Deck Segmentation and Yard Organization for Launched Bridges

deck segmentation and yard organization for incrementally launched bridges

Incremental launching of prestressed concrete bridges is a competitive construction method for a wide range of spans and bridge dimensions. The competitiveness of the construction method derives from inexpensive construction equipment and repetitive casting operations, and deck segmentation influences both these factors and the final economic result.

Deck segmentation depends on the bridge length and on the time available for its construction. This is true when, as in most cases, the deck segments are match-cast against each other in-place. In case of segmental precasting, time is less of a constraint as segment fabrication is a much faster assembly process.

The segments are match-cast behind the abutment along a projection of the launch trajectory, the entire deck is launched forward incrementally, and deck segmentation is therefore very flexible.

  • When the bridge includes a large number of short, equal spans, the segments can be as long as the whole span to reduce the number of construction joints and to simplify reuse of the same internal formwork.
  • As the span increases, the segments become shorter than the typical span to limit the cost of casting cell and delivery means for prefabricated rebar cages; in this case, the segments can be one-half or one-third of the span.
  • In short bridges or in the case of external post-tensioning, regardless of the span length, it may be convenient to use shorter segments, casting several segments on a continuous support, and post-tensioning and launching them as a whole.

When the number of segments composing the span is defined, the position of the construction joints is verified from the structural point of view (in relation to the self-weight counterflexure points in the final deck geometry on launch completion) and from the constructability point of view, as the core form of the box cell is launched with the segment and extracted backward through the joint section into the rebar cage of the new segment. The structural check is rarely a problem as the construction joints of cast-in-place launched bridges have continuous longitudinal reinforcement.

If the pier diaphragms are cast together with the rest of the section, the joints can be placed immediately before the diaphragm so that most of the core form can be recovered without obstruction. If the segments are cast in two phases (bottom slab and webs first, and later the top slab), the inner forms for webs and pier diaphragms are recovered with the tower crane, only the form table for the top slab is recovered by backward extraction, and the joints can be placed anywhere, provided that the pier diaphragms are tapered not to interfere with the form table.

As an alternative, the pier diaphragms can be cast in a second stage, and even after completion of launching, by means of casting pipes through the top slab. Finally, if the tendons of launch post-tensioning are spliced with couplers instead of by crossing and overlapping, the cross-sectional area of the couplers decreases the moment of inertia of the joint section, which then should be placed at the quarter span locations.

Deck Segmentation and Yard Organization for Launched Bridges (2001, ACI Concrete International) explores deck segmentation alternatives and the criteria that guide the choice of the most cost-effective solution. The second edition of Bridge Launching (2014, ICE Publishing) further expands the discussion of the topic. Both publications are indispensable sources of information and guidance for bridge owners, designers and constructors interested in the design and construction of prestressed concrete bridges built by incremental launching.

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