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Experimental testing on CFRP strengthened thin steel plates under shear loading
Thin–Walled Structures 109 (2016) 217–226
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Experimental testing on CFRP strengthened thin steel plates under shear loading ⁎
M. Khazaei Poula, , F. Nateghi-Alahib, X.L. Zhaoc a b c
Department of Civil, Architectural and Environmental Engineering, Drexel University, Philadelphia, PA 19104, USA Structural Engineering Research Center, International Institute of Earthquake Engineering and Seismology, Tehran, Iran Department of Civil Engineering, Monash University, Clayton, Vic. 3800, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Thin steel plate CFRP Shear Cyclic loading Energy absorption Stiffness
This paper presents an experimental study on thin steel plates strengthened by CFRP layers under shear loading. A series of laboratory tests were conducted on scaled one-story steel shear panel models with hinge type connections under cyclic quasi-static loading. One of the specimens is un-stiffened and selected as a reference sample of three others specimens. Steel infill plates are strengthened by CFRP layers with different numbers of layers and orientations. The results indicate that the strengthening of thin steel plate by CFRP layers increases ultimate strength, secant stiffness and energy absorption. Comparisons are made between the performance of CFRP and GFRP strengthened steel plates under cyclic shear loads. It was shown that both materials were effective at increasing stiffness and strength of the system. The increased ratio is similar for both FRPs when two layers are applied. However, adding additional two layers of CFRP does not provide extra benefit in terms of stiffness and energy absorption although there is still benefit for ultimate strength.
1. Introduction Application of fiber reinforced polymer (FRP) is a method for strengthening and repairing existing structures as well as for designing new structural systems. FRP is particularly well suited for strengthening due to its excellent mechanical properties, such as high strength, high stiffness, light weight, and flexibility to be formed into any shape. In recent years FRP has been successfully used for upgrading and retrofitting of different structural elements subjected to bending loading [5], compression loading e.g. [2,4,6], bearing force e.g. [8,9], and torsion loading [10], shear loading [11]. It has been also proved that FRP is efficient in repair or strengthening of reinforced concrete structures [12], particularly those structures that are vulnerable to extreme shear and torsional loading during strong earthquakes [13,14]. A brief summary is provided below. Many studies have been conducted on the strengthening of steel members under bending loading. FRP has been successfully applied on “I” section steel [5,7,11,16,17] beams, aluminum beams [17], and circular hollow section (CHS) beams [7,18] to improve bending capacity. The results showed that by strengthening the tension flange of metallic beam both the stiffness and ultimate load capacity of system were considerably increased. Applying CFRP plates on the web of steel beams, at the load points or support of beams, can also increase the
⁎
web buckling capacity [15]. The possible failure modes for the FRPstrengthened steel I-beams include failure of beam due to lateral buckling [20], debonding [21–23], and rupture of the laminate [21]. Recent studies on steel members with externally bonded CFRP under compression loading [2,4,21,51] showed that this method is effective to increase both stiffness and total capacity of steel members. Experimental studies on the behavior of steel square hollow section (SHS) tubes strengthened by CFRP [24,51] showed that the application of CFRP is effective at increasing the axial load capacity. They showed that restraint provided by CFRP can postpone the elastic buckling deformation in the FRP-strengthened columns. CFRP laminate can also be used to enhance the torsional strength of steel members. Tests on square hollow steel section under torsional loading [10] showed that using CFRP laminate can improve the ultimate strength of steel members. The optimum orientation angle for strengthening against cyclical torque is a combination of spiral and reverse-spiral CFRP wrap [10]. CFRP has been successfully used to strengthen rectangular hollow sections under end-bearing forces [19,25]. Using CFRP can change the failure mode from web buckling to web yielding and can increase restraints against web rotation [25]. Tests results on web buckling of light-steel beam subjected to end-bearing forces showed that CFRP strengthening can increase the web-buckling capacity and it is more
Corresponding author: [email protected] (M.Khazaei Poul). E-mail address: [email protected] (M. Khazaei Poul).
http://dx.doi.org/10.1016/j.tws.2016.09.026 Received 12 July 2016; Received in revised form 16 September 2016; Accepted 30 September 2016 0263-8231/ © 2016 Elsevier Ltd. All rights reserved.
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Nomenclature CRef CL2O1 CL2O2 CL4O3 GRef GL2O1 GL2O2 GL4O3
Reference test specimen Steel plate strengthened Steel plate strengthened Steel plate strengthened Reference test specimen Steel plate strengthened Steel plate strengthened Steel plate strengthened
for CFRP strengthened specimens by two layers of CFRP [Fiber orientation: 0° and 90°] by two layers of CFRP [Fiber orientation: 45° and −45°] by four layers of CFRP [Fiber orientation: 45° and −45°] for GFRP strengthened specimens by two layers of GFRP [Fiber orientation: 0° and 90°] by two layers of GFRP [Fiber orientation: 45° and −45°] by four layers of GFRP [Fiber orientation: 45° and −45°]
Fig. 1. Typical test set-up [dimensions in mm].
dissipation increased about 45–130%, whereas the increase was about 10%. This paper is a follow up study of [1], but focuses on CFRP strengthening of thin steel plates under cyclic shear loading. A series of laboratory tests were conducted to evaluate the effect of number of CFRP layers and the fiber orientations, different patterns of strengthening. Comparisons are made between GFRP and CFRP strengthened specimens under shear loading in terms of shear strength, secant stiffness and dissipated energy.
effective for those with large web depth-to-thickness ratio [26]. Steel plate shear walls (SPSWs) has been widely used as a lateral load resisting system to transfer wind and earthquake loads. SPSWs consist of a thin steel plate and the boundary elements [1]. After buckling of thin steel plate, shear loads are transferred due to formation of diagonal tension field in the steel web, which is known as tension field action. Extensive studies have been performed by researchers [24–48] on the performance of SPSWs. Thin steel plate in the SPSWs can be strengthened by adding layers of FRP laminate on both sides [1]. Similarly, in the FRP-strengthened systems the shear loads are transferred through diagonal tension field in the composite steel plate. This mechanism has been reported in the infilled structures as well [49,50]. Experimental [1] and numerical [35,44] studies on the GFRP-strengthened steel plate showed that this method of strengthening is effective for increasing strength, stiffness and energy absorption of thin steel plate under shear loading. Only a few studies have been conducted on the FRP-strengthened thin steel members under shear loading [1,35,45,46]. In 2012 [1] the first two authors of this paper investigated GFRP-strengthened thin steel plates under cyclic shear load. In 2016 Petkune and Donchev [43] studied GFRP and CFRPstrengthened steel plates under similar type of loading. The increased ultimate load carrying capacity varied from 24% to 120% in [1], whereas the increase was about 14% in [46]. The cumulative energy
2. Experimental set-up and test specimens 2.1. Test specimens and test set-up In order to investigate the effect of CFRP laminate on the shear behavior of thin steel plates, a series of laboratory tests were conducted on the scaled one-story steel shear panel models with hinge type connections under cyclic quasi-static loading. The detail of test setup and shear panel are shown in Fig. 1-a. The steel and composite plates were connected to the frame by utilizing two rows of high tension bolts. Because of rotational constraints at the edges of plates, the connection of thin plates to the frame can be considered as clamped (see Fig. 1-b). 218
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Fig. 2. Un-stiffened and CFRP-strengthened steel plate.
Table 1 Properties of infill plate in the experimental specimens. Specimen label
CRef CL2O1 CL2O2 CL4O3
Number of layers in composite infill plate
Thickness of laminate and steel infill plate
Steel plate
CFRP layer
Steel plate
CFRP layer
1 1 1 1
0 2 2 4
1 mm 1 mm 1 mm 1 mm
N/A 0.381 mm 0.381 mm 0.381 mm
Total thickness of infill plate
Orientation of CFRP layers (θ)
CFRP Type
1 mm 1.762 mm 1.762 mm 2.542 mm
– 0 # 90 45 # −45 45 & −45 # 45 & −45
– SikaWrap® Hex 230C SikaWrap® Hex 230C SikaWrap® Hex 230C
Table 2 Cured Laminate Sikawrap-230 with Sikadur 330 Epoxy.
