Nose-deck interaction in launched bridges

Nose-deck interaction in launched bridges

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 labor cost with similar investment
  • in longer bridges, it is an alternative to the use of movable scaffolding systems (MSS) and reduces investment with similar labor cost

Compared with segmental precasting, it may reduce both investment and the cost of post-tensioning.

The design of a launched bridge includes temporary stresses due to 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. 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 constant-depth deck geometry, and the launch nose controls self-weight bending and shear and the interaction between moment-of-inertia and the required level of launch post-tensioning.

Structures with so many load conditions require careful pre-sizing. Nose-Deck Interaction in Launched Bridges (1998, ASCE Journal of Bridge Engineering) illustrates how to optimize the nose-deck interaction with parametric design charts generated with a spreadsheet and closed-form equations. The approach discussed in the paper shows the effects of the relative length, weight and stiffness of the nose, streamlines and accelerates the design of launched bridges, 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), 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.

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