The behaviour of the infilled frame under seismic loading is very complex
and complicated. Since the behaviour is nonlinear and closely related
to the link between the frame and the infill, it is very difficult to
predict it by analytical methods unless the analytical models are supported
and revised by using the experimental data.
Due to the complex behaviour of such composite structures, experimental
research is of great importance to determine the strength, stiffness and
dynamic characteristics at each stage of loading.
In this study M30 concrete was adopted for beams and M50
concrete was adopted for column. High strength mix design according to
IS method was done to achieve the required strength.
A pre-cast foundation block available in the structural engineering laboratory
was used for fixing the specimen for testing. It was cast earlier in such
a way that the same unit could be used for testing any number of frames.
The foundation block had twelve numbers of bolt holes of 50 mm diameter
to enable the fixing of frame specimens to be tested. The fixity of foundation
block was obtained by fastening the foundation block with the structural
test floor by twelve number of 50 mm diameter rods. IS 1893 (part 1) (2002)
code was used to base shear calculation. Madan et al. (1997) carried
out modeling of masonry infill panels for structural analysis. Satyanarayanan
and Govindan (1989) have studied the effect of opening on the load carrying
capacity of the infilled frame. Kodur et al. (1998) presented a
numerical example that illustrates the steps associated with seismic design
of masonry infilled frames. The examples accounts for the effect of the
infill in all the design stages.
Chiou et al. (2006) have reported the structural behaviour of
a framed masonry wall subjected to in-plane monotonic loading is investigated
by a full-scale test and the method of discontinuous deformation analysis.
Lu (2002) has reported a comparative study of the nonlinear behaviour
of reinforced concrete multistory structures is carried out on the basis
of measured response of four six storey, three-bay framed structures,
namely a regular bare frame, a discontinuous-column frame, a partially
Perumal Pillai (1994) has studied the performance of RC frames with and
without brick-infill. The tests on two-quarter-scale, five storied frames
brought out the lose of ductility due to infilling. Govindan (1986) has
studied the behaviour of seven story infilled frames subjected to static
cyclic loading. The parameters included the effect of provision of reinforcement
in the plaster to improve the ductility of infilled frames.
Portland Pozzolana cement conforming to IS 269-1976 had been used for
concreting. Well-graded crushed aggregate and uniformly graded sand were
procured and used. TMT steel bars of 10 and 8 mm diameter had been used
as flexural reinforcement for columns and beams of the frames. TMT bars
of 6 mm average diameter had been used as shear reinforcement. Silica
fume was used as an admixture. Good quality, burnt clay bricks were procured
and used as infill in the frames. IS 13920 (1993) has been used for reinforcement
Equivalent diagonal strut approach: Paulay and Priestley (1992)
suggested treating the infill walls as diagonal bracing members connected
by pins to the frame members.
In this experimental study the effect of infills in the reinforced concrete
frame was considered as equivalent diagonal strut. The tests that have
been done by Stafford Smith (1962) on one-eighth scale models of steel
frames infilled with mortar and loaded eighth diagonally or back to back.
In his analysis, Stafford Smith has defined a relative stiffness factor,
which is based on the analysis of a beam resting on elastic foundation
as defined in equation:
||height of infill,
||thickness of infill,
||Modulus of Elasticity of infill,
||Youngs modulus of frame materials,
||height of column,
||moment of inertia of column.
Tests on steel frame models infilled with brick masonry have been conducted
by Holmes (1961, 1963). The infill is modeled by an equivalent compressive
strut of width equal to one-third the diagonal length of the infill and
the thickness of the strut is taken equal to that of the infill. This
equivalent frame model has been used for the elastic analysis of the infilled
Liauw and Kawn (1985) have proposed a plastic theory of integral infilled
frames based on the results of finite element analysis.
