FUNDAMENTAL STUDY ABOUT THE IMPACTS OF FIBER S ANGLE FOR THE STRENGTHENING OF STEEL STORAGE TANKS UNDER BENDING SHEAR LOAD

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 4, April 2018, pp , Article ID: IJCIET_09_04_167 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed FUNDAMENTAL STUDY ABOUT THE IMPACTS OF FIBER S ANGLE FOR THE STRENGTHENING OF STEEL STORAGE TANKS UNDER BENDING SHEAR LOAD Phan Viet Nhut Department of Architecture and Civil Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi, Japan Yukihiro Matsumoto Department of Architecture and Civil Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi, Japan ABSTRACT In recent years, a large number of cylindrical steel storage tanks (CSSTs) have appeared increasing signs of deterioration because of a large earthquake, corrosion or ageing degradation. In order to achieve the sustainable development of the economy and infrastructures, these existing tanks need to be strengthened. The conventional method often involves adding of heavy and bulky plates that are difficult for fixing and can interrupt the production. On the other hand, the use of carbon fiber reinforced polymers (CFRPs) with many outstanding characteristics such as lightweight, high strength, and high corrosion resistance to increase the load capacity and ductility of steel structures recently proved to be an economical and reliable strengthening solution. In this paper, the strengthening effects of the angle of carbon fiber on the increase of the load-carrying capacity of CSSTs will be investigated under bending shear loads by finite element analysis (FEA). The results show that the angled CFRPs have remarkable effects on the increase of ultimate strength of the tanks. Keywords: Cylindrical steel storage tanks, bending shear load, loading carrying capacity, strengthening, carbon fiber reinforced polymer Cite this Article: Phan Viet Nhut and Yukihiro Matsumoto, Fundamental Study about the Impacts of Fiber s Angle for the Strengthening of Steel Storage Tanks under Bending Shear Load, International Journal of Civil Engineering and Technology, 9(4), 2018, pp editor@iaeme.com

2 Fundamental Study about the Impacts of Fiber s Angle for the Strengthening of Steel Storage Tanks under Bending Shear Load 1. INTRODUCTION Large thin-walled CSSTs have been used in many fields of industry and infrastructures. They play important roles in the development of the whole production. The destruction of CSSTs will cause the interruption of the production process, even lead to disaster. However, a large number of CSSTs have appeared increasing signs of deterioration and reducing load-carrying capacity because of corrosion or ageing degradations. Moreover, the large earthquakes with highly recorded magnitudes occurred frequently in recent years, especially in the Pacific Ocean such as Tohoku-Chiho Taiheiyo-Oki earthquake 2011 (Japan), Chile Off Maule earthquake 2010, New Zealand Christchurch earthquake, etc. [1,2]. This causes the damage for not only degraded tanks but also existing healthy tanks. For achieving sustainable development of infrastructure and economy, these structures need to be strengthened to improve the performance and seismic capacity. The conventional method of repairing or strengthening of these existing structures often involves bulky and heavy plates that are difficult to fix and prone to corrosion [3]. Moreover, this method could cause business interruption and the damage of materials. In recent years, the use of carbon fiber reinforced polymers (CFRPs) with many outstanding characteristics such as lightweight, high strength, and high corrosion resistance to increase the load capacity and ductility of steel structures proved to be an economical and reliable strengthening solution [3]. In the previous analysis [4], the effects of unidirectional CFRP layers on the strengthening of CSSTs were investigated. However, the strengthening effects were trivial with the tanks having large ratios of inside tank s radius to the thickness and small ratios of the tank s height to the inside tank s radius. In this paper, the carbon fiber s angles will be considered in order to increase the strengthening effects of CFRP layers on CSSTs under bending shear load. The results will provide fundamental information of material design using FRPs to obtain rational strengthening effects. 2. MATERIAL PROPERTIES OF CFRP LAYERS AND STEEL STORAGE TANKS 2.1. CFRP layers CFRP layers contain carbon fiber layer and matrix. The volume fraction of fiber v f and of matrix v m are 50% and 50% respectively. One layer of CFRP and matrix has the thickness of mm. The longitudinal modulus of the CFRP fiber and matrix are E f = (MPa) and E m = 4100 (MPa) respectively [5]. From these parameters, the material properties of CFRP layers are determined by the composite rules. In this paper, four types of CFRP layers were used to strengthen CSSTs. Type-A and type-c have total circumferential CFRP layers. Type-B and type-d have total CFRP layers in (±45 o ) direction. Table 1 shows the evaluated properties of CFRP layers. In the table, E x and E y are the longitudinal and transverse elastic modulus of CFRP layers, G xy is the shear modulus of CFRP layers. E x and E y are also elastic modulus of CFRP layers in the circumferential and vertical direction in FEA Steel storage tanks The tanks use the typical mild steel with the material properties as shown in Figure 1. The yield stress of this steel is 235 (MPa) and the elastic modulus of steel E=205 (GPa). The stress-strain relationship of steel used for the tanks has the tri-linear shape. Von Mises stress (MPa) editor@iaeme.com

