 Open Access
 Authors : Musitefa Adem Yimer , Temesgen Wondimu Aure
 Paper ID : IJERTV9IS020110
 Volume & Issue : Volume 09, Issue 02 (February 2020)
 Published (First Online): 21022020
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Seismic Performance of Steel Fiber Reinforced Concrete BeamColumn Joints under the Variation of Column Axial Load
Musitefa Adem Yimer 1* , Temesgen Wondimu Aure 2a
1Department of Civil Engineering, Samara University, Samara, Ethiopia
2Department of Civil Engineering, Addis Ababa Science and Technology University, Addis Ababa, Ethiopia
Abstract: This study presents a finite element investigation of steel fiber reinforced concrete beamcolumn joints under cyclic loading with the variation of axial load. The aim of the study is to investigate the influence of axial load variations on the seismic behavior of steel fiberreinforced concrete (SFRC) beamcolumn joints. Nonlinear finite element analysis with a damaged plasticity model in ABAQUS/Standard is adopted. The finite element model is verified using experimental results conducted by other researchers. Six SFRC specimens with different column axial load ratios and a 2% volume fraction of steel fiber were simulated under reversed cyclic loading. The parameters investigated are maximum loadcarrying capacity, stiffness degradation, energy dissipation and failure mode. The results indicated that an increase of column axial load has a valuable influence to delay the initiation of cracks and damage accumulation, slightly improvement of the joint stiffness and improves the energy dissipation of joints at the initial stage of loading. Moreover, when the axial load level increases up to 50% of the column capacity, no cracks observed in the joint area and no change in the maximum loadcarrying capacity. However, when the axial load level of more than 50% of the column capacity, the cyclic stiffness decreased slightly due to the deterioration caused by crushing of concrete in column. Thus, the results revealed that the increase of column axial load improves the confinement of steel fiber reinforced concrete beamcolumn joints, however, a threshold limit could be required.
Keywords: Beamcolumn joints; Steel fiberreinforced concrete; Cyclic loading; Axial load variation; Nonlinear finite element analysis

INTRODUCTION
Moment resisting reinforced concrete frames are assemblies of beams and columns connected by beamcolumn joints. These frames should be adequate ductility, strength, energy dissipation and stiffness to resist the seismic loading without collapse [1]. Several studies [27] have established that SFRC is capable of improving the seismic behavior of reinforced concrete structural members, such as shear walls, beamcolumn joints, and exural members subjected to seismic loads. Based on earlier studies, the application of SFRC considerably improve the shear strength [8,9], the flexural strength and ductility [1012] and fracture toughness [13] of the reinforced concrete members. The use of SFRC as a minimum shear reinforcement for beams has been permitted in ACI 318 [14] following the research study by ParraMontesinos [15]. An experimental study of such joint behavior is not feasible to assess the effect of several parameters involved in joint behavior. Furthermore, because of the difficulties in integrating the compressive column axial loading in the experimental setup, it is common practice to assume the column axial load to be zero or constant in most of the researchs [16]. Nonlinear finite element analysis, however, can be viewed as one of a convenient and reliable solution to investigate such effects. In this study, the seismic behavior of SFRC beamcolumn joints under the variation of axial load is carried out using the finite element software ABAQUS/Standard. The finite element model is validated against existing SFRC beamcolumn joints tested by Choi and Bae [12]. After validation of the model, eight SFRC beamcolumn joint specimens under cyclic loading is investigated by varying the column axial load ratio.

RESEARCH SIGNIFICANCE
The use of steel fibers in RC members can offer a positive influence. However, due to the high expenses and restrictions of specimen fabrication, experimental tests for reinforced concrete structures need spending a great amount of time and money, especially countries not have advanced laboratories. Moreover, due to the complexity of incorporating the column axial load in the experimental test setup, it is common to assume the column axial load to be zero or constant in most of the investigations [16]. Hence, further study needs on SFRC beamcolumn joints with the variation of axial load. This study provides the finite element results on the seismic performance of SFRC beamcolumn joints with the variation of compressive column axial load.

FINITE ELEMENT MODEL
A three dimensional (3D) nonlinear nite element model of SFRC beamcolumn joints are developed using ABAQUS/Standard
[17] by considering both geometric and material nonlinearities.
Element types, meshes and boundary conditions
A threedimensional linear 8node brick elements (C3D8R) were employed for modeling of steel fiber reinforced concrete and steel plate. Twonode linear threedimensional truss elements (T3D2) was used to model steel reinforcements. A mesh size of 40
mm is used for the whole geometry for all elements. To apply the boundary conditions identical to the reference experimental test setup, the nodes at the surfaces of the lower and upper column ends are attached to a reference point using the coupling constraint. The loading steel plate at the tip of the beam was constrained in ydirection for the cyclic loading application. The boundary conditions, mesh and were set in the model as shown in Fig. 1.

(b) (c)
Fig. 1Boundary conditions, meshing and loading of the FE model


Interactions between Components
The embedded approach was adopted for the interaction between the SFRC and the steel reinforcement, which enforces displacement continuity and the perfect bond between the two elements. The interaction between concrete and reinforcement after cracking was incorporated only in a simplied way using the tension stiffening in the concrete model. This was adopted to approximately simulate load transfer between cracks over the rebar [18,19]. The interactions between the surfaces of the inelastic and elastic solid bodies representing the concrete material and the loading steel plate is modelled using tie constraints.

