The basic concept of Incremental Launching Method (ILM) is that approximately 15 to 30m long units of bridge segments are prefabricated under plant conditions behind an abutment and launched by means of sliding (on Teflon bearings), one segment at a time. Each current prefabricated segment is post-tensioned with the previously erected box girder segments and pushed in the direction of the bridge by launching equipment, which consists of a combination of hydraulic jacks acting vertically and horizontally and sliding bearings. The structural system continuously changes during the construction stage. The geometry, support condition and loading condition of the temporary structure of each stage vary from one stage to the next, without having any resemblance to the finally completed structure. In addition, bridges constructed by ILM are affected by time dependent material properties such as concrete strength (elasticity of modulus), dependent variables, and analysis results will deviate considerably from the true behaviors.
This example examines the procedure of carrying out a construction stage analysis for a PSC (pre -stressed or post-tensioned concrete) box bridge constructed by ILM reflecting the change of structural system.