LUSAS

For the permanent loadcase a survey carried out in 2007 showed that the central span of the bridge had deflected by around 330mm over the years due to the creep and sagging of the cables. Although the stretching of the cables didn’t change the tensions in them very much it did add considerably to the stresses in the deck when sagging at mid-span and when hogging at the towers. In the LUSAS assessment model the initial stress in the cable stays was adjusted to represent creep and the deck profile was matched to that of the 2007 survey. An as-built profile was also modelled to allow re-tensioning of the cables in the future.

For live loading, the traffic load definition was specific to West Gate Bridge and was derived from weigh-in-motion data where the weights of vehicles crossing the bridge were recorded and analysed to derive the extreme loading. Traffic loads were generated using traffic load optimisation software which required the creation of over 700 traffic loadcases to assess maximum stress along the box girder, and in the bearings and cables. For wind loading static loadcases were derived from the gust buffeting response of a spine model.

Because of the history of the bridge the handling of the construction loads was a particularly sensitive issue, made all the more difficult because the bridge was overstressed under the existing eight lane traffic loading. For reasons of safety, it was required to position concrete barriers along the bridge during the upgrade works which added considerable weight. It was decided to limit the loads during construction to the maximum theoretical 8 lane loading before the strengthening had started.