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CHAPTER 2: BEHAVIOR AND ANALYSIS OF INFILLED FRAMES
- STATE OF THE ART
The infill frames have drawn attention of several investigators in the recent past for their
inherent structural advantage. This chapter provides an overview of experimental and
analytical research in infilled frames during the last half century. The focus is kept on
steel and reinforced concrete infilled frames. Experimental investigations have been
conducted by several researchers using a wide range of testing scales, numbers of
specimens, infill materials, experimental set-ups and constraint studies. Several collapse
and damage patterns have been observed. Experimental research has been complimented
by the analytical attempts to model infilled frame behavior. Global and fundamental
models have been formulated. The infilled frame structure is however still difficult to
model, partly, due to a nonlinear phenomena associated with infills and with frame-to-
infill contact areas. There are no universally accepted design guidelines for the infilled
frames.
2.1 Opening remarks
Available literature attempts to evaluate the strength and stiffness of infilled frames. The
methods suggested by various authors are classified as follows:
i) Method based on concept of equivalent strut
ii) Method based on finite element analysis
iii) Method based on results of experimental investigations
iv) Method based on plasticity and collapse approach
v) General Studies on infilled frames
2.2 Methods based on concept of equivalent strut
Holmes (1961) proposed a method of determining the stiffness and strength of infilled
frames, wherein it was suggested that the infill be replaced by an equivalent strut of width
equal to one third of the diagonal length and thickness equal to that of infill. Steel frames
were modeled, with brick and concrete infill and compared the theoretical values of
ultimate loads with the experimental ones by equivalent strut approach. It was found that
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the theoretical values were differed by 2 to 8% with brick infill and 5% to 8% with
concrete infill from the experimental ones.
For diagonally loaded rectangular panel, Smith (1962) had tried two methods for finding
stresses. First one was based on the strength of material and the second method was
based on the theory of elasticity approach by considering finite difference approximation
to the infilled frames. For finding lateral stiffness and lateral strength, the same theory of
diagonal strut assumption had been suggested to find lateral strength from non-
dimensional parameter ‘λһ’. For lateral stiffness calculation, empirical formulae were
suggested by considering rigid frame and non-rigid frame and concluded that the results
of experimental and theoretical work differed by 15%.
Smith (1966) studied the behavior of square frames and tried to compare the theoretical
results with experimental ones and derived expressions for diagonal strength. It was
suggested that the concept, that the infill acts as a diagonal strut, as shown in Figure 2.1
of certain width along the loaded corners. To derive the effective width finite difference
method was used. A relationship was suggested between the diagonal stiffness and a
non-dimensional parameter ratio of relative stiffness of infill to the frame.
Figure 2.1 Smith’s equivalent strut
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Mainstone (1971) investigated the behavior of infilled frames under the full range of
restraint offered to an infill of different types of surrounding frames as well as the effect
of surrounding infills. The approach to the analysis of strength and stiffness of infilled
frames was based on the concept of diagonal struts. It was visualized that infill is to be
replaced by single or several struts as shown in Figure 2.2, depending upon the degree of
initial fit of the infill to the frame.
(a) Several struts (b) Single strut
(c)Initial stage of loading (d) Further stage of loadings
Figure 2.2 Infill replaced by single or several struts –Mainstone
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It was observed that if the construction is not supervised to ensure the joints between the
frame and infill, no reliance of the contribution of the infill could be made.
Smolira (1973) had emphasized the difficulties of an exact analysis for R.C.C. frames,
infilled with the brickwork and also presented a simplified approach of analysis on the
basis of the assumption of linear behavior of an equivalent strut. The width of the
equivalent strut was taken as 1/3 rd of the diagonal length.
Liauw (1979) had presented two analytical methods for the infilled frames with or
without connectors and had used the equivalent frame and equivalent diagonal strut
method respectively. He conducted the experiments on four storey steel frames with the
concrete infills with and without opening, under the lateral loads. He concluded that (i)
larger portion of the shear load was carried by the infill, which is stiffer than the frame.
On the other hand with the same infill stiffness, the stiffer frame carried a larger portion
of shearing load. (ii) The provision of the opening in the infill did not change the basic
behavior of the structure and the reduction in the strengths and stiffness were moderate.
(iii) In the frames with the connectors, slip and separation between the frames and the
infills were prevented.
Other researchers have proposed refinements or alternative definitions of the effective
width, w, of the equivalent diagonal strut. Based on the experimental and analytical data
Mainstone (1971) and Liauw et al. (1984) had proposed the expressions 2.1 and 2.2,
respectively. FEMA 306(1998) had recommended a modification of Mainstone’s formula
and proposed the equation 2.3.
w =0.16 (λh h) -0.3
ld 2.1
w =0.95 h cosθ / (λh h) 0.5
2.2
w =0.175 (λh h) -0.4
ld 2.3
where: ld is the diagonal length of the infill ,
λh = θ/4 Ef Ic h) 2.4
where,
Ei is the modulus of elasticity of the infill,
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t is the thickness of the infill,
Ef is the modulus of elasticity of the frame material,
Ic is the second moment of area of the column,
θ is tan-1 (h/l),
h and l are the height and the length of the infill,
These expressions for w yield unequal values of the effective width of the equivalent
diagonal strut. A fuller description and comparison for it was given by Chrisaffulli et al.
(2000).
