

partitions having relatively excellent thermal and sound insulation with high fire

resistance. In the areas where the burnt clay bricks are easily available, these infills are

made in brick masonry and in other areas, hollow or solid concrete blocks are used. The

behavior of reinforced concrete frames with brick masonry infills depend upon the

composite action of the frame and the infill. The structural response is quite complex as it

involves an interaction of infill behavior, reinforced concrete frames behavior and length

of contact between infill and frame.

Masonry panels, which contribute a large proportion of the mass of the infill-

frame, normally consist of anisotropic materials with a wide range of strength,

deformation and energy dissipation properties. Unlike other conventional materials such

as concrete and steel which have, to some extent, standard properties regardless of the

region (country) in which they are produced, masonry materials vary significantly from

one country to another based on the local constituent materials (the bricks and the mortar)

and workmanship. Different local materials are used to produce masonry units with

different shapes; they might be solid or hollow units.

2.2 Properties of Infill Walls

The presence of infills does not affect the structural response. This can be the case

if the infills are very light and flexible, or completely isolated from the RC frame, or so

brittle that a total failure is expected even for a moderate ground acceleration.



The infills are assessed to have a significant contribution on the response, and they

are expected to remain in the elastic range. In this case a linear elastic analysis can be

performed. The ductility capacity should be set to 1, unless inelastic structural wall

behaviour can be expected, with columns acting as tension or compression boundary

members, and the infill acting as a connecting shear element.

The infills are assessed to have a significant contribution to the response, and they

are expected to suffer significant damage during the seismic event. In this case the high

probability of the formation of a soft storey has to be recognized and taken into account.

2.3 Advantages of Infill Walls

It has been shown that there is a strong interaction between the infill masonry wall

and the surrounding frame, leading to:

Considerable increase of the overall stiffness (and, in many cases, higher base

shear force).

Increase of dissipated energy.

Redistribution of action-effects and, sometimes, unpredictable damages along the

frame.

Considerable reduction of the probability of collapse, even in cases of defective

infilled frames, when they are properly designed.

2.4 Design Practice of Infill Walls

The design of masonry infilled RC frame buildings is handled in different ways

across the world. Some of the prevalent design practices are:

Infills are adequately separated from the RC frame such that they do not interfere

with the frame under lateral deformations. The entire lateral force on the building is

carried by the bare RC frame alone.

Infills are built integral with the RC frame, but considered as non-structural

elements. The entire lateral force on the building is carried by the bare RC frame

alone. This is the most common design practice in the developing countries.

Infills are built integral with the RC frame, and considered as structural elements.

The in-plane stiffness offered by the infill walls is considered in the analysis of the



building. The forces from this analysis are used in the design of RC frame members

and joint.

2.5 Consideration of Infill Walls In Structural Design

The usual practice of ignoring the structural interaction between the frame and

infill in the structural design of infill-frames. This implies that the infill has no influence

on the structural behavior of the building except for its mass. This would be appropriate if

the frame and infill panel were separated by providing a sufficient gap between them.

However, gaps are not usually specified and the actual behaviour of infill frames observed

during past earthquakes shows that their response is sometimes wrongly predicted. Infill

frames have often demonstrated good earthquake-resistant behaviour, at least for

serviceability level earthquakes in which the masonry infill can provide enhanced stiffness

and strength. It should be noted that even where sufficient separation is provided at top

and ends of a panel, the panel will still tend to stiffen the supporting beam considerably,

concentrating frame potential plastic hinge regions in short hinge lengths at each end, or

forcing migration of hinges into columns, with a breakdown of the weak-beam, strong-

column concept.

Approximately 80% of the cost of damages of structures from earthquakes is due

to damage of the infill walls and to consequent damages of doors, windows, electrical and

hydraulic installations (Tiedeman, 1980). In spite of its broad application and its

economical significance Infill walls were not considered in the analysis and design due to

the following complexities:

Computational complexity: The particulated infill material and the ever changing

contact conditions along its interface to concrete, constitute additional sources of

analytical burden.

Structural uncertainties: The mechanical properties of masonry, as well as its

wedging conditions against the internal surface of the frame, depend very much on

local construction conditions.

