Strength Reduction Factor for Tubular Joints Under Combined Loading

Tubular joints are used in a frame structure such as an offshore jacket and are under combined loading in normal service condition. In the design of such frame structures, selecting safety factor (F_S) when determining dimensions for the joints generally depends on many factors such as previous experiences, material properties, fatigue performance analysis and history, etc. In this paper, the authors intend to show that understanding the interaction between the magnitude, direction, and combination of loading on the brace and chord would contribute to prediction of the strength of the tubular joint with a higher accuracy. Thus the ratio between the loading on the brace and chord could be used in determining the strength of the joint to a higher accuracy. The phenomenon of strength reduction for a tubular joint under combined loading in comparison with tubular joints under just single loading shall be explored and a definition for this phenomenon would be introduced. Hence, a more accurate F_S for the design of tubular joints under known combined loading can be selected which in turn would potentially be smaller than the F_S presented using the general F_S selection method without compromising reliability and safety of the structure. By reducing the F_S in this manner, many benefits would be potentially gained such as: 1. Reduced dimensions, hence reduced materials in the construction of the joints; hence easier fabrication. 2. Less potential deficiencies in the construction of members. 3. Lighter and more slender structures. 4. Potentially reduction in fabrication time and cost. This paper principally introduces and defines Strength Reduction Factor (Q_SRF) for Tubular Joints under Combined Loading Condition.