Shing et al. (2002) have also pointed out that the several researchers have reported a
wide difference in the conclusion that resulted in overestimations or underestimations of
effective widths compared to their experiments. Thus there is no definite conclusion
concerning the matter. The common agreement, however, is that the results indicate that
the width of the equivalent strut decreases when the parameter λh increases.
The major limitation of the equivalent diagonal strut analogy lies in its inability to give
the stress distribution in the panel or indeed in the frame. In a bid to overcome this
disadvantage, several researchers have proposed modifications to the single strut
idealization.
Chaar et al. (2002) conducted monotonic load tests on five half scale, single story, non-
ductile infill frames with one, two and three bays. Observed results were similar to those
of the earlier investigation with the infill, addition in the strength and stiffness to the bare
frames. Important information about the behavior and failure modes of the infill frames
was obtained from such experiments. These experiments provided the force-deformation
relationships and offered insights into determining the effect of infills on the lateral
strength and stiffness of infill frames. Experiments were performed on monotonically
loaded frames to reveal the effect of specific parameters such as openings, the contact
length between infill walls and bounding frame members, and the size and arrangement
of initial gaps.
El Dakhakhini et al. (2003) had adopted the limit analysis approach of Saneinejad et al.
(1995) to define the properties of struts in a three-diagonal strut model in order to assess
the stresses in the bounding frame.
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2.3 Methods based on finite element analysis
Mallick et al. (1967) had described a method, which makes the use of the finite element
concept. For the purpose of calculation, the actual frame with infinite degrees of freedom
was replaced by a system with finite number of degree of freedom as shown in Figure.
2.3.
(a) Rectangular infilled Frame (b) Possible finite
element idealization
Figure 2.3 Mallick and Severn ‘s idealization
At each node two degrees of freedom were permitted. Detailed procedure for the
analysis was given in their paper .The values of the stiffness obtained by the method
show good agreement with experimental values. The elements of the infill were taken to
be rectangular in shape as shown in Figure 2.4.
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Nodal and displacements for a rectangular element in plane stress
Figure 2.4 Finite element shapes
A finite element approach taking into the account the axial extension of frame members
was proposed by Barua et al. (1977) and a tightly fitted, homogeneous and elastic infill
was assumed in the analysis as well as in theoretical studies, matching the displacements
of frame and infill at finite number of nodes along the interface. In the experimental
work a series of mortar infilled model steel frames were tested.
Srinivas B. et al. (1977) performed a model analysis for a five storey reinforced masonry
infill and same frame models were selected and designed according to the IS 1893 codal
provisions. Infill walls are modeled by the equivalent strut approach and the bottom
storey of building kept open for considering the realistic behavior of the buildings and
nonlinear, static and dynamic analysis were performed. From the results it is clear that
the infill walls greatly contribute the stiffness to bear lateral loads, the storey response
and quantities such as storey shear and displacement decrease due to the infill.
A realistic criterion was proposed by Asteris (2008) to describe the frame- infill
separation under the lateral loads. Hashmi et al. (2008) study was based on the analytical
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investigation of the seismic performance and potential Seismic damage of masonry
infilled R.C. framed building due to earthquake. For that rational non-linear modeling
and displacement based analysis techniques were used. It was reported that the seismic
performance of the masonry infilled R.C. frame is adversely and significantly affected if
infill panels were discontinued in the ground storey resulting in a structural configuration
with an open storey generally termed as soft storey. It was also found if a storey is
partially infilled (in comparison to no infills), it decreases storey drift and deformations in
the column in the storey reducing the related damage to the columns and overall frame.
Arulselvan et al. (2007) conducted an experiment on the RC Infilled frame-RC plane
frame for interactions of the seismic Resistance. In this study the RC frame with middle
bay brick infilled representing five stories, three bays building in quarter scale has been
taken for the experimental investigation and the available method of theoretical analysis
and finite element analysis using ANSYS software for the frame has been carried out.
The conclusion was that, the strains measured in the infilled beams and the columns are
20% lesser than the bare frame up to failure of brick infill.
Lakshmi G.A. et al. (2008) conducted a detailed investigation by the numerical and
experimental study on strengthening of the beam-column joints under cyclic excitation
using FRP composites. In that study the three typical modes of failure namely flexural
failure of beam, shear failure of a beam and shear failure of a column were discussed.
Comparison was made in the terms of load carrying capacity. Three exterior beam-
column joint and sub assemblages were cast and tested under the cyclic loading. All the
three specimens were retrofitted using FRP materials and results were compared with the
control specimens. Finite element analysis has been carried out using ANSYS to
numerically simulate each of these cases. They concluded that the shear failure was very
brittle and hence retrofitting should be done in such a manner that the flexural failure
occurs in the beam.
Mondal et al. (2008) had studied the infill walls with central opening. For that a finite
element analysis has been carried out on single bay single storey, single bay two storey,
and Single bay storey with the infilled frame to examine the effects of central opening. It
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was proposed that a reduction factor can be utilized when a central window opening is
present to get a modified strength of a solid infilled frame.
According to Hashemi (2011) eccentrically braced frames have high stiffness against the
lateral loads such due to earthquake and perfect ability to absorb energy. Link beam, one
of the most important parts of the frame, guarantees the required stiffness and ductility of
the frame. In this study the behavior of eccentrically braced frames, role of stiffeners and
other parameters of the link beam on the ultimate strength and ductility of the frame were
considered using the finite elements model and ANSYS software.