Fiber direction Orthogonal to fiber direction
Elastic Modulus (MPa)
Tensile Strength (MPa)
Failure strain (%)
65,402 5876
894 27
0.0136 0.0046
2.2. Strengthening of steel plates In this study, four sets of laboratory tests were conducted. One of the specimens was un-stiffened steel plate with thickness of 1 mm and selected as a reference model for comparison. In the three others specimens, the steel plates were strengthened by CFRP. Uni-directional CFRP laminate was selected and applied on the steel plates. Nominal thickness of a layer of CFRP laminate and epoxy was equal to 0.381 mm. Three types of strengthening methods were considered. The details of strengthened steel plates are shown in Fig. 2. In the CL2O1 specimen, thin steel plate was strengthened by one layer of
Fig. 3. Strain-stress curve for steel plate, boundary element, Cured Laminate Sikawrap230 with Sikadur 330 Epoxy.
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Thin–Walled Structures 109 (2016) 217–226
200
200
150
150
100
100
50
50
Force (kN)
Force
(kN)
M. Khazaei Poul et al.
0 -50
0 -50
-100
-100
-150
-150
-200 -20
-15
-10
-5
0
5
10
15
-200 -20
20
Dis splacement (mm m)
-15
-10
-5
0
5
10
15
20
Displa acement (mm)
Fig. 4. CRef specimen.
Fig. 7. CL4O3 specimen.
200
orientation of fibers. Thin steel plate in the CL4O3 specimen was strengthened by two layers of CFRP laminate on each side and the main direction of CFRP laminate was oriented along the tension fields, Fig. 2-d. Details of all experimental specimens are presented in Table 1.
150 100
Force (kN)
50
2.3. Material properties
0
Tensile coupon tests were conducted to obtain the material properties of the steel plate and the boundary elements (UNP100). The strainstress curves for the steel materials are presented in Fig. 3. Unidirectional CFRP laminate, SikaWrap Hex 230 C, was selected and used for the strengthening. The Sikadur-330 epoxy adhesive was used to bond the CFRP sheet to the steel plate. The tensile strength and elastic modulus of Sikadur-330 were 30 MPa and 3.8 GPa, respectively. Average tensile strength and elastic modulus of a layer of CFRP laminate (CFRP sheet and adhesive) were provided by manufacturer based on average of 24 sample coupons and were equal to 894 MPa, and 65 GPa, respectively. Nominal thickness of a layer of CFRP laminate was 0.381 mm. Mechanical properties of a layer of CFRP laminate are listed in Table 2. Prior to applying the CFRP sheets, the steel plates were cleaned and sandblasted to roughen the surfaces and then the epoxy was applied on the steel plates. Afterward, CFRP sheets were carefully placed on the steel plates. In order to allow the adhesive reach its maximum strength, all the specimens were cured for two weeks based on manufacturer recommendation.
-50 -100 -150 -200 -20
-15
-10
-5
0
5
10
15
20
Displa acement (mm) Fig. 5. CL2O1 specimen.
200 150 100
Force (kN)
50 0 -50
2.4. Loading procedures
-100
To investigate debonding and crack growth on the strengthened specimens under loading and unloading forces, quasi-static load protocol was applied to the specimens along diagonal axes of specimen as shown in Fig. 1-a. Uniaxial and bi-axial methods of loading are widely used to determine mechanical properties of materials subjected to the in-plane shear loading [57–60]. In this study the first method was selected. There are two problems regarding to uniaxial method of loading, (1) bending of frame member and (2) extension of the frame member. To avoid axial and bending deformations of the boundary elements, relatively stiff boundary members were designed for the boundary elements. The loading protocol recommended by ATC-24 (1992) was used on this study [54]. The specimens were loaded until the failure of CFRP layers. A similar load pattern was used for all the tests. Tests were performed using a Schenk dynamic testing machine with capacity of 600 kN.
-150 -200 -20
-15
-10
--5
0
5
10
15
20 2
Dis splacement (mm m) Fig. 6. CL2O2 specimen.
CFRP plate on each side. The main direction of CFRP laminates in the CL2O1 specimen were oriented at +45° and −45° inclination with angle of tension fields as shown in Fig. 2-b. In the CL2O2 specimen, similar to CL2O1 specimen, thin steel plate was strengthened by one layer of CFRP laminate on each side, Fig. 2-c. In the CL2O2 specimen, the main direction of CFRP laminate was oriented along the direction of tension fields. The only difference between CL2O1 and CL2O2 was the 220
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Fig. 8. Behavior of CRef specimen under shear loads.
Fig. 9. Behavior of CL2O1 specimen under shear loads.
Fig. 10. Behavior of CL2O2 specimen under shear loads.
Figs. 4–9. The load displacement curves of the CRef specimen is shown in Fig. 4. Yielding strength and ultimate shear strength of the specimen was obtained to be 75 kN and 93 kN with the corresponding displacements of 4 mm and 13 mm, respectively. Signs of local tearing were observed at displacements larger than 11 mm as shown in Fig. 8-c. Overall, the specimen showed stable behavior under shear loading. CL2O1 specimen was strengthened by two layers of CFRP. Loaddeformation behavior of the specimen is presented in Fig. 5. At 7 mm displacement the specimen reached its maximum strength, which was 122 kN. The ultimate strength of the CL2O1 specimen was about 30%
3. Test results The results of four tests and observations are described in this section. General behavior of specimens including shear strength, secant stiffness and absorbed energy are presented and comparisons are made between the specimens. 3.1. Load-displacement behavior The load-displacement curves of all the specimens are presented in 221
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Fig. 11. Behavior of CL4O3 specimen under shear loads.
Table 3 Comparison of ultimate load carrying capacity.
CR ef: (Un-stiffened) CL2 2O1: (Strengthene ed by two layers of CFRP; @=0 & 90 0) CL2 2O2: (Strengthene ed by two layers of CFRP; @=+45 & -45 )
3
Cumulative Dissipated Energy (kN.m*10 )
0 30
5 25
CL4 4O3: (Strengthene ed by four layers of o CFRP; @=+45 & -45 )
Specimen Label
Ultimate Load (kN)
Ratio (Strengthened/ Unstrengthened)
Source
GRef GL2O1 GL2O2 GL4O3 CRef CL2O1 CL2O2 CL4O3
83 102 141 182 93 122 140 175
N/A 1.24 1.71 2.20 N/A 1.31 1.51 1.88
Ref. [1]
0 20
5 15
0 10
5
0
0.62
0.83
1.04
1.26
1.47
1.68 8
1.89
2.12
0 900
D Displacement t (m/m % )
0 800
Fig. 12. Cumulative dissipated energy of the specimens.
0 700 35
Secant stiffness (kN/mm)
Stress (MPa)
CR Ref: (Un-stiffened d) CL L2O1: (Strengthen ned by two layerss of CFRP; @=0 & 90 ) CL L2O2: (Strengthen ned by two layerss of CFRP; @=+4 45 & -45 )
30
This paper
CL L4O3: (Strengthen ned by four layerss of CFRP; @=+4 45 & -45 )
25 20
0 600
CFRP with Si kadur 330 Epoxy [Principal Direction] CFRP with Si kadur 330 Epoxy [Transvers se Direction] GFRP with Si kadur 330 Epoxy [Principal Direction] CFRP with Si kadur 330 Epoxy [Transvers se Direction]
0 500 0 400 0 300 0 200
15
100 0
10
0 0
0.5
1
1.5
2
2.5
Strain (% ( )
5
Fig. 14. Comparison between stress-strain behaviors of GFRP and CFRP [55,56]. 0
0.62
0.8 83
1.04
1.26
1.47
1.68 8
1.89
2.12
Displacement (m/m % )
larger than 7.5 mm, tearing in the CFRP laminate was observed, as shown in Fig. 10-c. By increasing the displacement, tearing and debonding started to expand. Similar to CL2O1 specimen, the failure of CFRP layer was due to transverse tearing. It was observed that tearing in the CL2O2 specimen initiated at displacements smaller than CL2O1 specimen. Once crack started to grow, the capacity of specimen started to decrease gradually. During the last cycles of loading, fibers rupture was observed at the drift ratios larger than 1.9%. Load-displacement curves of CL4O3 specimen are presented in Fig. 7. The hysteretic curves of the specimen were not symmetric between the positive and the negative loading directions. In the early cycles of loading the specimen exhibited a linear behavior. Ultimate strength in the positive and negative directions was 175 kN and 155 kN, respectively. At a displacement of 5.5 mm, crack started to grow. Crack path for the specimen are shown in Fig. 11-c and Fig. 11-d. In this specimen, both fiber rupture and transverse tearing were
Fig. 13. Secant stiffness of the specimens.
