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

The expanding use of prestressed concrete in offshore structures, both fixed and floating, in ever more hostile environments has generated intense interest in its fatigue endurance capabilities, even though, as far as it is known, no fatigue problems have arisen in actual structures. Although concrete does suffer progressive loss of strength with increasing number of cycles, a comparison of the Wo¨hler 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 high-cycle end of the spectrum. However, significant damage can occur at the low-cycle, high-amplitude end of the spectrum under a relative 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 splitting of the concrete. Cracking subjects the reinforcing and prestressing steel to cyclic tension. Loss of bond ensues and may lead to eventual fatigue failure. Adequate endurance can be ensured by prestressing, so as to avoid a large number of cycles extending into the crack opening range, and by the provision of adequate percentages of steel across the section plus transverse and confining steel. Cyclic membrane shear may produce diagonal tension cracking at about half the static strength. Conventional reinforcing in an orthogonal grid pattern is very inefficient in resisting such cracking. Crack widths grow rapidly. Repeated loading may lead to abrasion of concrete surfaces and failure of the steel due to combined axial force and bending. Vertical prestress is an efficient and practical method of resisting high-amplitude cyclic shear. For the typical concrete sea structure, high-cycle, low-amplitude, cumulative fatigue is not a significant problem. However low-cycle, high-amplitude fatigue requires consideration, expecially when there are numerous cycles into the tensile cracking range. In this latter case, fatigue of the steel and/or concrete may occur unless adequate amounts of steel are provided to ensure that crack widths and steel stresses are kept within allowable values.