- Email: [email protected]
Cyclic Shear Resistance of Expanded Beam-Column Joint
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
Procedia Engineering
Procedia Engineering 14 00(2011) (2011)1292–1299 000–000 www.elsevier.com/locate/procedia
The Twelfth East Asia-Pacific Conference on Structural Engineering and Construction
Cyclic Shear Resistance of Expanded Beam-Column Joint A. PIMANMASa* and P. CHAIMAHAWANb a
Sirindhorn International Institute of Technology, Thammasat University, Thailand b Department of Civil Engineering, Naresuan University Phayao, Thailand
Abstract This paper presents the strengthening technique for interior reinforced concrete beam-column joint based on joint expansion concept. The beam-column joint is expanded two-dimensionally by cast-in-situ concrete around the corners of the joint. Experiment has been conducted on interior beam-column specimens with expanded joint zone. Experimental results demonstrate a good performance of this method to upgrade the joint shear strength, energy dissipation and ductility. It has been found that by increasing the joint size, the joint shear failure can be prevented. The joint expansion is shown to reduce the joint shear stress. This method is effective to change failure mode from brittle joint shear failure to flexural failure in beams.
© 2011 Published by Elsevier Ltd. Keywords: Beam-column joint, joint shear failure, joint expansion, joint shear strength.
1. Introduction Several earthquakes have demonstrated many collapses of buildings due to the brittle failure of substandard beam-column joint. Beam-column joint is one of the most important structural components in the lateral load path of the structure. The failure of beam-column joint can be very disastrous because it can destroy column that is necessary to support gravity loads. The need to retrofit existing beam-column joints to resist earthquake excitation is therefore a critical consideration. Several methods for retrofitting beam-column joints have been proposed in the past. Concrete jacketing is one of the common techniques (Alcocer and Jirsa 1993). Other researchers (Ghabarah et al.
*
Corresponding author: Email: [email protected]
1877–7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.07.162
2
A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
1996) have attempted to strengthen beam-column joints by steel plates, angles and rods. However, there is a problem of corrosion and the need to fireproof the added steel elements. The use of fiber-reinforced polymer (FRP) materials has been investigated by several researchers (Gergely et al. 2000, Prota et al. 2004). The authors presented the joint strengthening method using joint enlargement technique (Pimanmas and Chaimahawan 2010). The existing joint in the frame is enlarged by cast in-situ concrete to increase the joint size. The enlargement is performed two-dimensionally so that it can be hidden in partitions in either transverse or longitudinal directions or both (Figure 1). This method is cost-effective because it uses conventional materials such as concrete and steel bars. In this paper, the study of shear resistance mechanism in the expanded joint is presented to identify the load resistance mechanism which can be used for design purpose.
Figure 1: Beam-column joint strengthened by planar joint expansion.