A reinforced concrete frame is modeled for the finite element sensitivity analysis by
Alam et al. (2012) followed the by direct differentiation method under both static and
dynamic loading cases. Later, the reliability analysis is performed to predict the seismic
behavior of the frame. However, the smooth materials show continuity in the response
sensitivity from elastic to plastic transition points. In the finite element reliability
analysis, the influence of smoothness behavior of reinforcing steel is carefully noticed
and reasonable reliability estimation can be achieved by using smooth material model
comparison to with bilinear material constitutive model.
Gupta Rachana et al. (2012) proposed that building damage by the earthquake action is a
serious problem. In this paper the seismically deficient structures were studied by
carrying out the Pushover analysis of the frame structures using SAP software. Building
gets deformed because of the lateral and seismic forces acting on the structure. Forces
increased as per the height of structure, low rise structures have a higher resonant
frequency and hence the lower frequency high rise structures are studied. The study of
various earthquake and pushover forces acting, the formation of hinges and their
implementation available in the literature evaluating the real strength of the structure and
damage assessment of the multi-storied building structures were done.
2.4 Methods based on results of Experimental Investigations
Wood (1958) investigated the infilled frames by conducting several tests on the concrete
encased steel frames with the brick and concrete infills. Experimental data was given on
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basis of the behavior of panel walls and on the stiffening and strengthening effects of
such panels on the resistance of structural frame works against the racking loads.
Benjamin et al. (1958, 1959) had tested many prototypes as well as models of R.C.C.
frames with plain and reinforced concrete infill walls. The foundations were considered
as rigid and results were reported category wise. It was observed that there was no scale
effect i.e. test can be performed on any scale model, results of scale models were found to
be consistent with the prototype.
They had mentioned two types of possible modes, (i) Tension columns failure mode,
which occurs when insufficient steel is provided at the junction between the tension
column and foundation (ii) Compression column failure mode, which is a function of
steel area to concrete strength and panel proportions. According to them the panel
proportions and panel reinforcement were main influencing factors for cracking load of
the shear wall.
Smith (1965) had discussed the cracking patterns for horizontal, vertical loading as well
as horizontal and vertical combined loading.
For the horizontal loading two possible modes of failure were given as, (i) Compressive
failure generating from loaded corner or infill, which may or may not be preceded
by(ii)Tension crack along compression diagonal of infill.
For vertical loading two possible modes were given as, (i) Vertical tension cracks from
beam to the foundation and (ii) General compressive failure of the whole infill roughly
along the plane parallel to the foundation.
He had also given the modes of failure for the combined loading. The failure modes were
same as those for the horizontal loading and the factors affecting occurrence of these
modes of failure were as follows: (i) Length to height ratio of the infill (ii) Frame-infill
relative stiffness’s and (iii) Compressive and tensile strength ratio of infill.
Mallick et al. (1971) had observed that an opening, centrally located, reduce the stiffness
and strength of infilled frames in comparison to solid infill panel. The effect of different
opening position on the lateral stiffness and strength of model infilled frame was studied.
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According to them, infilled frames with the shear connectors were more preferable than
those without the connectors.
It was recommended that the door opening should be provided in the centre of lower half
of the panel and window opening at mid height to the left or right half of the panel, nearer
to vertical edges of panel as far as possible as shown in Figure 2.5.
D- Door position W-Window position
Figure 2.5 Recommendations of D.V. Mallick and R.D. Garg for Door and Window
Barua et al. (1975) had adopted experimental approach for summarizing the results of
twenty one tests. They had given some new information regarding the influence of the
quality of brickwork on modes of failure, stiffness, strength and share of load between
frame and infill. In stating the mode of failure, they had observed that infill had failed
almost in all cases. They had observed the modes of failure as: Mode (1): Tensile crack
through bricks Mode (2): Shear cracking along mortar joints
According to them the most important consideration was that the mode of failure depends
upon the composition of cement mortar in the brickwork. Failure mode 1 was observed
for brick work with 1:3 and 1:4 cement mortar while, mode 2 was found when 1:6
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cement mortars was used. They did not come across the separation and slip between
frame and infill as well as crushing of brick at any stage of loading.
Mali et al. (1981) performed the test on R.C.C. Frames with brick as the infill. In
addition to racking load a uniformly distributed vertical load was applied, which caused
pre compression of wall. Cracking and failure behavior of infilled frames was reported
and initial stiffness was predicted by elastic analysis. The experimental values were
found to correlate well with theoretical values.
Riddingtion (1984) had considered the influence of initial gaps on the infilled frame
behavior. The investigation consists of a series of six full-scale tests on brick Work
infilled steel frames. From the results it was found that even a relatively small initial gap
used in the test would affect the behavior of the infilled frames. It was concluded that the
initial gaps should be avoided. For this purpose they suggested that the corners of infill
should be designed to prevent local crushing failure, this might be achieved by using high
strength mortar for corner joints or by placing the bricks tightly into frame corners
without a mortar joint.
