Chapter 4: The Construction Process of Segmental Bridges
segments can be attached to them, running along the chords of the girders. Precast segments are delivered to the girder by special heavy-duty vehicles.
If launching girders are built of high strength steel their weight can be reduced considerably. At the same time, however, larger deflections occur that are limited by additional support of the girders with a king post system with stay cables (Mathivat 1983).
Launching girder trusses can have triangular or rectangular cross-sections and can be constant in depth or higher towards the middle. They can be disassembled into parts that are connected with high-strength friction bolts (Mathivat 1983) for transportation, very similar to tower crane booms.
Most launching girders are overhead trusses that have three leg supports. The three legs are called rear, central, and guide leg. Some of these legs, often the guide leg, are not permanently fixed to the girder to allow the advancing movement as will be described below. Very often these legs form a bent above the superstructure, leaving space for the precast segments that are turned 90° sideward to be moved through the gap. Pivoting the whole launching girder around the rear support leg (Mathivat 1983) accommodates bridge superstructure curves in the horizontal plane.
A major feature of launching girders is their length in comparison with the span length of the bridge superstructure. Levintov (1995, p45) writes that launching girders composed of “single or double trusses may range from slightly longer than a span length to slightly longer than twice a span length.” Erection sequences for these two extremes shall be briefly described in the following paragraphs.
Launching Girder Slightly Longer Than One Span
Construction of the bridge superstructure with a launching girder about a span long is performed as follows. After advancement the girder rests with its rear leg on the cantilever tip at midspan and with its center leg on the next pier. Around this pier new segments will be placed with balanced cantilevering, filling the remaining half-span behind the pier and advancing the