The non-linear behaviour of infilled frames depended on the separation of masonry

infill panel from the surrounding frame.



The infill wall enhances considerably the strength and rigidity of the structure. It

has been recognized that frames with infills have more strength and rigidity in comparison

to the bared frames and their ignorance has become the cause of failure of many of the

multi-storeyed buildings. The main reason of failure is the stiffening effect of infilled

frame that changes the basic behavior of buildings during earthquake and creates new

failure mechanism.

2.6 Effects of Infill Walls

Unequal distribution of lateral forces in the different frames of a building –

overstressing of some frames.

Vertical irregularities in strength and stiffness – soft storey or weak storey as a

result higher interstorey drifts and higher ductility demands of RC elements of

the soft storey in comparision to remaining stories.

Horizontal irregularities – significant amount of unexpected torsional forces

since the centre of rigidity is moved towards the stiffer infilled frames of

increased stiffness and as a result occurrence of very large rotation and large

displacements in the extreme bare frames.

Inducing the effect of short column or captive column in infilled frame – a

captive column is full storey slender column whose clear height is reduced by

its part-height contact with a relatively stiff masonary infill wall, which

constraints its lateral deformation over the height of contact resulting in

premature brittle failure of columns, and

Failure of masonry infills – out of plane and in plane failure results which

become the cause of casualities.



CHAPTER 3

BEHAVIOUR OF INFILL WALLS

3.1General

Infills interfere with the lateral deformations of the RC frame; separation of frame

and infill takes place along one diagonal and a compression strut forms along the other.

Thus, infills add lateral stiffness to the building. The structural load transfer mechanism is

changed from frame action to predominant truss action (Fig. 1); the frame columns now

experience increased axial forces but with reduced bending moments and shear forces.

Fig.3.1 Change in lateral load transfer mechanism owing to inclusion of masonry infill walls.

Even when the infills are structurally separated from the RC frame, the separation

may not be adequate to prevent the frame from coming in contact with the infills after

some lateral displacement; the compression struts may be formed and the stiffness of the

building may increase. Infills possess large lateral stiffness and hence draw a significant

share of the lateral force. When infills are strong, strength contributed by the infills may

be comparable to the strength of the bare frame itself.



Masonry is mostly designed to “allowable stress” standard, although it may be

better to carry out the design under “ultimate strength” methods in terms of cost

effectiveness. However, in the case of designing a structure for ultimate strength, the

corresponding failure modes need to be known and the failure loads for different modes of

failure have to be computed in order to determine the ultimate capacity of the structure

and additionally the serviceability criteria have to be checked using working loads. At

moderate loading levels the infill of a nonintegral in-filled frame separates from the

surrounding frame and the infill acts as a diagonal strut (Fig. 2). As the racking load

increases, failure occurs eventually in either the frame or the infill. The usual mode of

frame failure is either due to tension in the windward column or due to shear on the

column or beams. However, if the frame strength is sufficient enough to prevent its failure

by one of these modes, the increasing racking load eventually produces failure of the

infill. In the most common situations, the in-plane lateral load applied at one of the top

corners is resisted by a truss formed by the loaded column and the infill along its diagonal

that connects the loaded corner and the opposite bottom corner. The state of stress in the

infill gives rise to a principal compressive stress along the diagonal and a principal tensile

stress in the perpendicular direction.

Fig 3.2 Equivalent diagonal strut



When infills are non-uniformly placed in plan or in elevation of the building, a

hybrid structural load transfer mechanism with both frame action and truss action, may

develop. In such structures, there is a large concentration of ductility demand in a few

members of the structure. For instance,

the soft storey effect (when a storey has no or relatively lesser infills than the

adjacent storeys),

the short-column effect (when infills are raised only up to a partial height of the

columns), and

plan-torsion effect (when infills are unsymmetrically located in plan)

cause excessive ductility demands on frame columns and significantly alter the collapse

mechanism. Another serious concern with such buildings is the out-of-plane collapse of

the infills which can be life threatening. Even when the infills are structurally separated

from the RC frame, the separation may not be adequate to prevent the frame from coming

in contact with the infills after some lateral displacement; the compression struts may be

formed and the stiffness of the building may increase.