According to Riddington et al. (1989) for an infilled frame to resist racking loading
efficiently, the infill needs to be fit tightly within the frame. If the frame is formed from
reinforced concrete, creep and shrinkage in the columns of the frame will result in
vertical load transfer onto a tightly fit infill. Since the degree of load transfer cannot be
accurately predicted and may vary with the age of structure, so it would be desirable to
prevent this load transfer. It had been suggested that this could be largely achieved if a
layer of lead was incorporated between the top of infill and the bottom of top beam. The
viscous elastic property of lead would allow the lead layer to creep at such a rate that it
can accommodate the shortening of columns- Experiments were performed on the half
scale infilled frames with and without lead layer- Results had indicated that lead layers
were capable of reducing the vertical load transfer from the columns to the infill.
The difficulties of an exact analysis for R.C frames infilled with brickwork has been
emphasized by M. Smolira(1973) .He has also presented a simplified approach of
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analysis on the basis of the assumption of linear behavior of equivalent strut. The width
of equivalent strut was taken as 1/3 rd of diagonal length.
The effectiveness of both the epoxy techniques studied by, French et.al (1990) to repair
interior joints that were moderately damaged due to inadequate anchorage of continuous
beam bars. For vacuum impregnation epoxy inlet ports were located at the bottom of each
beam and at the base of the column repair region. The vacuum was applied through three
hoses attached at the top of the repair region in the column. Both repair techniques were
successful in restoring over 85% of the stiffness, strength, and energy dissipation
characteristics of the original specimens. Severe bond deterioration in the repaired joints
occurred only one half-cycle earlier than in the original specimens. The main conclusion
was that the vacuum impregnation presents an effective means of repairing a large region
of damage at once and that it can be modified for the joints with fewer accessible sides.
To determine the importance of scale effect, Manos et al. (1993) tested 1/3 and 1/9 scale
similar single-bay, single-story URM infilled RC frames with the non-integral infill walls
under the cyclic loading. The researchers concluded that despite of some discrepancies,
the general cyclic behavior of URM infilled RC frames could be satisfactorily simulated
using small-scale models.
Karayannis et al. (1998) studied the effects of joint reinforcement arrangement on the
efficiency of epoxy repair by pressure injection. Eleven of the tested one-way exterior
joint specimens were repaired by the epoxy injection only and then retested. In these
specimens, cracks were observed both at the joint region and at the beam end during the
first cycles, but the failure was finally due to formation of hinge in the beam. After repair,
the specimens with two joint stirrups or column longitudinal bars crossed within the joint
exhibited only beam flexural failure with serious fragmentation of concrete at the beam
end and significant reduction in pinching of the hysteresis loops. The specimens with one
joint stirrup, however, exhibited the same failure mode before and after repair. The
increases in peak load and dissipated energy were 8 to 40% and 53 to 139%, respectively.
The change in stiffness varied between a 27% decrease and a 10% increase. The
variations in performance were partially attributed to the variations in being able to inject
epoxy successfully into the joint cracks.
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The results of the epoxy repair applications on one-way joints have shown that the
reliability of this technique in restoring the original characteristics of damaged joints is
questionable. The bond around the reinforcing bars, once destroyed, does not seem to be
completely restored by the epoxy injection and suggested to consider contribution of the
brick infill while analysis the frame for the lateral loads.
Al- Chaar et al. (2002) conducted an experiment on the behavior of masonry infilled non-
ductile reinforced concrete frames. In that study five similar, single storey structures with
non-ductile reinforced concrete frames and infill masonry panels subjected to in plane
loads were carried out. It was concluded that the reinforced concrete frames with brick
infill exhibit significantly higher peak and residual strength as well as initial stiffness
than bare R.C.C and the failure mechanism can be predicted by a combination of shear
strength, compressive strength and geometry of infill in concrete frames.
The research carried by Ngandu (2006) aims at providing a scientific basis for the
development of design guidelines for steel frames infilled with CASIEL walls, thus
providing stability to building frameworks. Although CASIEL infill walls were already
commonly found in building structures, the structural role it plays is frequently ignored.
This research postulates that such walls must be designed for rather than simply ignoring
or assuming the contribution of infill.
Amanat et al. (2006) conducted an experimental investigation on the use of Ferro cement
laminates for repairing masonry infilled RC frames. In that study they prepared a model
of a portal frame having masonry infill, the load was applied monotonically at the top of
the frame till the ultimate capacity was reached accompanied by substantial formation
and propagation of cracks. Then both the frames were repaired by Ferro cement coating.
After rehabilitation the frames were retested. They concluded that the Ferro cement
overlay is a highly effective method for strengthening the frame with masonry infill as
they showed higher capacity than the original one with less cracks and damages.
Dicleli et al. (2007) have developed chevron-braced frames in order to resist the
transverse dynamic loads. Eccentrically braced frames rely on the yielding of a link beam
between eccentric braces, which provides ductility and energy dissipation under dynamic
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loads. In a chevron-braced frame, energy dissipation solely depends on the nonlinear
cyclic response of the braces.
Bindu et al. (2008) conducted a detailed investigation on the performance of exterior
beam-column joints with inclined bars at joints under the cyclic loading. The effect of
inclined bars at the joint region was investigated. Four exterior beam column joints were
cast and tested under the cyclic loading. The performance of specimens which had joint
reinforcement with inclined bars was compared with the specimen without inclined bars.
It was concluded that the specimens with inclined bars show more ductility and energy
absorption capacity than the specimen without inclined bars.
