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1 INTRODUCTION
1.1 Performance of RC frame structures with
masonry infill walls
As shown (Kappos and Ellul 2000, Kose 2009, Ricci
et al. 2011, Asteris et al. 2015, Varum et al. 2017)
infill walls in reinforced concrete frame buildings
cause an ‗increase‘ in lateral stiffness, strength, and
energy dissipation capacity. Therefore, infills should
be taken into account during the design; however
this is not the case. This results in damage and fail-
ure of infill walls and sometimes RC elements dur-
ing the earthquakes (Dazio et al. 2009, Braga et al.
2011, Manfredi et al. 2014). Damage of masonry in-
fills may contribute significantly to economic losses
and cause considerable threats to human lives, even
in the case of infills in newly constructed buildings
(Hermanns et al., 2012). Vicente et al. (2012) and
Hermanns et al. (2012) pointed out that there is an
urgent need for improvements in the current ap-
proach for the verification and detailing of singular
points of infills. Whether this behaviour is favoura-
ble or not, the infill walls are usually the first ele-
ments to be damaged in seismic events. Villaverde
(1997) showed that the cost related to the failure of a
non-structural component in a building may easily
exceed the replacement cost of a building, due to the
loss of inventory, loss of business, repair and recon-
struction costs, downtime, injuries and casualties.
These facts have pushed research and development
of solutions that disable these negative effects result-
ing from the interaction between the masonry infill
walls and concrete structures.
1.2 Literature review
The behaviour of masonry infilled frames sub-
jected to in-plane lateral loads was investigated by a
number of researchers, both numerically and exper-
imentally (Mehrabi et al. 1996, Al-Chaar et al. 2002,
Shing and Mehrabi 2002, Drysdale and Hamid 2005,
Stylianidis, 2012; Morandi et al. 2014, Hak et al.
2017).
The behaviour of infill walls under out-of-plane
loads was examined by McDowell et al. (1956a,b),
Seismic behaviour of RC frames with uncoupled masonry infills having
two storeys or two bays
M. Marinković
Department of engineering mechanics and theory of structures, Faculty of Civil Engineering, University of
Belgrade, Belgarde, Serbia
C. Butenweg
Center for Wind and Earthquake Engineering (CWE), RWTH-Aachen University, Aachen, Germany
ABSTRACT: Reinforced concrete (RC) structures with masonry infills are widely used for several types of
buildings all over the world. However, it is well known that traditional masonry infills constructed with rigid
contact to the surrounding RC frame performed rather poor in past earthquakes. Masonry infills showed se-
vere in-plane damages and failed in many cases under out-of-plane seismic loading. As the undesired interac-
tions between frames and infills changes the load transfer on building level, complete collapses of buildings
were observed. A possible solution is uncoupling of masonry infills to the frame to reduce the infill contribu-
tion activated by the frame deformation under horizontal loading. The paper presents numerical simulations
on RC frames equipped with the innovative decoupling system INODIS. The system was developed within the
European project INSYSME and allows an effective uncoupling of frame and infill. The simulations are car-
ried out with a micro-modelling approach, which is able to predict the complex nonlinear behaviour resulting
from the different materials and their interaction. Each brick is modeled individually and connected taking in-
to account nonlinearity of a brick mortar interface. The calibration of the model is based on small specimen
tests and experimental results for one bay one storey frame are used for the validation. The validated model is
further used for parametric studies on two storey and two bay infilled frames. The response and change of the
structural stiffness are analysed and compared to the traditionally infilled frame. The results confirm the effec-
tiveness of the INODIS system with less damage and relatively low contribution of the infill at high drift lev-
els. In contrast to the uncoupled system configurations, traditionally infilled frames experienced brittle failure
at rather low drift levels.
Dawe and Seah (1989b), Asteris et al. (2017) and
Walsh et al. (2017).
However, in-plane/out-of-plane interaction was
noticed and investigated by a few authors (Hashemi
and Mosalam 2007, Di Trapani et al. 2017, Pasca et
al. 2017, Butenweg et al. 2019).
Recently, several researchers developed innova-
tive systems for improvement of behaviour of in-
filled frames (Verlato et al. 2016, Morandi et al.
2018, Preti et al. 2019).
Most of the studies considered one bay and one
storey frame, just a few authors investigated frames
with more bays and/or more storeys. Liauw and Lo
(1988) showed that there is significant increase of
ultimate load in a case of two bay frame in compari-
son to the one bay frame. Al-Chaar (1998) and Al-
Chaar et al. (2002) stated that the presence of stiffer
masonry infill wall enables the system of carrying
more load than the case of the bare frame. Moreover,
the stiffness increases with the increase of the number
of infilled bays of a multi bay frame.
