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Bridge incremental launching method

Incremental launching method relies on using the equipment and a small amount of worker. the superstructure is cast behind one of the abutments, one unit of the superstructure will be concreted first, then when the concrete hardened, a new segment will be connected to first one, followed the hardening of both units, the resultant structure will be pushed by the length of one unit as shown in figure 1. The concrete is well known for it is high compressive strength and low tensile strength, the launched superstructure will be under the effects of positive bending moment and negative bending moment, this will cause high tensile stress on superstructure top and bottom section which may cause concrete cracking and damaging, special measures should be taken to minimize the effects of tensile stresses on concrete. incremental launching method is suitable for bridges with 15 to 60 m span length



Figure 1








The area of casting yard behind the abutment shall be sufficient to perform casting and launching process, also the ground shall possess a high bearing capacity, if the bridge has a longitudinal gradient, it is preferred to set up the casting yard behind the lower abutment to avoid using of braking equipment, This method suitable for straight bridges or bridge with spatial curve, or bridge with constant curve horizontally and vertically.

The most suitable cross-section for this method is a single cell box section or double T beam, the span-to-depth ratio shall vary from 12 to 15,

Features of incremental launching method:

  1. Falsework is not required for this method, therefore there is no problem of passing over obstacles
  2. the superstructure made from 15 to 25 m segment length and there are no joints because segments are concreted against each other
  3. During the construction stage, the superstructure is centrally prestressed, to limit the tensile stresses produced by the bending moments. Small tensile stresses should be permitted (partial or limited prestressing), even if such stresses are not permitted in the completed structure; they considerably improve the economics of the method, without detracting from the safety of the structure.
  4. launching nose are fitted to superstructure during launching to reduce the effects of tensile stress due to cantilever moment
  5.  hydraulic jacks are used for launching of the superstructure and located at the abutment
  6. Special sliding bearing will be used at bridge support
  7. auxiliary supports may used between the piers when the span-to-depth ratio is high or when the span lenght is large

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