2. Experimental program 2.1. Specimens and material properties The experimental program consisted of five interior beam-column specimens, namely, J0, PJE1, PJE2, PJE3 and PJE4. Specimen J0 was an un-strengthened control specimen. Specimen PJE1 was another control specimen that was perfectly strengthened by joint expansion method. Specimens PJE2, PJE3 and PJE4 were strengthened by planar joint expansion with different sizes and shapes. All specimens had cruciform shape with beams and columns extending from joint faces to mid-length and mid-height, respectively. The control specimen has been designed according to the ACI318 building code (ACI318 2005) without considering earthquake force to represent a typical beam-column connection in low seismic areas. Figure 2 shows size and reinforcement details of the control specimen. The dimension of column and beam sections was 200 mm × 350 mm and 175 mm × 300 mm, respectively. Main and transverse
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reinforcements were 12 mm diameter deformed bars and 3-mm diameter plain mild steel, respectively. Table 1 shows the average tested yield and tensile strengths of reinforcing bars. The tested compressive strength of concrete measured from standard cylindrical specimens is given in Table 2. It should be noted that the control specimen had a substandard reinforcement detail typical of construction in low seismic zones. Particularly, it had a relatively low amount of stirrups in the end of members and contained no transverse reinforcements in the joint panel. The column longitudinal bars were lap spliced just above the floor with 300 mm lap length. 350 mm Steel Plate 20 mm thickness 175 mm
B-B
900 mm
C-C
4-DB12 3-Ø3@100mm 6-DB12
C
300 mm
B
300 mm
18-DB12 3-Ø3@100mm A-A
A
300 mm
A
175 mm
6-DB12 3-Ø3 @100mm 4-DB12
350 mm
740 mm
200 mm
260 mm
2-PC strands Ø 15.2mm 350 mm
6-Ø 5 @50mm
450 mm B 65 mm
530 mm
1045 mm 6-Ø 5 @50mm
40 mm
3-Ø 5 @50mm
4-Bolts? 20 1,500 mm
200 mm
C
1,500 mm
200 mm
3,400 mm
Figure 2: Dimension and reinforcing detail of control specimen J0
120 mm 300mm
DB12
100 mm
300 mm DB12
300 mm
300mm
120 mm
120mm Non-shrink grout RC planar joint enlargement
40 mm 100 mm RC planar joint enlargement (a) PJE1
(b) PJE2
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150 mm 300 mm
100 mm
DB12
200 mm
170 mm 150mm
150 mm
170mm
150mm
50mm 120mm
50mm
100 mm
50mm 120mm
40mm Non-shrink grout RC planar joint enlargement
40mm Non-shrink grout RC joint enlargement
DB12
(c) PJE3
(d) PJE4
Figure 3: Dimensions and reinforcing details of strengthened specimens. Table 1: Properties of reinforcing bar Type of reinforcing bar
Yield strength, MPa
Ultimate strength, MPa
Remark
DB12 SD40
480
614
DB12 SD40
552
706
for specimens J0, PJE1, PJE2 and PJE4 for specimen PJE3
wire -3mm
299
373
for all specimens
Table 2: Properties of concrete Compressive strength fc′, MPa Top column Beam/ joint Bottom column Planar enlargement
Control specimen J0 24.2 27.3 23.7 -
Specimen PJE1
Specimen PJE2
Specimen PJE3
Specimen PJE4
29.3 28.3 28.5 27.8
28.7 28.7 30.7 27.2
27.5 28.2 23.7 24.9
25.1 24.9 27.5 24.8
The size and reinforcement details of strengthened specimens were identical to the control specimen except the expanded area. The joint details of specimens PJE1-PJE4 are shown in Figure 3. For all strengthened specimens, the width of the enlarged area was 175 mm, which was equal to the beam’s width. The expanded area was 300 mm x 300 mm square in specimen PJE1; 300 mm x 300 mm triangle in specimen PJE2; 200 mm (vertical) x 300 mm (horizontal) triangle in specimen PJE3; and 150 mm x 150 mm triangle in specimen PJE4. In each enlargement, 8-DB12 (12 mm diameter deformed bars) were placed as dowel bars to attach the new concrete to beam and column. 2.2 Test set-up, load history and instrumentation The test set-up and boundary conditions are shown in Figure 4. Specimens were pushed laterally at top of the column by a 500 kN hydraulic actuator which was reacted against a strong reaction frame. The ends of beams were supported by rollers that allowed free horizontal movement to simulate the lateral drift. The bottom end of the column was supported by a hinge which allowed no movement in any direction. To