Appa Rao et al. (2008) conducted detailed investigation on the performance of RC beam-
column joints strengthened by various schemes subjected to the seismic loads. In this
study different strengthening methods such as steel jacketing, fiber wrapping and
providing haunch elements were discussed. The important design parameters such as
joint shear strength and energy dissipation capacity for various schemes were discussed.
The conclusion was to enhance the strength, stiffness and energy dissipation; it lacks the
proper placement and arrangement of FRP sheets and strips, hence it could not improve
the joint shear strength. The numerical studies revealed that the haunch element had
significant reduction of shear force and bending moment in the frame members leading to
significant reduction of the joint shear force.
Tsonos et al. (2008) conducted a detailed investigation on effectiveness of CFRP jacket
and RC jacket in post-earthquake and pre earthquake beam-column sub assemblages. The
feasibility and technical effectiveness of high strength fiber jacket system and reinforced
jacket system were discussed. Four exterior beam-column joint sub assemblages were
tested under the cyclic loading. The inference was that, in the case of post-earthquake,
specimens retrofitted with RC jacket is more effective but in the case of pre earthquake
both retrofitting technique shows equal effectiveness.
Gupta et al. (2009) had studied the behavior of non-integral infilled frames for lateral
loads. The experiment is carried out on bare frames using hollow steel frame section.
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Mainly, their study explores the bare frame behavior in single storey, two storey
construction with single and double bay along with the effect of aspect ratio in the single
storey non-integral frames.
The aim of the study of V. Bhikshma et al. (2010) was to determine the suitability of
epoxy resin material type to be used in RCC beams for repairing and restoring good
strength and for considering the economical aspects. Hardened concrete specimens were
tested for compression and flexural test. The results of these experiments show that the
beams repaired using epoxy resin material (EXPACRETE SNE1) gave higher increase in
the ultimate load carrying capacity than other epoxy resin materials. The flexural strength
increased significantly, about 15 percent for concrete beams repaired with the epoxy resin
material (EXPACRETE SNE1) compared to other epoxy resin materials. Deflections
were lesser in reinforced concrete beams with epoxy resin compared to the conventional
concrete beams. Though reinforced concrete beams repaired with epoxy resins is costlier
comparatively, it is cheaper than reconstructing the structure.
K.G.Viswanath et al. (2011) concluded that many existing reinforced concrete buildings
need retrofit to overcome deficiencies to resist seismic loads. The use of steel bracing
systems for strengthening or retrofitting seismically inadequate reinforced concrete
frames is a viable solution for enhancing earthquake resistance. Steel bracing is
economical, easy to erect, occupies less space and has flexibility in the design to meet the
required strength and stiffness. In the present study, the seismic performance of RC
buildings rehabilitated using concentric steel bracing is investigated.
Mohammadi et al. (2011) conducted experiment on methods to improve the infilled
frame ductility. In that study the first stage was aimed at discovering the methods to
increase the infilled frame’s ductility by testing six single storey single bay infilled steel
frames. The results show that supplying a sliding fuse in the infill significantly increases
ductility , however, it presented some problem that can be avoided by improving fuse
configuration. The second stage was conducted with two specimens with an improved
sliding the fuse configuration in which fuse sliding strength on the behavior of infilled
frame were studied. It was concluded that the fused infills were five times more ductile
than regular infill panels. They had a well-defined failure mode with a negligible cyclic
26
stiffness degradation and strength deterioration. It also had a high resistance in the
transverse direction even, after collapse and can be adjusted and designed for required
strength.
The results of analytical and experimental investigations were studied by Jayaguru et al.
(2011) on one-third scaled, two-bay, and two-story RC frames with partial infill in the
bottom story subjected to lateral cyclic loads. A local retrofit strategy of strengthening of
RC structural elements with GFRP composites was adopted. Test results indicated that
the retrofitted frame exhibited significantly high ultimate strength and stiffness than the
control frame (frame without retrofit). Doni (2012) performed tests on braced and
retrofitted R.C.C. frames up to collapse and reported useful experimental results.
2.5 Methods based on plasticity and collapse approach
Before discussing the details of design it is necessary to know about "Plastic theory",
which is appropriate approach to design the steel structures. It serves two functions
"Economy as well as simplicity" .In elastic design the factor of safety is provided on the
basis of yield stress which is not a consistent indication of safety with respect to ultimate
capacity of a member, in other words if yield point is attained at a single location it does
not mean the collapse of the member. Steel has a unique property called ductility, due to
which it is able to absorb large deformations beyond the elastic limit without fracture.
Due to plastic deformation during strain hardening of the material, the particle which is
less stressed will be brought into the action, so the structure is actually able to resist
larger loads. This method, which utilizes this reserve strength, is called as plastic method
of analysis. The fundamental aspect is that the margin of safety is same as in the case of
elastic theory.
Wood (1978) studied the full-scale models, till destruction. From these tests wood
indentified four collapse modes.
(1) Shear mode, S (2) Shear rotation mode, SR
(3) Diagonal compression mode, DC and
(4) Corner crushing mode, CC
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Combining Nelson’s idealized plastic yield criterion for concrete membranes which are,
either crushing at constant yield stress or cracking at zero tensile stress and using
standard plastic theory for frame work, wood predicted S, SR, DC and CC in proper order
of decreasing relative frame strengths. Composite action at collapse was assumed to
involve frame bending, wall cracking and Wall crushing. The first two of these suited the
plastic theories almost ideally but wall crushing gave some trouble because limitation of
yield strain. Hence Wood suggested a penalty factor to lower the effective crushing stress
of masonry. Both upper and lower bound solutions for different collapse modes
described earlier were also derived. The collapse load of panel was given as
2.5
Where ‘ ’ is the plastic moment of the frame connection,
is the crushing strength of the infill,
‘t’ is the thickness of the infill panel,
‘l’ is the length of the infill panel and
‘h’ is the height of the infill panel.