more than the reference specimen (CRef). By increasing the displacement, the crack started to grow in the CFRP laminate along the transverse direction of fibers (see Fig. 9-c and Fig. 9-d). Initial signs of tearing and debonding were observed at displacements larger than 7.5 mm. Once crack started to grow, the capacity of specimen started to decrease rapidly and debonding between CFRP and steel plate took place. No fiber rupture was observed during the test. In the CL2O2 specimen, fibers were aligned along the direction of tension fields. Load-deformation behavior of CL2O2 specimen is shown in Fig. 6. Ultimate strength of CL2O2 specimen was equal to 140 kN, which was 53% more than the CRef and 15% more than CL2O1 specimens. At displacements about 6–7 mm, debonding was observed between CFRP and steel plate as shown in Fig. 10-b. At displacements 222
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Fig. 15. Comparison between CFRP and GFRP strengthened steel plate under shear force.
223
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Secant stiffness (kN/mm)
3.3. Energy absorption
CRef: (Un-stiffen ed) CL2O1: (Strength hened by two layerrs of CFRP; @=0 & 90 ) CL2O2: (Strength hened by two layerrs of CFRP; @=+4 45 & -45 )
35
A comparison was made to investigate the effect of CFRP laminate in dissipation of energy. In Fig. 12, the cumulative dissipated energy versus displacement charts for all the specimens are provided. As it can be seen, the total amount of dissipated energy in CL2O1 and CL2O2 specimens were larger than that for the un-strengthened specimen. By changing the orientation of the fibers, little change was observed in the amount of dissipated energy. The difference in the amount of dissipated energy in CL2O1 and CL2O2 was between 2% and 7%. It was expected the CL4O3 specimen should dissipate more energy among the specimens. However, due to diagonal rupture of fibers (see Fig. 11), the energy dissipation in this specimen was less than other strengthened specimens, but still higher than the un-strengthened specimen.
CL4O3: (Strength hened by four layers of CFRP; @=+4 45 & -45 ) GRef: (Un-stiffen ed)
30
GL2O1: (Strength hened by two layerrs of GFRP; @=0 & 90 ) GL2O2: (Strength hened by two layerrs of GFRP; @=+4 45 & -45 )
25
GL4O3: (Strength hened by four layers of GFRP; @=+4 45 & -45 )
20 15 10 5 0
.64
.86
1.07 1.28 1.5 1.71 Diisplacement (m/m ( %)
1.93
2.25
Fig. 16. Comparison between change in secant stiffness of the specimens.
3.4. Secant stiffness GRef: (U Un-stiffened) GL2O1: (Strengthened byy two layers of GFR RP; @=0 & 90 )
3
Cumulative Dissipated Energy (kN.m*10 )
35 30
Secant stiffness is defined as the tangent stiffness at each cycle for each specimen. The comparison of secant stiffness of the specimens is presented in Fig. 13. It is evident that CFRP laminate can significantly increase the secant stiffness. Among the specimens, the CL4O3 specimen had the highest secant stiffness. The comparison between CL2O1 and CL2O2 showed that the fiber orientation has significant effect on the shear stiffness. The difference between the secant stiffness of CL2O1 and CL2O2 varies from 22% to 30% for the range of displacement up to 1.68%. By applying the CFRP layers along tension fields (O2 orientation), the maximum secant stiffness can be obtained. For the larger displacements, due to progressive failure of CFRP laminates, the difference in secant stiffness of the specimens reduces.
GL2O2: (Strengthened byy two layers of GFR RP; @=+45 & -45 ) GL4O3: (Strengthened byy four layers of GFR RP; @=+45 & -45 ) CRef: (U Un-stiffened)
25
CL2O1: (Strengthened byy two layers of GFR RP; @=0 & 90 ) CL2O2: (Strengthened byy two layers of GFR RP; @=+45 & -45 ) CL4O3: (Strengthened byy four layers of GFR RP; @=+45 & -45 )
20 15 10 5 0
0.6
0..8
1
1.2 1.4 4 1.6 Displacementt (m/m % ) D
1.8
2
2.2
Fig. 17. Comparison in term of cumulative dissipated energy in the specimens strengthened by CFRP and GFRP laminates.
4. Comparison with GFRP strengthening Both CFRP and GFRP have been widely used to upgrade and strengthen steel members. Basically, in term of strength, CFRP has higher strength than GFRP. In contrast, GFRP can experience larger strain before failure of fibers. Therefore, it is expected that GFRP strengthened steel members exhibit more ductile behavior. The comparison of stress-strain behavior of studied CFRP and GFRP laminate is presented in Fig. 14. In this section, the comparison was made in terms of ultimate strength, secant stiffness and energy absorption of CFRP and GFRP-strengthened specimens. The details of GFRP strengthened specimens under shear loading are available in Ref. [1].
observed. Due to rupture of CFRP laminates and debonding, shear strength of the specimen was reduced at larger displacements. 3.2. Failure modes Different types of failure modes can take place in FRP-strengthened steel structures. Generally, the failure modes in the FRP-strengthened steel structures can be categorized as follows [1,53]: Mode-1. steel and adhesive interface failure Mode-2. FRP layer delamination
4.1. Ultimate strength Mode-3. rupture in the direction perpendicular to the main direction of the FRP laminate (transverse tearing)
The comparison of ultimate load carrying capacity of both CFRP and GFRP strengthened specimens is presented in Table 3. It can be seen that the increase in ultimate load carrying capacity is significant for both GFRP and CFRP strengthening. The increase ratio is almost the same for both FRPs. It should be noted that the CRef has slightly higher value because of 1 mm steel plate was used compared with 0.9 mm steel plate used in GFRP testing series. The increase in ultimate load found in this study and in [1] is much higher than those (11% to 14%) reported in [46]. It can be seen from Fig. 15 that the behavior of CFRP and GFRP is quite similar when the number of layers are two. For two layers of FRP strengthening with orientation 1, the envelop curves are very close to each other. For two layers of FRP strengthening with orientation 2, the CFRP curve is higher for deflection up to 10 mm. When 4 layers FRPs are used, CL4O3 specimen reached the peak at a displacement of 5 mm, whereas specimen GL4O3 reached its peak around 12 mm while the two peak loads are almost the same. It seems that for CFRP and GFRP, adding another 2 layers can still provide additional benefit in terms of ultimate strength.