Das and Murthy (2004a, b) have reported the design of five reinforced
framed buldings with brick masonry infills for the same seismic hazard
in accordance with the applicable provision given in Eurocode 8, Nepal
building code 201 and Indian seismic code (with and without ductile detailing)
and the equivalent braced frame method given in the literature. Sobaih
and Abdin (1988), have presented a numerical method for the analysis of
infilled frame subjected to earthquake excitation. The method is based
on the concept of equivalent struts used to idealize the infill panels.
General: The experimental investigation consisted of testing one-fourth
full size model of five-storey, three-bay, reinforced concrete frame with
central portion infill under static lateral cyclic load. The frame members
are designed in such a way that plastic hinges in beams are realized before
the failure of columns. The frames were cast with quality raw materials
using wooden mould; sufficient care was taken to ensure quality control
and they were cured as per the norms. The RC frame was analysed and designed
in STAADPRO software and checked with the conventional way. The foundation
portion of the frame was provided with holes to anchor the specimen to
strong test floor, so as to test the frame as a vertical cantilever. The
arrangement of the wooden mould was in such a way that the frame and its
foundation portion can be cast monolithically.
Concreting and curing: High strength concrete was used for columns.
For mixing of concrete an electrically operated concrete mixer was used
and the concrete was placed immediately after mixing. Needle vibrator
of 25 mm diameter was used for compaction of concrete. The casting was
done at a stretch. Twelve numbers of boltholes of 50 mm diameter were
provided in the footing portion of the frame at the same locations as
that in foundation block. Companion specimen such as cubes, cylinders
and prisms were cast for all mixes. The specimen was covered with wet
gunny bags and was kept moist by periodical sprinkling of water for a
period of 21 days from the day of casting. The companion specimens were
also cured for the same period as that of the frames.
Brickwork construction: For infilling the central bay of the frame
with brick masonry, cement mortar 1: 4 with a water cement ratio of 0.55
was used. The panel size at the bottom storey of the infilled specimen
was 800x675 mm and it was 800x600 mm for other stories. The thickness
of the brick masonry panel was 100 mm. The curing for brickwork was done
for 7 days. The frames and the control specimens were tested after the
stipulated period of curing.
Test set-up: The models were tested as vertical cantilevers under
a cyclic loading program. The schematic diagram of test set-up is presented.
It consists of the following arrangements,
||Instruments for measuring deflection (LVDT, Deflectometer etc.)
||Rigid body rotation of foundation block
Loading arrangement: Lateral cyclic loading was applied at first
storey, third storey and fifth storey levels in line with the beams. The
reaction frame, which is used for loading arrangements, is rigidly fixed
to the test floor. Double acting hydraulic jacks (push pull jack) of capacity
500 KN were used at required levels. Loads were applied from the push
pull jack at first storey, third storey and fifth storey levels of the
frame. A common console controlled all the three jacks. Pressure gauges
were used to measure the applied load. For the application of load through
jacks two numbers of hand operated oil pumps were used for applying load
in the reverse direction, if needed. The lateral movement of the test
frames at the ultimate load stages was avoided by providing suitable guides
using mild steel pipes.
Instruments for measuring deflection: LVDT (Linear Variable Differential
Transformer) of least count 0.01 mm was used for measuring deflections
at all storey levels during the initial stages of loading. When the use
of LVDT needed frequent resetting the LVDT was removed and disc-type displacement
meters of least count 0.1 mm were used. The LVDT/ displacement meters
were connected to slotted angles that were in turn connected to the fixed
type to steel reaction frame available.
Testing of frames: Figure 1 shows the complete
test setup adopted for the frame model. The effectiveness of instrumentation
set up and the loading were checked in the beginning by loading and unloading
the frame with small loads (of the orders of 0.5 KN at all the three load
points) till all the readings was repeatable.
The frame was subjected to equivalent static lateral cyclic loading.