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A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
represent existing gravity load on the column, the axial load of 16.5% column axial capacity was applied to the column by means of vertical prestressing bars. The column was pushed and pulled with incremental drift ratios of ± 0.25%, ± 0.50%, ± 0.75%… as shown in Figure 5. The loop was repeated twice at each drift ratio. The measurements consisted of 1) horizontal force and displacement at the top of column; 2) flexural rotations in beam and column; 3) shear deformation in beam, column and joint panel; 4) rocking angle at the interface between joint face and beam; and 5) strains in longitudinal steels and stirrups in beam and column. 350 mm
Support Frame
Reference Frame
φ15.2mm PC strands
500 kN MTS Actuator
Reaction Frame
Dial gage
2080 mm
740 mm 300 mm 545 mm Steel Support
15 mm
Bottom Pin Support
300mm 1500 mm
1500 mm 3000 mm
Figure 4: Test set-up
6
Drift ratio(%)
4 2 0 -2 -4 -6 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 Loop numbers
Figure 5: Displacement history
5
6
A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
3. Test results and Discussions 3.1. Behavior of specimens and failure types The photos of specimens after the test are shown in Figure 6. The control specimen J0 failed by concrete crushing in the joint panel (Figure 6a). On the contrary, another control specimen (PJE1) failed by flexural yielding in beam with moderately ductile performance (Figure 6b). The behavior and failure of specimens PJE2 and PJE3 were generally close to those of specimen PJE1, except the early occurrences of horizontal cracks at the construction joints in PJE2 and PJE3 (Figure 6c). However these cracks did no actively widen in subsequent cycles because of dowel bars that attached the expanded zones to the main specimen. As for PJE4, the failure was crushing in the joint panel as well as in the expanded area (Figure 6e). Substantial damage occurred in the joint panel and the expanded zone.
(a) control specimen J0
(b) specimen PJE1
(d) specimen PJE3
(e) specimen PJE4
(c) specimen PJE2
Figure 6: photo of specimens after test
3.2. Hysteretic load-displacement relations and energy dissipation The hysteretic load-displacement relations of all specimens are shown in Figure 7. The column shear force versus drift ratio of the control specimen (Figure 7a) demonstrated elastic behavior when drift ratio was less than 0.50%. The hysteresis loops were pinched indicating low energy dissipation. The specimen reached the maximum load of 72 kN at 1.75% drift. The ductility ratio, defined as the ratio of the maximum deformation (failure point) to the elastic limit deformation (yield point) was calculated to be 2.0.
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The column shear force versus drift ratio of specimens PJE1-PJE3 is shown in Figure 7b to 7d. During 0.0% to 0.5% drift, the behavior was elastic with the initial stiffness approximately 10% higher than that of the control specimen. The maximum load was 98.3 kN, 97.6 kN, 112 kN for specimens PJE1 to PJE3, respectively. The corresponding drift at peak load occurred at around 1.5% drift. The maximum load of specimen PJE3 was higher than other specimens because steel bars used in this specimen had higher yield and tensile strengths. The hysteresis loops were obviously wider than that of the control specimen. The ductility ratio was calculated to be 3.0, 2.5 and 3.0 for specimens PJE1 to PJE3, respectively. As for specimen PJE4, the hysteretic loops were not as wide as those of specimens PJE1-PJE3. The maximum load was 94.7 kN at 2.5% drift. The ductility ratio was calculated to be 2.5. All strengthened specimens except PJE4 failed by beam flexural failure with moderate ductility. The failure points of specimens PJE1, PJE2, PJE3 and PJE4 are 3.0%, 2.5%, 3.0% and 2.5%, respectively. The slightly inferior performance of specimen PJE2 may be due to the triangular enlargement that provided lower confinement compared with square enlargement in specimen PJE1 and thickened ended enlargement in specimen PJE3. The square enlargement and the thickened part may also be beneficial to suppress buckling and fracture of beam bars and prevent spalling of concrete cover.