Load Application Method and Cyclic Loading Protocol
In this study, the loading on the beam tip is adopted as in the experimental setup by Choi and Bae [12]. Additionally, the ACI 374
[20] recommendation for reverse cyclic loading protocol is used with the same loading history as reported in the experimental study, as showed in Fig. 2. The reversed cyclic load is applied in terms of displacementcontrolled step, where the displacement is determined from the drift ratio (%) based on the following expression:Drift ratio (%) = Ã— 100 (1)
Where and are the applied displacement at the beam tip and length of the beam from column face to the application point of the cyclic displacement, respectively. Analogous to the test loading condition, two loading steps are dened in the finite element modeling. The rst step is the axial load step where the axial load is applied at the column top surface as a pressure load in a load controlled mode and kept constant in the second step. Then, the reverse cyclic loading is applied in the second step on the beam end at a distance of 1800 mm from the beamcolumn interface.
15
10
Drift ratio (%)
Drift ratio (%)
5 Drift ratio in percentage
5%
2.5 3.5%
7.5%
10%
0 0.2%0.25% 0.35%0.5%
0.75% 1%
1.5% 2%
3.6
5
4.5
6.3
9 13.5 18
27 36 45
63
10
Displacement in mm
90
135
180
15
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42
Cycle number
Fig. 2Schematic representation of the loading history based on ACI 374 used by Choi and Bae [12].

Analysis Approach
Nonlinear finite element analysis of the beamcolumn joint specimens was performed in ABAQUS/standard by considering both geometric and nonlinearities with viscose regularization. In the static general analysis procedure of structural components, an appropriate solution algorithm is needed to solve the nonlinear equations. NewtonRaphson equilibrium iteration provides convergence at the end of each load increment within tolerance limits for all degree of freedoms in the model [18,19]. Automatic increment with a large number of increments and a smalltime step size are used to improve the convergence rate.

Material modeling
The concrete damaged plasticity (CDP) model available in ABAQUS was adopted for the nonlinear material behavior of steel fiber reinforced concrete in compression and cracking in tension. The CDP model uses the concepts of compressive crushing and tensile cracking to represent the inelastic behaviors of quasibrittle material. The plasticity of the steel reinforcement and the elastic behavior of the steel plate were considered in the finite element modeling. The CDP model is eective for the analysis of concrete structures under monotonic, cyclic, and dynamic loading [18,19,21]. The CDP model was developed by Lubliner et al.
[22] and later refined by Lee and Fenves [23]. In uniaxial compressive loading, the CDP model performs linearly up to the value of the initial yield is reached, then followed by a stress hardening and strain softening beyond the ultimate stress. After the calibration of the CDP model parameters on the experimental specimen, JNR2BTR, tested by Choi and Bae [12], the considered parameters in the CDP model for all specimens were presented in Table 1.Table 1 The adopted input parameters in the CDP model
Plasticity Parameters
Values
Dilation Angle ()
32o (calibrated)
Viscosity Parameter, Âµ
0.0025 (calibrated)
Shape factor ()
0.8 (calibrated)
Stress ratio (0 )
0
1.16 (default value)
Eccentricity ()
0.1 (default value)
According to ACI 544.3R93 [24] report, normal weight SFRC with a fiber content up to 2% by volume has a density in the same range as normal concrete of 2306 to 2403 kg/m3. Also, according to the ACI 544.1R96 [25] report, the density and Poissons ratio of SFRC are generally taken as same range as to those of normal concrete when the volume percentage of steel fiber is up to 2%. In this study, the Poisson's ratio and density were taken as 0.2 and 2400 kg/mÂ³, respectively, for steel fiber reinforced concrete material modeling based on the above recommendations.

Modeling of SFRC in Compression
Several investigators have proposed models for the characterization of the compressive stressstrain behavior of SFRC [2631] all have relatively similar approximations of the SFRC behavior under uniaxial compression. Generally, steel fibers only have minor effects on the ultimate compressive strength of concrete, slightly increasing or decreasing its magnitude, depending on the characteristics of the fibers [25,32]. However, the use of steel fiber in concrete significantly enhances the descending branch of the compressive stressstrain curve. Due to this, a compressive constructive model of SFRC differs other than normal concrete compressive model was still employed. In this study an attempt was made in the validation section from different SFRC compressive stressstrain model and the model proposed by Lee et al. [31] was adopted. Based on this constitutive model, the behavior of the SFRC was assumed to be elastic linear up to reaching 0.4 . After this point, the plastic behavior was defined
using the following equations.
(/0)
= [
= [
1+(/0)
] (2)
Where; for prepeak: = = 1
1( )
0
/0
1.0 (3)
For postpeak: = (
0
0.064
)
[1 + 0.882 ( )
0.882
] (4)
A= 1 + 0.723 (
)
0.957
/
> 1.0 (5)
0
Where;
= (0.0003
+ 0.0018) 0.12 (6)
0
= (367
+ 5520) 0.41 (7)
Where is the uniaxial compressive stress of SFRC, is compressive strength of SFRC, is the initial elastic modulus of SFRC, is the uniaxial compressive strain of SFRC, 0 is strain of SFRC at peak stress, is the volume fraction of steel fiber,
is length steel fiber and is the diameter of steel fiber. As mentioned in previous section, hooked steel fiber with a length of 30 mm and a diameter of 0.5 mm was used in this study for consistency of the referenced experimental study by Choi and Bae [12]. Equation (8) which is proposed by Ou et al. [33] was used to estimate the compressive strength of SFRC because it gives an approximately equal value with the referenced experimental test data reported by [12].
= + 2.35 (8)
Where, is the compressive strength of normal concrete; is compressive strength of SFRC and is the fiber reinforcing
index which expressed as:
=
(9)
The ultimate strain for SFRC was set to the value = 0.02 according to the work reported by Wang [34] for > 0.5%.

