For several years, there has been major concern on the part of the designers of offshore structures that commonly-used column strength design formulas might in some instances be inadequate, while in other cases leading to overly conservative column selections. Realizing this, the American Petroleum Institute initiated sponsorship in 1974 of a major research project at Lehigh University aimed at the development of correct tubular column strength prediction tools.
The data obtained during the experimental portion of this research work is described herein. The ten fabricated tubular steel columns tested were subjected to axial, static loading under essentially pin-ended conditions. Column behavior under loading, as well as ultimate column strengths, were determined.
In addition, the distribution of residual stresses in these columns received particular attention. Various techniques were applied to measure these stresses. Circumferential residual stresses, due to forming of a flat plate into a cylindrical shape, were measured along with the longitudinal residual stresses resulting from the welding of the long seams needed to complete the fabricated cylinder. These residual stresses are believed to have a significant effect on the behavior and strength of a fabricated, tubular column; hence, they must be considered in any complete theoretical analyses of the column.
The results of the ten columns tested (at diameter-to-thickness ratios of 48 and 70 and with a L/r range of 39 to 83) are presented herein in graphical format. To provide the reader with a better appreciation of the column strengths attained, the CRC-AISC ultimate column strength curve has also been added on the figures summarizing the test results.
The development of the associated theoretical column behavior predication tools is now underway at Lehigh University. It is hoped that with the completion of these that analytical predictions of the behavior of fabricated, tubular columns will be possible.
Concurrent with the theoretical work, interpretative studies are underway to determine which of the many parameters that could affect the behavior and strength of this type of column are really significant for design purposes.
The ultimate goal of this entire effort is the development of rational design guidance describing the strength and behavior of axially loaded, fabricated tubular columns.
The use of fabricated, tubular columns is a growing phenomenon in structural engineering. Currently, columns of relatively large diameters are commonly in use in offshore structures and have increasing applications in various other civil engineering structures.
Two primary design problems emerge with the use of these columns. On one hand, the designer is faced with an immediate lack of reliable design guidance. The lack of experimental evidence on probable strengths levels for these columns hinders specification-writing bodies in their attempts to provide designers with safe, but relatively economical, column design guidance. On the other hand, there is also a major lack of knowledge about the behavior of such tubulars under various axial loading situations.
