ABSTRACT

Experimental results are described on the inelastic behavior of two one-sixth scale models of an X-braced tubular steel offshore platform subjected to cyclic loads simulating the effects of severe earthquake ground motions. The dimensions of the 29-ft. (8.8 m) tall test specimens are based on a representative production facilities platform designed according to API wind and earthquake criteria applicable to Southern California. Two types of detailing are investigated: in one specimen the nominal tube diameter-to-wall thickness (D/t) ratio is 48, while it is 33 in the other. Hysteretic response for individual bracing members as well as for the frames as a whole are presented and interpreted. The effect of D/t ratio on deterioration of the strength, stiffness and energy dissipation capacity of the specimens under inelastic cycling is examined. Based on the results presented, implications for current design and analysis practices are briefly discussed and recommendations for future analytical and experimental research are offered.

INTRODUCTION

Current API recommendations stipulate that both strength and ductility must be considered when designing a fixed offshore tower to be located in an area of seismic risk. l That is, the structure must be sufficiently strong so that it will not yield during moderate ground shaking comparable in intensity to that considered in the design of important onshore structures. It must also be sufficiently ductile to remain stable during a rare and unusually intense earthquake. To satisfy this latter requirement, a designer must be able to demonstrate that the tower can, prior to becoming unstable, absorb at least four times the energy developed during application of the strength level design criteria. Fortunately, it may not be necessary to design for these increased energy absorption demands on an elastic basis, if the tower can be detailed to absorb and dissipate this energy through ductile inelastic deformations. While this design concept has been successfully used in the building industry, its applicability to tubular steel offshore structures must be thoroughly investigated since these structures incorporate a number of unique features not found in building construction and relatively little is known about their inelastic seismic behavior.

A number of tests have recently been performed to determine the inelastic behavior of reduced scale models of tubular members of the type used in offshore construction. 2, 3 .While such data are useful in evaluating current analytical models, 4,5 test results indicate that brace inelastic behavior is sensitive to the type of end supports used. Tests of single braced panel sub assemblages can help clarify some of the effects of end conditions on brace behavior. 6 However, analyses indicate that the distribution of member forces within a panel is sensitive to the inelastic behavior experienced in adjacent panels.7. Thus, to evaluate and improve seismic-resistant design and analysis methods, it is desirable to test frames consisting of several braced panels.

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