optimizing the structural performance of concrete … the structural performance of concrete ......
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51st ASC Annual International Conference Proceedings Copyright 2015 by the Associated Schools of Construction
345
Optimizing the Structural Performance of Concrete
Elements against Seismic Excitation by Placing Crack-like
Joints: Infill Wall
Mohammad Saied Andalib, PhDc and Mehdi Tavakolan, Prof.
University of Tehran
Tehran, Iran
Armin Aziminejad, Prof.
Islamic Azad University, Science & Research Branch
Tehran, Iran
Fayaz Rahimzadeh Rofooei, Prof.
Sharif University of Technology
Tehran, Iran
A new constructible and low-cost method is introduced in the present study for optimizing the
structural behavior of concrete elements against seismic excitation. The method is based on placing
specific cracks named “Crack-like Joints” (CLJ) in the concrete components and evaluating its effect
on the member’s performance. The method is applied on infill walls against earthquake excitation.
The infill wall considered in this study is a concrete wall embedded in a reinforced concrete frame.
Infill walls can have significant positive effect on the structure’s resistance, hardness and overall
performance against earthquake, if used properly. The finite element models used in this study have
been modeled finely with the ABAQUS software. The results of the verification model matches
appropriately with the previous experimental study. The results show that by placing CLJs suitably
in the middle of an infill wall, the sudden diagonal failure during earthquake excitation would
diminish. In the presence of a suitable CLJ in the infill wall, the force transition is spread through
the wall and the stiffness and strength deterioration during lateral loading on the frame is extremely
reduced.
Keywords: Concrete Infill Wall, Crack-like Joints, Weakening Technique, Seismic Excitation
Introduction
Most techniques for protecting concrete structures against earthquake excitations are able to theoretically achieve a
targeted structural performance; however excessive costs, invasiveness and constructability are still main issues for a
wider implementation. The most common procedures to improve the seismic performance of existing buildings are
the following:
- Strengthening produced by adding (or by reinforcing) lateral elements, which lead to a reduction of deformations
and displacements but lead to an increase in accelerations in the yielding structures.
- Base isolations change the dynamic properties of structures, reducing the seismic acceleration and drift but
increasing the total displacement.
- Supplemental Damping devices reduce lateral displacements, but do not change substantially the amount of
seismic acceleration in the inelastic structures.
In the recent decade, researchers have proposed a counter-intuitive but rational seismic retrofit strategy of
selectively weakening a structural system (Reinhorn, Viti et al. 2005; Ireland, Pampanin et al. 2006; Kam and
Pampanin 2009). Although, weakening reinforced concrete walls by segmenting it vertically has been done by
Ireland, Pampanin et al. 2006 to improve its ductility, but the literature has a gap in weakening infill walls in order
to prevent its brittle failure during earthquake excitation. Also, the literature has not comprehensively studied
different possible methods of weakening the structures in order to enhance their behavior against earthquake
excitations. This study proposes placing crack-like joints in infill walls to enhance the infill wall’s behavior under
cyclic earthquake excitations. It should be noted that the application of the proposed method is not only limited to
infill walls.
One of the main problems of using infill walls in reinforced concrete frames is its brittle failure during sweeping
earthquake excitation. Usually, when the use of infill walls becomes necessary, it is isolated from the surrounding
frame in order to prevent its influence on the structure’s behavior. This is while infill walls can have a significant
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346
and positive effect on the structure’s resistance, hardness and overall performance if used properly. Currently,
specific infill walls are introduced to enhance the structure’s performance, but they often use expensive materials
and construction methods, which often makes them economically unfeasible in wide application.
Different techniques can be developed for strategic weakening of a structure. A typical method with this regard is
“weakened plane joints” which is a plane through a section of a concrete element without any cohesive bind
between the two sides of the concrete. A crack-like joint could be like a thin elliptic void through a wall’s thickness.
This void could be constructed by placing a piece of foam similar to the designed crack, in the desired location prior
to shotcrete or concrete work. The geometry of the crack is such that, it shall be closed under the ultimate
compressive strength without causing failure in the element. This method would reduce the stiffness of the element
before the closure of crack and add compressive strength to the element after its closure. Additionally, this crack
would change the force distribution through continuous massive elements such as walls. This method of weakening
is easily constructible whether the concrete work is in-situ or precast and also does not charge considerable costs to
the construction.