Mode-4. rupture in the main direction of FRP layers Mode-5. joints failure Mode-6. steel yielding; (ductile mode) The results showed multiple failure modes for FRP-strengthened steel plate under shear loading. For all strengthened specimens, after buckling of composite plates, some local bond failures at the interface of steel plate and adhesive (Mode-1) were observed. In CL2O1 and CL2O2 specimens which were strengthened by two layers of CFRP, at drift ratios larger than 1.05% transverse tears (Mode-3) were observed in the CFRP layers. In CL2O1 specimen fiber rupture was noticed. However, in CL2O2 specimen, at drift ratios larger than 1.9% local rupture of fibers (Mode-4) were observed. In CL4O3 specimen, which was strengthened by two orthogonal layers of CFRP on each side, at drift ratios larger than 0.8% fiber ruptures (Mode-4) were observed. By increasing the displacement, rupture of fibers started to grow and the shear strength of specimen started to decrease. 224
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4.2. Stiffness Comparisons of secant stiffness of the CFRP and GFRP strengthened specimens are presented in Fig. 16. It can be seen that the behavior of CFRP and GFRP is quite similar when the number of layers are 1 and 2 on each side. This is in consistent with that observed from Fig. 15 reported in Section 4.1. For specimens with two layers of FRP strengthening on each side, the secant stiffness is very close to each other, i.e. when comparing CL2O1 and GL2O1 or comparing CL2O2 and GL2O2. Fig. 16 also reveals that CFRP strengthening is more sensitive to the fiber orientation, orientation 2 gives larger secant stiffness than orientation 1. When four layers of FRPs are used, GL4O3 specimen demonstrated a large increase in secant stiffness compared with those with two layers of GFRP. However, the increase in secant stiffness of CL4O3 is not significantly higher than those with two layers of CFRP. Therefore, adopting 4 layers of GFRP can still gain extra benefit in terms of increasing secant stiffness. In the case of CFRP, the additional two layers does not provide additional benefit.
• •
Research is being conducted to simulate the behavior of CFRP and GFRP strengthened thin steel plates under cyclic shear loading using an analytical model or a finite element analysis. Acknowledgement The authors wish to acknowledge the support of the International Institute of Earthquake Engineering and Seismology (IIEES).
4.3. Energy absorption Cumulative dissipated energy values of the CFRP and GFRP strengthened specimens are presented Fig. 17. The results show that both methods of strengthening are effective at increasing the total amount of dissipated energy. In terms of energy, GFRP strengthened specimens showed better results and dissipated more energy. It can be seen from Fig. 17 that fiber orientation does not affect cumulative dissipated energy when two layers of CFRP are applied. The cumulative dissipated energy for strengthening with four layers of CFRP is less than that with two layers of CFRP. This can be explained by looking at Fig. 15 where load carrying capacity of CL4O3 starts to drop after a deflection of 5 mm due to CFRP rupture. On the contrast, additional cumulative dissipated energy was obtained for 4 layers of GFRP strengthening when compared with 2 layers of GFRP strengthening. The increase in energy absorption found in this paper (around 14– 22%) and in [1] (about 45–130%) is much higher than that (10%) reported in [46].
References [1] F. Nateghi-Alahi, M. Khazaei-Poul, Experimental study of steel plate shear walls with infill plates strengthened by GFRP laminates, J. Constr. Steel Res. 78 (2012) 159–172. [2] K.A. Harries, A.J. Peck, E.J. Abraham, Enhancing stability of structural steel sections using FRP, Thin-Walled Struct. 47.10 (2009) 1092–1101. [4] A. Shaat, A.Z. Fam, Slender steel columns strengthened using high-modulus CFRP plates for buckling control, J. Compos. Constr. 13.1 (2009) 2–12. [5] P. Colombi, C. Poggi, An experimental, analytical and numerical study of the static behavior of steel beams reinforced by pultruded CFRP strips, Compos. B: Eng. 37.1 (2006) 64–73. [6] N. Silvestre, D. Camotim, B. Young, On the use of the EC3 and AISI specifications to estimate the ultimate load of CFRP-strengthened cold formed steel lipped channel columns, Thin-Walled Struct. 47 (10) (2009) 1102–1111. [7] A. Lenwari, T. Thepchatri, P. Albrecht, Debonding strength of steel beams strengthened with CFRP plates, J. Compos. Constr. 10.1 (2006) 69–78. [8] C. Wu, X.L. Zhao, W.H. Duan, Design rules for web crippling of CFRP strengthened aluminium rectangular hollow sections, Thin-Walled Struct. 49.10 (2011) 1195–1207. [9] X.L. Zhao, P. Phiphat, Tests on CFRP strengthened aluminium RHS subject to end bearing force, Advances in FRP Composites in Civil Engineering, Springer Berlin Heidelberg, 2011, pp. 911–914. [10] N.A. Chahkand, M.Z. Jumaat, N.R. Sulong, X.L. Zhao, Experimental and theoretical investigation on torsional behaviour of CFRP strengthened square hollow steel section, Thin-Walled Struct. 68 (2013) 135–140. [11] R. Sen, L. Liby, G. Mullins, strengthening steel bridge sections using CFRP laminates, Compos. B: Eng. 32.4 (2001) 309–322. [12] P. Balaguru, A. Nanni, J. Giancaspro, FRP Composites for Reinforced and Prestressed Concrete Structures: A Guide to Fundamentals and Design for Repair and Retrofit, CRC Press, 2008. [13] M. Amjadian, A. Kalantari, A.K. Agrawal, Analytical study of the coupled motions of decks in skew bridges with the deck-abutment collision, J. Vib. Control (2016). http://dx.doi.org/10.1177/1077546316659781. [14] M. Amjadian, A. Agrawal, Rigid-body motion of horizontally curved bridges subjected to earthquake-induced pounding, J. Bridge Eng. (2016) 04016090. [15] X.L. Zhao, R. Al-Mahaidi, Web buckling of lightsteel beams strengthened with CFRP subjected to end-bearing forces, Thin-Walled Struct. 47.10 (2009) 1029–1036. [16] M. Tavakkolizadeh, H. Saadatmanesh, Repair of damaged steel-concrete composite girders using carbon fiber-reinforced polymer sheets, J. Compos. Constr. 7.4 (2003) 311–322. [17] Y. Zhou, H. Fan, K. Jiang, M. Gou, N. Li, P. Zhu, Y. Tu, Experimental flexural behaviors of CFRP strengthened aluminum beams, Compos. Struct. 116 (2014) 761–771. [18] J. Haedir, M.R. Bambach, X.L. Zhao, R.H. Grzebieta, Strength of circular hollow sections (CHS) tubular beams externally reinforced by carbon FRP sheets in pure bending, Thin-Walled Struct. 47.10 (2009) 1136–1147. [19] D. Fernando, T. Yu, J.G. Teng, X.L. Zhao, CFRP strengthening of rectangular steel tubes subjected to end bearing loads: effect of adhesive properties and finite element modelling, Thin-Walled Struct. 47.10 (2009) 1020–1028. [20] H.E. Sallam, S.S. Ahmad, A.A. Badawy, W. Mamdouh, Evaluation of steel I-beams strengthened by various plating methods, Adv. Struct. Eng. 9.4 (2006) 535–544. [21] D. Linghoff, R. Haghani, M. Al-Emrani, Carbon-fibre composites for strengthening steel structures, Thin-Walled Struct. 47.10 (2009) 1048–1058. [22] R. Haghani, M. Al-Emrani, A new design model for adhesive joints used to bond FRP laminates to steel beams: part B: experimental verification, Constr. Build. Mater. 30 (2012) 686–694.
5. Conclusions Experimental studies were carried out on CFRP-strengthened steel plates under quasi-static loading. The results showed that using CFRP laminate can be effective at improving the behavior of thin steel plate under shear loading. The following observations were made based on the limited test results:
•
• • • •
terms of increasing the ultimate load carrying capacity and secant stiffness. Fiber orientation does not affect cumulative dissipated energy when two layers of CFRP on each side are applied. Both GFRP and CFRP strengthening significantly increase the ultimate load carrying capacity. The increase ratio is almost the same for both FRPs. Adopting 4 layers of GFRP can still gain extra benefit in terms of increasing load carrying capacity, secant stiffness and cumulative dissipated energy. In the case of CFRP, the additional two layers does not provide additional benefit for stiffness and energy absorption because the failure is governed by CFRP rupture.
