13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- energy. Under extreme conditions they are expressed as large versus small; fast versus slow; thick versus thin; high versus low. The extreme conditions are those that deviate more appreciably from the mean or the average. They are regarded as being ‘least’ reliable, while the opposite being ‘most’ reliable. Application of the least variance approach will be made to investigate the reliability aspects of the fatigue crack growth behavior in the wires of Runyang cable-stayed bridge. Dual scale fatigue crack growth model is employed. Only cable that undergoes the most severe tension under traffic will be singled out to be explored for the purpose of reliability analyses. 2. Problem statement of cable-stayed portion of Runyang bridge The Runyang Bridge [9] consists of two major bridges that link the south bank and the north bank. The south bridge is a suspension bridge with a main span of 1,490 meters. The north bridge is a cable-stayed bridge with a main span of 406 meters. Keep in mind that current research is focused on the cable-stayed portion, compare Fig.1. The cables are mounted onto the two towers and anchored at the girders of the bridge. Due to the load variations, tightening and loosening of the cables is expected. Only tightening case for reliability analyses will be discussed in the current work. 2.1. Cable forces with and without traffic There are 104 cables used to support the deck with the help of towers. Due to the symmetry, only a total of 52 cables on each side need to be shown here. The position of each cable is shown and numbered from left to right in Fig.2. Note the largest forces prevail in cable no.3 and no.50. Therefore only cable no.3 will be chosen to demonstrate the reliability of fatigue crack growth behavior. A detailed description of cable construction can be referred to Fig.3. The cable consists of multiple high strength steel strands arranged in a bundle and encased in a polyethylene pipe filled with cement grout. Since the polymer carries little load and their resistance can be neglected. Each cable consists of 37 stranded steel ropes and each of the ropes contains 7 galvanized steel wires. Hence, a total of 259 galvanized steel wires construct one cable. With the diameter of steel wire being 5mm, the stress calculation thus is executed based on the effective section areas as follows Fig.1 Cable-stayed portion of Runyang Bridge
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