Background
Preliminary suggestions regarding the use of strategic weakening to improve the performance of a structure can be
found in FEMA-273 (FEMA, 1997), FEMA-356 (FEMA, 2000). The concept for an alternative seismic retrofit
strategy referred to as a “selective weakening” approach which focuses on protecting undesirable seismic response
mechanisms by first strategically weakening specific elements within a structure was discussed by Pampanin in
2006(Pampanin 2006). Also Reinhorn, 2005 offered a new procedure to retrofit existing structures subjected to
seismic excitation by weakening the structure and using supplemental damping devices(Reinhorn, Viti et al. 2005).
Weakening a structure will reduce the seismic demand while at the same time changing the inelastic mechanism
according to capacity design principles in order to achieve an overall higher performance level. In a second phase, to
achieve a complete retrofit solution other currently available and applicable retrofit techniques can be used in
combination with the selective weakening strategy to upgrade the weakened structure to the desired and controlled
level of capacity. Comprehensive experimental and numerical study is done on “selective weakening” (SW) retrofit
strategy for earthquake vulnerable existing RC frames with particular focus on the exterior beam-column (b-c)
joints(Kam and Pampanin 2009; Kam, Pampanin et al. 2010).
Infill Walls
Walls are inseparable parts of buildings which their capacity can be used in the structural design. However, the
seismic behavior of building systems with infill walls has not yet been well known (Pavese and Bournas 2011).
Implementation of infill walls in buildings, without considering them in the design process changes the mechanism
of failure and generally the structural behavior against earthquakes. For a structure which has been designed
according to the regulations and its performance is guaranteed to a certain extent, changing the force transmission
paths by using influential walls is often undesirable. For example, a comprehensive report by PEER (Pacific
Earthquake Engineering Research Center) was released in 2007 that explained the effect of brick walls in concrete
frame buildings. The results from this research indicate that the existence of brick walls in the structure, increases its
stiffness, reduces the time period and increases the seismic attenuation. These changes generally increase the level
of resistance and displacement demand in the structure’s seismic design. Brick walls also change the force
transmission path and the strength demand between the peripheral frame elements. It is worth mentioning that the
walls act as lateral bearing members and absorb greater amount of inertia in the earthquake. Finally, the new force
transmission path increases the strength demand in the ceiling diaphragm and therefore it must be designed stronger
(Hashemi and Mosalam 2006). Most of the above-mentioned phenomena are considered undesirable for a flexural
frame building that has been designed according to seismic codes.
As mentioned previously, extensive research is done for rehabilitating structures using infill walls, or designing infill
walls as one of the structure’s lateral bearing system. For example, Billington et al. (2004) did laboratory and
analytical study on an innovative infill wall system for retrofitting building frames against earthquake loads. The
infill walls in this study consist of prefabricated panels with a new ECC concrete material. The results showed a
51st ASC Annual International Conference Proceedings Copyright 2015 by the Associated Schools of Construction
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dominant flexural failure mode for the walls and also the material used in the walls increased the systems resistance
compared to normal concrete, more than 45% (Billington and Yoon 2004).
The failure mode of concrete panels under gradually increasing axial load experiments has shown to be crushing at
the foot instead of buckling(Pavese and Bournas 2011). Also laboratory studies on reinforced concrete walls under
pure axial load has shown negligible effect of reinforcement on the wall’s final strength and only walls with a height
to thickness ratio bigger than 20 were reported to fail of buckling (Pillai and Parthasarathy 1977). Therefore, as the
height to thickness ratio in this study’s modeling is approximately 18, and as the software results also approve, the
buckling of panels is not a matter of concern.
Concrete Sandwich Panels
This study uses a specific type of concrete panels which is usually known as 3D, prefabricated, precast or sandwich
concrete panels(Rezaifar, Kabir et al. 2008; Pavese and Bournas 2011). However, the prefabricated characteristic of
these panels is not a matter of concern in this study; therefore we shall name it concrete sandwich panel (CSP).
CSP’s consist of a double reinforced concrete layer and a polystyrene insulating core with variable thicknesses. The
polystyrene foam layer acts as the concrete mold and an acoustic-thermal insulation. The steel texture of the panel is
woven automatically in the factory. The connection of the two concrete layer reinforcements which look like truss
members is named “shear connectors” as seen in Figure 1. If there is enough shear connectors in terms of resistance
and stiffness for transmitting the shear strength caused by the panels bending, the behavior of these composite
panels is named “Fully Composite”. The thickness of the polystyrene layer varies from 4 to 20 cm. Technically the
minimum thickness of the concrete layers should be 4 cm with at least 200 kg/cm2 strength (Kabir and
Hasheminasab 2002).