Modeling of SFRC in Tension
There are a number of different constitutive models developed by researchers to represent the tensile behavior of SFRC [27,35 37]. In this study, the model proposed by Lok and Xiao [36] was employed to represent the tensile behavior of SFRC. Furthermore, a number of researchers [38,39] conducted a nonlinear finite element analysis of fiber reinforced concrete elements by using this model and found the Lok and Xiao [36] for SFRC material behavior in tension resulted in precise predictions. The uniaxial tensile stressstrain relationships of SFRC by Lok and Xiao [36] model shown in Fig.3 and which expressed by Eqs. (10)(11):
2
2
= [2 ( ) ( ) ] for 0
(10)
0
0
0
= [1 (1 ) ( 0 ) ] for
(11)
10
0
1
= for 0 (12) Where; is the ultimate uniaxial tensile strength of SFRC, 0 is the corresponding ultimate tensile strain, is the residual strength from the strain, 1, as shown in Fig. 3. These values are defined by [40] as follows:
=
(13)
=
1
(14)
1
Where is the fiber orientation factor in a three dimensional (3D) case. Hannant [41] used the value of as 0.50, Soroushian and Lee [42] taken as 0.405 and Lok and Xiao [36] taken as 0.50 for slabs and 0.405 for beams. In the present work, is taken as
0.50. The value of modulus elasticity of steel fiber ( ) 200 GPa used in this study recommended by Amin and Gilbert [43]. According to Lok and Xiao [40], the initial tangent modulus of SFRC in tension and in compression is assumed to be equal, and the ultimate tensilestrain 0, corresponding to expressed as:
0
= 2
0
(15)
where, 0 is the initial tangent modulus of steel fiber reinforced concrete.
For hookedtype steel fibers, the value of bond stress ( ) 6.8 MPa is used in this study as suggested by Lim et al [44]. Equation (16) which is the correlation between splitting tensile strength () and compressive strength () of SFRC proposed by [45] was used to estimate the splitting tensile strength () of SFRC:
= 0.21()0.83 (16)
However, the CDP model requires the uniaxial tensile strength as an input. Therefore, the relation is given in Eq. (17) based on Eurocode 2 [46] recommendation was used to convert the splitting tensile strength into a uniaxial one.
= 0.9 (17)
Fig. 3Tensile stressstrain behavior of SFRC used in FE model by Lok and Xiao [36].

Steel Reinforcement and Steel Plate Modelling
The mechanical properties of reinforcing bars were employed to represent the longitudinal reinforcement and shear stirrups in numerical modeling were exactly the same as the material properties reported by Choi and Bae [12] as presented in Table 2 and Poissons ratio of 0.3.
Table 2 Mechanical properties of reinforcing bars used by Choi and Bae [12].
Bar ID
()
()
(/)
()
D10
400
436.9
0.0022
517.0
D13
400
400.0
0.0020
472.3
D13
500
505.8
0.0025
521.8
D19
500
558.7
0.0028
656.1
D25
400
471.7
0.0024
587.6
Note: is specified minimum yield strength, is measured yield strength, is yield strain, and is ultimate tensile stress. The tensile stressstrain of reinforcing bars was assumed to be elastic with corresponding Youngs modulus and Poissons ratio. The nonlinear behavior of steel bars was modeled as a bilinear elasticplastic material using a strain hardening ratio of 0.01, as recommended by Kachlakev and Miller [47] and it is used by many researchers [48,49]. The typical bilinear stressstrain model of reinforcement bars is shown in Fig. 4. In order to avoid the concentration of stresses on the concrete, premature failure or cracking at the cyclic loading point a 20 mm thick steel plates were used at this loading point. This loading steel plate is modeled with elastic properties steel material having an elastic modulus (Es) of 200 GPa, Poissons ratio of 0.3, and no yielding strength to avoid any premature failure in the steel plates.
Fig. 4 Bilinear stressstrain model for reinforcing steel bars by Kachlakev and Miller [47].