3.2 Parameters Influencing The Behavior of The Infilled Frame

It is very difficult to determine specific parameters which will influence the

behavior of the infilled frame, as many factors are involved in this type of structure.

However, it is normally acknowledged that mechanical and geometric attribution of frame

and infills, as well as constructive quality and workmanship, are very important factors. In

the following section, some major parameters will be briefly discussed.

3.2.1 The Relative Stiffness between the Infills and Surrounding Frame

Stafford Smith [1966,1969] proposed a dimensionless parameter λh, which expresses

the relative stiffness of the infill panel to the frame:

λh h Emt sin 2θ

4EcIchm (3.1)

Where t - thickness and height of the masonry panel,

hm- the height of the masonry panel,

θ - the inclination of the diagonal of the panel,

Em and Ec are the modulus of elasticity of the masonry and the concrete,


4


Ic - the moment of inertia of the columns.

The smaller values of λh indicate that the frame is much stiffer than the infill panel.

Empirical equations have been proposed to evaluate the stiffness, the lateral strength or

the contact length between the frame and infill panel as a function of parameter λh [Konig,

1991], [Liauw, et al., 1984], [Mainstone, 1971], [Shrive, 1983], [Stafford Smith, et al

1969, 1978]. It has been observed that for low values of λh (the frame is stiffer than the

infill panel), the lateral strength of the infilled frame increases, as does the contact length

between the panel and the frame.

3.2.2 Strength of the Masonry Infill

The compressive and shear strength of masonry panels depends on the properties

of their constitutive materials, mortar and bricks, such as the hydration conditions at the

mortar –brick interface, the characteristics of the brick surface and compressive strength

of the mortar. A better quality of the constitutive materials leads to a stronger masonry,

but not necessarily to increase the lateral strength of the infilled frame, as excessively

strong masonry will result in the premature failure of the frame members. Tests [Dawe, et

al 1989], [Liauw, 1988] indicate that the use of poor quality mortar will considerably

reduce the force at initial cracking and the lateral strength of the infilled frame, and vice

versa. Solid masonry units usually show a higher compressive strength when compared

with that of the hollow masonry units.

3.2.3 Characteristics of the Reinforced Concrete Frame

The concrete area of the members and the amount of the longitudinal and

transverse reinforcement are the most important parameters which can affect the response

of infilled frames. Benjamin and Williams [1958] observed that the variation of concrete

and steel area does not influence the stiffness in the uncracked stage. However, the

strength of the composite structure depended on the resistance of the frame to bending

moment, axial force and shear.

Experimental results [Fiorato, et al, 1970], [Meli, 1994], [Kato, et. al, 1992],

[Valiasis, 1989] show that the amount of longitudinal reinforcement at the column had

only a limited effect on increasing the ultimate capacity of the frame structure, but it

significantly increased the ductility that could be attained. The amount of transverse



reinforcement of the columns can change the response of infill framed in the ultimate

stage. It is necessary to provide sufficient transverse reinforcement, especially in columns,

to sustain large deformation without a brittle shear failure.

3.2.4 Gaps between Infills and Surrounding Frame

Gaps between the masonry panel and the surrounding frame can develop in non-

integral infilled frames due to the shrinkage of the mortar and constructive defects. The

presence of vertical gaps between columns and the masonry panel provides slackness and

reduced stiffness in the initial stage. When the gaps are very small, the effect on the

response is negligible, because these gaps close rapidly when the lateral force is applied

and the diagonal strut mechanism develops. However, if the gap is large, the effect of

vertical gaps can significantly modify the response of the structure. The horizontal gaps

between the panel and beam can also change the response of the structure significantly.

3.3 Structural behaviour

The structural behaviour of the following infill provisions is discussed:

i) Fully infilled frame.

ii) Partially infilled frame.

iii) Infill frame with opening.

iv) Soft storey.

3.3.1. Behaviour of fully infilled frame

The structural behaviour of an infill-frame can be divided into two parts:

a) In-plane behaviour and

b) Out-of-plane behaviour.

The simultaneous effect of in-plane and out-of-plane loading has usually been

ignored in the research conducted to date, although in actual earthquakes this effect will

usually be present.



