In the erection of large bridges the structure is successively analysed at each critical stage in the sequence. Invariably such structures are erected by fir.st building the primary structural members, either by cantileverin~ them forward or lifting them into position, and following this by adding such secondary members as cantilevers, cross members and concrete slabs. The effect of these secondary members is to change the structural characteristics of the bridge and consequently the analysis must be able to tahe account of this history throughout the sequence.
The LEAP suite of programs has been used on several major bridges to analyse and develop viable erection schemes and, using FRIDA Y, to assess the critical stages and identify the most highly stressed members.
This paper describes in a general way the methods used, and shows examples of the procedure for the particular case of the large cable-stayed West Gate Bridge in Australia.
It is necessary to analyse the critical stages in the development of erection schemes for large bridge structures to assess the stresses arising in the structure from the various loadings to which it is subjected. These may be the self weight of the structure itself (its dead load), the weights of the equipment needed to erect the bridge -cranes, derricks and ancillary works -and the forces arising from meteorological conditions--wind and temperature effects. The erection stages of a large bridge are often the most critical from the point of view of bridge safety. The problem of completing the main span by cantilevering from each side may well induce stresses in the critical sections in excess of those from traffic when the bridge is in service.
Economy of construction means that the additional strengthening installed for the erection conditions must be at a minimum. It is thus essential to analyse and assess the erection conditions extremely carefully. Each step in the sequence must be considered, from the first to the last stage.
The only practicable way of undertaking this formidable task is to use computer-aided design, with structural analysis programs that can deal with all aspects of the sequence.
COMPUTER PROGRAM
Many computer programs have been written to analyse structural frameworks; generally these have assumed a Flint and Neill, Portland House, Stag Place, London SW1E 5BH linear stress-strain relationship in the material and the solution is based on a stiffness method. One such program in the civil engineering field is the LEAP suite, which is outlined in the Appendix.