Murlikrishna (1980) and Natu (1981) had performed the experiments on model infilled
frames up to collapse. The behavior of the infill frames had been studied with respect to
two parameters.
(1) Span to depth ratio
(2) Infill material (brick and concrete infill)
Mild steel I sections were used for the frame works in both the studies, tension cracks
were observed parallel to compression diagonal. A method was proposed to estimate the
collapse load theoretically. Results indicate that theoretical values deviated from
experimental values by 1.9% to 19.73% for brickwork and 5.3% to 15.8% for concrete
infill.
Liauw et al. (1983) had proposed a plastic theory for the analysis of integral infilled
frames. This theory considered redistribution of stress on concrete and the shear strength
28
at the infill frame interface at collapse was taken into the account. This theory is
applicable to both single storey and multi-storey integral infilled frame and comparison
of theoretical values fairly equals to experimental results.
Non-integral infilled frame problem was overcome by introduction of material with
strong bond or shear connectors at the frame-infill interface. Four failure modes were
suggested as shown in Figure 2.6.
(a) Mode 1 (b) Mode 2
(b) Mode 3 (d) Mode 4
Figure 2.6 Liauw and Kwan’s idealized crack pattern
(1) Mode 1- corner crushing with failure in columns and infill/beam connection.
(2) Mode 2- corner crushing with failure in beams and infill/columns connection.
(3) Mode 3- diagonal crushing with failure in infill/beam connection.
(4) Mode 4- diagonal crushing with failure in infill/column connection.
29
Actual collapse mode was the one, which gave the smallest value of collapse shear
strength.
Dravid (1983) had studied the infilled frame with and without opening. He observed that
corners of the opening in the infill were the weak points. A method based on plastic
theory was proposed to estimate collapse load of infilled frames with a central door
opening. Results indicate that theoretical values deviate by 3.39% to 21.25% from the
experimental ones.
Saneinejad et al. (1995) proposed a strut model which was based on attainment of
plasticity in the infill at the loaded corners. From their experimental and finite element
analysis they concluded that frame –infill interaction is associated with the shear forces
that may be evaluated closely using equations.
Fh = μα2 Ch 2.6
and ,Fl = μ C l 2.7
where, Fh and Fl are shear forces at the column–to-wall and beam–to-wall contacts,
respectively,
Ch and C l are normal forces at the column–to-wall and beam– to-wall contacts,
respectively,
μ , is the coefficient of friction between the steel frame and the concrete infill panel, and
α ,is the panel aspect ratio defined as h/l
α =h/l ≤ 1
From these contact forces equilibrium considerations of a concrete infill panel yielded
equation for the collapse load, H.
c t(1- αc) αc h + τb t αb l+2(Mpj+ Mj)/ h 2.8
where: αc is the ratio of the column contact length to the height of the column,
αb is the ratio of the beam contact length to the span of the beam,
c and b are the normal contact stresses at the face of the column and beam,
respectively,
τb is the uniform shear contact stress on the face of the beam,
h and l are the column height and the beam span, respectively,
30
Mpj is the minimum of the beam’s, column’s and the frame Connection’s plastic moment
and,
Mj is the frame connection’s plastic moment.
Collapse loads determined by the method of Saneinejad et al. (1995) were shown to be
consistently closer to the experimental values than the earlier method.
Deodhar et al. (1996) have studied the effect of reinforcement on ultimate strength of
infilled frames. A mathematical model has been proposed to estimate the ultimate load
of the infilled frames and the contribution of the infill has been reported.
The main objective of Liao (2010) study was to develop and validate a seismic design
methodology for RC SMF which is able to produce structures with predictable and
intended seismic performance. Based on the performance limit states of target drift and
desired yield mechanism, this design methodology accounts for inelastic structural
behavior directly and practically eliminates the need for assessment or iteration by
nonlinear static or time-history analysis after initial design.
Indian buildings built over the past two decades are seismically deficient as per Vijaya
kumar et al. (2012) because of lack of awareness regarding seismic behavior of the
structures. This paper aims to evaluate selected existing reinforced concrete building in
zone – III to conduct Pushover Analysis. The analysis shows the pushover curves,
capacity spectrum, plastic hinges and performance level of the existing building. The
non-linear static analysis gives better understanding and more accurate seismic
performance of buildings as progression of damage or failure can be traced.
2.6 General studies on Infilled frames
The study of Popov et al. (1975) state that the effectiveness of the epoxy repair is limited
by an access to the joint and that epoxy cannot be effectively introduced into the joints
surrounded by transverse beams and floor slab. This limitation can possibly be overcome
by further advances in the vacuum impregnation technique. A high level of skill is
required for satisfactory execution of such techniques, and application may be limited to
the ambient temperature.
