Fatigue Crack Growth Rate of Cable-Stayed Portion of Runyang Bridge: Part I – Cable Crack Growth Due to Disproportionate Cable Tightening/Loosening and Traffic Loading

Relatively little is known about the fatigue crack growth behavior of bridge cables. Such information can become increasingly more important in time for the cable-stayed bridges whose structural integrity can be greatly affected by the degradation of the individual cable material due to aging as well as defect growth. Moreover, crack initiation and propagation are intimately related such that they have to be treated as one of the same process. This requires a dual scale fatigue crack growth model involving both micro- and macro-cracking where the interaction of three micro/macro material, geometric and load parameters is considered. Based on the design data for the stresses in the 52 cables of the Runyang cablestayed bridge, three typical cables referred to as #28, #38 and #50 are selected for analyzing fatigue crack growth under traffic and no traffic conditions. The initial tension in the cable can vary by using α=1 as the reference such that deviation from the design condition may be regarded as tightening and loosening. Under traffic, initial tightening and loosening of the cable can enhance and impede fatigue crack growth, respectively. When traffic is removed, initial tightening and loosening of the cable can impede and enhance fatigue crack growth. Opposing effects of fatigue crack growth are observed for cables with and without traffic. These results are discussed in terms of crack growth rate and crack length as a function of the fatigue cycles assuming that the cable has a life of two million cycles. Substantial variance in fatigue crack growth is found. Crack length can vary from a few mm to 50 mm and more before reaching the onset of rapid fracture. The stiffness of the cable is also affects crack growth. Each cable is found to behave as a structure of its own. Replacement of cables appears to be eminent during the life span of cable-stayed bridge because of the wide variance of fatigue lives of the cables. This means that some cables will fail while others may still have ample remaining life.