Chapter 4: The Construction Process of Segmental Bridges
supports, e.g. additional temporary towers for long spans or stay cables also generate stresses in the superstructure. Another factor to be considered is the prestressing tendons that are installed in the concrete members to withstand stresses during construction and under service. More longitudinal forces can be caused by the erection itself, e.g. horizontal jacking forces during incremental launching. Formwork and supporting installations, e.g. the forms and frames in a launching girder also impose live loads, as well as the fresh concrete that it carries.
Finally, environmental influences also create changing loads on the structure, e.g. wind, snow, and temperature gradients. Extreme events, such as floods, storms, and earthquakes can also hit a structure during construction and may need to be considered in the calculations. Apart from these Acts of God, accidents may happen. Podolny and Muller (1982) note that to prevent a scenario such as falling of a form traveler during cantilevering, inspections are necessary. They also note that critical fixtures, e.g. suspension rods that reach through the superstructure and anchor bars that hold the traveler, need to have a large safety margin and may be provided in double numbers. In general, “cast-in-place cantilever construction has established an extremely good safety record” (Podolny and Muller 1982, p482).
The aforementioned extreme load cases are mostly considered with a lower factor of safety than for service conditions (ACI 1995). The reason for this approach lies in the reduced probability of occurrence during the relatively short construction period in comparison with the total duration of service for which the bridge structure is designed. More explanation for this rationale lies in the fact that although the resistance is reduced during construction the structure produces less danger for the general public, as it has not been opened for traffic by then. Furthermore, the bridge under construction is under direct control of engineering personnel on site that can immediately take appropriate measures if necessary to ensure safety of further construction works.
When looking at the structure and its behavior during construction, a striking similarity with the model presented in Section 3.5.1 appears. In fact, when looking at any structure the four main elements are geometry of the structural system, structural details and restraints from boundary conditions, material properties, and loads. For analysis of how a structure behaves while being under construction the same elements need to be taken into consideration.