3.3.1.1 In-plane behavior

Various in-plane examples of unreinforced masonry walls subjected to seismic

lateral loads can be seen in several text books (e.g. Paulay and Priestley, 1992). The in-

plane capacity of the wall depended on the relative strength of the masonry and the

mortar. The level of the axial load significantly controls the type of failure. There are

several failure conditions for in-plane infill walls due to the form of construction and the

combine effects of axial load and bending, as follows (Tomaževič, 1999)

Sliding shear failure, along head or bed joint because of low normal stresses

and/or low friction coefficients, which may be due to poor quality of the mortar .Shear

failure takes place where the principal tensile stresses developed in the wall under a

combination of vertical and horizontal loads, exceeds the tensile strength of masonry

materials. The crack propagation either follows the mortar joints or passes through the

masonry units, or both. Shear failure should be avoided as it will cause a limited/lower

ductility for infill walls. The strength and stiffness of the infill wall will degrade rapidly

following formation of a diagonal shear crack.

Fig 3.3 Comparison between the actions in a bare-frame and infill-frame

Flexural failure, crushing of compressed zones at the ends of the infill wall usually

takes place, indicating the flexural mode of failure. It happens when the shear resistance

still strong enough when compared to the shear demand. The performance of Uinfill wall

is linear-elastic before the flexural tension stresses at the wall heel exceed the tensile



strengths capacity, or diagonal tension or bed joint shear stresses exceed the shear strength

capacities.

Based upon truss action (interaction) in an infill-frame system as shown in Figure

3, the idea of a strut model was first proposed by Polyakov (1956). In this method, the

infill panel is replaced by one (or more) compressive diagonal(s) in the frame as shown in

Figure 2. Opposite diagonals represent the infill panel as the direction of the lateral load

changes. It is important to note that the diagonal properties are heavily empirical.. In order

to consider the local effects of the infill on the frame, e.g. shear and moment actions in

beams and columns, the triple strut model and the single eccentric strut model are

recommended.

3.3.1.2 Out-of-plane behaviour

The out-of-plane behaviour of infill-frames has been investigated since the 1950s.

As reported by Shing and Mehrabi (2002), many studies (Angel 1994, Mander et al. 1993;

Bashandy et al. 1995; and Flanagan 1999) on out-of-plane behaviour of infill -

framesindicate that infill panels restrained by frames can develop significant out-of-plane

resistance as a result of arching effect. The out-of-plane strength of a masonry infill is

mainly dependent on its slenderness. If an “x” pattern of cracks develops under both

inplane and out-of-plane loading, this implies that there may be some substantial

deterioration in either in or out-of plane strength under the loading in the opposite

direction (Angel 1994). It is shown by Angel (1994) that the out-of-plane strength

deterioration may reach as much as 50% for infill panels with high slenderness ratio where

they have already been cracked under lateral in-plane loading.

Calvi et al. (1996) and Costley and Abrams (1995), reported that the flexural

tension strength at the wall heel does not limit lateral strength. The limit lateral strength of

the wall depends on the diagonal tension, bed-joint sliding, toe crushing or rocking

system. Correspondingly, for slender walls that the shear strength capacity will not

exceed; the URM wall will start to rock about its toe. The wall can still transfer the shear

through the friction at the wall toe and also depends on the axial compressive force. The

rocking system can be advantageous system for strengthening an existing URM wall. The

rigid body of the wall will rotate about its toe and displace to quite large a drift with

limited crack damage and predictable performance.Dept. of Civil Engineering 14 BIT, Bangalore


Based on the results of tests conducted by (Angel, 1994), the following behaviour

can be expected due to different values of slenderness ratio:

a) Crushing along the edges for low hm/t (where hm and t are the height and thickness of

the infill panel, respectively);

b) Snap-through (small effect of arching) for high hm/t i.e. approximately between 20 and

30 (this limit depends on the crushing strain of the masonry which usually varies between

0.002 and 0.005).

Fig.3.4 Out of plane loading

Regarding the out-of-plane behaviour of masonry (bare) walls, it has been shown that they

exhibit substantial out-of-plane displacement capacity and hence more ductile behaviour

than is conventionally accepted (Griffith et al. 2007). A comprehensive study on the

damping of masonry walls in out-of-plane (on-way) flexure can also be found in Lam et

al. (2003). The displacement-based method of design has been used by Magenes et al.

