S.P. Chiew

Static Strength of Tubular T-Joints with Reinforced Chord under Axial Compression

A typical tubular T-joint is made up of a chord member and a brace member, and the brace member is connected to the chord surface with full penetration welds. As the stiffness of the chord in the radial direction is much weaker than that of the brace member in the axial direction, failure occurs easily at the weld toe on the chord surface when the chord is subjected to static axial load in the brace axial direction. To improve the static strength of a tubular T-joint, the chord near the weld toe can be reinforced locally by increasing its thickness. To study the effect of the chord reinforcement on the static strength of a tubular T-joint, 48 T-joint models with different reinforcements have been analyzed using the finite element method. The effects of the chord thickness and the length of the reinforced chord on improving the static strength of tubular joints have been investigated. It is found that the static strength can be greatly improved by increasing the chord thickness near the intersection. However, the increased chord thickness should not be much greater than the original chord thickness to avoid changing of the failure mode. On the other hand, it is not effective to improve the static strength by increasing the length of the reinforced chord. The effects of the geometrical parameters and the chord thickness on the static strength of the T-joints are also studied parametrically by analyzing another 240 T-joint models. Finally, a parametric equation is presented, and its accuracy in predicting the static strength of the tubular T-joint subjected to axial compression is verified through error analysis against the numerical results.

Stress intensity factors for a surface crack in a tubular T-joint

Abstract This paper describes a systematic modelling of a general cracked tubular Y-joint commonly found in offshore structures. The Y-joint under consideration may contain either a through-thickness or surface crack which can be of any length and located at any position along the brace–chord intersection. This is particularly significant, as it has always been found in practice that the initiation site of a surface crack does not always start from the saddle or crown position when tubular joints are subjected to a complex loading condition. The geometrical model developed in the work described in this paper includes the weld detail which is compatible with the American Welding Society (AWS) standard [1] . Based on this geometry, well-graded finite element (FE) meshes were generated for a T-joint, which is a specific type of Y-Joint, to obtain the stress intensity factors (SIFs) for a surface semi-elliptical crack along the crack tip using quarter-point elements. The numerical analysis indicated that the FE models generated are appropriate to the geometry of the joints since converging values of SIF were obtained as the meshes used were refined. The accuracy of the Mode I, Mode II and Mode III SIFs demonstrates that the proposed model is reliable.

Neural network-based estimation of stress concentration factors for steel multiplanar tubular XT-joints

Abstract The hot-spot stress method for fatigue design of tubular joints relies on the accurate predictions of the stress concentration factors (SCF) at the brace to chord intersection areas. At present, SCFs are predicted based on established empirical equations. An alternative approach using a neural network-based model has been developed in this paper to estimate the SCFs of multiplanar tubular XT-joints. The neural network software, Stuttgart Neural Network Simulator, was used for the purpose. To train and test the network, an SCF database was built up using the finite element method (FEM). The database covers a wide range of geometrical parameters for the XT-joints. Three axial load cases were considered. The geometrical properties of the tubular joints were used as the training input data. The FEM SCFs are used as the training output data. Different network configurations are also tested for the best possible results. The results show that a trained neural network can indeed predict the SCFs for the various load cases with a higher level of accuracy.

Strain concentrations at intersection regions of a multiplanar tubular DX-joint

Abstract A large-scale steel multiplanar tubular Double X-joint (DX-joint) specimen was tested such that its in-plane and out-of-plane braces were subjected to three different load cases of pure axial forces. The strain gradients at the joint intersection regions were measured and consequently the hot spot strain locations, and hence, the strain concentration factors (SNCFs) were determined. The shapes of the SNCF distributions and the peak SNCFs determined in the tests were found to be in good agreement with those obtained by finite element (FE) analyses, except for the load case where all the braces were subjected to axial tensile forces. In addition, the results at the peak hot-spot saddle locations were compared with those predicted using authors FE analyses, Efthymious influence function (IF) method (Efthymiou M. Development of SCF formulae and generalized influence functions for use in fatigue analysis. Proceedings of OTJ Conference, Surrey, UK, October 1988) and Romeijns equations (Romeijn A, Puthli RS, De Koning CHM, Wardenier J. Stress and strain concentration factors of multiplanar joint made of circular hollow sections. Proceedings of the Second International Offshore and Polar Engineering Conference, San Francisco, USA, June 1992:384–93). Due to the multiplanar stiffening effect of the loaded out-of-plane braces, the IF method (Efthymiou M. Development of SCF formulae and generalized influence functions for use in fatigue analysis. Proceedings of OTJ Conference, Surrey, UK, October 1988) grossly overestimated the test results in the case where all the braces were subjected to axial tensile forces.

