Design Formulas of Concrete Filled Circular Steel Tubes Reinforced by Carbon Fiber Reinforced Plastic Sheets

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1 Available online at Procedia Engineering 14 (2011) The Twelth East Asia-Paciic Conerence on Structural Engineering and Construction Design Formulas o Concrete Filled Circular Steel Tubes Reinorced by Carbon Fiber Reinorced Plastic Sheets J. W. PARK a, Y. K. HONG b, G. S. HONG b, J. H. KIM c and S.M. CHOI a* a Department o Architectural Engineering, University o Seoul, Korea b School o Architecture, Hong-Ik University, Korea c Research Institute o Industrial Science & Technology, Kyonggido, Korea Abstract Concrete-illed steel tube structure has advantages such as excellent deormation capacity, energy absorption and ireresistance. However, CFT lumn has the drawback o local buckling at steel tube due to the deterioration o ninement eect ater steel tube yield. To solve the problem, CFT lumn system reinorced by CFRP(Carbon Fiber Sheets) will be introduced and the structural behaviors o CFRP reinorced lumns will be mpared with those o the current CFT lumns. The main experimental parameters are diameter-thickness (D/t) and number o carbon FRP sheet layers. 10 specimens were prepared with the main experimental parameters. Axial mpression test were nducted and axial behavior maximum strength capacity, and ductility capacity were analyzed between CFRP reinorced lumns and the current CFT lumns. To propose o design ormula or FRP reinorced CFT lumns, ninement eectiveness eicient o nined ncrete were proposed based on the experimental results. Finally, base on the eicient, the axial design ormula or FRP reinorced CFT lumns were proposed and mpared the design strength values with the experimental strength values Published by Elsevier Ltd. Open aess under CC BY-NC-ND license. Keywords: CFT, FRP, Carbon Fiber Sheet, Coninement eect 1. Introduction The ncrete inside CFT lumns signiicantly improves Concrete Filled Steel Tubular (CFT) * Corresponding author: smc@uos.ac.kr Published by Elsevier Ltd. Open aess under CC BY-NC-ND license. doi: /j.proeng

2 J.W. PARK et al. / Procedia Engineering 14 (2011) lumns are widely used in structures such as high-rise buildings, bridge piers etc. CFT lumns have recently beme well known because o their beneit in nstruction and their structural advantages. In terms o nstruction, because the steel tubes can be used as a ormwork, enomic eects can also be expected in nstruction ields. In terms o structural aspects, when a CFT lumn is subjected to an axial mpression load, the inill ncrete laterally expands and the expansion is nined by a circular steel tube. Consequently, the ncrete illed inside circular steel tube eatures a 3-axis-stress, which improves the mpressive strength o the lumn. The ductility capacity o CFT lumns is also improved over that o the steel tube lumns due to the inill ncrete. However, the CFT lumn has disadvantages. In terms o nstruction, the beam-lumn nnections or CFT rames are mplicated, causing an increase in nstruction sts. In terms o structural aspects, ater the yielding o steel tubes, the transverse ninement o a CFT rame to inill ncrete is drastically reduced. Local buckling may our at the steel tubes ollowed by the crushing o inill ncrete (Sakino 1981) and the local buckling can reduce the strength o the CFT lumns. Previously, reinorcement with an additional layer o steel plate was employed in nstruction engineering to mitigate the ailure at the lower part o lumns caused by cyclic lateral load (Xiao 2002). Stieners or steel plates were used to ntrol the local buckling o CFT lumns. However, reinorcement with steel plates has drawbacks; increased weight due to the additional layer o steel plate, the mplicated process, changes in material property due to steel plate welding, and deteriorated durability against climatic changes. The necessity or new reinorcement materials has emerged in order to solve these problems. In recent years, research has been carried out on FRP strengthening or structural members. FRP have a high strength-to-weight ratio and high rrosion resistance. This property also provides a signiicant advantage in site handling, reducing nstruction st. For strengthening CFT lumns, additional transverse ninement by carbon iber sheets (CFS) is designed or the potential plastic regions to improve the structural loading capacity. Xiao (2005) nducted axial and cyclic tests or the circular CFT lumns nined by CFS. From the test results, providing additional ninement by CFS can increase axial loading and ductility capacity. For the cyclic tests, seismic capacity also improves because delaying local buckling ourred at the end o the lumn by additional nining CFS. Park (2009, 2010) nducted axial and cyclic tests or rectangular CFT lumns nined by CFS. From the tests, the ninement by carbon iber sheets delayed local buckling and the nining eect became obvious as the number o layers increased. Additional ninement o the area surrounding local buckling by carbon iber sheets did not result in a signiicant improvement in the load capacity and stiness o the lumns. From the previous research, FRP mposites have been used or strengthening CFT lumns. This application provides the most eective solution or strengthening CFT lumns. In this study, axial tests or the current and FRP nined circular CFT lumns were nducted. A total o 10 specimens were prepared and tested under ncentric axial mpression loads. The main variables are depth-thickness ratio (D/t) and the number o CFS layers and the test results will be presented. Finally, to apply use FRP nined CFT lumns in the nstruction ield, the design equation or FRP nined CFT lumns should be necessary. Thereore, in this study, The stress strain model o ncrete reinorced with carbon iber sheets which is used in ACI mminute 440 was used to derive axial load capacity design ormula. Finally, simpliied axial design ormula will be presented rom nined ncrete equation which is obtained rom Xiao and Park data. 2. Test plan and result 2.1. Test plan 10 specimens were abricated, as shown in table 1. The main specimen parameters were depth-