(1997) to study the in plane behaviour of masonry walls. The displacement-based method

has also been used for the analysis/assessment of out-of-plane behaviour and stability of

masonry walls and validated by experimental results (Doherty et al. 2002). A similar

approach, considering the effect of amplification of the acceleration at different levels and

the P-delta effect is proposed by Priestley et al. (2007).



3.3.2 Behaviour of Partially Infilled Frames

In majority of hospitals, academic institutions and commercial complexes, partial

infills are provided to attain light within the rooms. It is observed that such walls on one

hand contribute in enhancing the lateral stiffness of the structure while on the other hand

they play ironic role with an adverse effect called "short column effect".

The term short column effect is defined as the effect caused to the full storey

slender column whose clear height is reduced by its part height contact with a relatively

stiff non-structural elements such as a masonry infill, which constrains its lateral

deformation over the height of contact. The column which gets its effective height reduced

due to such partial infill walls is termed captive column, or in general, the short column.

The shear required to develop flexural yield in the effectively shortened column is

substantially higher than shear required developing in full length column. If the designer

has not considered the short column effect, shear failure may occur before flexural yield

and often fail in brittle manner. The cracking in captive column generally initiates from

window headers and sill level. The short column effect arises mostly due to accidental

modification to the original structural configuration by restricting its freedom to deform

laterally due to the presence of non-structural elements that partially confine it. The non

structural elements keep some portion of the column captive and only the free portion of

the column can deform laterally.

From the available earthquake damage reports, with few exceptions worldwide,

numerous cases of captive column have occurred. The direct visual effect is on the column

itself however, the cause usually is due to the non-structural elements which impose the

pattern of response to the earthquake motions different from the expected behavior of the

column itself without the non structural elements. Few damages due to short column effect

on building structures are shown in the following photographs (Fig. 6 and 7). The need to

study on short column effect for lateral loading may also be justified by observing the

photographs.



Fig.3.5 Captive column failure

When the floor slab moves horizontally during an earthquake, the upper ends of

these columns undergo the same displacement. However, the stiff walls restrict the

horizontal movement of the lower portion of a short column and get deformed by the full

amount over the short height adjacent to the window opening. The regular columns get

deformed over the full height. Since the effective height over which a short column can

freely bend is small, it offers more resistance to horizontal motion and thereby attracts a

larger force as compared to the regular column. As a result, the short column sustains

more damage. Such problems originate in the architectural designs of most of the

buildings. Contractors often add partial height walls between columns at the request of the

building owners (after the building is occupied), without taking any consent of the

involved architect or engineer. Thus, the designers and contractors should understand the

problem to avoid short column effect.


Fig.3.6 Lateral deformation in bare frame

Fig.3.7 Lateral deformation in partial infilled frame


It is observed from the various research works that there is no doubt that the infill

walls contribute in enhancing the structural strength. However, the contribution of partial

infill walls must be well identified so that while analyzing models for real structures, the

composite action of the frame and infill would be realized. The lessons from the past

earthquakes also indicate that partial infilled frame structures are vulnerable to ground

motions and if there is a method identified to model such structures, the earthquake hazard

to structure would be reduced significantly.

When it is not possible to avoid short columns, this effect must be considered

either in the analysis or during the design of column member. From the knowledge that

there would be significant increase in shear in the case of captive column the Indian

Standard IS:13920-1993, for ductile detailing of RC structures, requires special confining

reinforcement to be provided over the full height of columns that are likely to sustain short

column effect. The special confining reinforcement (i.e., closely spaced closed lateral ties)

must extend beyond the short column into the column vertically above and below by a

certain distance. So far, exact scenario regarding analysis for captive columns is yet to be

understood for précised responses on shear and bending and the convenient modeling

method is also not postulated for such structures. More researches need to be done, and in

fact by experimentation the validation would be better quantified.

3.3.3 Behaviour of Infill Frames With Openings

In most cases, door or window openings are provided in masonry infill panels

because of the functional and ventilation requirements of buildings.

Introducing openings in an infill wall alters its behavior and adds complexity in behavior.