3 Phan Viet Nhut and Yukihiro Matsumoto Figure 1 Material properties of steel Table 1 The properties of CFRP layers Thickness (mm) E x (MPa) E y (MPa) G xy (MPa) Poisson ratio Type-A Type-B Type-C Type-D FINITE ELEMENT ANALYSIS D finite element models 3D nonlinear analysis (LUSAS package) was used to analyze the failure buckling modes and the ultimate strength of CFRP-strengthened CSSTs subjected to internal pressure under bending shear load. The internal pressure with the ratios between tensile hoop stress to yield stress of tank s material σ h /σ y = 0.5 and 0.7 was analyzed. In total, there were 10 models in finite element analysis as shown in Table 2. Figure 2 Half models in finite element analysis In these models, the tank's walls were simulated with 3D quadratic solid elements and the tank's roofs were used the surface elements with rigid materials. These rigid elements can help the load transfer from the top of the tanks to the tanks easily. Mesh sizing and the number of analysis iterations play important roles in the analysis. The mesh sizing and the analysis iteration increment must small enough so that the convergence can occur. In addition, small mesh sizing assures the accuracy of the analysis results and collapse behavior of the tanks. In this analysis, half models were applied for all the tanks because of symmetrical conditions. In the tank's walls, there were 170 elements in the circumferential direction, 125 elements in the height direction, and 2 elements in the thickness direction. The mesh sizing was dived smaller at the bottom of the tank so that the convergence of the analysis occurred easily editor@iaeme.com

4 Fundamental Study about the Impacts of Fiber s Angle for the Strengthening of Steel Storage Tanks under Bending Shear Load In this paper, the analysis tank has the parameters h/l/r/t (mm) = 6200/5000/7490/10. In these parameters, h is the distance between the lower edge of the tank to point where horizontal loading is applied, r is the inside radius of the tank, l is the height of the tank, and t is the thickness of the tank. 8 models with different types of CFRP-strengthened layers were analyzed as shown in Table 2. Models from A1 to A5 were subjected to the internal pressure with σ h /σ y =0.5, while models from B1 to B5 were subjected to the internal pressure with σ h /σ y =0.7. The analysis will be stopped when getting one of two conditions. Firstly, the maximum displacement at the top of the tank s wall was constrained to the value of one percent of the tank s height (50mm). This is because the big displacement may affect the equipment connected to the tanks. Secondly, in the cases of strengthening with CFRP layers, the analysis results will consider the damage of CFRP layers. The analysis will be stopped when the strain in the CFRP layers reaches the breaking strain of CFRP layers in fiber direction (1.4%). This breaking strain value was calculated from the properties of CFRP layer [5]. Table 2 The models in FEA Model Strengthening type A1, B1 Without strengthening A2, B2 Type-A A3, B3 Type-C A4, B4 Type-B A5, B5 Type-D 3.2. Boundary conditions The tanks were fixed at the bottom surface, two surfaces on the tank's walls and two lines on the tank's roofs were applied symmetrical conditions as shown in Figure Loading conditions The loads were applied to two steps. In the first step, only the internal pressure was applied to the analysis. After finishing this step, the results were fixed and then the horizontal loading was assigned. The values of internal pressure were defined according to the ratios of σ h /σ y. From the tensile hoop stress σ h, the values of the internal pressure were defined by the equation below. The values of internal pressure were (MPa) for σ h /σ y =0.5 and (MPa) for σ h /σ y =0.7. p = σ t / r h 1 4. RESULTS AND DISCUSSIONS 4.1. Buckling Modes Figure 4 shows the buckling modes and Von Mises stress of tanks. The main buckling mode of tanks without strengthening is elephant foot bulge (EFB) and shear buckling with σ h /σ y =0.5; whereas the failure mode was only EFB with σ h /σ y =0.7. EFB usually appears at the bottom of the tanks (20% of the height of the tanks). The EFB occurs easily at the bottom if the ratio σ h /σ y is larger editor@iaeme.com

5 Phan Viet Nhut and Yukihiro Matsumoto Figure 4 Von Mises stress and deformation of the tanks. Unit: MPa and mm. When strengthening with CFRP layers, the EFB at the bottom of the tanks was limited and the shear buckling occurred at the tank s walls. The appearance density of shear buckling depends on the thickness of CFRP-strengthened layers and the ratio σ h /σ y editor@iaeme.com

