Elsevier

Marine Structures

Volume 77, May 2021, 102936
Marine Structures

Local joint flexibility of tubular T/Y-joints retrofitted with GFRP under in-plane bending moment

https://doi.org/10.1016/j.marstruc.2021.102936Get rights and content

Highlights

  • 139 3-D FE models are generated and analyzed to investigate the Local Joint Flexibility (LJF) of the tubular T/Y-joints reinforced with fiber reinforced polymer (FRP).

  • The effect of the FRP layers (number, length, and orientation), the brace inclination angle (θ), and the connection geometry (θ, β, and γ) is investigated.

  • A design equation is proposed to determine the fLJF of the T/Y-joints reinforced with FRP.

Abstract

In the present paper, the Local Joint Flexibility (LJF) of tubular T/Y-joints retrofitted with Glass Fiber Reinforced Polymer (GFRP) under IPB moment is studied and discussed. For this aim, a finite element (FE) model was generated and verified with the results of available experimental data and parametric formulas. Afterward, a set of 158 finite element (FE) models was created to evaluate the efficacy of the FRP sheets (number, length, and orientation), the brace inclination angle (θ), and the non-dimensional parameters (β, τ, and γ) on the LJF coefficient (fLJF) and the fLJF ratio of the retrofitted to the associated un-retrofitted joint. In the FE models, the efficacy of the weld profile and the contact between the FRP and the steel members (chord, weld, and brace) was considered. Also, analysis of variance (ANOVA) is used to identify the most dominant parameters which affect the fLJF ratios. Results showed that in the retrofitted joints, the increment of the FRP sheet number results in the notable drop of the fLJF. But, the efficacy of the FRP sheet orientation on the fLJF can be ignored. Despite the considerable efficacy of the FRP sheets on the behavior of the tubular joints, there was not any study on the LJF in the joints retrofitted with FRP. Hence, after an extensive parametric study, the results were used to derive a parametric equation for determining the fLJF of T/Y-joints retrofitted with FRP. Moreover, the derived equation was checked according to the UK DoE acceptance criteria.

Introduction

Circular hollow section (CHS) members are extensively applied as the primary components in offshore tubular lattice structures [1]. The CHSs are connected together to form a tubular connection where the profiled ends of secondary members (the braces) are welded onto the circumference of the main member (the chord) [2]. For tubular joints, the local joint flexibility (LJF) happens simply when the joint is under basic loadings such as IPB moment [3]. The LJF causes the growth of the nominal stresses, deflections, and variation the natural frequencies of the structure. In addition, LJF can simply cause plastic failure of the chord wall [4,5].

So far, several methods are proposed for enhancing tubular joints, such as FRP method [[6], [7], [8]], collar plate method [[9], [10], [11]], doubler plate method [[12], [13], [14]], external ring method [15,16], internal ring method [17,18], grout method [19,20], joint can method [21]. Fig. 1 shows a Y-connection retrofitted FRP.

In spite of the important role of the LJF in evaluating the global static and dynamic responses of tubular connections, the LJF of the connections with FRP has not been investigated. Consequently, there is an essential demand for further investment so that more detailed guidelines on the LJF of the connections retrofitted with the FRP which can be formulated.

In this work, in the first step, a FE model was generated. In the FE model, the weld along the chord/brace junction was generated. Besides, using ANSYS contact ability, the interface between the bottom face of the FRP and the outer face of the chord, weld, and brace wall was modeled. In the next step, the precision of the generated FE model was evaluated with the data of three experimental tests and nine models produced by the equation derived by Fessler et al. [22] and three experimental tests carried out by Lesani et al. [23]. After that, 158 FE models were generated to evaluate the efficacy of the FRP sheets (number, length, and orientation), the brace angle (θ), and the joint geometry (θ, β, and γ) on the fLJF of the FRP retrofitted connections (fLJF,frp), and the ratios of the fLJF,frp to the fLJF of the associated un-retrofitted connection (fLJF,u). According to the results of FE models, a full fLJF database was performed and finally, an equation was proposed. This equation reflects the efficacy of the FRP, the brace inclination angle, and connection geometry on the fLJF,frp. Also, the formula is checked with the acceptance standard proposed by the UK DoE [24].

Section snippets

Literature survey

The following two paragraphs evaluate the research works carried out on the LJF of tubular connections, and the research works conducted on the joints retrofitted with FRP.

Determination of the fLJF

The LJF of tubular joints defines as the displacement attributed to the local chord deformation occurred by a unit load. To determine the LJF at the joint junction, the rotation of the joint due to the overall displacement must be removed from the total occurred rotation. The local rotation at the joint can be calculated without taking the beam movement. It obtains the distortion of the circular section which is oval shape in IPB moment [2]. The LJF determines as indicated in Eq. (1).LJF=φIPBMIP

Details of FE models

In this work, the weld along the chord/brace junction was modeled as stated by the American Welding Society [53]. The details of the weld profile are depicted in Fig. 3 and Table 1. The same as other research works conducted on the LJF in tubular joints [3,[33], [34], [35], [36]], and [[39], [40], [41]], the properties of the weld are the same as the properties of the brace. Also, the SOLID186 element and SHELL 281 element were applied to model the steel members and FRP sheets. A sub-zone mesh

Validation of FE results

The accuracy of FE results should be validated. As far as the authors are aware, no experimental/FE/theoretical data of fLJF for joints retrofitted with the FRP is available in the literature. Due to this reason, the following works are utilized to evaluate the accuracy of the FE model. Three experimental Y-joints in IPB moment reported by Fessler et al. [22], nine Y-joints in IPB moment produced by the formula suggested by Fessler et al. [22], and three experimental T/Y-joints retrofitted with

Characteristics of the parametric study

A total of 158 FE models were created, in the FE software package ANSYS19.0, to evaluate the efficacy of the FRP sheet number, FRP sheet length, FRP sheet orientation, connection geometries, and the angle between brace and chord on the fLJF of the retrofitted T/Y-joints in IPB moment. The parameters include; FRP sheet number (N), FRP length coefficient (η), FRP sheet orientations, the diameter proportion between brace and chord (β), the proportion of the radius to wall thickness of the chord (γ

Parametric formula for the prediction of the fLJF values of the joints under IPB load

Until now, no equation is available to determine the fLJF of connections retrofitted with FRP. Therefore, in this section, by using the results of FE analyses and the statistical software package, SPSS V22, a parametric equation is proposed for this problem.fLJF,GFRP = N−0.178η−0.017β−2.14τ−0.48γ2.025θ1.54; R2 = 0.971Where fLJF,GFRP is the fLJF of the T/Y-connections with FRP. N shows the GFRP sheet numbers. η shows the FRP length coefficient and it is equal to δFRP/d. R2 is the determination

Conclusions

158 FE models were produced to evaluate the efficacy of FRP sheets (length, number, and orientation), the brace inclination angle (θ), and the joint geometry (β, θ, τ, and γ) on the local joint flexibility coefficient (fLJF) of the T/Y-connections retrofitted with FRP in IPB moment. The main conclusions of this paper are as follows:

  • (1)

    Validation of the FE results using experimental data and empirical equations showed that the FE model can well predict the fLJF of T/Y-joint retrofitted with FRP

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge the useful comments of anonymous reviewers on the draft version of this paper.

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