Furthermore, due to the presence of openings in infill panels, the lateral strength and

effective stiffness of infilled frames is reduced

Mallick and Garge experimentally investigated the effect of opening position on

lateral stiffness of infilled frames with and without shear connectors. The conclusion was

that if an opening is provided at either end of the loaded diagonal of an infilled frame

without shear connectors, the strength and stiffness are reduced by about 75 and 85%–

90%, respectively when compared to those of a similar infilled frame with

solid infill panel. Also, it has been recommended that the best location for a window or

door opening is at the center of the infill panel. Mosalam et al. reported that the presence



of openings reduces solid infill panel stiffness values by about 40% for lateral loads below

the cracking load level. Also, openings in infill walls lead to a more ductile behavior while

ultimate load capacities of solid infills and infills with windows are similar.

They found that the location of the opening as near to the edge of the infill as

possible provides an improvement on the performance of the infilled frame. Also, it was

observed that the energy dissipation is more significant in the case of the larger piers

where a better distribution of cracks in the wall is developed. Kakaletsis and

Karayannis experimentally investigated the effect of masonry infill compressive strength

and openings on failure modes, strength, stiffness and energy dissipation of infilled RC

frames under cyclic loading. They found that infills with openings and strong masonry can

significantly improve the performance of RC frames.

Fig.3.8 Cracking pattern of infill wall with central opening

3.3.4 Soft storey

The base floors of the existing buildings are generally arranged as garages or

offices. No walls are built in these floors due to its prescribed usage and comfort

problems. But upper floors have walls separating rooms from each other for the residential

usage. In these arrangements, the upper floors of most buildings are more rigid than their

base floors. As a result, the seismic behaviors of the base and the upper floors are

significantly different from each other. This phenomenon is called as the weak-story

irregularity. Weak stories are subjected to larger lateral loads during earthquakes and



under lateral loads their lateral deformations are greater than those of other floors so the

design of structural members of weak stories is critical and it should be different from the

upper floors.

Kirac et al. (2010) studied the seismic behavior of weak-story. Calculations were carried

out for the building models which are consisting of various stories with different storey

heights and spans. Some weak-story models were structural systems of existing buildings

which were damaged during earthquakes. It was observed that negative effects of this

irregularity could be reduced by some precautions during the construction stage. Also

some recommendations were presented for the existing buildings with weak-story

irregularity.

One of the main reasons of failure of structures due to earthquakes is discontinuity of

lateral force resisting elements like bracing, shear wall or infill in the first story as show

conceptually in Fig. 11. So first story act as soft story, in this case columns are imposed to

large deformation and plastic hinges are formed at top and bottom of the element.

Conceptual figure is obtained from actual earthquake observation as shown in fig. 12. This

phenomena is so-called story mechanism (severe drift of the story). Most of these

buildings have collapsed. The upper stories have infills and consequently their stiffness is

much more than the first story.

Fig.3.9 Schematic view of soft story mechanism



Fig.3.10 Soft story failure in a building during earthquake (Italy 1976)

The performance of a building in earthquake is shown in Fig. 12. This building is RC

structure and has parking in the first story; there is no infill in the parking story.

Deformations are localized in the first story and the columns of this story undergo large

deformation, passing collapsed limit (4% of height).



Chapter 4

FAILURE MECHANISM OF INFILLED WALLS

4.1 General

The type of failure that will occur in an infilled frame is normally difficult to

predict, depending on several factors, such as the relative stiffness of the frame and infill

panel, the strength of components and the dimensions of the structure. It is however hard

to isolate those factors, which may impose an influence on the behaviour of the structure,

as they are interacting with each other. The mechanisms of failure affecting the

components of the infilled frames are referred to as failure modes in a general sense.

Sometime the final failure of the structure results from one principle mode; while in other

cases a combination of different modes may lead to failure. In this chapter, the major

failure modes and influential factors will be the focus under discussion. The failure of

masonry panels can develop by debonding of the mortar joints, cracking or crushing of

masonry units or a combination of these. The occurrence of the different types of failure

depends on the material properties and stress state induced in the panel. Crisafulli [1997]

summarized the different modes of failure which may occur in masonry.