MODEL VALIDATION
Existing steel fiber reinforced concrete beamcolumn joint specimen which experimentally tested by other researchers was used for model validation.
4.1. Details of experimental specimen used for validation
To investigate the behavior of SFRC beamcolumn joints numerically, the performance of the nonlinear finite element models developed in this paper are validated against experimental specimen tested by Choi and Bae [12], namely JNR2BTR was selected. According to their study, the specimen represented the firstfloor exterior beamcolumn joint (at a scale of twothird) subassemblies of a 10story office building. The specimen, JNR2BTR, is designed without any hoops in the joint panel region and made with a 2% volume fraction of steel fiber in concrete, but the beam and column are reinforced by hoops based on ACI ASCE Committee 352 [50]. Hooked steel fiber with a length of 30 mm and a diameter of 0.5 mm was used in the experimental study in consistence with ACI 318 [51] as shown in Fig. 5. The columns had a crosssection of 300 mm Ã— 300 mm, with a height of 3000 mm. The beam had a crosssectional dimension of 250 mm Ã— 375 mm with lengths of 1900 mm up to the contraflexure point in all subassemblages. The reinforcement details and geometry of the validated specimen is displayed in Fig. 6.
Fig. 5Size and shape of steel fiber used in an experimental study by Choi and Bae [12].
Fig. 6Details of the specimen JNR2BTR tested by Choi and Bae [12].
Furthermore, the geometrical properties, variables and details as presented in Table 3. The compressive and indirect splitting tensile strength for the SFRC reported in the experimental study are presented in Table 4. In experimental study, beamcolumn joint specimens were loaded using a hydraulic actuator under cyclic loading applied tip of the beam as a displacement controlled. The axial load was applied on the top of the column.
Table 3 Crosssections, variables and details of specimens tested by Choi and Bae [12].
Specimen
Beam
Column
(%)
Crosssection (mm)
Bars (mm) (top & bottom)
Stirrups (mm)
Cross section (mm)
Bars (mm)
Stirrups (mm)
Ã—
Ã—
JNR2BTR
0.1
250 X 375
4D25
D10@70
300 X 300
4D19+2D13
D13@60
2
Note: P is the applied axial load to column, is the cylindrical compressive strength of the concrete steel fiber concrete, is the area of the column, is the diameter of reinforcement bar; is width of beam, is overall depth of beam, is width of column, is overall depth of column, and is volume fraction of steel fiber. (Note: 1 mm = 0.0394 in.)
Table 4Mechanical properties of concrete tested by Choi and Bae [12]
Concrete ID
(%)
()
()
()
SFRC2
2
31826
54.7
6.5
Note: is volume fraction of steel fiber, is the modulus of elasticity of SFRC, is compressive strength of cylinder and
is splitting tensile strength of SFRC.

Finite element model predictions
As described before in the summary of experimental work the specimen, JNR2BTR, which were experimentally tested by Choi and Bae [12], was selected to validate the developed finite element model. The accuracy of the nonlinear finite element model
was evaluated by comparing the nonlinear finite element analysis (NLFEA) results of this specimens with experimental result in terms of loaddisplacement response and failure pattern.

LoadDisplacement Response
The loaddisplacement envelope curves obtained from nonlinear finite element analysis compared with the experimental results of the specimen JNR2BTR are displayed in Fig. 7 (ab). The higher stiffness in finite element models may result from more microcracks reducing the stiffness of the experimental test. It is obvious that more microcracks are presents in test, while finite element models do not include these microcracks [52]. The loaddisplacement curve predicted by the NLFEA shows a very good agreement with that of the experimental result. The average maximum load obtained from the NLFEA analysis was 3.35% lower than the average maximum load reported from the experimental study (refer Table 5 for comparison).
150
100
0
Load (kN)
Load (kN)
0
50
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
JNR2BTR
JNR2BTR
150
100
50
Load (kN)
Load (kN)
0
50
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
JNR2BTR
JNR2BTR
100
150
NLFEAHysteretic Curve TestHysteretic Curve
100
TestEnvelope Curve NLFEA envelope Curve
TestEnvelope Curve NLFEA envelope Curve
150
200 150 100 50 0 50 100 150 200
200 150 100 50 0 50 100 150 200
Displacement (mm)
Displacement (mm)

Hysteretic curves (b) Envelope curves Fig. 7Loaddisplacement hysteretic and envelope curves of JNR2BTR [12].
From the above figures, the ABAQUS models show the envelope curve precisely. However, the hysteretic loops of the ABAQUS models as compared to the test results are exhibit a fatpinching distance as expected from the embedded method of concrete and reinforcement modeling. This pinching distance clearly described that the tension stiffening approximate consideration of bond slip effect in the modelling due to the adoption of the embedded (perfect bond) method to simulate the bond between reinforcements and concrete.
The error and overall model accuracy predicted by NLFEA is compared with the test result and presented in Table 5. In order to express the overall model accuracy, and accompanying average underestimation or overestimation of the NLFEA, the error (%) and mean model accuracy [M (%)] estimated based on the relation given in (Behnam et a., 2018) and it dened as:
Error(%) = 
 Ã— 100 (18)
Mean model accuracy, (%) =
Ã— 100 (19)
It can be seen in Table 5 that in all cases, the model prediction of the joint diagonal cracking load and maximum load lead to an error below 5%, which once again shows that the nonlinear finite element simulated response of the specimens was in a very good agreement with the results reported from the experimental study.
Table 5 Diagonal cracking load and maximum load comparisons of NLFEA prediction with the experimental results of the two specimens.


Failure Pattern
Specimen
Average load at joint diagonal cracking (kN)
Prediction
Average maximum Load (kN)
Prediction
NLFEA
Test
Error (%)
M (%)
NLFEA
Test
Error (%)
M (%)
JNR2BTR
85.31
88.21
3.29
96.71
99.73
103.19
3.35
96.65
Fig. 8 and Fig. 9 shows the comparison of crack patterns in specimens JNR2BTR obtained by the nonlinear finite element simulation and reported from the experimental study at the maximum load and at failure load stage, respectively. These responses
clearly demonstrate that there is a very good agreement between the experimental and nonlinear finite element prediction crack patterns. This numerical result also further approves the accuracy of the nonlinear finite element model and shows strong capability in predicting the general behavior of both the reinforced concrete and steel fiber reinforced concrete specimens. Therefore, the model can be used as an effective and reliable numerical tool for further studies on SFRC beamcolumn joints. Based on the comparisons, it can be concluded that the developed finite element model can successfully predict the seismic behavior of SFRC beamcolumn joints with high accuracy.

Experimental (b) NLFEA (ABAQUS) Fig.8Comparison of crack patterns at maximum load stage of JNR2BTR [12].


