Elsevier

Marine Structures

Volume 61, September 2018, Pages 46-61
Marine Structures

Structural behavior of tubular X-joints strengthened with collar plate under axially compressive load at elevated temperatures

https://doi.org/10.1016/j.marstruc.2018.03.012Get rights and content

Highlights

  • The effect of the joint geometry and collar plate size were investigated on static strength of X-joints subjected to brace compression at elevated temperatures.

  • The welds joining of the collar plates to the chord wall and also, the contact between the collar plates and the chord member were modeled.

  • A new design equation was proposed to determine the ultimate capacity of X-joints strengthened with collar plates subjected to brace compression at elevated temperatures.

Abstract

This paper investigates elevated temperature resistance of circular hollow section X-joints reinforced with collar plates subjected to brace compression. A finite element (FE) model was developed and the results were validated against the data available from seven experimental tests. Also, the results of the present FE model were compared with the numerical results of other researchers. Afterwards, several 3-D FE-models of the collar plate reinforced X-joints were generated to investigate the effect of joint geometry and collar plate size on the initial stiffness, ultimate strength, and failure mechanisms at different elevated temperatures. Results showed that the use of the collar plates in X-joints at elevated temperatures (200 °C, 400 °C, and 600 °C, and 800 °C) leads to the significant increase of the initial stiffness and ultimate strength. Also, the increase of the plate length results in the less reduction in ultimate strength, at all elevated temperatures. But, the effect of the plate thickness on the reduction factor can be ignored. Moreover, the decrease of the β in the plate reinforced X-joints at elevated temperature can lead to the shifting of the maximum plastic strain from joint intersection to the toe of the weld joining the collar plate to chord member. The parametric investigation was followed by the non-linear regression analysis to propose a parametric equation for determining the ultimate strength of the collar plate reinforced X-joints under brace compression at elevated temperatures.

Introduction

In recent years, extensive researches have been carried out to investigate the performance of tubular joints subjected to extreme loading conditions, such as impact, fire, and blast. Among these hazardous events, fire is one of the most difficult design conditions in offshore tubular structures, because steel material is sensitive to the elevated temperatures [l]. It is important that the performance of the tubular joints at elevated temperatures is thoroughly understood and reliable techniques are accessible to determine their capacities [2].

This paper investigates the effect of the geometrical parameters on the initial stiffness, ultimate strength, and failure mechanisms of the collar plate reinforced tubular X-joints subjected to brace compression load at elevated temperatures. Tan et al. [1] investigated structural performance of un-reinforced tubular T-joints in fire condition. Ozyurt et al. [2] numerically investigated the ultimate strength of T-, Y-, X-, N- and non-overlapped K-joints at elevated temperatures. Gao et al. [3] investigated T-joints reinforced with collar plates in a fixed axial load at elevated temperatures. They investigated fire endurance time, critical temperature, and failure pattern. Shao et al. [4] discussed two methods for determining the static strength of tubular T-joints at elevated temperatures. Nassiraei et al. [5] experimentally investigated the static behavior of the collar plate reinforced X-joints in brace compression. Van der Vegte et al. [6] and Choo et al. [7,8] carried out some experimental and numerical tests on the static behavior of the collar plate reinforced T-joints subjected to brace load. Shao [9] experimentally investigated the effect of the collar plate on the static strength of tubular T-joints. Choo et al. [10,11] numerically investigated the advantages of the collar plates on the static capacity of the X-joints. Nassiraei et al. [[12], [13], [14]] studied the effect of the collar plate size and the joints geometry on the static capacity of T/Y-joints in three different types of loading. Shao et al. [15,16] indicate that collar plate reinforced T-joints have a more ductile characteristic than the corresponding unreinforced joint. Feng et al. [17] and Chen et al. [18] studied the advantages of the collar plate on the static capacity of SHS T- and X-joints. Qu et al. [19] studied the advantages of the collar plates on the dynamic behavior of CHS T-joints. Berkhout [20] and, Cai and Shao [21] indicated that the collar plates have the remarkable effect on the decrease of the SCF values.

Past studies indicate that the collar plate reinforcement method is very valuable for reinforcing tubular joints and it is among the most useful methods for enhancing the static strength at ambient temperature. But, there is not any study on the static capacity of the tubular X-joints reinforced with collar plates at elevated temperatures. Moreover, no formula is accessible to determine the ultimate strength of CHS X-joints reinforced with collar plates at elevated temperatures. Hence, there is an urgent need for further research detailed information and instructions on the static evaluation of the collar plate reinforced joints at elevated temperatures can be formulated. Fig. 1 depicts a tubular X-joints reinforced with the collar plates.

In the present paper, in the first step, the present FE model was proposed and verified by the experimental data provided by the present authors in Ref. [5] and Tan et al. in Ref. [1]. Also, the results of the present FE model were compared with the numerical result published by Ozyurt et al. [2]. After that, 360 3-D FE-models were generated and analyzed (Fig. 1) to investigate the effect of the collar plate size and joint geometry on the static strength of X-joints at elevated temperatures. Finally, a new equation was proposed for the determining of the ultimate strength of X-joints reinforced with collar plates subjected to compressive load at elevated temperatures.

Section snippets

Material nonlinearity

In this study, isotropic hardening plasticity model is used to define nonlinear material behavior. In addition, von Mises failure yield criterion is applied in the numerical modeling. The yield stress of the chord member, collar plates, and brace members at ambient temperature is assumed 325 N/mm2. In addition, Young's modulus (E) at ambient temperature and Poisson's ratio (ν) were taken to be 210 GPa and 0.3, respectively. The elevated temperature stress-strain curves were defined according to

Verifications of the FE model

In this section, the results of present developed FE model are validated with experimental data provided by the present authors in Ref. [5] and Tan et al. in Ref. [1]. After that, the results of present developed FE model are compared with the numerical results of published by Ozyurt et al. in Ref. [2].

Details of parametric study

A parametric study was carried out to investigate the effect of geometrical parameters on the initial stiffness, ultimate strength, and failure mechanisms of the collar plate reinforced X-joints under compressive load at elevated temperature. The geometrical parameters include; the ratio of chord diameter to twice of chord thickness (γ), diameter ratio between brace and chord (β), the collar plate thickness factor (τc = tc/T), and the collar plate length factor (η = δc/d). To evaluate the

Formulation for static capacity of X-joints reinforced with collar plates

Up to now, there has been no formula for the determining of the ultimate strength of CHS joints reinforced with the collar plates at elevated temperatures. In the current research, a new parametric equation is proposed for determining the ultimate strength of collar plate reinforced X-joints under compressive load at elevated temperature.

In order to develop the parametric formula, multiple nonlinear regression analyses were implemented by the statistical software package, SPSS. Values of the

Conclusions

360 3-D FE-models, verified against the experimental data, were analyzed to investigate the performance of X-joints reinforced with the collar plates subjected to brace compression at elevated temperatures. Main conclusions are summarized as follows:

  • The increase of the temperature leads to the remarkable decrease of the initial stiffness and ultimate strength of the collar plate reinforced X-joints.

  • The increase of collar plate length and collar plate thickness leads to the increase of the

Acknowledgement

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

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