31
The effectiveness of vacuum impregnation epoxy inlet ports techniques studied by
French et al. (1990) to repair interior joints of the beams and columns moderately
damaged due to inadequate anchorage of continuous bars of the beam. It was concluded
that the vacuum impregnation is an effective method of repairing large regions of damage
with fewer reachable sides. As all cracks were accessible, the cracks within the deficient
RC frames were repaired by grouting, pouring and using brush for epoxy application.
Murty et al. (1995) review the IS 1893:2002 .It contains a discussion on the Clauses that
are confusing or vague and needs clarification immediately. The typographical and
editorial errors were pointed out. Most Indian buildings will be soft storey buildings as
per code definition simply because the ground storey height is usually different from that
in the upper storey. Hence, the definition of soft storey needs a review. It was suggested
to allow more variation between stiffness of adjacent storey before terming a building as
a soft storey building. For instance, IS 1893 allows for more variation in the weight of the
adjacent floors, as compared to the NEHRP code, before terming a building as having
mass irregularity. A similar approach is needed for the definition of the soft storey
buildings.
In the 1999 Kocaeli (Turkey), an earthquake caused complete failure of the first storey or
the bottom two storeys of the buildings. Matjaz et al. (2001) demonstrated that a soft
storey mechanism is formed in such structural systems if the intensity of ground motion
is above a certain level. It is likely that the collapse will occur if the global ductility of the
bare frames, as well as the ductility of the structural elements is low and if the infill walls
are relatively weak and brittle.
Maheri (2005) reviewed the results of some recent works conducted by the authors on
new methods of retrofitting the RC frames. On the local retrofit of RC members, it
includes the work on the application of a new high performance fiber-reinforced
composite material. The composite can be applied either as a wet mix to the desired
thickness or attached as precast sheets or strips to the face of the member using a suitable
epoxy adhesive. The suitability of this technique of retrofit to enhance the strength and
ductility of the retrofitted member compared with other methods of local retrofit, such as
steel plates and FRP laminates, is discussed. Other works reviewed in this paper include
32
those carried out recently on the global retrofit of RC frames using direct internal steel
bracing. It was reported that such bracings can increase the yield strength capacities and
reduce the global displacements of the frames to the desired levels.
The objective of Murat et al. (2005) work was the collection of current information on
repair and strengthening of none seismically designed joints so that the engineers and
researchers may more efficiently proceed to develop improved seismic retrofits. Each
method of repair or strengthening is reviewed with emphasis on its performance and
relative advantages and disadvantages with respect to the application details, required
labor, and range of application.
In their research, Dicleli et al. (2005) had studied the seismic response of a new braced
steel frame type and a new design philosophy that will focus on minimizing the damage
to the essential structural members and ensuring satisfactory post-earthquake
performance of steel buildings under service loads with minimum rehabilitation costs.
The proposed frame type is composed of beams and columns with rigid connections,
chevron braces and a conventional energy-dissipating member called the shear element
connected between the braces and the beam. Nonlinear static pushover and seismic
analyses are conducted to assess the performance of the proposed EEDBF compared to
the conventional CBFs and MRFs.
Consequently, numerous research studies have been initiated in recent years to improve
the performance of the bracing systems through the use of high performance materials;
Berman (2007) described an experimental and analytical investigation into the use of
members with hollow rectangular cross-sections as eccentrically braced frame links that
do not require lateral bracing. Using cross-sectional plastic analysis, the plastic shear and
moment strength for a general tubular section with different web and flange yield
strengths and thicknesses are derived. Equations are derived for maximum flange
compactness ratio and minimum web stiffener spacing to prevent flange and web
buckling. A proof-of-concept experiment involving a large scale eccentrically braced
frame with a tubular link is described. The link has a hybrid tubular cross-section
composed of webs and flanges of different thicknesses, with full-penetration groove
welds. Experimental results indicate that the link reached a rotation 0.15 radian and
33
almost twice the current 0.08 radian limit for wide-flange links. An investigation of the
fracture surface indicated that flange fracture did not initiate in the full-penetration weld
used to assemble the shape, but rather in the heat-affected-zone of the flange adjacent to a
fillet weld used to connect a stiffener to the flange. Finally, a finite element model of the
link is developed using shell elements, and reasonable agreement with the experimental
results is observed.
In a comprehensive parametric study carried out in order to establish design guidelines
for favorable tradeoffs between damping benefits and the associated stiffness and
strength penalties in an FRP V-type joint by Ioannis et al. (2008). The results are
compared with the corresponding tradeoffs for a V-type joint made from conventional
materials.
Kaveh et al. (2010) studied the problem of layout optimization for X-bracing of steel
frames using the Ant System (AS). A new design method is employed to share the
gravity and the lateral loads between the main frame and the bracings. An algorithm is
developed which is called optimum steel design, an optimization method based on an
approximate analysis is also developed for layout optimization of braced frames. Davis et
al. (2010) proposed rigorous nonlinear dynamic analysis (NDA) and also simplified the
equivalent static analysis for open ground storey buildings.
The stress distribution for the relative stiffness had been studied by Rao (2011) with
aluminum frame and araldite AY103 with a hardener HY 951 as infill material. It was
concluded that the mutual interaction of the frame and the in-fill plays an important role
in controlling the stiffness and strength of the in-filled frame.