(a) Experimental (b) NLFEA (ABAQUS)
Fig. 9Comparison of crack at the failure stage of specimen JNR2BTR [12].


PARAMETRIC STUDY
5.1 Specimens description
The level of axial load is described as a percentage of ( ), where ( ) is the area of column crosssection and ( ) is
compressive strength of steel fiber reinforced concrete having 2% volume faction of steel fiber. The column axial load is applied as pressure compressive stress. Eight different column compressive axial load levels: (0.0, 0.05, 0.1, 0.2, 0.3, 0.5, 0.7 and 0.75)
Ã— ( ); were modeled and simulated. Based on the amount of axial load ratio, these eight specimens designated as JS4SFRC2
A0, JS4SFRC2A05, JS4SFRC2A10, JS4SFRC2A20, JS4SFRC2A30, JS4SFRC2A50, JS4SFRC2A70 and JS4SFRC2
A75, respectively. The column and beam crosssection, length and reinforcement details are identical to the validated specimen (JNR2BTR) except the number of stirrups in joints panel zone. The joint reinforcement details of the parametric study specimens are as shown in Fig. 10. The description of the specimens with axial load variations are reported in Table 6.
Fig. 10: Joint reinforcement details of all specimens. (Note: Dimensions in mm).
Table 6: Variables and details of specimens used in this study.
Note: P is the applied axial load to column, is the compressive strength steel fiber reinforced concrete, is the area of column, is diameter of reinforcement bar; is width of beam, is overall depth of beam, is width of column, is overall depth of column, and is volume fraction of steel fiber.

RESULTS AND DISCUSSIONS OF THE PARAMETRIC STUDY
The main parameters investigated in this study due to the effects of axial load variation on the joint seismic behavior are load carrying capacity, crack patterns, energy dissipation, and stiffness degradation responses under cyclic loading.
6.1 Effects of Column Axial Load on LoadCarrying Capacity
The hysteretic loops obtained from the nonlinear finite element simulation of specimens with different column axial load ratio are shown in Fig. 11 (ah). The increasing of column axial load level from 0% to 20% of column capacity resulted in an increase of some extent (approximately 1.3%) of the peak loadcarrying capacity of the specimens. However, when the axial load level of more than 20% of the column capacity, the lateral strength deteriorate slightly due the development of greater compressive shear stress in the joint area. As displayed in Fig.13, the specimens with column axial loads exhibited early strength than the specimen simulated without axial load. The specimen which has not column axial load (JS4SFRC2A0) recorded its average maximum loadcarrying loadcapacity of 103.46 kN. The specimens JS4SFRC2A05, JS4SFRC2A10, JS4SFRC2A20, JS4SFRC2 A30, JS4SFRC2A50, JS4SFRC2A70 and JS4SFRC2A75, are exhibited their average maximum load of 103.59, 103.95, 104.79, 104.23, 104.38, 104.35 and 104.74 kN, respectively.
150
100
50
Load (kN)
Load (kN)
0
50
100
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0
P/fcs'Ag = 0
150
100
Load (kN)
Load (kN)
50
0
50
100
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0.05
P/fcs'Ag = 0.05
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
(a) JS4SFRC2A0 (b) JS4SFRC2A05
Drift ratio (%)
150
100
50
Load (kN)
Load (kN)
0
50
100
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0.1
P/fcs'Ag = 0.1
150
100
Load (kN)
Load (kN)
50
050
100
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0.2
P/fcs'Ag = 0.2
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
(c) JS4SFRC2A10 (d) JS4SFRC2A20
Figure 11: Loaddisplacement hysteretic responses of all simulated specimens (continued).
150
100
Load (kN)
Load (kN)
50
0
50
100
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0.3
P/fcs'Ag = 0.3
150
100
50
Load (kN)
Load (kN)
0
50
100
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0.5
P/fcs'Ag = 0.5
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
(e) JS4SFRC2A30 (f) JS4SFRC2A50
Drift ratio (%)
150
100
50
Load (kN)
Load (kN)
0
50
100
Drift ratio (%)
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0.7
P/fcs'Ag = 0.7
150
100
50
Load (kN)
Load (kN)
0
50
100
10 8 6 4 2 0 2 4 6 8 10
P/fcs'Ag = 0.75
P/fcs'Ag = 0.75
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
150
200 150 100 50 0 50 100 150 200
Displacement (mm)
(g) S4SFRC2A70 (h) S4SFRC2A75 Figure 13: Loaddisplacement hysteretic responses of all simulated specimens.
The average maximum loadcarrying capacity of the specimens under different column axial load levels are exhibited in Fig.12.
120
Average maximum load (kN)
Average maximum load (kN)
90
60
30
0
0 10 20 30 40 50 60 70 80
Axial load ratio (%)
Fig.12: Average maximum load of simulated specimens. (Note: Average value of the maximum load in the push and pull direction are considered).

Influence of Column Axial Load Level on Damage and Failure Pattern

Joint Cracking Behavior
Fig.13 to Fig.14 shows the cracking pattern in the joint observed at different stages of loading for specimens with different axial load ratio. The specimens with higher column compressive axial load level were established to delay and reduce the occurrence of cracking in the beamcolumn joint area in compared to the specimens simulated without column axial load and under low column axial load. Fig.13 illustrates the joint cracks observed in the specimens at the first loading cycle of 1.0 % drift. In specimens under low axial load (JS4SFRC2A05 and JS4SFRC2A10), the diagonal cracks appeared in this loading stage were very low intensity compared to the specimen without axial load (JS4SFRC2A0). However, no joint cracking was observed in the
specimens under axial load levels more than 10% of the column axial capacity (> 0.1 ). This confirmed that high axial load
would delay the formation of joint cracks and it was beneficial for joint confinement.