Applying CFRP laminates can significantly increase yield strength, ultimate strength, and secant stiffness. Fiber orientation is an important factor in the shear strengthening. The highest shear strength and stiffness can be obtained by applying the principal orientation of CFRP laminates along the direction of the tension fields. When strengthening the steel plate by a layer of CFRP laminate on each side, after debonding, transverse shear tearing was observed at the drift larger than about 1%. At drifts larger than 1.9% fiber failures were initiated. By adding two layers of CFRP laminate on each side, transverse shear tearing can be avoided. The failure in the CFRP-strengthened specimen under shear loading is caused by a combination of several modes. The first failure mode that has been observed is debonding between the steel plate and the adhesive. CFRP-strengthening method is useful for increasing the dissipated energy of thin steel plate under shear loading. By changing the angle of fibers, little change was observed in the amount of dissipated energy. For CFRP strengthening, orientation 2 is better than orientation 1 in 225
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[23] J. Deng, M.M. Lee, Behaviour under static loading of metallic beams reinforced with a bonded CFRP plate, Compos. Struct. 78.2 (2007) 232–242. [24] M.R. Bambach, M. Elchalakani, Plastic mechanism analysis of steel SHS strengthened with CFRP under large axial deformation, Thin-Walled Struct. 45.2 (2007) 159–170. [25] X.L. Zhao, D. Fernando, R. Al-Mahaidi, CFRP strengthened RHS subjected to transverse end bearing force, Eng. Struct. 28.11 (2006) 1555–1565. [26] X.L. Zhao, R. Al-Mahaidi, Web buckling of lightsteel beams strengthened with CFRP subjected to end-bearing forces, Thin-Walled Struct. 47.10 (2009) 1029–1036. [27] A. Astaneh-Asl, Seismic behaviour and design of steel shear walls, Steel TIPS Report, Structural Steel Educational Council, Moraga, CA, 2001. [28] R. Sabelli, M. Bruneau, Design Guide 20: Steel Plate Shear Walls, American Institute of Steel Construction, Chicago, IL, USA, 2007. [29] L.J. Thorburn, G.L. Kulak, C.J. Montgomery, Analysis of steel plate shear walls, Structural Engineering Report No. 107, Department of Civil Engineering, University of Alberta, Edmonton, Alberta, Canada, 1983. [30] P.A. Timler, G.L. Kulak, Experimental study of steel plate shear walls, Structural Engineering Report, University of Alberta, Canada, No. 114, 1983. [31] M. Elgaaly, Thin steel plate shear walls behavior and analysis, Thin-Walled Struct. 32 (1998) 151–180. [32] J.W. Berman, M. Bruneau, Plastic analysis and design of steel plate shear walls, J. Struct. Eng. 129 (11) (2003) 1148–1156. [33] S. Sabouri-Ghomi, C. Ventura, H.K. Kharrazi, Shear analysis and design of ductile steel plate walls, J. Struct. Eng. 131 (6) (2005) (June 1). [34] M.H.K.Kharrazi, Rational Method for Analysis and Design of Steel Plate Walls (Ph.D. dissertation), Vancouver (Canada), University of British Columbia, 2005. [35] M. Khazaei-Poul, F. Nateghi-Alahi, Theoretical and numerical study on the strengthened steel plate shear walls by FRP laminates, Int. J. Eng. IJE Trans. C: Asp. 25 (1) (2012). [36] A.S. Lubell, H.G.L. Prion, C.E. Ventura, M. Rezai, Un-stiffened steel plate shear wall performance under cyclic loading, J. Struct. Eng. Vol. 126 (4) (2002) 453–460. [37] V. Caccese, M. Elgaaly, R. Chen, Experimental study of thin steel-plate shear walls under cyclic load, ASCE J. Struct. Eng. 119 (2) (1993) 573–587. [38] R.G. Driver, G.L. Kulak, D.J.L. Kennedy, Cyclic tests of four-story steel plate shear wall, ASCE J. Struct. Eng. 124 (1998) 112–120. [39] S. Sabouri-Ghomi, T.M. Roberts, Nonlinear dynamic analysis of steel plate shear walls including shear and bending deformations, Eng. Struct. 14.5 (1992) 309–317. [40] D. Vian, M. Bruneau, K.C. Tsai, Y.C. Lin, Special perforated steel plate shear walls with reduced beam section anchor beams. I: experimental investigation, J. Struct. Eng. 135.3 (2009) 211–220. [41] M.M. Alinia, A study into optimization of stiffeners in plates subjected to in- plane shear loads, Thin-Walled Struct. 43.5 (2005) 845–860. [42] P.M. Clayton, J.W. Berman, L.N. Lowes, Subassembly testing and modeling of self-
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Experimental testing on CFRP strengthened thin steel plates under shear loading ⁎
M. Khazaei Poula, , F. Nateghi-Alahib, X.L. Zhaoc a b c
Department of Civil, Architectural and Environmental Engineering, Drexel University, Philadelphia, PA 19104, USA Structural Engineering Research Center, International Institute of Earthquake Engineering and Seismology, Tehran, Iran Department of Civil Engineering, Monash University, Clayton, Vic. 3800, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Thin steel plate CFRP Shear Cyclic loading Energy absorption Stiffness
This paper presents an experimental study on thin steel plates strengthened by CFRP layers under shear loading. A series of laboratory tests were conducted on scaled one-story steel shear panel models with hinge type connections under cyclic quasi-static loading. One of the specimens is un-stiffened and selected as a reference sample of three others specimens. Steel infill plates are strengthened by CFRP layers with different numbers of layers and orientations. The results indicate that the strengthening of thin steel plate by CFRP layers increases ultimate strength, secant stiffness and energy absorption. Comparisons are made between the performance of CFRP and GFRP strengthened steel plates under cyclic shear loads. It was shown that both materials were effective at increasing stiffness and strength of the system. The increased ratio is similar for both FRPs when two layers are applied. However, adding additional two layers of CFRP does not provide extra benefit in terms of stiffness and energy absorption although there is still benefit for ultimate strength.
1. Introduction Application of fiber reinforced polymer (FRP) is a method for strengthening and repairing existing structures as well as for designing new structural systems. FRP is particularly well suited for strengthening due to its excellent mechanical properties, such as high strength, high stiffness, light weight, and flexibility to be formed into any shape. In recent years FRP has been successfully used for upgrading and retrofitting of different structural elements subjected to bending loading [5], compression loading e.g. [2,4,6], bearing force e.g. [8,9], and torsion loading [10], shear loading [11]. It has been also proved that FRP is efficient in repair or strengthening of reinforced concrete structures [12], particularly those structures that are vulnerable to extreme shear and torsional loading during strong earthquakes [13,14]. A brief summary is provided below. Many studies have been conducted on the strengthening of steel members under bending loading. FRP has been successfully applied on “I” section steel [5,7,11,16,17] beams, aluminum beams [17], and circular hollow section (CHS) beams [7,18] to improve bending capacity. The results showed that by strengthening the tension flange of metallic beam both the stiffness and ultimate load capacity of system were considerably increased. Applying CFRP plates on the web of steel beams, at the load points or support of beams, can also increase the
⁎
web buckling capacity [15]. The possible failure modes for the FRPstrengthened steel I-beams include failure of beam due to lateral buckling [20], debonding [21–23], and rupture of the laminate [21]. Recent studies on steel members with externally bonded CFRP under compression loading [2,4,21,51] showed that this method is effective to increase both stiffness and total capacity of steel members. Experimental studies on the behavior of steel square hollow section (SHS) tubes strengthened by CFRP [24,51] showed that the application of CFRP is effective at increasing the axial load capacity. They showed that restraint provided by CFRP can postpone the elastic buckling deformation in the FRP-strengthened columns. CFRP laminate can also be used to enhance the torsional strength of steel members. Tests on square hollow steel section under torsional loading [10] showed that using CFRP laminate can improve the ultimate strength of steel members. The optimum orientation angle for strengthening against cyclical torque is a combination of spiral and reverse-spiral CFRP wrap [10]. CFRP has been successfully used to strengthen rectangular hollow sections under end-bearing forces [19,25]. Using CFRP can change the failure mode from web buckling to web yielding and can increase restraints against web rotation [25]. Tests results on web buckling of light-steel beam subjected to end-bearing forces showed that CFRP strengthening can increase the web-buckling capacity and it is more
Corresponding author: [email protected] (M.Khazaei Poul). E-mail address: [email protected] (M. Khazaei Poul).
http://dx.doi.org/10.1016/j.tws.2016.09.026 Received 12 July 2016; Received in revised form 16 September 2016; Accepted 30 September 2016 0263-8231/ © 2016 Elsevier Ltd. All rights reserved.