4.2 Modes of Failure

The mode of failure of an infilled building depends on the relative strengths of

frame and infill (Table 1). And, its ductility depends on the (a) infill properties, (b)

relative strengths of frame and infill, (c) ductile detailing of the frame when plastic

hinging in the frame controls the failure, (d) reinforcement in the infill when cracking in

infills controls the failure, and (e) distribution of infills in plan and elevation of the

building.



Weak Infill Strong Infill

Weak Frame -

Diagonal cracks in infill.

Plastic hinges in columns.Frame with Weak Joints

and Strong Members Corner crushing of infillsCracks in beam-column joints

Diagonal cracks in infill

Strong Frame Horizontal sliding in infills -

Fig.4.1 Failure modes in masonry infills

4.2.1 Shear Cracking

Cracking in the masonry panel due to shear stresses is the most common type of

failure which is mainly controlled by shear strength of the mortar joints (bond strength and

coefficient of friction), the tensile strength of the masonry units and the relative values of

the shear and normal stress. Depending on these parameters, the combination of shear



stresses with vertical axial stresses can produce either cracks crossing the masonry units or

debonding along the mortar joints (also termed as shear friction failure). The ratio of

normal stress fn, to the shear stress τ, can be utilized to approximately represent stress state

along the diagonal when subjected to lateral forces.

fn hm Aspect ratioτ Lm

Where, hm is the height of the masonry panel;

Lm is the length of masonry panel.

4.2.1.1 Stepped Cracking Along the Mortar Joints.

When the mortar joints are weak in comparison with the masonry units or when

the shear stress predominates over the normal stress (low to medium aspect ratio),

cracking usually occurs by debonding along the mortar joints. Figure 4.2 illustrates the

case in which one or two large cracks formed along the diagonal with stepped pattern.

This mode of cracking is widely observed and regarded as the most common type of

failure.

Fig.4.2 Shear cracking with stepped cracks



4.2.1.2 Horizontal Sliding Along the Mortar Joints

A different mechanism has also been observed, in which the panel fails by shear

due to the formation of a horizontal crack, as shown in Figure 4.3. This type of failure was

reported by Fiorato et al. [1970], Brokken and Bertero [1981, 1983] in tests of infilled

reinforced concrete frames. Tests results indicate that the major crack usually starts a few

courses below the upper loaded corner and continues diagonally downwards to

approximately the centre of the panel. Then the cracks propagate horizontally. The Sliding

Shear (SS) mode, which represents horizontal sliding shear failure through bed joints of a

masonry infill is associated with infill of weak mortar joints and a strong frame.

Fig.4.3 Shear cracking with horizontal sliding

When the direction of the force is reserves, the horizontal crack increases its

length, crossing the panel. The relative dimension of the masonry units and the infill panel

may be one major factor contributing to the formation of the horizontal cracks. When the

following equation is satisfied, the horizontal crack could form:

hm 2bLm d

Where d and b are the length and height of masonry unit.



4.2.1.3 Cracking Due to Diagonal Tension

This type of cracking usually occurs when the mortar joints are strong in

comparison with masonry or when the normal stress predominates over the shear stress

(medium to high aspect ratio). The Diagonal Compression (DC) mode, which represents

crushing of the infill within its central region, as shown in Fig. This mode is associated

with a relatively slender infill, where failure results from out-of-plane buckling of the

infill.

Fig.4.4 Shear cracking due to diagonal tension

4.2.2 Compression Failure

Failure of masonry due to compression has been observed following two

mechanisms, resulting from the different stress states which develop in the infill panel at

the loaded corner and along the diagonal.

4.2.2.1 Crushing of the Loaded Corner

Compressive failure can occur in the region close to the loaded corners, where a

biaxial compression-compression stress state develops due to the lateral loading. Figure



4.5 illustrates this case. This type of failure would be rare to occur in masonry infilled

reinforced concrete frames, due to the weakness of shear and diagonal tension failure

[Smith and Carter, 1969]. The Corner Crushing (CC) mode, which represents crushing of

the infill in at least one of its loaded corners is usually associated with in-filled frames

consisting of a weak masonry infill panel surrounded by a frame with weak joints and

strong members.

Fig.4.5 Compressive failure due to crushing of the loaded corners

4.2.2.2 Compressive Failure of the Diagonal Strut

This mechanism is associated with diagonal cracking as illustrated in Figure 4.4.