80
80
Column shear force (kN)
120
Column shear force (kN)
120
40 0
40 0
-40
-40 Yield strength Maximum strength Failure point Yield of beam bar
-80
-120
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%)
Yield strength Maximum strength Failure point Yield of beam bar
-80
3.0
-4.5
4.5
-3.0
(a) Control specimen J0
3.0
4.5
(b) Specimen PJE1
120
80
80
40 0
Column shear force (kN)
120
Column shear force (kN)
-1.5 0.0 1.5 Drift ratio(%)
40 0
-40
-40 Yield strength Maximum strength Failure point Yield of beam bar
-80
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%) (c) Specimen PJE2
3.0
4.5
Yield strength Maximum strength Failure point Yield of beam bar
-80
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%) (d) Specimen PJE3
3.0
4.5
A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
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Column shear force (kN)
120 80 40 0
-40 Yield strength Maximum strength Failure point Yield of beam bar
-80
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%)
3.0
4.5
(e) Specimen PJE4 Figure 7: Column shear force versus drift ratio
4. Conclusions This paper presents a joint strengthening method by expanding the joint area. Experiment is conducted to investigate the shear strength of specimens with expanded joint. Control specimen is a substandard beamcolumn joint with non-ductile reinforcement details. With sufficiently large size, the joint enlargement is effective to reduce shear stress transmitted in the joint panel. The energy dissipation can also significantly increase as evidenced by wider hysteresis loops. The failure mode is changed from brittle joint shear failure to moderately ductile flexural failure in beams. The plastic hinge is moved from column face to the edge of enlargement. On the other hand, if the size of enlarged area is small, the failure mode is concrete crushing in the joint panel and expanded areas. 5. Acknowledgments The authors are very grateful to Thailand Research Fund (TRF) for providing the research fund RMU4880022 to carry out the research, and to Asian Institute of Technology (AIT) for providing test facilities. References [1] [2] [3] [4] [5] [6]
Alcocer SM, Jirsa JO. Strength of Reinforced Concrete Frame Connections Rehabilitate by Jacketing. ACI Struct J, 1993, 90(3), pp. 249-261. Ghobarah A, Aziz TS, and Biddah A. Seismic rehabilitation of reinforced concrete beam-column connections, Earthquake Spectra, 1996, 12(4), pp. 761-780. Gergely I, Pentelides CP, and Reavely LD. Shear Strengthening of RCT-Joints Using CFRP Composites, J of Composites for Constr, 2000, 4(2), pp. 56-64. Prota A, Nanni A, Manfredi G, and Cosenza E. Selective upgrade of underdesigned reinforced concrete beam-column joints using carbon fiber reinforced polymers, ACI Struct J, 2004, 101(5), pp. 699-707. Pimanmas A and Chaimahawan P. Shear strength of beam-column joint with enlarged joint area, Engineering Structures, 2010, 32(9), pp. 2529-2545. ACI Committee 318. Building code requirements for reinforced concrete and commentary (ACI318-05/318R-05). Detroit: American Concrete Institute; 2005.
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Procedia Engineering
Procedia Engineering 14 00(2011) (2011)1292–1299 000–000 www.elsevier.com/locate/procedia
The Twelfth East Asia-Pacific Conference on Structural Engineering and Construction
Cyclic Shear Resistance of Expanded Beam-Column Joint A. PIMANMASa* and P. CHAIMAHAWANb a
Sirindhorn International Institute of Technology, Thammasat University, Thailand b Department of Civil Engineering, Naresuan University Phayao, Thailand
Abstract This paper presents the strengthening technique for interior reinforced concrete beam-column joint based on joint expansion concept. The beam-column joint is expanded two-dimensionally by cast-in-situ concrete around the corners of the joint. Experiment has been conducted on interior beam-column specimens with expanded joint zone. Experimental results demonstrate a good performance of this method to upgrade the joint shear strength, energy dissipation and ductility. It has been found that by increasing the joint size, the joint shear failure can be prevented. The joint expansion is shown to reduce the joint shear stress. This method is effective to change failure mode from brittle joint shear failure to flexural failure in beams.
© 2011 Published by Elsevier Ltd. Keywords: Beam-column joint, joint shear failure, joint expansion, joint shear strength.
1. Introduction Several earthquakes have demonstrated many collapses of buildings due to the brittle failure of substandard beam-column joint. Beam-column joint is one of the most important structural components in the lateral load path of the structure. The failure of beam-column joint can be very disastrous because it can destroy column that is necessary to support gravity loads. The need to retrofit existing beam-column joints to resist earthquake excitation is therefore a critical consideration. Several methods for retrofitting beam-column joints have been proposed in the past. Concrete jacketing is one of the common techniques (Alcocer and Jirsa 1993). Other researchers (Ghabarah et al.