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Nomenclature CRef CL2O1 CL2O2 CL4O3 GRef GL2O1 GL2O2 GL4O3
Reference test specimen Steel plate strengthened Steel plate strengthened Steel plate strengthened Reference test specimen Steel plate strengthened Steel plate strengthened Steel plate strengthened
for CFRP strengthened specimens by two layers of CFRP [Fiber orientation: 0° and 90°] by two layers of CFRP [Fiber orientation: 45° and −45°] by four layers of CFRP [Fiber orientation: 45° and −45°] for GFRP strengthened specimens by two layers of GFRP [Fiber orientation: 0° and 90°] by two layers of GFRP [Fiber orientation: 45° and −45°] by four layers of GFRP [Fiber orientation: 45° and −45°]
Fig. 1. Typical test set-up [dimensions in mm].
dissipation increased about 45–130%, whereas the increase was about 10%. This paper is a follow up study of [1], but focuses on CFRP strengthening of thin steel plates under cyclic shear loading. A series of laboratory tests were conducted to evaluate the effect of number of CFRP layers and the fiber orientations, different patterns of strengthening. Comparisons are made between GFRP and CFRP strengthened specimens under shear loading in terms of shear strength, secant stiffness and dissipated energy.
effective for those with large web depth-to-thickness ratio [26]. Steel plate shear walls (SPSWs) has been widely used as a lateral load resisting system to transfer wind and earthquake loads. SPSWs consist of a thin steel plate and the boundary elements [1]. After buckling of thin steel plate, shear loads are transferred due to formation of diagonal tension field in the steel web, which is known as tension field action. Extensive studies have been performed by researchers [24–48] on the performance of SPSWs. Thin steel plate in the SPSWs can be strengthened by adding layers of FRP laminate on both sides [1]. Similarly, in the FRP-strengthened systems the shear loads are transferred through diagonal tension field in the composite steel plate. This mechanism has been reported in the infilled structures as well [49,50]. Experimental [1] and numerical [35,44] studies on the GFRP-strengthened steel plate showed that this method of strengthening is effective for increasing strength, stiffness and energy absorption of thin steel plate under shear loading. Only a few studies have been conducted on the FRP-strengthened thin steel members under shear loading [1,35,45,46]. In 2012 [1] the first two authors of this paper investigated GFRP-strengthened thin steel plates under cyclic shear load. In 2016 Petkune and Donchev [43] studied GFRP and CFRPstrengthened steel plates under similar type of loading. The increased ultimate load carrying capacity varied from 24% to 120% in [1], whereas the increase was about 14% in [46]. The cumulative energy
2. Experimental set-up and test specimens 2.1. Test specimens and test set-up In order to investigate the effect of CFRP laminate on the shear behavior of thin steel plates, a series of laboratory tests were conducted on the scaled one-story steel shear panel models with hinge type connections under cyclic quasi-static loading. The detail of test setup and shear panel are shown in Fig. 1-a. The steel and composite plates were connected to the frame by utilizing two rows of high tension bolts. Because of rotational constraints at the edges of plates, the connection of thin plates to the frame can be considered as clamped (see Fig. 1-b). 218
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Fig. 2. Un-stiffened and CFRP-strengthened steel plate.
Table 1 Properties of infill plate in the experimental specimens. Specimen label
CRef CL2O1 CL2O2 CL4O3
Number of layers in composite infill plate
Thickness of laminate and steel infill plate
Steel plate
CFRP layer
Steel plate
CFRP layer
1 1 1 1
0 2 2 4
1 mm 1 mm 1 mm 1 mm
N/A 0.381 mm 0.381 mm 0.381 mm
Total thickness of infill plate
Orientation of CFRP layers (θ)
CFRP Type
1 mm 1.762 mm 1.762 mm 2.542 mm
– 0 # 90 45 # −45 45 & −45 # 45 & −45
– SikaWrap® Hex 230C SikaWrap® Hex 230C SikaWrap® Hex 230C
Table 2 Cured Laminate Sikawrap-230 with Sikadur 330 Epoxy.
Fiber direction Orthogonal to fiber direction
Elastic Modulus (MPa)
Tensile Strength (MPa)
Failure strain (%)
65,402 5876
894 27
0.0136 0.0046
2.2. Strengthening of steel plates In this study, four sets of laboratory tests were conducted. One of the specimens was un-stiffened steel plate with thickness of 1 mm and selected as a reference model for comparison. In the three others specimens, the steel plates were strengthened by CFRP. Uni-directional CFRP laminate was selected and applied on the steel plates. Nominal thickness of a layer of CFRP laminate and epoxy was equal to 0.381 mm. Three types of strengthening methods were considered. The details of strengthened steel plates are shown in Fig. 2. In the CL2O1 specimen, thin steel plate was strengthened by one layer of
Fig. 3. Strain-stress curve for steel plate, boundary element, Cured Laminate Sikawrap230 with Sikadur 330 Epoxy.
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200
200
150
150
100
100
50
50
Force (kN)
Force
(kN)
M. Khazaei Poul et al.
0 -50
0 -50
-100
-100
-150
-150
-200 -20
-15
-10
-5
0
5
10
15
-200 -20
20
Dis splacement (mm m)
-15
-10
-5
0
5
10
15
20
Displa acement (mm)
Fig. 4. CRef specimen.
Fig. 7. CL4O3 specimen.
200
orientation of fibers. Thin steel plate in the CL4O3 specimen was strengthened by two layers of CFRP laminate on each side and the main direction of CFRP laminate was oriented along the tension fields, Fig. 2-d. Details of all experimental specimens are presented in Table 1.
150 100
Force (kN)
50
2.3. Material properties
0
Tensile coupon tests were conducted to obtain the material properties of the steel plate and the boundary elements (UNP100). The strainstress curves for the steel materials are presented in Fig. 3. Unidirectional CFRP laminate, SikaWrap Hex 230 C, was selected and used for the strengthening. The Sikadur-330 epoxy adhesive was used to bond the CFRP sheet to the steel plate. The tensile strength and elastic modulus of Sikadur-330 were 30 MPa and 3.8 GPa, respectively. Average tensile strength and elastic modulus of a layer of CFRP laminate (CFRP sheet and adhesive) were provided by manufacturer based on average of 24 sample coupons and were equal to 894 MPa, and 65 GPa, respectively. Nominal thickness of a layer of CFRP laminate was 0.381 mm. Mechanical properties of a layer of CFRP laminate are listed in Table 2. Prior to applying the CFRP sheets, the steel plates were cleaned and sandblasted to roughen the surfaces and then the epoxy was applied on the steel plates. Afterward, CFRP sheets were carefully placed on the steel plates. In order to allow the adhesive reach its maximum strength, all the specimens were cured for two weeks based on manufacturer recommendation.
-50 -100 -150 -200 -20
-15
-10
-5
0
5
10
15
20
Displa acement (mm) Fig. 5. CL2O1 specimen.
200 150 100
Force (kN)
50 0 -50
2.4. Loading procedures
-100
To investigate debonding and crack growth on the strengthened specimens under loading and unloading forces, quasi-static load protocol was applied to the specimens along diagonal axes of specimen as shown in Fig. 1-a. Uniaxial and bi-axial methods of loading are widely used to determine mechanical properties of materials subjected to the in-plane shear loading [57–60]. In this study the first method was selected. There are two problems regarding to uniaxial method of loading, (1) bending of frame member and (2) extension of the frame member. To avoid axial and bending deformations of the boundary elements, relatively stiff boundary members were designed for the boundary elements. The loading protocol recommended by ATC-24 (1992) was used on this study [54]. The specimens were loaded until the failure of CFRP layers. A similar load pattern was used for all the tests. Tests were performed using a Schenk dynamic testing machine with capacity of 600 kN.