JS4SFRC2A0 (b). JS4SFRC2A05
(c) JS4SFRC2A10 (d). JS4SFRC2A20
(e) JS4SFRC2A30 (f) JS4SFRC2A50
Fig.13: Joint cracking behavior of the specimens at 1% drift ratio (continued).
(g) JS4SFRC2A70 (h) JS4SFRC2A75 Fig.13: Joint cracking behavior of the specimens at 1% drift ratio.
With further increase of loading cycles, the specimens without axial load (JS4SFRC2A0) and under low axial load levels (JS4 SFRC2A05 and JS4SFRC2A10) exhibited a significant diagonal shear cracking in the joint core zone and flexural caking in the beam plastic hinge region. However, only flexural cracks observed at the edges of the beamcolumn junction extended into the beam plastic hinge region for specimens with column axial load level more than 10% of the column capacity. For example, joint cracking patterns at the first cycle of 5% drift ratio showed in Fig.14.

JS4SFRC2A0 (b) JS4SFRC2A05 Fig.14: Joint cracking behavior of all the specimens at 5% drift ratio (continued).



(c) JS4SFRC2A10 (d) JS4SFRC2A20
(e) JS4SFRC2A30 (f) JS4SFRC2A50
(g) JS4SFRC2A70 (h) JS4SFRC2A75 Fig.14: Joint cracking behavior of all the specimens at 5% drift ratio.

Effects of Column Axial Load on Stiffness Degradation
Fig. 15 illustrates the peaktopeak stiffness of all specimens from each first cycle out of three loading cycles at each drift ratio. The specimens with higher axial load ratios were found to have a higher peaktopeak stiffness than specimens with low axial load ratios in early drift ratios. It was observed that stiffness degradation of specimens had a critical influence only in the initial loading cycles and the stiffness values after a 2.5% drift ratio of loading were approximately in the same value in specimens with different axial load ratios, but specimens under very high axial load are slightly greater stiffness degradation than specimens with low axial load.
10
9
8
7
Stiffness (kN/mm)
Stiffness (kN/mm)
6
JS4SFRC2A0
5 JS4SFRC2A05
JS4SFRC2A10
4 JS4SFRC2A20
JS4SFRC2A30
3 JS4SFRC2A50
JS4SFRC2A70
2 JS4SFRC2A75
1
0
0 1 2 3 4 5 6 7 8 9 10 11 12
Drift ratio (%)
Fig. 15: Stiffness degradation of specimens.

Effects of Column Axial Load on Energy Dissipation Capacity
The plastic energy dissipation of the specimens is presented in Fig. 16. It can be observed that the specimens with column axial
load level up to 0.3 caused a decrease of the plastic energy dissipation compaired to the specimen without column axial
load. However, it was observed that the decreasing rate plastic energy of the specimens with column axial load level more than
0.1 has greater than compared to the specimen with a column axial load of 0.1
and less.
500000
450000
Plastic dissipated energy (kN.mm)
Plastic dissipated energy (kN.mm)
400000
JS4SFRC2A0 JS4SFRC2A05 JS4SFRC2A10 JS4SFRC2A20
350000
350000
JS4SFRC2A30 JS4SFRC2A50
JS4SFRC2A70
300000
250000
JS4SFRC2A75
200000
150000
100000
50000
0
0 1 2 3 4 5 6 7 8 9 10 11 12
Drift ratio (%)
Fig. 16: Plastic energy dissipation of all specimens.
For instance, specimens JS4SFRC2A05 and JS4SFRC2A10 are dissipating less plastic energy at drift ratios of 7.5% by only 0.15% and 0.43%, respectively, compared to the specimen JS4SFRC2A0. In addition, specimens JS4SFRC2A20, JS4SFRC2 A30, JS4SFRC2A50, JS4SFRC2A70 and JS4SFRC2A75, are displayed less plastic energy at drift ratios of 7.5% by 4.25%, 4.6%, 4.0%, 3.0%, and 1.79%, respectively, compared to the specimen JS4SFRC2A0. This can be indicated that the increasing of column axial load levels are slightly adverse effect on energy dissipation due to axial failure and local crushing of concrete.


CONCLUSIONS
Based on the nonlinear finite element analysis results of the simulated beamcolumn joint specimens to explore the effect of axial load variations on the seismic behavior of steel fiber reinforced concrete exterior beamcolumn joints, the following conclusions can be drawn:

The results indicated that an increase of axial load has a valuable influence to delay the initiation of cracks and damage accumulation. Moreover, when the axial load ratio increases up to 50% of the column capacity, no cracks observed in the joint area, only flexural cracks observed at the beamcolumn interface and number of cracks in the beam increases with increase of axial load.

ncreasing the column axial load level from 5 to 30% of the column axial capacity can lead to increase slightly the maximum loadcarrying capacity of the beamcolumn joints. However, increasing of column axial load level more than 30%, no significant change in the maximum loadcarrying capacity of the joints compared with no axial load.

Increasing of column axial load levels enhances the energy dissipation capacity of joints at initial stage of loading, however, slightly adverse effect when the loading stages further increases due to axial failure and local crushing of concrete.