*
Corresponding author: Email: [email protected]
1877–7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.07.162
2
A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
1996) have attempted to strengthen beam-column joints by steel plates, angles and rods. However, there is a problem of corrosion and the need to fireproof the added steel elements. The use of fiber-reinforced polymer (FRP) materials has been investigated by several researchers (Gergely et al. 2000, Prota et al. 2004). The authors presented the joint strengthening method using joint enlargement technique (Pimanmas and Chaimahawan 2010). The existing joint in the frame is enlarged by cast in-situ concrete to increase the joint size. The enlargement is performed two-dimensionally so that it can be hidden in partitions in either transverse or longitudinal directions or both (Figure 1). This method is cost-effective because it uses conventional materials such as concrete and steel bars. In this paper, the study of shear resistance mechanism in the expanded joint is presented to identify the load resistance mechanism which can be used for design purpose.
Figure 1: Beam-column joint strengthened by planar joint expansion.
2. Experimental program 2.1. Specimens and material properties The experimental program consisted of five interior beam-column specimens, namely, J0, PJE1, PJE2, PJE3 and PJE4. Specimen J0 was an un-strengthened control specimen. Specimen PJE1 was another control specimen that was perfectly strengthened by joint expansion method. Specimens PJE2, PJE3 and PJE4 were strengthened by planar joint expansion with different sizes and shapes. All specimens had cruciform shape with beams and columns extending from joint faces to mid-length and mid-height, respectively. The control specimen has been designed according to the ACI318 building code (ACI318 2005) without considering earthquake force to represent a typical beam-column connection in low seismic areas. Figure 2 shows size and reinforcement details of the control specimen. The dimension of column and beam sections was 200 mm × 350 mm and 175 mm × 300 mm, respectively. Main and transverse
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reinforcements were 12 mm diameter deformed bars and 3-mm diameter plain mild steel, respectively. Table 1 shows the average tested yield and tensile strengths of reinforcing bars. The tested compressive strength of concrete measured from standard cylindrical specimens is given in Table 2. It should be noted that the control specimen had a substandard reinforcement detail typical of construction in low seismic zones. Particularly, it had a relatively low amount of stirrups in the end of members and contained no transverse reinforcements in the joint panel. The column longitudinal bars were lap spliced just above the floor with 300 mm lap length. 350 mm Steel Plate 20 mm thickness 175 mm
B-B
900 mm
C-C
4-DB12 3-Ø3@100mm 6-DB12
C
300 mm
B
300 mm
18-DB12 3-Ø3@100mm A-A
A
300 mm
A
175 mm
6-DB12 3-Ø3 @100mm 4-DB12
350 mm
740 mm
200 mm
260 mm
2-PC strands Ø 15.2mm 350 mm
6-Ø 5 @50mm
450 mm B 65 mm
530 mm
1045 mm 6-Ø 5 @50mm
40 mm
3-Ø 5 @50mm
4-Bolts? 20 1,500 mm
200 mm
C
1,500 mm
200 mm
3,400 mm
Figure 2: Dimension and reinforcing detail of control specimen J0
120 mm 300mm
DB12
100 mm
300 mm DB12
300 mm
300mm
120 mm
120mm Non-shrink grout RC planar joint enlargement
40 mm 100 mm RC planar joint enlargement (a) PJE1
(b) PJE2
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150 mm 300 mm
100 mm
DB12
200 mm
170 mm 150mm
150 mm
170mm
150mm
50mm 120mm
50mm
100 mm
50mm 120mm
40mm Non-shrink grout RC planar joint enlargement
40mm Non-shrink grout RC joint enlargement
DB12
(c) PJE3
(d) PJE4
Figure 3: Dimensions and reinforcing details of strengthened specimens. Table 1: Properties of reinforcing bar Type of reinforcing bar
Yield strength, MPa
Ultimate strength, MPa
Remark
DB12 SD40
480
614
DB12 SD40
552
706
for specimens J0, PJE1, PJE2 and PJE4 for specimen PJE3
wire -3mm
299
373
for all specimens
Table 2: Properties of concrete Compressive strength fc′, MPa Top column Beam/ joint Bottom column Planar enlargement
