Chapter 4: The Construction Process of Segmental Bridges
to keep the bending moments in the superstructure smaller. Mostly the launching nose has a length of about 60 % of the bridge spans (Podolny and Muller 1982). Another way of reducing the bending moments is to implement temporary towers between the bridge piers. These towers need to be able to take the horizontal forces that arise from launching.
On top of all supports, including abutments, piers, and temporary towers temporary sliding bearings are installed during construction that will later be replaced with the permanent ones. Stainless steel plates are installed on the bearings. While the superstructure is advanced, Neoprene pads coated with Teflon and reinforced with steel plates are inserted between concrete and steel to reduce friction (Liebenberg 1992). Very low friction coefficients of 2% or less can be achieved with this method.
Several advantages make incremental launching a very competitive erection method. As with any cantilevering method it leaves the site below completely unobstructed during construction. Only for very long spans temporary towers or cable stays from above as supports are needed. Except for these the equipment necessary is reduced to the jacking mechanism, the adjustable stationary casting bed, and temporary sliding bearings, all of which may possibly be reused, which reduces the capital investment considerably. Podolny and Muller (1982) furthermore mention the cost savings due to avoidance of segment transportation and heavy construction equipment. They also point at less maintenance cost due to the higher prestressing of the superstructure. The controlled casting and curing conditions allow steady and quick construction progress.
Bridges that are erected with the incremental launching method should, according to Podolny and Muller (1982), have a constant cross-section, especially in depth, and have a straight superstructure. It is possible to accommodate small variations in alignment and horizontal and vertical curvatures provided that they have a constant radius. Close control of the bridge geometry during casting and launching is very important. Sloping grades at the bridge site are also accommodated, in this case “the launch is usually in the downward direction”, more than 2 % slope would require a retarding mechanism to stop the movement of the superstructure (Liebenberg 1992, p165).