-150 -200 -20
-15
-10
--5
0
5
10
15
20 2
Dis splacement (mm m) Fig. 6. CL2O2 specimen.
CFRP plate on each side. The main direction of CFRP laminates in the CL2O1 specimen were oriented at +45° and −45° inclination with angle of tension fields as shown in Fig. 2-b. In the CL2O2 specimen, similar to CL2O1 specimen, thin steel plate was strengthened by one layer of CFRP laminate on each side, Fig. 2-c. In the CL2O2 specimen, the main direction of CFRP laminate was oriented along the direction of tension fields. The only difference between CL2O1 and CL2O2 was the 220
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Fig. 8. Behavior of CRef specimen under shear loads.
Fig. 9. Behavior of CL2O1 specimen under shear loads.
Fig. 10. Behavior of CL2O2 specimen under shear loads.
Figs. 4–9. The load displacement curves of the CRef specimen is shown in Fig. 4. Yielding strength and ultimate shear strength of the specimen was obtained to be 75 kN and 93 kN with the corresponding displacements of 4 mm and 13 mm, respectively. Signs of local tearing were observed at displacements larger than 11 mm as shown in Fig. 8-c. Overall, the specimen showed stable behavior under shear loading. CL2O1 specimen was strengthened by two layers of CFRP. Loaddeformation behavior of the specimen is presented in Fig. 5. At 7 mm displacement the specimen reached its maximum strength, which was 122 kN. The ultimate strength of the CL2O1 specimen was about 30%
3. Test results The results of four tests and observations are described in this section. General behavior of specimens including shear strength, secant stiffness and absorbed energy are presented and comparisons are made between the specimens. 3.1. Load-displacement behavior The load-displacement curves of all the specimens are presented in 221
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Fig. 11. Behavior of CL4O3 specimen under shear loads.
Table 3 Comparison of ultimate load carrying capacity.
CR ef: (Un-stiffened) CL2 2O1: (Strengthene ed by two layers of CFRP; @=0 & 90 0) CL2 2O2: (Strengthene ed by two layers of CFRP; @=+45 & -45 )
3
Cumulative Dissipated Energy (kN.m*10 )
0 30
5 25
CL4 4O3: (Strengthene ed by four layers of o CFRP; @=+45 & -45 )
Specimen Label
Ultimate Load (kN)
Ratio (Strengthened/ Unstrengthened)
Source
GRef GL2O1 GL2O2 GL4O3 CRef CL2O1 CL2O2 CL4O3
83 102 141 182 93 122 140 175
N/A 1.24 1.71 2.20 N/A 1.31 1.51 1.88
Ref. [1]
0 20
5 15
0 10
5
0
0.62
0.83
1.04
1.26
1.47
1.68 8
1.89
2.12
0 900
D Displacement t (m/m % )
0 800
Fig. 12. Cumulative dissipated energy of the specimens.
0 700 35
Secant stiffness (kN/mm)
Stress (MPa)
CR Ref: (Un-stiffened d) CL L2O1: (Strengthen ned by two layerss of CFRP; @=0 & 90 ) CL L2O2: (Strengthen ned by two layerss of CFRP; @=+4 45 & -45 )
30
This paper
CL L4O3: (Strengthen ned by four layerss of CFRP; @=+4 45 & -45 )
25 20
0 600
CFRP with Si kadur 330 Epoxy [Principal Direction] CFRP with Si kadur 330 Epoxy [Transvers se Direction] GFRP with Si kadur 330 Epoxy [Principal Direction] CFRP with Si kadur 330 Epoxy [Transvers se Direction]
0 500 0 400 0 300 0 200
15
100 0
10
0 0
0.5
1
1.5
2
2.5
Strain (% ( )
5
Fig. 14. Comparison between stress-strain behaviors of GFRP and CFRP [55,56]. 0
0.62
0.8 83
1.04
1.26
1.47
1.68 8
1.89
2.12
Displacement (m/m % )
larger than 7.5 mm, tearing in the CFRP laminate was observed, as shown in Fig. 10-c. By increasing the displacement, tearing and debonding started to expand. Similar to CL2O1 specimen, the failure of CFRP layer was due to transverse tearing. It was observed that tearing in the CL2O2 specimen initiated at displacements smaller than CL2O1 specimen. Once crack started to grow, the capacity of specimen started to decrease gradually. During the last cycles of loading, fibers rupture was observed at the drift ratios larger than 1.9%. Load-displacement curves of CL4O3 specimen are presented in Fig. 7. The hysteretic curves of the specimen were not symmetric between the positive and the negative loading directions. In the early cycles of loading the specimen exhibited a linear behavior. Ultimate strength in the positive and negative directions was 175 kN and 155 kN, respectively. At a displacement of 5.5 mm, crack started to grow. Crack path for the specimen are shown in Fig. 11-c and Fig. 11-d. In this specimen, both fiber rupture and transverse tearing were
Fig. 13. Secant stiffness of the specimens.
more than the reference specimen (CRef). By increasing the displacement, the crack started to grow in the CFRP laminate along the transverse direction of fibers (see Fig. 9-c and Fig. 9-d). Initial signs of tearing and debonding were observed at displacements larger than 7.5 mm. Once crack started to grow, the capacity of specimen started to decrease rapidly and debonding between CFRP and steel plate took place. No fiber rupture was observed during the test. In the CL2O2 specimen, fibers were aligned along the direction of tension fields. Load-deformation behavior of CL2O2 specimen is shown in Fig. 6. Ultimate strength of CL2O2 specimen was equal to 140 kN, which was 53% more than the CRef and 15% more than CL2O1 specimens. At displacements about 6–7 mm, debonding was observed between CFRP and steel plate as shown in Fig. 10-b. At displacements 222
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Fig. 15. Comparison between CFRP and GFRP strengthened steel plate under shear force.
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Secant stiffness (kN/mm)
3.3. Energy absorption
CRef: (Un-stiffen ed) CL2O1: (Strength hened by two layerrs of CFRP; @=0 & 90 ) CL2O2: (Strength hened by two layerrs of CFRP; @=+4 45 & -45 )
35
A comparison was made to investigate the effect of CFRP laminate in dissipation of energy. In Fig. 12, the cumulative dissipated energy versus displacement charts for all the specimens are provided. As it can be seen, the total amount of dissipated energy in CL2O1 and CL2O2 specimens were larger than that for the un-strengthened specimen. By changing the orientation of the fibers, little change was observed in the amount of dissipated energy. The difference in the amount of dissipated energy in CL2O1 and CL2O2 was between 2% and 7%. It was expected the CL4O3 specimen should dissipate more energy among the specimens. However, due to diagonal rupture of fibers (see Fig. 11), the energy dissipation in this specimen was less than other strengthened specimens, but still higher than the un-strengthened specimen.
CL4O3: (Strength hened by four layers of CFRP; @=+4 45 & -45 ) GRef: (Un-stiffen ed)
30
GL2O1: (Strength hened by two layerrs of GFRP; @=0 & 90 ) GL2O2: (Strength hened by two layerrs of GFRP; @=+4 45 & -45 )
25
GL4O3: (Strength hened by four layers of GFRP; @=+4 45 & -45 )
20 15 10 5 0
.64
.86
1.07 1.28 1.5 1.71 Diisplacement (m/m ( %)
1.93
2.25
Fig. 16. Comparison between change in secant stiffness of the specimens.
3.4. Secant stiffness GRef: (U Un-stiffened) GL2O1: (Strengthened byy two layers of GFR RP; @=0 & 90 )
3
Cumulative Dissipated Energy (kN.m*10 )
35 30
Secant stiffness is defined as the tangent stiffness at each cycle for each specimen. The comparison of secant stiffness of the specimens is presented in Fig. 13. It is evident that CFRP laminate can significantly increase the secant stiffness. Among the specimens, the CL4O3 specimen had the highest secant stiffness. The comparison between CL2O1 and CL2O2 showed that the fiber orientation has significant effect on the shear stiffness. The difference between the secant stiffness of CL2O1 and CL2O2 varies from 22% to 30% for the range of displacement up to 1.68%. By applying the CFRP layers along tension fields (O2 orientation), the maximum secant stiffness can be obtained. For the larger displacements, due to progressive failure of CFRP laminates, the difference in secant stiffness of the specimens reduces.