Control specimen J0 24.2 27.3 23.7 -
Specimen PJE1
Specimen PJE2
Specimen PJE3
Specimen PJE4
29.3 28.3 28.5 27.8
28.7 28.7 30.7 27.2
27.5 28.2 23.7 24.9
25.1 24.9 27.5 24.8
The size and reinforcement details of strengthened specimens were identical to the control specimen except the expanded area. The joint details of specimens PJE1-PJE4 are shown in Figure 3. For all strengthened specimens, the width of the enlarged area was 175 mm, which was equal to the beam’s width. The expanded area was 300 mm x 300 mm square in specimen PJE1; 300 mm x 300 mm triangle in specimen PJE2; 200 mm (vertical) x 300 mm (horizontal) triangle in specimen PJE3; and 150 mm x 150 mm triangle in specimen PJE4. In each enlargement, 8-DB12 (12 mm diameter deformed bars) were placed as dowel bars to attach the new concrete to beam and column. 2.2 Test set-up, load history and instrumentation The test set-up and boundary conditions are shown in Figure 4. Specimens were pushed laterally at top of the column by a 500 kN hydraulic actuator which was reacted against a strong reaction frame. The ends of beams were supported by rollers that allowed free horizontal movement to simulate the lateral drift. The bottom end of the column was supported by a hinge which allowed no movement in any direction. To
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A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
represent existing gravity load on the column, the axial load of 16.5% column axial capacity was applied to the column by means of vertical prestressing bars. The column was pushed and pulled with incremental drift ratios of ± 0.25%, ± 0.50%, ± 0.75%… as shown in Figure 5. The loop was repeated twice at each drift ratio. The measurements consisted of 1) horizontal force and displacement at the top of column; 2) flexural rotations in beam and column; 3) shear deformation in beam, column and joint panel; 4) rocking angle at the interface between joint face and beam; and 5) strains in longitudinal steels and stirrups in beam and column. 350 mm
Support Frame
Reference Frame
φ15.2mm PC strands
500 kN MTS Actuator
Reaction Frame
Dial gage
2080 mm
740 mm 300 mm 545 mm Steel Support
15 mm
Bottom Pin Support
300mm 1500 mm
1500 mm 3000 mm
Figure 4: Test set-up
6
Drift ratio(%)
4 2 0 -2 -4 -6 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 Loop numbers
Figure 5: Displacement history
5
6
A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
3. Test results and Discussions 3.1. Behavior of specimens and failure types The photos of specimens after the test are shown in Figure 6. The control specimen J0 failed by concrete crushing in the joint panel (Figure 6a). On the contrary, another control specimen (PJE1) failed by flexural yielding in beam with moderately ductile performance (Figure 6b). The behavior and failure of specimens PJE2 and PJE3 were generally close to those of specimen PJE1, except the early occurrences of horizontal cracks at the construction joints in PJE2 and PJE3 (Figure 6c). However these cracks did no actively widen in subsequent cycles because of dowel bars that attached the expanded zones to the main specimen. As for PJE4, the failure was crushing in the joint panel as well as in the expanded area (Figure 6e). Substantial damage occurred in the joint panel and the expanded zone.
(a) control specimen J0
(b) specimen PJE1
(d) specimen PJE3
(e) specimen PJE4
(c) specimen PJE2
Figure 6: photo of specimens after test
3.2. Hysteretic load-displacement relations and energy dissipation The hysteretic load-displacement relations of all specimens are shown in Figure 7. The column shear force versus drift ratio of the control specimen (Figure 7a) demonstrated elastic behavior when drift ratio was less than 0.50%. The hysteresis loops were pinched indicating low energy dissipation. The specimen reached the maximum load of 72 kN at 1.75% drift. The ductility ratio, defined as the ratio of the maximum deformation (failure point) to the elastic limit deformation (yield point) was calculated to be 2.0.
