Self-weight bending and shear vary continuously during the incremental launching process. 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 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. Optimizing the nose-deck interaction with a parametric spreadsheet 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 entire launch stress analysis should pre-sizing ultimately turn out inadequate.
In 23 pages, Control of Construction Stresses in Launched Bridges introduces the parameters that govern the nose-deck interaction, explores an analytical model of the nose-deck interaction and a step-by-step procedure for its optimization, and explains the use of parametric design charts for control of self-weight bending and shear in the front region of the deck by means of the relative length, weight and stiffness of the nose.
The spreadsheet enclosed to the eManual 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.
Spreadsheet and eManual will become your productivity tools for value engineering and the pre-award design of incrementally launched bridges, will show you the effects of the relative length, weight and stiffness of the launch nose on deck pre-sizing, and will avoid expensive trial-and-error use of structural software programs during the pre-sizing process.