Launch Bearing Misalignment in Launched Bridges

Geometry tolerances are unavoidable in incrementally launched bridges:

• Prestressed-concrete decks are cast incrementally behind the abutment and the geometry tolerances of the casting cell affect the final geometry of the deck.
• Steel girders are launched without the concrete slab and are therefore cambered. A cambered girder violates the principles of rigid extrusion, and the effects of geometry tolerances are amplified. Additional geometry-related stress irregularities result from tolerances of planarity and field splices in the bottom flanges.
• The launch bearings are positioned with long-range surveying, and elevation errors do occur as well.
• The neo-flon pads may be expelled laterally from the launch bearings, inserted too late to avoid squeezing, or crushed during launch due to excessive wear and tear.

For all these reasons, Section 5.14.2.4.6b of AASHTO-LRFD specifies that a launched bridge must be designed for 2.5mm vertical misalignment of adjacent launch bearings. But how to analyze these effects?

In 11 pages, the eManual explains how to analyze the effects of misaligned launch bearings with a parametric spreadsheet and closed-form equations. The approach is discussed for the general case of a prestressed-concrete box girder and can be easily extended to steel U-girders, braced I-girders and prestressed composite box girders with steel corrugated-plate webs.

The eManual explores the analytical approach discussed in Chapter 3.4.2 of the second edition of Bridge Launching (2014, ICE Publishing) and includes a companion spreadsheet validated by finite-element analysis (also included). The spreadsheet will help you to analyze the effects of torsion and distortion without a shell-element model, to optimize rebar design for control of transverse bending, and to streamline and accelerate the design of launched bridges.

• Introduction
• Effects of launch bearing misalignment
• Model validation with finite-element analysis
• Symbols
• References

Spreadsheet and eManual are fundamental productivity tools for the preliminary and final design of incrementally launched bridges and the neo-flon pads and launch bearings used for their construction.