The loading sequences in the beginning for all the frames were almost
same. The load increment for each cycle was 2.50 KN at the initial stages
i.e., before initial cracking and 5.00 KN in the later cycles i.e., after
the first cracking. The deflections at all storey levels were measured
at each increment or decrement of load. The strains in steel, concrete
and infill were monitored at maximum load of each cycle and at unloading
conditions of frame (i.e., when the load is released fully) during all
cycles of loading.
The formation and propagation of cracks, hinge formation and failure
pattern have been recorded. The deflectometer readings for calculating
error due to rigid body rotation of foundation block were also recorded.
The load cycles were continued till loading was about two to 4 h and around
6 days of continuous testing. The concrete cubes were tested for 3rd,
7th and 28th days strength as per IS 516 - 1964. The brick prisms
were tested under compression and for modulus of elasticity.
||Test set up of frame
Testing procedure: To start with, the frame was loaded with small
loads and then unloaded to check the effectiveness of the instrument setup
and loading. This process was repeated till the readings were consistent.
The frame was subjected to equivalent static cyclic loading. A predetermined
static loading was applied to the frame. The lateral loads were applied,
by the use of hydraulic jacks, at first, third and fifth stories. At each
increment of load level, the readings on deflectometer were measured at
all floor levels. Strains in steel, concrete and bricks were measured
only at maximum and zero position of loading of cyclic loading. The cracks
in the beams, columns and brick works were also observed and marked in
the frame simultaneously.
Investigation of brick infilled RC frame: The frame was cast,
cured for 21 days lifted and erected on the test bed. Brickwork, in 1:
4 cement mortar, was constructed on the next day and was cured for a period
of seven days. The various parameters like stiffness degradation, ductility
factor and energy dissipation were considered for study of the behaviour
of the frame. Theoretical analysis using non-linear finite element method
was carried out and the results of these analyses have been compared with
the experimental results.
Loading and load-deflection behaviour: The strains in steel, concrete,
column-beam joint, column-infill joint and infill were measured at maximum
load of each cycle and at unloading conditions of frame (i.e., when the
load is released fully). The formation and propagation of cracks, hinge
formation and failure pattern have been recorded.
||Load vs deflection
The deflectometer readings for calculating error due to rigid body rotation
of foundation block were also recorded.
The ultimate base shear was reached in the thirteenth cycle of loading.
After reaching ultimate load, post ultimate cycles were performed to study
the behaviour of the brick infilled RC frame after ultimate cyclic load
till the final collapse of the frame. The displacement due to rigid body
rotation of the footing and the foundation block were incorporated in
the calculation of net deflection.
The top story deflection versus base shear (Table 1)
diagram is presented in Fig. 2 for 1 to 13 cycles of
loading, i.e., up to maximum load. From the hysteretic curve assuming
bilinear behaviour, the yield deflection Δy was found to be 18.0
Theoretical analysis based on non-linear finite element analysis were
carried out and the results of experimental studies were plotted as maximum
base shear of each cycle vs top story deflection up to ultimate base shear
shown in Fig. 3. From the Fig. 3 it
can be seen that the top storey deflection values for some of the initial
load cycles obtained from finite element analysis are higher than the
experimental values, the theoretical deflection values are lower than
the experimental values. At the ultimate base shear the top storey deflection
was found to be 90.47 mm (experimental value) where as it was obtained
as 56.175 mm from the non-linear finite element analysis.
In the FEA the brick masonry is assumed as homogeneous and isotropic
material as assumed for concrete, but brick masonry is a two - phase material
with brick units and mortar joints. For more precise representation of
the brick masonry, individual modeling of brick, motor and the interface
is required, which is laborious and practically not possible always. The
type of loading the frame has been subjected to in experimental investigation
(Cycle loading) and the considered in analytical study (monotonic loading)
would have been one of the reasons for the difference in the values other
than those stated above.
||Base shear vs top storey deflection
||Load cycle vs deflection
||Base shear vs deflection
When the maximum base shear of each cycle is plotted against top storey
deflection (Fig. 4) there exist a good relation between
the base shear and the top storey deflection whose R2 value
works out to be greater than 0.95 (very good relation). Also when the
maximum base shear of each cycle is plotted against residual top storey
deflection (Fig. 4), there exist a good relation between
the base shear and the residual top storey deflection whose R2
value works out to be to be greater than 0.35 (very good relation).
