Assessment of the Bill Emerson Memorial Bridge - FTP Directory ...

1. Introduction1.1. GeneralCable-stayed bridge is an elegant, economical and efficient structure and now it isbecoming more and more popular throughout the world. In the past several decades, theUnited States has witnessed a rapid development in the construction of this type ofbridges. Due to its good characteristics in earthquake resistance, it has even become thefirst choice of the construction of a bridge in seismic zones with high risks (Hu et al.,2006). With the rapid progress in analysis tools and construction technologies, the mainspan of cable-stayed bridges has been pushed much longer in recent years. For example,the length of the main span is 605 m (1985 ft), 886 m (2907 ft), and 890 m (2907 ft) forQingzhou Bridge (Ming River, China), Pont de Bridge (Normandie, France), and TartaraBridge (Hiroshima, Japan), respectively. With ever increasing span lengths, cable-stayedbridges behave in a more complicated way, and often become more susceptible toenvironmental effects. The fundamental characteristics such as stiffness of structuralmembers, variation of cable forces, and stability of structural systems play a more criticalrole in the safety evaluation of these bridges.Although a cable-stayed bridge seems subjected to high stresses under gravity loads, it isgenerally sensitive to dynamic loadings resulting from earthquakes, winds and movingvehicles. In these cases, the condition of a large span cable-stayed bridge must beassessed to ensure the smooth operation and safety during its life span. One way to assessa structure is to observe changes in vibration characteristics such as natural frequencies,damping ratios and mode shapes of bridges. Those changes, if properly identified andclassified, can provide a viable means for damage detection of the structure (Roebling etal., 1996; Ren et al., 2005; Wang et al., 2007). The other way of structural assessment isto conduct extensive dynamic analyses in frequency domain (Allam and Datta, 1999) tounderstand the behavior of the structure by comparing load/displacement withstrength/ductility.Field tests provide an effective way of characterizing a cable-stayed bridge structure forits mechanic and dynamic properties (Hu et al., 2006). They can be performed underthree types of loadings: harmonic excitation, initial disturbance, and ambient excitation.In harmonic/force vibration tests, bridges are excited by a shaker or other artificialmeans. In this case, both input and output can be obtained. By using a known forcingfunction, many of the uncertainties associated with data collection and processing can beavoided. For large-scale bridge structures, however, generating significant vibrationrequires the use of a heavy shaker or other equipment, which often makes this methodimpractical. Free vibration tests are carried out by suddenly releasing a heavy load ormass appropriately connected to the bridge. The induced free vibration decays and energydissipates as a result of friction or heat generation. The free vibration records can beanalyzed to determine the properties of the bridge structure. Both forced vibration andfree vibration are excited by the use of an artificial means with no traffic on the bridgeduring tests. This requirement often causes great inconvenience for existing bridges. As aresult, ambient vibration tests are preferred in many applications. They take advantage ofthe vibration sources available during regular operations, including wind and earthquake1