Figure 1 shows damaged two bay infilled Rc
frame. Therefore the aim of this study was to inves-
tigate the behaviour of two storey and two bay in-
filled frames. Additional novelty of this paper is that
it shows the findings on uncoupled infilled frames
too. The system for uncoupling presented in
Marinković & Butenweg (2019a) was numerically
investigated using already calibrated and validated
model (Marinković & Butenweg 2018). Ridington &
Smith (1977) investigated three bay frame and con-
cluded that the behaviour is dependent on the distri-
bution of loading.
Figure 1. Damage of two bay infilled frame during the Mw=6.4
earthquake on 26.11.2019. in Albania.
2 UNCOUPLED INFILL/FRAME CONNECTION
2.1 Description of uncoupled connection
From 2013 until 2017, the European Project IN-
SYSME (Innovative Systems for Earthquake Re-
sistant Masonry Enclosures in RC Buildings) has
been funded within the 7th Framework Programme
by the European Commission. The research present-
ed here is part of this project and its aim was devel-
oping a constructive measure that solves the above
mentioned problems and provides its simple applica-
tion in practice, thus enabling engineers to apply the
system easily and without any complicated numeri-
cal models. The objective of the study was the de-
velopment of a new system for the improvement of
seismic safety of infills made of bricks. And this was
done, so at the end of the project system INODIS
(Innovative Decoupled Infill System), for improving
the seismic behaviour of masonry infilled reinforced
concrete frames was developed and patented on the
European level.
The conceptual idea is to uncouple infill by applying
elastomers between RC frame and infill panel such
that the brittle behaviour of the infill walls will be
avoided. This way activation of infill walls due to
RC frame in-plane deformations is postponed to
higher drifts, thus disabling high stresses in both RC
frame and infill wall. The elastomer bearings are de-
signed to allow the design drift of the RC frame
without inducing damages to the infill wall. Moreo-
ver, due to the viscoelastic behaviour of the elasto-
meric joints, overall damping capacity of the build-
ing is enhanced. Furthermore, the load transfer
mechanism of out-of-plane load is limited by uncou-
pling, and to overcome this, alternative mechanisms
for the out-of-plane load transfer have been provided
with the shear key and U-shaped elastomeric profile.
More detailed description of the INODIS system and
its performances tested experimentally are given in
Marinković & Butenweg (2019b) and Marinković &
Butenweg (2018)
3 NUMERICAL MODELLING
3.1 Overall approach
A three dimensional finite element model was
developed (Figure 2) to investigate the strength, lat-
eral displacement and stress distribution throughout
the infill wall. Finite element analyses were conduct-
ed using software Abaqus (2013). Some simplifica-
tions are introduced in the model, such as not taking
into account bond slip effect directly by defining in-
teraction between reinforcement and concrete. Also
material model used for bricks does not take into
consideration orthotropic behaviour of masonry and
mortar joints were modelled using interaction con-
tact between the bricks instead of full size continu-
um elements.
According to recommendations (Abaqus, 2013),
three-dimensional 8-node hexahedral continuum fi-
nite elements, with the reduced integration (C3D8R)
are most appropriate for the explicit dynamic anal-
yses and they are used for modelling concrete, bricks
and elastomer. For reinforcement, truss elements
(T3D2) have been used and they are embedded in
solid concrete elements.
In the next sections just a rough description of
numerical modelling approach is given, while de-
tailed description can be found in Marinković (2018)
and Marinković & Butenweg (2019).
Figure 2. Geometry and finite element mesh of the numerical
model of infilled frame (Marinković 2018).
3.2 Material modelling
For material definition, built-in material models
in Abaqus (2013) were used to describe the behav-
iour of concrete, masonry units, reinforcing steel and
elastomer. Similar to the approach used by Stavridis
and Shing (2010), smeared crack models are en-
forced on the concrete and brick elements. The con-
stitutive model adopted for concrete is the concrete
damage plasticity (CDP) model for quasi-brittle ma-
terial implemented in Abaqus (2013). The model is a
continuum, plasticity-based, damage model suitable
for quasi brittle materials.