Increasing the column axial load level resulted slightly improvement of the joint stiffness, this is due to the confining action provided by axial load against joint shear and the development of cross diagonal cracks was suppressed. However, when the axial load level of more than 50% of the column capacity, the cyclic stiffness decreased slightly due to the deterioration caused by local crushing of concrete in column. Thus, the results indicated that the increase of column axial load improves the confinement of the SFRC beamcolumn joints, however a threshold limit could be required.
REFERENCES

T. Paulay and M. J. N. Priestley, Seismic design of reinforced concrete and masonry buildings. New York: Wiley. 1992.

Y. Jiuru, T.; Chaobin, H.; Kaijian, Y.; and Yongcheng, Seismic Behavior and Shear Strength of Framed Joint Using Steel Fiber Reinforced Concrete,
Journal of Structural Engineering, vol. 118, no. 2, pp. 341358, 1992.

J. Gustavo, ParraMontesinos, S. Peterfreund, and S. Chao, Highly damagetolerant beamcolumn joints through use of highperformance fiber reinforced cement composites, ACI Structural Journal, vol. 102, no. 3, pp. 487495, 2005.

N. Ganesan, P. V. Indira, and M. V. Sabeena, Behaviour of hybrid fibre reinforced concrete beamcolumn joints under reverse cyclic loads, Materials and Design, vol. 54, pp. 686693, 2014.

D. Kheni, R. H. Scott, and A. Deb, S. K. and Dutta, Ductility enhancement in beamcolumn connections using hybrid fiberreinforced concrete, ACI Structural Journal, vol. 112, no. 2, pp. 167178, 2015.

R. . Chidambaram and P. Agarwal, Seismic behavior of hybrid fiber reinforced cementitious composite beamcolumn joints, Materials and Design, vol. 86, pp. 771781, 2015.

M. hossein Saghafi and H. Shariatmadar, Enhancement of seismic performance of beamcolumn joint connections using high performance fiber reinforced cementitious composites, Construction and Building Materials, vol. 180, pp. 665680, 2018.

S. H. Cho and Y. I. Kim, Effects of Steel Fibers on Short Beams Loaded in Shear, ACI Structural Journal, vol. 100, no. 6, pp. 765774, 2003.

and J. K. W. Hai H. Dinh, Gustavo J. ParraMontesinos and Results, Shear Behavior of Steel FiberReinforced Concrete Beams without Stirrup Reinforcement, vol. 107, no. 5, pp. 597606, 2010.

P. S. Song and S. Hwang, Mechanical properties of highstrength steel fiberreinforced concrete, Construction and Building Materials, vol. 18, no. 9, pp. 669673, 2004.

A. Thomas, J; Ramaswamy, Mechanical properties of steel fibre concrete, Journal of Materials in Civil Engineering, vol. 19, no. 5, pp. 385392, 2007.

C. S. Choi andBae.II, Effectiveness of steel fibers as hoops in exterior beamtocolumn joints under cyclic loading, ACI Structural Journal, vol. 116, no. 2, pp. 205219, 2019.

P. H. Bischoff, Tension Stiffening and Cracking of Steel FiberReinforced Concrete, Journal of Materials in Civil Engineeringrials in Civil Engineering, vol. 15, no. 2, pp. 174182, 2003.

ACI Committee 318, Building Code Requirements for Structural Concrete and Commentary (ACI 318R08). 2008.

G. ParraMontesinos, Shear strength of beams with deformed steel fibers, Concrete InternationalDetroit, vol. 28, no. 11, p. 57, 2006.

Li and Leong, Experimental and Numerical Investigations of the Seismic Behavior of HighStrength Concrete BeamColumn Joints with Column Axial Load, ASCE Journal of Structural Engineering, 2014.

Simulia, ABAQUS Version 2018: Dassault Systemes Simulia Corp: 2017. 2017.

M. A. Najafgholipour, S. M. Dehghan, A. Dooshabi, and A. Niroomandi, Finite element analysis of reinforced concrete beamcolumn connections with governing joint shear failure mode, Latin American Journal of Solids and Structures, vol. 14, no. 7, pp. 12001225, 2017.

H. Behnam, J. S. Kuang, and B. Samali, Parametric finite element analysis of RC wide beamcolumn connections, Computers and Structures, vol. 205, pp. 2844, 2018.

ACI Committee 374, Guide for testing reinforced concrete structural elements under slowly applied simulated seismic loads (ACI 374.2R13), American Concrete Institute, Farmington Hills, MI, p. 22, 2013.

J. G. Stoner, Finite element modelling of GFRP reinforced concrete beams, 2015.

J. Lubliner, J. Oliver, S. Oller, and E. Onate, A PlasticDamage Model for Concrete, International Journal of Solids and Structures, vol. 25, no. 3, pp. 299326, 1989.

J. Lee and G. L. Fenves, Plasticdamage model for cyclic loading of concrete structures, Journal of Engineering Mechanics, vol. 124, no. 8, pp. 892 900, 1998.

ACI Committee 544, Design Considerations for Steel Fiber Reinforced Concrete, ACI Structural Journal, vol. 88, no. Reapproved 1999, 1988.

ACI Committee 544.1R96, StateoftheArt Report on Fiber Reinforced Concrete. (ACI 544.1R96), C American Concrete Institute, Farmington Hills, MI, p. 66, 2002.

P. Ezeldin, A. and Balagurur, Normal and h i g h – s t r e n g t h fiberreinforced concrete under compression, Journal of Materials in Civil Engineering, vol. 4, no. 4, pp. 415429, 1992.

J. Barros, J.A.O. & Figueiras, Flexural Behaviour of Steel Fibre Reinforced Concrete: Testing and Modelling, Journal of Materials in Civil Engineering, vol. 11, no. 4, pp. 331339, 1999.