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The column shear force versus drift ratio of specimens PJE1-PJE3 is shown in Figure 7b to 7d. During 0.0% to 0.5% drift, the behavior was elastic with the initial stiffness approximately 10% higher than that of the control specimen. The maximum load was 98.3 kN, 97.6 kN, 112 kN for specimens PJE1 to PJE3, respectively. The corresponding drift at peak load occurred at around 1.5% drift. The maximum load of specimen PJE3 was higher than other specimens because steel bars used in this specimen had higher yield and tensile strengths. The hysteresis loops were obviously wider than that of the control specimen. The ductility ratio was calculated to be 3.0, 2.5 and 3.0 for specimens PJE1 to PJE3, respectively. As for specimen PJE4, the hysteretic loops were not as wide as those of specimens PJE1-PJE3. The maximum load was 94.7 kN at 2.5% drift. The ductility ratio was calculated to be 2.5. All strengthened specimens except PJE4 failed by beam flexural failure with moderate ductility. The failure points of specimens PJE1, PJE2, PJE3 and PJE4 are 3.0%, 2.5%, 3.0% and 2.5%, respectively. The slightly inferior performance of specimen PJE2 may be due to the triangular enlargement that provided lower confinement compared with square enlargement in specimen PJE1 and thickened ended enlargement in specimen PJE3. The square enlargement and the thickened part may also be beneficial to suppress buckling and fracture of beam bars and prevent spalling of concrete cover.
80
80
Column shear force (kN)
120
Column shear force (kN)
120
40 0
40 0
-40
-40 Yield strength Maximum strength Failure point Yield of beam bar
-80
-120
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%)
Yield strength Maximum strength Failure point Yield of beam bar
-80
3.0
-4.5
4.5
-3.0
(a) Control specimen J0
3.0
4.5
(b) Specimen PJE1
120
80
80
40 0
Column shear force (kN)
120
Column shear force (kN)
-1.5 0.0 1.5 Drift ratio(%)
40 0
-40
-40 Yield strength Maximum strength Failure point Yield of beam bar
-80
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%) (c) Specimen PJE2
3.0
4.5
Yield strength Maximum strength Failure point Yield of beam bar
-80
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%) (d) Specimen PJE3
3.0
4.5
A. PIMANMAS and P. CHAIMAHAWAN / Procedia Engineering 14 (2011) 1292–1299 Author name / Procedia Engineering 00 (2011) 000–000
8
Column shear force (kN)
120 80 40 0
-40 Yield strength Maximum strength Failure point Yield of beam bar
-80
-120 -4.5
-3.0
-1.5 0.0 1.5 Drift ratio(%)
3.0
4.5
(e) Specimen PJE4 Figure 7: Column shear force versus drift ratio
4. Conclusions This paper presents a joint strengthening method by expanding the joint area. Experiment is conducted to investigate the shear strength of specimens with expanded joint. Control specimen is a substandard beamcolumn joint with non-ductile reinforcement details. With sufficiently large size, the joint enlargement is effective to reduce shear stress transmitted in the joint panel. The energy dissipation can also significantly increase as evidenced by wider hysteresis loops. The failure mode is changed from brittle joint shear failure to moderately ductile flexural failure in beams. The plastic hinge is moved from column face to the edge of enlargement. On the other hand, if the size of enlarged area is small, the failure mode is concrete crushing in the joint panel and expanded areas. 5. Acknowledgments The authors are very grateful to Thailand Research Fund (TRF) for providing the research fund RMU4880022 to carry out the research, and to Asian Institute of Technology (AIT) for providing test facilities. References [1] [2] [3] [4] [5] [6]
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