After the cracks occur, the tensile stressed along the diagonal are relieved and the masonry

between the cracks behaves like small prisms axially loaded. The Diagonal Cracking

(DK) mode, which is seen in the form of a crack across the compressed diagonal of the

infill panel and often takes place with simultaneous initiation of the SS mode. This mode

is associated with a weak frame or a frame with weak joints and strong members in-filled

with a rather strong infill.



4.2.3 Flexural Cracking

In those cases where flexure effects are predominating, such as multistory infilled

frames, and the columns of the frame are very weak, flexure cracks can open in the tensile

side of the panel due to the low tensile strength of the masonry [Leuchars et al., 1973].

The Frame Failure (FF) mode, which is seen in the form of plastic hinges

developing in the columns or the beam-column connections. This mode is associated with

a weak frame or a frame with weak joints and strong members in filled with a rather

strong infill.



CHAPTER 5

CONCLUSIONS

Many structural engineers ignore such infills when assessing the seismic

vulnerability of these frames. Consequently, more research is needed to evaluate the

strength and stiffness of masonry-infilled frames with openings.

1) Masonry infill wall panels increase strength, stiffness, overall ductility and energy

dissipation of the building.More importantly, they help in drastically reducing the

deformation and ductility demand on RC frame members.

2) The better collapse performance of fully-infilled frames is associated with the

larger strength and energy dissipation of the system, associated with the added

walls.

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

columns. However, in the case of infilled frame with a soft ground story, the shear

forces acting on columns are considerably higher than those obtained from the

analysis of the bare frame.

4) A classification of the failure modes (crack patterns) enhances considerably the

understanding of the earthquake resistant behavior of in-filled frames and leads to

improved comprehension of their modelling, analysis and design.

5) Because of high stiffness of the infill walls, considering them as structural

elements leads the initial stiffness of structures to increase. Such elements show

high strength at the first step of seismic loading, but by reaching to the maximum

strength, the infill walls fail and high loss of strength occurs in small drifts.



REFERENCES

1) Agarwal, P. & Shrikhande, M. (2006). “Earthquake Resistant Design of Structures”,

Prentice Hall of India Pvt. Ltd., India.

2) Mulgund, G. V. & Kulkarni, A. B. (2011). “Seismic Assesement Of Rc Frame

Buildings With Brick Masonry Infills”, International Journal Of Advanced Engineering

Sciences And Technologies, Vol No. 2, Issue No. 2, 140 – 147.

3) Pauley, T. and Priestley, M.J.N. (1992). “Seismic design of reinforced and masonry

buildings” Wiley Interscience Inc., USA.

4) I.S. 1893(Part I)-2002, “Criteria for Earthquake Resistant Design of Structure, General

Provisions and Buildings”, Bureau of Indian Standards, New Delhi.

5) Das, D., Murty, C. V. R. (2004). “Brick masonry infills in seismic design of RC framed

buildings: Part 1 –Cost implications” The Indian Concrete Journal, vol78 No7: 39-43.

6) Asteris, P.G., Kakaletsis, D. J., Chrysostomou, C. Z. & Smyrou, E. E. (2011).

Electronic Journal of Structural Engineering 11(1) , 11-20.

7) Pradhan, P. M., Pradhan, L.P., & Maskey, R.K. (2012). “ A Review On Partial Infilled

Frames Under Lateral Loads”, Kathmandu University Journal Of Science, Engineering

And Technology, VOL. 8, No. I, 142-152.

8) Murty C V R, and Jain S K, (2000). "Beneficial Influence of Masonry Infill Walls on

Seismic Performance of RC Frame Buildings", Proceedings of the Twelfth World

Conference on Earthquake Engineering, Auckland, New Zealand, Paper No. 1790.

9) Wijanto L. S., (2007). “Seismic Assessment of Unreinforced Masonry Walls”, A thesis,

University of Canterbury Christchurch, New Zealand.

10) ZHANG B., (2006). “Parametric Study On The Influence Of Infills On The

Displacement Capacity Of Rc Frames For Earthquake Loss Estimation” A

Dissertation, European School For Advanced Studies In Reduction Of Seismic Risk.


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