M. C. Nataraja, N. Dhang, and A. P. Gupta, StressStrain Curve for Steel Fibre Reinforced Concrete Under Compression, Cement and Concrete Composites, vol. 21, pp. 383390, 1999.

D. Dupont, Modelling and experimental validation of the constitutive law and cracking behavior of Steel Fiber Reinforced Concrete, Heverlee, Catholic Universty of Leuven, 2003.

R. N. Bencardino, F., Rizzuti, L., Spadea, G. & Swamy, Stressstrain behavior of steel fiberreinforced concrete in compression, ASCE Journal of Materials in Civil Engineering, vol. 20, no. 3, pp. 255263, 2008.

J. H. Lee, S.C., Oh and J. Y. Cho, Compressive behavior of fiberreinforced concrete with endhooked steel fibers, Materials, vol. 8, no. 4, pp. 1442 1458, 2015.

ACI Committee 544.3R93, Guide for Specifying, Proportioning, Mixing, Placing, and Finishing Steel Fiber Reinforced Concrete (ACI 544.3R93),
American Concrete Institute, Farmington Hills, MI, p. 10, 1998.

K. C. Ou, Y.C., Tsai, M.S., Liu, K.Y., Chang, Compressive Behavior of SteelFiberReinforced Concrete with a High Reinforcing Index, Journal of Materials in Civil Engineering, vol. 24, no. 2, pp. 207215, 2012.

C. Wang, Eperimental Investigation of the Behavior of Steel Fiber Reinforced Concrete, Masters Thesis, University of Canterbury, p.165, 2006.

J. A. O. Barros and J. A. Figueiras, Model for the analysis of steel fibre reinforced concrete slabs on grade, Computers and Structures, vol. 79, no. 1, pp. 97106, 2001.

T. S. Lok and J. R. Xiao, Flexural strength assessment of steel berreinforced concrete, Journal of Materials in Civil Engineering, vol. 11, no. 3, pp. 188196, 1999.

H. Tlemat, K. Pilakoutas, and K. Neocleous, Modeling of SFRC using inverse finite element analysis, RILEM Materials and Structures, vol. 39, no. 286, pp. 221233, 2006.

S. M. B. S. Mohsin, Behaviour Of FibreReinforced Concrete Structures Under Seismic Loading, Department of Civil and Environmental Engineering Imperial College London, no. April, p. 340, 2012.

A. Abbas and S. S. Mohsin, Numerical modeling of fibrereinforced concrete, no. 2006, pp. 20052006, 2010.

T. S. Lok and J. R. Xiao, Tensile behaviour and momentcurvature relationship of steel fibre reinforced concrete, Magazine of Concrete Research, vol. 50, no. 4, pp. 359368, 1998.

D. J. Hannant, Fibre cements and fibre concretes. 1979.

P. Soroushian and C. D. Lee, Distribution and orientation of fibers in steel fiber reinforced concrete, ACI Materials Journal, vol. 87, no. 5, pp. 433 439, 1990.

A. Amin and R. I. Gilbert, Steel fiberreinforced concrete beamsPart I: Material characterization and inservice behavior, ACI Structural Journal, vol. 116, no. 2, pp. 101111, 2019.

T. Y. Lim, P. Paramasivam, and S. L. Lee, Analytical Model for Tensile Behavior of SteelFiber Concrete, ACI Materials Journal, vol. 84, no. 4, pp. 286298, 1987.

B. W. Xu and H. S. Shi, Correlations among mechanical properties of steel fiber reinforced concrete, Construction and Building Materials, vol. 23, pp. 34683474, 2009.

Eurocode 2, Design of Concrete Structures: Part 11: General Rules and Rules for Buildings, CEN, London, 2004.

T. Kachlakev, D. I.; Miller, Finite Element Modeling of Reinforced Concrete Structures Strengthened with FRP Laminates, Oregon Department of Transportation Research Group & Federal Highway Administration, Washington, DC, USA, p. 113, 2001.

A. M. A. Ibrahim, M. F. M. Fahmy, and Z. Wu, 3D finite element modeling of bondcontrolled behavior of steel and basalt FRPreinforced concrete square bridge columns under lateral loading, Composite Structures, vol. 143, pp. 3352, 2016.

A. Raza, Q. U. Z. Khan, and A. Ahmad, Numerical investigation of loadcarrying capacity of GFRPreinforced rectangular concrete members using CDP model in abaqus, Advances in Civil Engineering, vol. 2019, 2019.

ACIASCE Committee 352, Recommendations for Design of BeamColumn Joints in Monolithic Reinforced Concrete Structures (ACI 352R02),
American Concrete Institute, Farmington Hills, MI, p. 37, 2002.

ACI Committee 318, Building Code Requirements for Structural Concrete (ACI 31814) and Commentary (ACI 318R14), American Concrete Institute, Farmington Hills, MI, p. 520, 2014.

A. R. Mohamed, M. S. Shoukry, and J. M. Saeed, Prediction of the behavior of reinforced concrete deep beams with web openings using the finite ele ment method, Alexandria Engineering Journal, vol. 53, no. 2, pp. 329339, 2014.

V. G. Haach, A. LÃºcia Homce De Cresce El Debs, and M. Khalil El Debs, Evaluation of the influence of the column axial load on the behavior of monotonically loaded R/C exterior beamcolumn joints through numerical simulations, Engineering Structures, vol. 30, no. 4, pp. 965975, 2008.