The relationships established can be utilized for estimating the lateral
loads the buildings of similar nature has been subjected to and the total
deflection the building
might have experienced from the residual deflection of the top storey.
For obtaining a generalized relationship suitable for estimation of the
above values for buildings varying heights, study of similar nature has
to be carried out.
Load carrying capacity: Separation cracks were first noticed in
the 2nd storey left bay brick infill in the 8th cycle. In the 13th cycle,
separation cracks and bed joint cracks and brick infill failure were found
in the first and 2nd floor panels and also cracks developed at the junction
of first floor beam and leeward column. The different types of cracks
witnessed during various loading stages are shown in Fig.
Stiffness degradation: The stiffness of the infilled frame for
various load cycles were calculated and presented in Table
2 and the variation of stiffness with respect to load cycles is shown
in Fig. 5. The theoretical maximum stiffness, the experimentally
observed stiffness at cracking load and stiffness at service load are
also found out. The stiffness of the brick infilled RC frame was found
to decrease from 46.5 KN mm-1 during first cycle to 2.57 KN
mm-1 during the thirteenth (final) cycle of loading. Stiffness
in the 4, 5, 6, 12 and 13th cycles of loading were greater than that in
the previous cycles. This may be due to the strain hardening effect of
tension steel and non-uniform closing of cracks on the compression side
especially in brickwork. The theoretical maximum stiffness was 23.67 KN
mm-1 and it was 8.667 KN mm-1 at the cracking load.
At service load (50% of ultimate load) it was 7.263 KN mm-1.
Ductility factor: The ductility factor (μ) for three bay
five storey RC frame, central bay infilled with brick was calculated.
The first yield deflection (Δy) for the assumed bi-linear load-deflection
behaviour of the frame was obtained as 1.1 mm. The ductility factor value
μ = (Δ1/Δy) for the various load cycles of the frame were
worked out and presented in Table 3 and the variation
of ductility factor with load cycles is shown in Fig. 6.
The cumulative ductility factor for various load cycles is also presented
in Table 4 and the variation of Cumulative ductility
factor with respect to the load cycles
is shown in Fig. 7. The cumulative ductility factor
was found to increase from 0.227 to 79.91 during the thirteenth cycle
||Stiffness vs load cycle
||Load cycle vs ductility factor
||Stiffness degradation curve
Energy dissipation capacity: The energy dissipation capacity of
the frame during various load cycles was calculated similar to RC frame
as the sum of the area under the hysterisis loops from the base shear
versus top storey deflection diagram obtained. The energy dissipation
capacity during first cycle of loading was 0.0026 KN m and that during
13th cycles was 2.15 KN m. The energy dissipation capacity values calculated
for all cycles are given in Table 5 and the variation
of energy dissipated by the frame during each cycle is shown in Fig.
8. The variation of cumulative energy dissipated by frame is shown
in Fig. 9.
||Load cycle vs cumulative ductility factor
||Ductility factor for the frame vs load cycle number
||Cumulative ductility factor for the frame
||Energy dissipation capacity and cumulative energy dissipation
||Energy dissipation capacity of the frame
||Cumulative energy dissipation capacity of the frame
Behaviour and failure modes: The first crack was witnessed in
the second storey left bay brick infill when the base shear was 43.2 KN.
The cracking occurred during loading reflect the fact that the infilled
frame behaved as an integral unit. At failure, the infilled frame exhibited
spalling of brick fragments. The formation of plastic hinges in the floor
beams observed after severe cracking of brickwork in the ground storey.
The leeward shear also in addition to compression because of the diagonal
strut effect of the infill. At the junction of the diagonal strut, leeward
column and foundation, the leeward column suffered shear and local buckling.