GL2O2: (Strengthened byy two layers of GFR RP; @=+45 & -45 ) GL4O3: (Strengthened byy four layers of GFR RP; @=+45 & -45 ) CRef: (U Un-stiffened)
25
CL2O1: (Strengthened byy two layers of GFR RP; @=0 & 90 ) CL2O2: (Strengthened byy two layers of GFR RP; @=+45 & -45 ) CL4O3: (Strengthened byy four layers of GFR RP; @=+45 & -45 )
20 15 10 5 0
0.6
0..8
1
1.2 1.4 4 1.6 Displacementt (m/m % ) D
1.8
2
2.2
Fig. 17. Comparison in term of cumulative dissipated energy in the specimens strengthened by CFRP and GFRP laminates.
4. Comparison with GFRP strengthening Both CFRP and GFRP have been widely used to upgrade and strengthen steel members. Basically, in term of strength, CFRP has higher strength than GFRP. In contrast, GFRP can experience larger strain before failure of fibers. Therefore, it is expected that GFRP strengthened steel members exhibit more ductile behavior. The comparison of stress-strain behavior of studied CFRP and GFRP laminate is presented in Fig. 14. In this section, the comparison was made in terms of ultimate strength, secant stiffness and energy absorption of CFRP and GFRP-strengthened specimens. The details of GFRP strengthened specimens under shear loading are available in Ref. [1].
observed. Due to rupture of CFRP laminates and debonding, shear strength of the specimen was reduced at larger displacements. 3.2. Failure modes Different types of failure modes can take place in FRP-strengthened steel structures. Generally, the failure modes in the FRP-strengthened steel structures can be categorized as follows [1,53]: Mode-1. steel and adhesive interface failure Mode-2. FRP layer delamination
4.1. Ultimate strength Mode-3. rupture in the direction perpendicular to the main direction of the FRP laminate (transverse tearing)
The comparison of ultimate load carrying capacity of both CFRP and GFRP strengthened specimens is presented in Table 3. It can be seen that the increase in ultimate load carrying capacity is significant for both GFRP and CFRP strengthening. The increase ratio is almost the same for both FRPs. It should be noted that the CRef has slightly higher value because of 1 mm steel plate was used compared with 0.9 mm steel plate used in GFRP testing series. The increase in ultimate load found in this study and in [1] is much higher than those (11% to 14%) reported in [46]. It can be seen from Fig. 15 that the behavior of CFRP and GFRP is quite similar when the number of layers are two. For two layers of FRP strengthening with orientation 1, the envelop curves are very close to each other. For two layers of FRP strengthening with orientation 2, the CFRP curve is higher for deflection up to 10 mm. When 4 layers FRPs are used, CL4O3 specimen reached the peak at a displacement of 5 mm, whereas specimen GL4O3 reached its peak around 12 mm while the two peak loads are almost the same. It seems that for CFRP and GFRP, adding another 2 layers can still provide additional benefit in terms of ultimate strength.
Mode-4. rupture in the main direction of FRP layers Mode-5. joints failure Mode-6. steel yielding; (ductile mode) The results showed multiple failure modes for FRP-strengthened steel plate under shear loading. For all strengthened specimens, after buckling of composite plates, some local bond failures at the interface of steel plate and adhesive (Mode-1) were observed. In CL2O1 and CL2O2 specimens which were strengthened by two layers of CFRP, at drift ratios larger than 1.05% transverse tears (Mode-3) were observed in the CFRP layers. In CL2O1 specimen fiber rupture was noticed. However, in CL2O2 specimen, at drift ratios larger than 1.9% local rupture of fibers (Mode-4) were observed. In CL4O3 specimen, which was strengthened by two orthogonal layers of CFRP on each side, at drift ratios larger than 0.8% fiber ruptures (Mode-4) were observed. By increasing the displacement, rupture of fibers started to grow and the shear strength of specimen started to decrease. 224
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4.2. Stiffness Comparisons of secant stiffness of the CFRP and GFRP strengthened specimens are presented in Fig. 16. It can be seen that the behavior of CFRP and GFRP is quite similar when the number of layers are 1 and 2 on each side. This is in consistent with that observed from Fig. 15 reported in Section 4.1. For specimens with two layers of FRP strengthening on each side, the secant stiffness is very close to each other, i.e. when comparing CL2O1 and GL2O1 or comparing CL2O2 and GL2O2. Fig. 16 also reveals that CFRP strengthening is more sensitive to the fiber orientation, orientation 2 gives larger secant stiffness than orientation 1. When four layers of FRPs are used, GL4O3 specimen demonstrated a large increase in secant stiffness compared with those with two layers of GFRP. However, the increase in secant stiffness of CL4O3 is not significantly higher than those with two layers of CFRP. Therefore, adopting 4 layers of GFRP can still gain extra benefit in terms of increasing secant stiffness. In the case of CFRP, the additional two layers does not provide additional benefit.
• •
Research is being conducted to simulate the behavior of CFRP and GFRP strengthened thin steel plates under cyclic shear loading using an analytical model or a finite element analysis. Acknowledgement The authors wish to acknowledge the support of the International Institute of Earthquake Engineering and Seismology (IIEES).
4.3. Energy absorption Cumulative dissipated energy values of the CFRP and GFRP strengthened specimens are presented Fig. 17. The results show that both methods of strengthening are effective at increasing the total amount of dissipated energy. In terms of energy, GFRP strengthened specimens showed better results and dissipated more energy. It can be seen from Fig. 17 that fiber orientation does not affect cumulative dissipated energy when two layers of CFRP are applied. The cumulative dissipated energy for strengthening with four layers of CFRP is less than that with two layers of CFRP. This can be explained by looking at Fig. 15 where load carrying capacity of CL4O3 starts to drop after a deflection of 5 mm due to CFRP rupture. On the contrast, additional cumulative dissipated energy was obtained for 4 layers of GFRP strengthening when compared with 2 layers of GFRP strengthening. The increase in energy absorption found in this paper (around 14– 22%) and in [1] (about 45–130%) is much higher than that (10%) reported in [46].
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5. Conclusions Experimental studies were carried out on CFRP-strengthened steel plates under quasi-static loading. The results showed that using CFRP laminate can be effective at improving the behavior of thin steel plate under shear loading. The following observations were made based on the limited test results:
•
• • • •
terms of increasing the ultimate load carrying capacity and secant stiffness. Fiber orientation does not affect cumulative dissipated energy when two layers of CFRP on each side are applied. Both GFRP and CFRP strengthening significantly increase the ultimate load carrying capacity. The increase ratio is almost the same for both FRPs. Adopting 4 layers of GFRP can still gain extra benefit in terms of increasing load carrying capacity, secant stiffness and cumulative dissipated energy. In the case of CFRP, the additional two layers does not provide additional benefit for stiffness and energy absorption because the failure is governed by CFRP rupture.
Applying CFRP laminates can significantly increase yield strength, ultimate strength, and secant stiffness. Fiber orientation is an important factor in the shear strengthening. The highest shear strength and stiffness can be obtained by applying the principal orientation of CFRP laminates along the direction of the tension fields. When strengthening the steel plate by a layer of CFRP laminate on each side, after debonding, transverse shear tearing was observed at the drift larger than about 1%. At drifts larger than 1.9% fiber failures were initiated. By adding two layers of CFRP laminate on each side, transverse shear tearing can be avoided. The failure in the CFRP-strengthened specimen under shear loading is caused by a combination of several modes. The first failure mode that has been observed is debonding between the steel plate and the adhesive. CFRP-strengthening method is useful for increasing the dissipated energy of thin steel plate under shear loading. By changing the angle of fibers, little change was observed in the amount of dissipated energy. For CFRP strengthening, orientation 2 is better than orientation 1 in 225
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