ULTIMATE STRENGTH OF TUBULAR JOINTS SUBJECTED TO COMBINED LOADS (FATIGUE, STATIC, OFFSHORE STRUCTURES)

Nine tests were conducted on double-tee (DT) tubular joints subjected to various combinations of axial load, in-plane bending and out-of-plane bending in the branch. These tests along with three reference tests (axial load alone, in-plane bending alone, and out-of-plane bending) were used to study joint interaction and to develop equations for joint design. It was found that the branch load inter- action was best represented by the following equation: P/P(,u) + (M/M(,u))(,OPB)('1.2) + (M/M(,u))(,IPB)('2.1) = 1.0. The equation of the type used in Supplement 1 to the 13th Edition of the American Petroleum Institute (API) Specification, namely P/P(,u) + 2/(pi) arcsine SQRT.((M/M(,u))(,IPB)('2) + (M/M(,u))(,OPB)('2)) = 1.0 was found to be slightly unconservative when P(,u) and M(,u) are based on experimental results. When API predictions of P(,u) and M(,u) are used, the arcsine equation was very conservative in many instances because M(,u) is underestimated.;The behavior of the connection was critically analyzed in order to develop a design oriented analytical solution for ultimate strength. Several empirical equations exist which yield reasonable predictions of ultimate strength but do not provide insight into the way a joint resists applied load. A ring model was used to develop an analytical solution for the ultimate axial and bending strengths of the DT connection. The solution yielded an estimate of ultimate axial load within 20 percent of that measured. The ring model does not model shell behavior well, which is important in resisting IPB bending loads. The process of developing a solution for ultimate strength revealed that the strength of the chord adjacent to the joint resists a significant amount of applied bending and axial branch load.;Strain gage measurements were used to determine the SCF at the toe of the weld for axial load, in-plane bending and out-of-plane bending. For axial load and in-plane bending, the SCF were 31.7 and 3.9, respectively, which compare favorably with predictions of 34.9 and 4.38 from finite element solutions. For out-of-plane bending, however, the load-stress response was nonlinear with the SCF increasing when the nominal stress increases. The maximum measured SCF for OPB was 18.9 for a nominal branch bending stress of 2.8 ksi.