Figure 1: Schema of a concrete sandwich panel
Methodology
The finite element (FE) “Abaqus/CAE 6.11-PR3” modeling software is used in this study due to its robustness and
special capabilities in modeling the material’s nonlinear behavior in comparison to similar software. For example
some of the main modeling specifications used in this study is stated in Table 1.
Table 1
Specifications used in the modeling by ABAQUS
Modeling Item Selected value or option
Analysis method Explicit. Although using the implicit method is easier but the current method is
useful and comes in handy for the upcoming dynamic modeling.
Concrete element type
C3D8R: This is a 3D element with 8 nodes and reduced integration. The use of
three-dimensional elements instead of shell elements gives the ability to model
the concrete layer’s peripheral contact behavior.
Reinforcement modeling element B31: This is a 3D element in the space consisting of two nodes with elasto-
plastic behavior and embedded in the whole solid parts.
Concrete material model Damaged Concrete Plasticity
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Is the polystyrene insulation layer
modeled? Yes
Friction coefficient between
concrete contacts 0.8 according to (Mansur, Vinayagam et al. 2008)
Number of elements through the 4
cm concrete layer 2
Interaction of rebar and concrete Concrete is modeled independently from the rebar and for considering their
interaction a software’s option named “Tension Stiffening” has been defined
Compressive behavior of concrete Kent & Park’s Model (Park and Harik 1987)
Contact surface formulation Kinematic contact method
Concrete Plasticity
Specifications
Dilation
Angle A 45 degree caused the model to validate with the relevant experimental results
Eccentricity 0.1
Fb0/fc0 1.12
K 0.67
Viscosity
parameter 0
Loading method Displacement Control Pushover
The “standard cubic sample’s strength” used for this modeling is calculated by dividing the “standard cylindrical
sample strength” to 1.25(Park and Gamble 1999). Concrete reaches its ultimate tensile strength in the tensile strain
10-4, and it has a linear behavior up to 70% of this strain which is shown in Figure 2(a) (Park and Harik 1987). In
Figure 2(b) the brittle behavior of the steel mesh is shown in tension. This brittle behavior is due to pretensioning
and carbonizing the steel meshes in order to increase its strength.
(a) (b)
Figure 2: The force-deflection curve in tension, (a) Concrete (Park and Harik 1987); (b) Panel’s
steel mesh (Kabir and Hasheminasab 2002).
Regarding the fact that intensive cyclic loading of a concrete segment will damage it and decrease its stiffness, the
two indicators dc & dt are defined as the fraction of concrete’s destruction respectively in pressure & tension. These
indicators can have values from 0 to 1, which the bigger values show more destruction. For example, when the
concrete firstly reaches its maximum strength, dc is equal to zero and thereafter its value is computed by the formula
1-σ/σCmax. FE modeling practitioners suggest selecting 80% and 99% respectively for the maximum damage in
compression and tension of concrete. If these values are exceeded in some location of the model, the location will be
colored in red as shown in Figure 3.
Figure 3: Guide to the colors used in the modeling outputs.
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The gravity load is imposed within the first seconds in the modeling and afterwards the lateral load is applied. At the
instant of applying the lateral load, the structure endures acceleration due to change in the speed of loading and that
the method of analysis is explicit. An option named “smooth step” is applied through the loading amplitude of the
ABAQUS software, to resolve such undesirable effect.
Model Validation
In order to validate the FE modeling, a reliable experiment chosen through the literature is modeled with the highest
details possible by ABAQUS. Validating CSP’s FE model under flexural loading is more challenging than other
types of loading due to the composite behavior of these panels. Figure 4 shows the experiment setup and further
details on the experiment can be found in Benayoune and Samad’s paper in 2008. Due to the lack of experiments on
CSPs as infill walls under lateral loading, the experiment setup shown in Figure 4 is chosen as the sample for this
study’s modeling validation. Figure 5 shows the comparison between the force-deflection results of the experiment
and FE modeling in this study. Both of the compared curves show the initiation of crack and final strength in
approximately 11 kN and 21 kN, respectively and their corresponding displacements also match commensurately.
However, the FE result shows a sudden drop in the force after the initial cracking which is due to a phenomenon
called “snap back”. This incident is not shown in the experimental results due to the limited number of test output
points.
Figure 4: The experiment setup for model validation (Benayoune, Samad et al. 2008).
