Incremental launching is a competitive construction method for medium-span prestressed concrete bridges. Compared with other techniques for in-place casting:
- in short bridges it is an alternative to the use of falsework and reduces the cost of labor with similar investment;
- in longer bridges it is an alternative to the use of movable scaffolding systems (MSS) and reduces investment with similar cost of labor.
Compared with segmental precasting, it may reduce both investment and the cost of post-tensioning.
The design of a launched bridge is affected by temporary stresses arising from its movement over fixed bearings. The increasing length of the front cantilever and recovery of the elastic deflection at nose landing at the next pier govern the envelopes of self-weight bending and shear, and within those envelopes, the deck cross-sections cyclically migrate from peak negative bending and shear when they are over the piers to peak positive bending when in midspan.
The envelopes of self-weight bending and shear are more demanding in the front deck region and govern deck pre-sizing. Without a launch nose, negative bending at the root of the front cantilever would be 6 times higher than in the rear pier regions, and shear would be double. Launchability criteria require a cylindrical constant-depth deck geometry, and the launch nose controls self-weight bending and shear and the interaction between cross-sectional moment-of-inertia and the required level of launch post-tensioning.
Structures that present so many load conditions require careful pre-sizing. Nose-Deck Interaction in Launched Prestressed Concrete Bridges (1998, ASCE Journal of Bridge Engineering) illustrates the techniques of optimizing the nose-deck interaction through several graphs, while the analysis of special cases may be carried out by means of a spreadsheet. The design approach explored in the paper shows the effects of the relative length, weight and stiffness of the nose, streamlines and accelerates the design of a launched bridge, and minimizes the risk of remaking the launch stress analysis should pre-sizing ultimately turn out inadequate.
The design optimization approach explored in the paper has been further expanded in the second edition of Bridge Launching (2014, ICE Publishing) and in the eManual Control of Construction Stresses in Launched Bridges (23 pages & spreadsheet), which also includes the Excel spreadsheet for immediate productivity with the optimized pre-sizing of incrementally launched bridges. The spreadsheet draws parametric design charts of positive and negative bending in the front span and at the nose-deck joint and has been time-tested in the design of several launched bridges. Closed-form equations lead to excellent match with the results of the final launch stress analysis with structural analysis programs.