The Axial Strength and Behavior of Cylindrical Columns

Abstract

This paper describes tests of large-diameter, fabricated, tubular columns with essentially pin-ended conditions. Measurements of residual stresses were made in addition to the behavior, strength, and manner of failure of these members. The maximum strength of the columns is compared with the CRC basic column-strength curve. Introduction The use of fabricated tubular columns is a growing phenomenon in structural engineering. Currently, columns phenomenon in structural engineering. Currently, columns with relatively large diameters are commonly used 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 strength levels for these columns hinders specification writers 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. Realizing the foregoing deficiencies, several designers of offshore platforms began a critical review of this situation in the early 1970's under the auspices of the API's Task Group on Fixed Platform Criteria. One of the major outgrowths of this review was the recognition of the need for more basic information regarding the probable ultimate strengths and corresponding design probable ultimate strengths and corresponding design strengths of axially loaded, fabricated tubular columns. This led the API to fund a major research project at Lehigh U. in 1974. Included in the Lehigh program is an experimental investigation to provide much-needed testing data; a theoretical investigation into column behavior throughout the entire range of loading up to and beyond the ultimate load; and the development of the necessary background information to allow specification writers to properly assess both the strength and behavior of properly assess both the strength and behavior of fabricated, tubular columns. Selection of the column dimensions for testing was predicated on the desire to force inelastic column buckling failures, as opposed to situations where local buckling might play a major part in the failure. This paper discusses the experimental phase of the research program. Included in the investigation was an experimental determination of residual stresses in a typical fabricated tubular column used in offshore oil structures, and the testing of 10 full-scale columns under axial load and pin-ended conditions. Scope and Design of Specimens Fabricated tubular columns commonly used on offshore structures are usually fabricated from flat steel plate that is formed into circular elements, such that the width of the plate becomes the height of the element. The ends of the plate are then longitudinally joined by welding. The completed element is known as a "can" and is generally about 3 m (10 ft) long. By joining a number of cans end to end with circumferential or girth welds, a column of any length may be formed. The longitudinal welds in the cans are staggered, and it is common for the longitudinal weld in one can to be about 180 degrees out of phase with the longitudinal weld in the next can. JPT P. 239