High- and Low-Cycle Fatigue Behavior of Prestressed Concrete in Offshore Structures

Abstract

The expanding use of prestressed concrete in offshore structures, both fixed and floating, in increasingly hostile environments has generated intense interest in its fatigue endurance capabilities, even though no known fatigue problems have arisen in actual structures. Although concrete does suffer progressive loss of strength with increasing number of cycles, a comparison of the Wohler curves developed on the basis of laboratory tests with the probable distribution of compressive stresses during a service life in an environment such as the North Sea shows extremely low cumulative usage at the highcycle end of the spectrum. However, significant damage can occur at the low-cycle, high-amplitude end of the spectrum under a relatively small number of cycles of very high magnitude. This damage is displayed by a reduction in stiffness and by rapidly increasing axial and lateral strains that lead to cracking and spalling.Repeated cycling into high compressive ranges causes a substantial increase in creep, reducing the effective prestress. Confining reinforcement resists lateral deformation and delays compressive fatigue failure. Cycling into the tensile range a large number of times can produce cracking due to tensile fatigue at about half the static tensile strength. Cracking also can occur due to overload, accident, construction procedures, and thermal strains. Repeated excursions of submerged concrete into the crack-opening range leads to pumping of water in and out of the crack and hydraulic wedging, leading to concrete splitting.Cracking subjects the reinforced and prestressed steel to cyclic tension. Loss of bond ensues and may lead to eventual fatigue failure. Adequate endurance can be ensured by prestressing to avoid a large number of cycles extending into the crack -opening range and by providing adequate percentages of steel