Figure 5: Comparing the force-deflection curves of the FE modeling & the experimental outputs.
The study’s modeling
Figure 6 represents the embedment of the panel in the Reinforced Concrete (RC) frame and also Table 2 expresses
the modeling properties of the panel. A 5 cm gap is considered in Figure 6(b) for the spacing between the infill wall
and the frame columns. A frame in the base elevation of a conventional four-story building design is chosen for the
modeling in this study. Usually the frames in the lowest building story are more vulnerable to earthquake
excitations. Such frame has a rigid footing connection and a uniform surface lateral load is applied at the top of the
upper beam.
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Table 2
Alternative material properties in the model
Component Poisson
Ratio
Elasticity
Module (kg/cm2)
Yield
Strength (kg/cm2)
Tensile
Strength (kg/cm2)
Compressive
Strength (kg/cm2)
Rebar 0.3 2,000,000 4,500 - -
Frame Concrete 0.2 200,000 - 20 200
Panel Concrete 0.15 150,000 - 10 180
(a) (b)
Figure 6: (a) Panel sections (units in mm); (b) Panel scheme in the RC frame.
Loading of the Bare Frame
The bare frame chosen for the placement of the infill wall in this study is analyzed under a pushover loading and its
force-deflection behavior is shown in Figure 7. A flexural failure with a maximum strength of approximately 16 tonf
occurs according to the analytical results and also the deterioration of the system’s strength is fairly low.
Figure 7: Bare frame force-displacement curve
Loading of the Composite Frame (CF)
The phrase “composite frame” is used for the companion system of the frame and infill wall together. According to
Figure 8(b), after 0.18 cm displacement, the two corners of the panel partially fail and the system exits the linear-
elastic mode. After the system reaches its maximum strength, the diagonal failure initiates and then a sudden
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resistance drop occur after the peak strength. When the diagonal failure of the infill wall is completed, suddenly
acceleration is imposed to the system and then a residual strength of approximately 25 tonf is preserved.
(a) (b)
Figure 8: (a) The contour of dt in the CF at the peak load; (b) Force-displacement curve of CF.
Loading of the Composite Frame with Crack-Like Joints (CF-CLJ)
Many placements and geometrical configurations were tested in this study for the crack, in order to achieve a
general perception of the composite frame’s behavior plus the crack. According to Figure 9(a)&(b) a cross crack at
the middle of the wall was chosen. The void due to the crack is only applied on the concrete and the steel mesh is
left intact. The opening of the crack is equal to sqrt(2)/2 cm and is closed after approximately 1 cm lateral
displacement of the frame. This closure is observed in Figure 9(b) and results in an additional compressive strength
according to Figure 9(d). Figure 9(c) shows the spreading of force distribution at the peak lateral strength in
comparison to Figure 8(a). The sudden strength deterioration occurred in the previous systems no longer happens,
but rather a smooth degradation in the lateral stiffness is observed. Also, after a small drop in the strength at a lateral
displacement of 1 cm, the CLJ increases the strength and the system uses its reserved strength capacity.
(a) (b)
(c) (d)
Figure 9: (a) Cross crack placement; (b) Closure of the CLJ; (c) The contour of dt in the CF-CLJ
at the peak load; (b) Force-displacement of the CF-CLJ.
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Conclusion
In the recent decade, researchers have proposed a seismic retrofit strategy of selectively weakening a structural
system. However, the literature has a gap in weakening infill walls in order to prevent its brittle failure during
earthquake excitation. Also, the literature has not comprehensively studied different possible methods of weakening
the structures in order to enhance their behavior against earthquake excitations. This study offers a new constructible
and low-cost method for optimizing the structural behavior of concrete elements against seismic excitation. The
method is based on placing specific cracks named “Crack-like Joints” (CLJ) in concrete components and evaluating
its effect on the member’s performance. The method is applied on infill walls against earthquake excitation. The
force-deflection results and the amount of destruction in the concrete show that by placing CLJs suitably in the
middle of an infill wall, the brittle failure during earthquake excitation will diminish. In the presence of a suitable
CLJ in the infill wall, the force transition is spread through the wall and the stiffness and strength deterioration
during lateral loading on the frame is extremely reduced. Future works can consider other arrangements of CLJs in
structural components and evaluate the structures behavior against seismic loads. Furthermore, the general strategy
of weakening the structure in order to enhance its behavior against seismic loads is not limited to using weakened
plane joints or CLJs; others methods can also be developed.
References
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