3 2918 J.W. PARK et al. / Procedia Engineering 14 (2011) thickness ratio (D/t) and the number o CFS layers. For the circular section, D/t ratios ranged rom 21 to 33. The circular steel tubes were 139.8mm in diameter and 620mm in length. The thicknesses o steel tubes were 3.2mm, 4.5mm and 6.6mm. The design ncrete strength was 36MPa and 28-day target ncrete strength was 37.5MPa. The yield strength and tensile strength o 3.2mm, 4.5mm and 6.6mm steel were 365MPa, 334MPa and 301MPa, respectively. The numbers o CFS layers nining the CFT lumns were zero, one, and three in the test. However, one circular specimen (C4F-2) reinorced with 2 layers o carbon iber sheets was also abricated to observe the behavior dierent rom those reinorced with 1 and 2 layers. The design thickness and tensile strength o the carbon iber sheets provided by the manuacturer were 0.111mm and 3500 MPa, respectively. Table 1: Parameter plan Types Specimen Size(mm) D/t y CFS (MPa) layers C3N D C3F-1 D C3F-3 D C4N D Circular C4F-1 D C4F-2 D C4F-3 D C6N D C6F-1 D C6F-3 D Test setup The specimen setup is shown in Fig 2. Specimens were placed in a 3000kN UTM. 4 LVDTs were placed inside the top and bottom end plate to monitor axial deormation as shown in Fig 1. Axial deormations were obtained rom the average value o the LVDTs at each rner. To observe lateral deormation, 4 LVDTs were set at the middle part o the specimens as shown in Fig 1. A lumn load was applied at a very slow rate in order to careully observe the local buckling behavior o the CFT. Figure 1: Test setup

4 J.W. PARK et al. / Procedia Engineering 14 (2011) Specimen ailure procedure and Load displacement relationship The expansion o steel tube uld be seen on the entire body o specimen. As the steel tubes expanded, CFS was able to nine the expansion o the steel tubes. However, when the lateral displacement reached the ultimate tensile strain o CFS, CFS was ruptured and the local buckling zone then gradually expanded. Finally the local buckling was observed as shown in Figs 2 and 3. (a) C6N (b) C6F-3 Figure 2: Final ailure shape o specimens The load-displacements o specimens are shown in Fig 3. (a) D/t=44 (b) D/t=31 (c) D/t=21 Figure 3: Load-displacement curve o each specimen At the early loading stage, the current CFT specimen s behavior was linear elastic up to peak load and then the load dropped. Finally, axial displacement increased without deterioration o the load capacity as nstant deormation progressed. For the CFRP nined circular CFT lumn, the load linearly increased similarly to the current CFT lumns. However, CFRP nined circular CFT lumns had a bilinear curve, unlike the CFT lumns. In the irst linear stage, the behavior o the nined CFT was similar to that o the current CFT. In the send linear stage, CFRP ninement was activated and the load linearly increased. Ater maximum load point, the load dramatically dropped when CFRP was ruptured. 3. Design ormula o FRP strengthened circular Ct lumns 3.1 ACI 440 de