In an attempt to investigate the effect of soft storey for multi-storey reinforced concrete
building frame, four building models (3, 6, 9 and 12 storey) with identical building plan
were analyzed by Amin et al. (2011). Equivalent diagonal struts were provided, as
suggested in FEMA-273, in place of masonry to generate infill effect. Earthquake load
was provided at each diaphragm’s mass centre as a source of lateral load as set forth by
the provision of BNBC, 1993. Soft storey level was altered from ground floor to top floor
for each model and equivalent static analysis was carried out using ETABS 9.6 analysis
package. Results show a general changing pattern in lateral drift irrespective to building
34
height and location of soft storey. Inter-storey drift ratio was found increasing below the
mid storey level and maximum ratio was obtained where the soft storey was located.
Bahalkar (2011) had performed experiments by using the bracings on soft storied steel
frames subjected to lateral loads. It was reported that bracings are effective for such
frames.
Kurt et al. (2012) used the hybrid simulation method to achieve a realistic evaluation of
the effectiveness of different retrofit methods by conducting tests on a 1/2 scale, two
story, three bay, low rise infilled RC building in built, CFRP retrofitted and precast
concrete panel.
As per Agarwal Pankaj and Shrikhande Manish(2006) ,the failure of multi-storied
buildings is basically due stiffening effect of infill panels which is cause of (i) unequal
distribution of lateral forces in the different frames ;(ii)soft storey or weak storey;(iii)
short column or captive column effect; (iv) torsion forces; (v)cracking of infill walls.
2.7 Summary of literature review
Past and present research in infilled frames has been motivated by using brickwork infills
in resisting blast loads, providing stability to the tall buildings, rehabilitation of masonry
structures and in seismic engineering .In this chapter experimental research and modeling
strategies used in the study of infilled frames have been reviewed.
Smith (1966) had conducted several tests on infill frames in an elastic region only.
Benjamin (1959) stated that diagonal reinforcing was less effective than rectangular
reinforcing without any reason. Smolira (1973) adopted one third rule for width of
diagonal strut is same as Holmes (1961) and is empirical. Barua et al. (1975) had adopted
experimental approach but all formulae were empirical. Wood (1978) and Liuaw et al.
(1983) had used the plastic theory to understand and theoretically estimate the ultimate
strength of infilled frames but only with the masonry infill. From the experimental
investigations, the following observations have been made.
i. The stiffness and strength of the frames are significantly improved by use of
infills.
ii. Infills basically act as a form of bracing.
35
iii. Diagonal tensile cracking, shear sliding in the joints and crushing of the infill
have been observed as modes of damage. Formation of plastic hinges in the steel
and reinforced concrete frames with reinforced infills take place.
iv. The failure mode and stiffness of the infilled frames depends upon frame-to-wall
stiffness ratio, frame-to-wall interface conditions and material characteristics.
Other factors that affect the behavior of infilled frames are openings in the infill
panels, type of loadings, number of bays and number of storeys.
v. Epoxy can be used as a binding material which has been widely used for patching
or repairing surface defects of different types of concrete structures. It forms a
good bond with old concrete surface and rebar which is one of the prime
requirement of a good repair works.
Several universal and fundamental models have been used to further know and
predict the behavior of the infilled frames. Even then, most of the current design
codes all over the world do not contain design procedure for this type of structure.
2.8 Problem statement
There is very less work available in the literature, dealing with the proper solutions for
steel frames, shear walls and R.C. frames with sound theoretical background for ultimate
strength. No walls are provided in the building frames of multi-storey at stilt level for
parking and free movement of users, which makes it a soft storey at that particular level.
Performance of such buildings is poor towards the lateral loads due to earthquake in
comparison to the upper storey which have infilled frames, especially in the regions of
moderate to high seismicity. A lesser amount of experimental and analytical work is
available, with the use of bracing and infill combined, to achieve the desirable ultimate
strength and overcome the collapse or damage problem during earthquake.
The existing rules for designing such structures, till date does not reflect completely the
past observations and only some empirical rules have been adopted, namely inducing
higher seismic coefficients whenever certain regularity criteria are not satisfied. IS
1893:2002, IS 13920:1993 and other international codes have provision of a load factor
multiplied to the lateral loads to obtain the design load for the columns of such soft
36
storeys. For that usually extra reinforcement and size of column are provided in addition,
in comparison to the upper storey. It may lead to inefficient and uneconomical designs. By
limiting the side sway of the building frames using bracings with partial infill walls, cost
of construction can be curtailed along with practicability.
Experimental investigation needs to provide a scientific basis for the development of
lateral behavior and theoretical methods for the ground storey of soft storey framed
buildings of steel and R.C. frames with bracings and partial infill of cement mortar and
concrete up to the collapse.
To economize the design of the boundary elements, a proper solution for the R.C.C. shear
walls with the provision of steel bracings is required. Generally partially collapsed
structures are repaired and reused. Such frames repaired by epoxy resin after collapse
should be tested in order to know, up to what extent strength is recovered. So,
accordingly, experimental works are carried out and simplified equations for the
prediction of the ultimate loads have been proposed and evaluated in light of numerical
results.
2.9 Concluding remarks
Finally, as pointed out in the chapter 1, the common practice of ignoring the influence of
infills in the structural design and that they may be inaccurate, uneconomical and may
pose unacceptable risks to the structure, finishes and fittings. There is a need to develop
practical design guidelines for these infilled frames. This research, therefore, represents
an application for a relatively new but considerably prevalent method for soft storey and
shear wall construction. The next chapter deals about experimental set-up.