This initiated the final collapse. Second and third storey brick works
were slightly damaged, but the fourth and fifth storey bricks were intact.
However separation cracks between frame and infill occurred in all storey
panels. The infilled frame at failure stage is shown in Fig.
Finite element analysis: Asteris (2003) has reported a new finite
element technique for the analysis of brickwork infilled plane frames
under lateral loads. Ghosh and Amde (2002) have reported the design of
infilled frames to resist lateral loads on buildings in terms of their
failure modes, failure loads and initial stiffness using procedures proposed
by previous authors and verified. This verification is made by comparing
the results of the analytical procedures of the previous authors with
those of a new finite element model for infilled frames, which are verified
by using experimental results. The influence of the masonry infill panel
opening in the reduction of the infill frames stiffness has been investigated
by means of this technique. Non - Linear finite element analysis has been
carried out using ANSYS software. The RC members of the frame have been
modeled with SOLID 65 element (Reinforced concrete element) elements,
the infill was modeled with SOLID 45 element (Brick element) and the connection
between the RC element and infill was made with LINK 10 element (link
element) available in the elements library of the ANSYS software. The
deflected shape of the finite element model of RC frame at ultimate load
is shown in Fig. 13. The top story deflection was 56.173
mm from the finite element analysis where as it was 90.47 mm from the
experimental study. Separation of infill is visible in the bottom three-storey
panels of the frame in the tension side, which was identical to that of
the experimental behaviour of the frame.
||Failure pattern of frame
||Diagonal crack formation of brick infill in the bottom
||Deflected shape of the frame
In the literature review, the works were carried out in the performance
of RC frames with and without brick-infill. The tests on two quarter-scale,
five storied frames brought out the loss of ductility due to infilling.
Study of the nonlinear behaviour of reinforced concrete multistory structures
on the basis of measured response of four six story, three-bay framed
structures, namely a regular bare frame, a discontinuous-column frame,
a partially masonry-infilled frame. Behaviour of seven story infilled
frames subjected to static cyclic loading.
In this study, three bays, five stories from with central bay brick infill
under static cyclic loading were carried out. The combined effect of infill
and bare frame action at one stretch were obtained in this study. The
effects of ductility, energy dissipation capacity, stiffness, load carrying
capacity, stress-strain behaviours differ significantly. The behaviours
in the infill bay were not same in the plain bay. Load carrying capacity
and stiffness of the infill bay were much more than the plain frame. These
different studies were obtained in this frame. The differences in behaviours
of the column with infill in one side and without infill on the other
side were studied. The behaviour of frame with central bay with diagonal
effect and the other two bay without diagonal effect have been carried
The frame developed beam and column hinging near beam column interfaces
before they reached their maximum story shear force and they eventually
failed due to joint shear, exhibiting successive strength drops. The frame
exhibited diagonal crack failure at brick infill in the bottom story at
a relatively slow rate of increase before they started to break down.
After collapse of brick infill in the bottom story, it was act as a soft
story. The contribution of brick infill was significant up to breaking.
It is recommended that brick infill should be considered in the analysis
of moment frames, especially in the lateral load resisting systems. The
strains measured in infilled beams and columns are 20% lesser than bare
frame beams upto failure of brick infill. The windward column of the infilled
frame in the bottom story was affected to shear and local buckling severely.
It is suggested that windward column should be strong enough in the ground
floor. It was found that the bottom story columns were more severe than
the other stories. Very minute cracks were found at the infill in the
top story. The leeward shear also in addition to compression because of
the diagonal strut effect of the infill initiated the final collapse of
the frame. The diagonal cracks were found in the infills. After this diagonal
crack, the infill in the bottom story was inactive. Also the infill in
the bottom story was separated from the beam after the formation of diagonal
crack. The slope of the failed leeward column in the bottom story was
high than the other leeward columns in the top stories.