5 2920 J.W. PARK et al. / Procedia Engineering 14 (2011) When a circular steel tube or FRP sheet is illed with ncrete and is under mpressive loads, while the ncrete undergoes lateral expansion, hoop tensile orce o the steel tube or FRP provides ninement eect and lateral ninement is distributed evenly. For circular CFT lumns nined with FRP, stressstrain curves o circular test pieces nined with FRP not those o unnined ones should be used. While many studies have been nducted or the stress-strain curves o the ncrete nined by FRP, the ACI mmittee 440 uses equation (1) suggested by Mander (1998). This equation was originally developed or ninement provided by steel jacket. l l [ ] (1) Here,, and mean mpressive strength o nined ncrete, mpressive strength o unnined ncrete. In this study, because the specimens was simultaneously nined by steel tube and FRP sheet, lateral ninement pressure(l) o FRP reinorced CFT lumn can be expressed by equation (2). 2 st d 2 t d s FRP FRP l (2) Here, d is diameter o inilled ncrete 3.2 Simpliied nined ncrete model Acrding to the precious researches (Richart 1928, Lam 2002), strength models or nined ncrete take the ollowing orm equation (3). 1 k l 1 (3) Where, k 1 is the ninement eectiveness eicient and the lateral pressure l is obtained rom equation (2) In this study, the ninement eicient has been calculated based on 11 tests data on the mpressive strength o FRP strengthened circular CFT lumns rom researches o Xiao(2005) and Park(2009). The result is expressed by equation (4). l (4) 3.3 Proposed axial design ormula By using the ACI 440 des, the axial design ormula can be expressed as ollowings. P oc A F s y A 0.85 c (5) Where, can be obtained rom the equation (1) Compressive load capacity o a FRP-reinorced CFT circular lumn (Poc) obtained rom the ncrete model nined by FRP is as shown in equation (6).

6 J.W. PARK et al. / Procedia Engineering 14 (2011) P oc A F s y A c (6) The results o each equation obtained rom equation (5) and (6) are listed in table 2. Figure 4: Simpliied nined ncrete model o FRP strengthened CFT lumn Table 3: Comparison o test results and proposed equation results Specimen Pexp (kn) Poc(kN) Poc(kN) error (eqn 5) (eqn 6) error CCFT-2L CCFT-2L CCFT-4L CCFT-4L C3F C3F C4F C4F C4F C6F C6F Avg By using nined ncrete stress-strain curve, the mparative analysis with existing test data showed that the values obtained rom the test and suggestion were similar to each other. 4. Conclusion This paper presents a new CFT lumn system which is nined by CFRP. Additional ninement is provided or ntrolling the local buckling ourring at the surace o steel tubes. With the 10 specimens, axial loading tests were nducted. The main specimen parameters were the depth-thickness ratio (D/t) and the number o CFRP layers. The design equation was derived by using FRP and steel tube nined ncrete mpressive stress block. In summary, the ollowing nclusions were drawn in this study. 1) For FPR reinorced CFT lumn, the specimen was laterally expanded as loading intensiied. As the

7 2922 J.W. PARK et al. / Procedia Engineering 14 (2011) steel tubes expanded, CFS uld nine the expansion o the steel tubes. However, when the lateral displacement reached the ultimate tensile strain o the CFS, the specimen ruptured and the local buckling zone then gradually expanded. 2) CFRP nined CFT lumns uld signiicantly increased the axial loading capacity due to the additional ninement eect by FRP. However, ductility capacity was decreased because o FRP rapture and sudden deterioration in load capacity. 3) The design ormula or CFT lumn additionally nined with CFRP was suggested. The nined ncrete strength model o ncrete reinorced with CFRP which is used in ACI mminute 440 and simpliied nined ncrete model were also used in this study to derive axial load capacity design ormula. 5. Acknowledgments This work was supported by the National Research Foundation o Korea(NRF) grant unded by the Korea government(mest) (No. RoA ). Reerences [1] AISC. Steel Construction Manual Vol 2, 13th ed., American Institute o Steel Construction; [2] ACI 440R-96, (2002), Guide or the Design and Construction o Externally Bonded FRP Systems or Strengthening Concrete Structures, ACI Committee 440, 2002, pp [3] J.G.Teng., J.F. Chen., S.T. Smith., L. Lam. FRP strengthened RC structures, John Wiley & Sons, Inc; [4] F. E. Richart., A. Brandzaeg., R. L. Brown. A Study o the Failure o Concrete under Combined Compressive Stresses., University o Illinois, Engineering Experimental Station, Illinois., U.S.A; [5] Sakino, K., Tommi, M. Hysteretic Behavior o Concrete Filled Square Steel Tubular Beam-Columns Failed in Flexure. Transactions o the Japan Concrete Institute 3(6), 1981, pp [6] L. Lam., G. Teng., (2002), Strength Models or Fiber-Reinorced Plastic-Conined Concrete, Journal o Structural Engineering, ASCE, 128(5); [7] Xiao, Y., He. W., Choi. K.K. Conined Concrete-Filled Tubular Columns. Journal o Structural Eng, ASCE, 131(3), 2005, pp [8] J.W.Park., Y.K.Hong., G.S.Hong., S.M.Choi., (2009), Experimental study on ncrete steel circular tubes nined by carbon iber sheet under axial mpression loads. Journal o the earthquake engineering society o Korea, 2009, 13(5), pp