Chapter 4: The Construction Process of Segmental Bridges
A major drawback of this method is found in the previous description. The bridge superstructure will take considerable loads during construction, since the heavy launching girder rests with one leg at the midspan cantilever tip.
Launching Girder Slightly Longer Than Two Spans
Construction of the bridge superstructure with a launching girder about two spans long does not incur the aforementioned detrimental load condition. At all stages the load-carrying girder legs will be located above piers. This is shown in Figure 4-6. In the normal placement position the launching girder rests with its rear leg above a previous pier and with its central leg above a free pier. It is easily possible to also support it at the guide leg once the third pier table has been placed. Placement of the segments will then proceed on both sides of the pier table in the middle of the girder. To speed up construction, the girder can be equipped with two crane devices to place segments on both sides simultaneously. After the remaining gap in the superstructure has been closed and the cantilever has grown into the next span, the girder is advanced one complete span. During advancement the girder rests on the guide and central leg that remain on the newly finished pier and the one that lies ahead. This second way of employing launching girders is the more recent technique (Mathivat 1983).
Even longer launching girders have been used in construction, as reported by Mathivat (1983). Due to the long span required for launching girders and the mechanical parts, such as crane and advancement devices, launching girders can be quite costly. If possible, launching girders should be adaptable for reuse or should be rented. Launching girders can reach lengths of more than 150 m, depending on the requirements of the bridge spans, and weights of up to about 400 t (Mathivat 1983). In addition to these specialized, expensive and heavy pieces of construction equipment it is also possible to use simple lifting devices that are located at the cantilever tip. In the case of the Linn Cove Viaduct, which has been presented in Section 220.127.116.11, a derrick was mounted to the bridge superstructure that placed segments as they were delivered by truck (Anon. 1984). Other types of deck-mounted equipment are imaginable and mentioned by Levintov (1995, p44), e.g. “a longitudinal beam fitted with lifting tackle and winches.”