Background to New Formulae for the Ultimate Limit State of Tubular Joints
Authors: Billington Colin J. , Lalani Minaz , Tebbett Ian E. 
Published: October 2008

Tubular joints constitute one of the main problems and high cost areas in the design, construction and maintenance of steel offshore structures and have been the subject of considerable research effort. Design certification of offshore structures has led to the production of many codes of-practice rules, regulations, guidance notes and published research data. Recently a study of the various design documents has been funded by the UK Department of Energy, with particular emphasis on the static strength of joints. This study concluded that no two published design documents make identical recommendations for the calculation of static strength and that there are major inconsistencies, particularly with regard to the inherent levels of safety. Therefore a complete review of available static strength test data has been undertaken.

This paper presents the derivation of new formulae which describe the ultimate limit state of tubular joints. The formulae are based on a statistical analysis of over 200 carefully screened static test results. The format of the equations is based on brace member loads and they are therefore both directly compatible with computer based analysis methods and amenable to hand calculation. Appropriate margins of safety are also considered and direct comparisons with existing design formulae are presented.

Finally, the ranges of application of the formulae are compared to typical joint configurations and loading. This leads to the identification of areas where future research in the ultimate limit state should be directed.

Introduction

The large majority of steal offshore structures consist of three dimensional frames composed of cylindrical steel members with hollow sections which provide the best compromise to the requirements of low drag coefficient, buoyancy and high strength-to-weight ratio. The hollow sections also provide for the possibility of using the inner space for transportation, fire protection, or additional strength. The principal design and fabrication problem for this form of structure concerns the welded joints between members. Typical simple tubular joints and the notation system used in this paper are shown in Figure 1. The word 'simple' refers to joints without overlap of brace members and without the use of gussets, diaphragms, stiffeners or grout.

Design certification of such offshore structures has led to the production of many codes of practice, rules, regulations, guidance notes and published research data concerning tubular joints. Recently, a study of the various design documents has been carried out for the Underwater Engineering Group and funded by the UK Department of Energy. This study included an exhaustive review of available static strength test data since major inconsistencies exist in current design documents for the calculation of static strength particularly with regard to the inherent levels of safety.

This paper presents the derivation of new formulae which describe the ultimate limit state of tubular joints. A statistical analysis has been carried out to obtain characteristic strength formulae for various joint configurations for axial tension, axial compression, in-plane bending and out-of-plane bending loads.