The elastic response is assumed to be linear and
isotropic. It can be assessed through the use of mod-
ulus of elasticity and Poisson‘s ratio. Modulus of
elasticity is taken from the experimental tests, while
the value of 0.2 is used for Poisson‘s ratio as sug-
gested in Eurocode 8 (EN 1998-1, 2004). CDP mod-
el allows describing separately the nonlinear tensile
and compressive behaviour of plain concrete. In
compression, the stress-strain curve (Figure 3) is
first linear until 0.4fcm according to Eurocode 2 (EN
1992-1-1, 2004). Beyond this point, concrete is in
the plastic region in which plastic strain is input to
define the stress-strain relationship in the finite ele-
ment model in Abaqus (Abaqus, 2013). For this part,
expression given in Eurocode 2 (EN 1992-1-1, 2004)
is used, which can be regarded as a specialisation of
the non-linear stress-strain curve according to Sargin
(1971). This curve is defined only up to the nominal
ultimate strain. To reach experimental levels of de-
formation, the curve is extended with respect to the
equation proposed by Pavlović (2013). The behav-
iour in tension is defined using a fracture energy cri-
terion and a stress-displacement curve (Figure 4) in-
stead of a stress-strain curve. The relationship
between the crack width and the corresponding ten-
sion stress is based on an equation proposed by
Hordijk (1992).
The same as for concrete material, the constitu-
tive model for masonry used in this study is concrete
damage plasticity (CDP) model. Stavridis and Shing
(2010) recommended that the material characteristics
of masonry units based on that of masonry prisms
should be used, rather than that of individual brick
units. This approach has been accepted in this study.
Also, the approach according to Stavridis and Shing
(2010) is used to generate the required stress curves
in both compression and tension.
Figure 3. Stress-strain curve for concrete in com-
pression (Marinković 2018).
Figure 4. Stress-displacement curves for concrete
in tension (Hordijk 1992).
Elastomer material used for uncoupling infill wall
from the RC frame is rubber-based material, with
hyperelastic behaviour. Since the material used is
highly compressible, it can be characterized as elas-
tomeric foam. Therefore, it was decided to use hy-
perfoam material available in Abaqus (2013) to
model its behaviour.
3.3 Contact definition
All the joints between the bricks, both vertical
and horizontal, as well as joints between the frame
and infill have been defined using general contact
with the specified interaction properties. Three inter-
action properties have been defined, with the first
defined as global property assignment to the all ele-
ments that are in contact. For this interaction, ―hard‖
contact normal behaviour is defined together with
penalty friction formulation with the friction coeffi-
cient being 0.6. This means that just compression
stresses and frictional forces are transferred, when
the surfaces are in contact, without having any ten-
sile strength. Since head joints did not have mortar
applied, global property assignment has been used
for them.
Second interaction property was defined to repre-
sent bed joint behaviour and it was assigned to the
horizontal surfaces of the bricks being in contact.
This interaction property, beside ―hard‖ contact and
penalty friction assignment, contains surface-based
cohesive interaction.
Third interaction property is interaction at the in-
fill/frame connection, for which the same approach
is used but with the reduced characteristics (Table
1). For more details check Marinković (2018).
Table 1. Values used for interaction definition.
Bed joint
Interaction
Value
Frame/infill
Interaction
Value
knn [GPa/m]
35.2
knn [GPa/m]
35.2
kss [GPa/m]
1.48
kss [GPa/m]
1.48
ktt [GPa/m]
1.48
ktt [GPa/m]
1.48
tn [MPa]
0.19
tn [MPa]
0.06
ts [MPa]
0.15
ts [MPa]
0.05
tt [MPa]
0.15
tt [MPa]
0.05
Gn [N/m]
20
Gn [N/m]
1
Gs [N/m]
20
Gs [N/m]
10
Gt [N/m]
20
Gt [N/m]
10
η [-]
2
η [-]
2
μ [-]
0.7
μ [-]
0.7
4 NUMERICAL SIMULATIONS
4.1 Two storey frame
Numerical model was used for simulating in-
plane behaviour of infilled RC frames. First, the
model was calibrated and validated according to the
experiments (Butenweg et al. 2019) on full scale
bare frame (BF), traditionally infilled frame (TIF)
and infilled frame with the INODIS system (IIF).
Traditionally infilled frame was constructed in a way
that connection between frame and infill is made
with the mortar.
The whole process of calibration and validation
of the model is described in details in Marinković
(2018). This model is used as a base for the simula-
tions shown in this paper. The model is extended to
the frames of two stories and two bays.
Table 2. Forces and drift levels for two storey model.