Advanced Numerical Modeling of Cracked Tubular K Joints: BEM and FEM Comparison

A critical aspect in the design of tubular bridges is the fatigue performance of the structural joints. The estimation of a fatigue crack life using the linear elastic fracture mechanics (LEFM) theory involves the calculation of stress intensity factors (SIF) at a number of discrete crack depths. The most direct way is to carry out modeling by either the finite-element method (FEM) or the boundary-element method (BEM). For tubular joints commonly found in tubular bridges and off-shore structures, due to the complicated geometry resulting from the tube intersections and welding, the construction of the numerical model often becomes a complex process. This paper presents two different model construction techniques that have been developed independently at the Swiss Federal Institute of Technology (EPFL) and the Nanyang Technological University (NTU), Singapore, that are based in the BEM and the FEM, respectively. The SIF values obtained by these two methods are compared. It is found that as long as consistent geometric models are employed, compatible SIF values can be obtained by both approaches. The best and the most consistent values are obtained for the deepest point along the crack front and should be used for fatigue-life computations. DOI: 10.1061/(ASCE)BE.1943-5592.0000274.

Stress Intensity Factor Solutions for Semi-Elliptical Weld-Toe Cracks in Tubular K-Joints

Stress intensity factors (SIFs) are generally used to predict the remaining life of cracked tubular K-joints used in the offshore structures. The accuracy of SIFs depends very much on accurate modelling of the surface crack. In this paper, a new mesh generation method is proposed for any cracked tubular K-joint where five types of elements are adopted in modelling the surface crack so as to avoid high elements aspect ratio around the crack front. The mesh of any K-joint is obtained by merging the mesh of several distinct regions, and each region is generated separately according to different requirements. To check the accuracy of the computed SIFs, two large scale K-joint specimens were tested to failure under fatigue loading conditions. The alternating current potential drop (ACPD) technique is used to monitor the rate of crack propagation of the surface crack located at the crown position. The experimental test results are then used to validate the numerical results. Generally, they agree well and the relative errors are acceptable.

The ultimate behaviour of cracked square hollow section T-joints

Publisher Summary The chapter proposes an approach to predict the ultimate strength of cracked square hollow section (SHS) tubular (T) joints. The ultimate static strength of tubular joints is usually calculated at the design stage based on empirical formulae incorporating the joint geometry, loading mode, and materials strength. However, fatigue cracks are detected in some aging structures that tend to reduce the static strength. Methods for predicting the loss of strength of cracked tubular joints are therefore important in practice. Very few published information is available concerning the residual strength of SHS joints containing cracks. To develop guidelines on assessing the static strength of fatigue-cracked SHS joints, a range of numerical analyses and full-scale tests are carried out on cracked and uncracked T-joints. The nonlinear elastic-plastic finite element (FE) technique is employed successfully for calculating the plastic collapse loads of uncracked and cracked T-joints under axial load at the brace end.

Debonding failure model for FRP retrofitted steel beams

Publisher Summary The chapter describes debonding failure as an important consideration for fiber-reinforced polymer (FRP) -steel structures. Premature bond failure can prevent the strength of the FRP from being fully utilized. A bond failure model for FPR-steel structures comprising shear and normal strain/strain energy density components is proposed in the chapter. With its incorporation into the finite element analysis (FEA), the ultimate load of a FRP retrofitted steel beam can be predicted, and the debonding failure process can be successfully simulated. Bonding a FRP laminate externally to the steel beams is a promising strengthening method. However, debonding the FRP laminate is a dominant failure mode for such strengthened beams. Based on the results of the FRP-steel adhesive joint experiments and subsequent analyses, an adhesive failure model is proposed in the chapter. The progressive debonding failure of the FRP retrofitted beam can be simulated by incorporating the proposed bond failure model into its finite element analysis. The predicted failure load agreed well with the experimental measurements.

Use of Alternative Steel in Building Steelwork Design to BS 5950

In April 2008, the Building and Construction Authority of Singapore releases design guide BC 1: 2008 to the industry and alternative steel materials which are manufactured to non-BS standards are now permitted in building structural steelworks designed to BS 5950. This paper examines the BC 1 requirements for material performance and quality assurance vis-à-vis the use of such alternative steel materials in design. A list of certified alternative steel materials complying with the minimal essential material performance requirements is compiled. These certified materials are treated as per normal without any restriction or downgrading in design if they are produced by steel mills who can meet the quality assurance requirements. Emphasis is now placed on our stockholders and traders to source and import into Singapore steel materials from audited mills that have in place stringent factory production control system and proper mill test certification. All key stakeholders – manufacturers, stockholders, traders, auditors, fabricators and engineers have a duty and are expected to play a role in maintaining quality in material usage in Singapores steel construction industry.

Numerical modelling of square tubular T-joints with surface crack at corner

Publisher Summary The chapter discusses the numerical modeling of square tubular T-joints with a surface crack at corner. Fracture mechanics method is proven a reliable approach for the assessment of a component with crack-like defect. Central to the fracture mechanics method is the stress intensity factor (SIF) that gives the magnitude of the elastic stress field at the crack tip of an existing crack. The SIF is a function of the applied stress field, crack length, and geometry, and it is used in conjunction with a crack growth law, such as the Paris equation. To get the accurate SIF for cracked tubular joints, one accurate model that can properly simulate the tubular joints with cracks is needed. Large number of researchers are involved in the modeling of tubular joints with cracks.