Geometric Design of Launched Bridges

geometric design of launched bridges

Incremental launching is a versatile construction method with a wide range of applications. Limited at first to the construction of bridges with simple geometry, it has eventually been used successfully for the construction of steel and prestressed-concrete (PC) bridges with increasing geometric complexity.

A bridge built by incremental launching is subject to geometry constraints. PC decks and steel girders are supported under the webs during launch, and the first and most obvious limitation results from the deck geometry itself. Varying-depth decks are unfit to incremental launching because of the irregular profile of their soffit. Varying-width decks are also conceptually unfit, but they can be launched by progressively shifting launch bearings and guides laterally to support the deck under the webs. The use of skidding bearings and guides is technically simple but requires wide support areas for the skidding shoes and causes lateral load eccentricity in piers and foundations. In most practical cases, therefore, incremental launching construction is used in combination with a cylindrical geometry of the deck.

During the launch, a cylindrical deck is a continuous beam supported on launch bearings and restrained laterally by launch guides. Vertical misplacement of launch bearings and lateral misplacement of launch guides cause hyperstatic effects and stress redistribution within the deck and accelerated wear and tear of the launch systems. Excessive misalignment of launch bearings and guides may lead to structural distress and failure, and bearings and guides must therefore be tightly aligned with the surfaces of the deck and the launch nose they will come in contact with during the launch.

The launch alignment is controlled through tolerance restrictions on the support geometry and the deck geometry. Launch bearings and guides must be positioned correctly along the launch trajectory of the deck, as their misalignment relative to the launch trajectory causes hyperstatic effects and stress redistribution related to the support geometry. The launch trajectory is calculated based on the theoretical alignment of deck centerline and the theoretical geometry of the cross-section, and excessive construction tolerances in deck centerline, cross-sections, or both, cause hyperstatic effects and stress redistribution related to deck geometry.

Launching a perfect continuous beam over misaligned launch bearings would cause the same effects as launching an irregular beam on perfectly aligned bearings. Geometry control during launch, in other words, involves a combination of controls on launch bearings and guides and the deck regions that come in contact with them during launch; the other deck regions are unaffected.

A PC deck built by incremental launching is cast segmentally behind the abutment; a steel girder is assembled segmentally. A cylindrical geometry is set for the casting cell of a PC deck and the assembly supports of a steel girder, and launch bearings and guides are positioned on the piers along projections of that cylindrical geometry. The launch nose acts as a structural extension of the deck during launch and its bottom flanges must also be aligned with the common cylindrical solid.

The allowable geometries can be defined mathematically by considering the production of identical segments. For a rigid body (deck and launch nose) to slide within another rigid body (the launch alignment provided by launch bearings and guides), the solid must be superimposable onto itself by translation, rotation, or rotation-translation. The only lines that are superimposable by rigid displacement are the rectilinear segment (translation), the arc of circle (rotation) and the circular helix, which combines rotation and translation through the pitch of the helix.

The circular helix is rarely used, and the rectilinear segment is a geometric degeneration of an arc of circle. Most launched bridges, therefore, involve a circular geometry for the deck centerline.

A sequence of individually-launchable deck centerlines is not launchable if the rigid displacement of the sequence generates areas of non-overlapping. For example, a deck that includes a rectilinear segment followed by an arc of circle is not launchable, even though the two deck segments would be launchable if handled individually. Launching the two deck sections from the opposite abutments for central closure can solve the problem, as different launchable lines may be used for the two sections of the deck.

Transverse deck stability requires at least two support lines during launch. In most cases the deck is supported under each web, and the number of support lines increases from two (single-cell PC box girders, steel U-girders for single-cell composite box girders, and twin steel I-girders for composite decks) to three or more. The launch bearings are located under each web, and the launchability criteria are extended from deck centerline to the individual support lines to avoid longitudinal redistribution of bending and shear, and torsion and distortion of the cross-section.

Geometric Design of Launched Bridges establishes the geometric criteria for bridge launchability and explores the use of the launch cone for the geometric design of launched bridges made of steel and prestressed-concrete construction.

The eManual discusses the general case of conic launch trajectories and explains how simpler deck geometries can be attained by geometric degeneration of the launch cone. The use of elliptical launch trajectories is thoroughly explained, along with the impacts of crossfall transitions and geometry anomalies in the launch nose. Extensive guidance is also provided for the generation of the most appropriate conic geometry of the deck via 3D extrusion of the deck cross-section with AutoCAD and MicroStation.

The structure of chapters will guide you through a progressive learning experience for early familiarity and productivity with the use of the launch cone. You will discover launch trajectories for curved bridges that you would never have thought about…!

If you are interested in the design and construction of incrementally launched bridges, you should seriously consider this extremely inexpensive guide.