6 Fundamental Study about the Impacts of Fiber s Angle for the Strengthening of Steel Storage Tanks under Bending Shear Load 4.2. The effects of carbon fiber s angle on the strengthening of CSSTs Figure 5 shows the maximum horizontal loading P max obtained from FEA in the cases of tanks. It is clear that the maximum loads increased when strengthening the tanks with CFRP layers in all the cases. The increasing levels of the maximum loads depend on the values of internal pressure and types of CFRP layers. Figure 6 shows the relationship between the horizontal loading and the displacement at the top of the tank's wall for all the models. In the cases of tanks without CFRP strengthening, the loading decreased rapidly after the tanks reached the maximum load capacity. This is very dangerous for the working of the structures. However, when strengthening with CFRP layers, the loading decreased slowly and more stable. This is the very important thing for the buckling restraint of the tanks under the loading, especially seismic loading. Figure 5 The maximum loading and strengthening effects of CFRP layers Figure 6 The loading-displacement relationships Concerning about the effects of fiber s angle on the increase of ultimate strength of the tanks, the strengthening effects of different types of CFRP layers have close relationships with the buckling types of the tanks. Firstly, because the buckling mode of the tank without strengthening is EFB and shear buckling in the case of σ h /σ y =0.5 while only EFB occurs with σ h /σ y =0.7, the circumferential CFRP layers (type-a and type-c) have remarkable effects in models B2 and B3 than models A2 and A3. This is because the circumferential CFRPstrengthened layers are unidirectional materials; therefore, the strengthening effects of CFRP layers against EFB on the circumferential directions were clear. Secondly, the buckling modes changed after strengthening the tanks with CFRP layers. EFB was restrained and shear buckling happened more in the tanks walls. This is the reason why angled-cfrp layers (type-b and type-d) had more strengthening effects clearly than circumferential CFRP layers. Moreover, because the thickness of type-d is higher than type-c, the effects of shear buckling restraint of models A5 and B5 are better than models A4 and B editor@iaeme.com

7 Phan Viet Nhut and Yukihiro Matsumoto In all models, the strengthening effects were better with the higher values of internal pressure (σ h /σ y ). The maximum strengthening effect was found at model B5 (91.9%) and strengthened with angled-cfrp layers. 5. CONCLUSION Finite element analysis was used to investigate the impacts of fiber s angle on the effectiveness of the strengthening of CSSTs by CFRP layers. From the analysis results, some main points were concluded as follow. In the cases of tanks without strengthening, elephant foot bulge and shear buckling are the failure modes of the tank with the low value of internal pressure (σh/σy = 0.5); whereas only elephant foot bulge appears when the tank subjected to high value of internal pressure (σh/σy = 0.7). Elephant foot bulge is restrained and shear buckling occurs more when CFRP layers strengthen the tanks. The types of buckling modes will decide the strengthening effect of different types of CFRP layers. The ultimate strength of the tanks increased when strengthened by CFRP layers. The increasing levels of the ultimate strength depend on the types of CFRPstrengthened layers and values of internal pressure. In all the cases, the angled- CFRP layers have significant effectiveness than circumferential CFRP layers because the main buckling modes after strengthening are shear buckling. The strengthening effects are clearly seen with the tanks subjected to higher values of internal pressure. 6. FUTURE WORKS The results are only analyzed in one kind of tank in this paper. It is necessary to investigate with many types of tanks in the future to clear the strengthening effects of fiber s angle on the increase of ultimate strength of the tanks. ACKNOWLEDGEMENTS This work was supported by JSPS KAKENHI Grant Number 17K REFERENCES [1] Reconnaissance Report on the 2010 Chile off Maule Earthquake and Reconnaissance Report on the 2011 New Zealand Christchurch Earthquake: Architectural Institute of Japan (AIJ), September [2] Preliminary Reconnaissance Report of the 2011 Tohoku-Chiho Taiheiyo-Oki Earthquake: Architectural Institute of Japan (AIJ), [3] X Zhao. FRP-Strengthened Metallic Structures (CRC Press), chapter 1, pp [4] P, V, Nhut. and Y, Matsumoto. The Effects of Carbon Fiber Reinforced Polymer Strengthening on Cylindrical Steel Storage Tanks under Bending Shear Load. Proceedings of International Conference on Building Materials and Construction (ICBMC), [5] High-performance carbon fiber Torayca, Torayca cloth. [6] I. Tegos, N. Giannakas and T. Chrysanidis. Cross-Correlation of Stresses in the Transverse Reinforcement under Shear Load and Confinement. International Journal of Civil Engineering and Technology, 8(1), 2017, pp editor@iaeme.com