First a model with the two stories is made, with
the height twice as the basic one storey frame. The
same size of the beams, columns and bricks is used
as described in Marinković (2018). Now, the hori-
zontal displacement load is applied on the beam at
the top of the second storey. Vertical forces of 200
kN are applied on each column at the top beam of
the second storey.
The effect of infill uncoupling from the frame can
be seen (Figure 5) in the case of two storey frame
too. Here, it is obvious that the frame with the INO-
DIS system (IIF) is less activated in-plane than tradi-
tionally infilled frame (TIF) and therefore reaches
higher in-plane drifts before collapse. TIF model is
characterized with high stiffness and brittle failure at
low in-plane drifts. In contrary, IIF model has stiff-
ness slightly higher than BF. This stiffness increases
with the increase of in—plane drift, due to the high
compression of the elastomer used for uncoupling.
The same trend can be seen on Figure 6, where base
shear force is plotted against the interstorey drift of
the second storey.
The results of the numerical simulations on the
two storey frames are given in Table 2, where again
it can be seen that IIF and BF have reached high in-
plane drifts while TIF is heavily damaged at quite
lower in-plane drifts. This is especially pronounced
at the first storey.
First visible crack is the moment of appearance f
the first crack in infill wall (for TIF and IIF) or in
RC frame (for BF). Figure 7 and Figure 8 show that
at the level of maximum force resistance of the TIF
model is several times more damaged than IIF mod-
el.
Figure 5. Comparison of force-displacement curves for intersto-
rey drift of the first storey.
First crack
Max. load
Max. displacement
F
[kN]
1st storey
drift
[%]
2nd storey
drift
[%]
F
[kN]
1st storey
drift
[%]
2nd storey
drift
[%]
F
[kN]
1st storey drift
[%]
2nd storey drift
[%]
BF
32.6
0.17
0.17
65.4
2.40
2.55
62.4
2.98
3.97
TIF
134.8
0.27
1.37
169.1
0.64
2.21
142.1
0.72
2.71
IIF
86.41
1.68
1.71
126.2
4.65
3.61
104.2
3.95
3.98
Figure 6. Comparison of force-displacement curves for intersto-
rey drift of the second storey.
Figure 7. Deformed shape with the tensile damage distribution
at the level of maximum load for TIF model.
Figure 8. Deformed shape with the tensile damage distribution
at the level of maximum load for IIF model.
4.2 Two bay frame
For the modelling of the two bay frame, one bay
frame model validated according to experimental re-
sults (Marinković 2018) is used. The model is just
extended in the length by adding one infill wall and
one column at the end of it. Everything else was kept
the same.
Figure 9 confirms beneficial effect of application
of the INODIS system, which can significantly re-
duce infill/frame interaction, thus allowing reaching
high in-plane drifts with low or no damage.
Tensile damage distribution of the simulated
specimens is presented in Figure 10. Here it can be
seen that infill wall at maximum load of specimen
IIF is almost no damaged in contrast to TIF speci-
men, even high in-plane drifts reached for TIF are
much lower than for IIF. This again shows the ad-
vantage of application of the INODIS systems,
which provides infill/frame uncoupling and thus re-
duces damage in infills.
Summary of the results is given in Table 3 where
it can be seen that traditional system exhibits first
crack at the very low drift and reaches quite low
maximal drift.
Figure 9. Comparison of force-displacement curves for two bay
frames.
Table 3. Forces and drift levels for two bay model.
First crack
Max. load
Max. displacement
F
[kN]
drift
[%]
F
[kN]
drift
[%]
F [kN]
drift [%]
BF
62.0
0.16
126.2
1.89
124.2
3.02
TIF
193.4
0.06
329.2
1.51
265.45
1.69
IIF
278.5
2.40
283.4
2.42
246.81
3.00
5 CONCLUSION
The paper presents results of numerical simulations
on two bay and two storey frames, with the focus on
uncoupled masonry infill walls. The aim of the paper
was to extend experimental campaign of testing of
the INODIS system used for infill/frame uncoupling.
The extension was done by investigating the behav-
iour of two storey and two bay frames. This was
done using already developed and validated numeri-
cal model.
Results show that INODIS system is effective not
on just one bay one storey frame, but also in a case
of more bays and stories. It highly reduces infill ac-
tivation, thus the stiffness of the system is several
times lower than traditionally infilled frame. And
models with the uncoupling systems always reached
higher in-plane drifts.
For further work, numerical studies on the build-
ing levels using macro modeling approach, is
planned in order to study the effects of the INODIS
system on the global level. This will move us a one
step closer to the design concept for application of
INODIS system in the design of RC frame buildings
with masonry infills.
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