Orthotropic deck
An orthotropic panel is used as a deck in the bridge device, which consists of a steel plate which is reinforced on the bottom with welded steel profiles, in the longitudinal and transverse direction. The orthotropic plate is used as a cover plate of box girder bridges or plate girder bridges, steel construction, as a track, suspension bridges and cable-stayed bridges, as well as movable bridges. Stiffeners prevent local bending, so that the loads can be absorbed better selective and transmitted to the supporting structure. The Orthotropic plate is compared with other equivalent constructions particularly easy, but expensive to manufacture and sensitive to errors in the detailed design. It is also referred to as lightweight steel roadway.
- 4.1 Selected examples of bridges with orthotropic deck 4.1.1 Examples from the German-speaking 4.1.1.1 Germany
- 4.1.1.2 Austria
- 4.1.1.3 Switzerland
Origin of the term
The name is formed from the two orthogonal technical terms of the strength of materials and anisotropic. The metal plate without stiffening elements would have an isotropic behavior, that is, they would show regardless of the loading direction, respectively the same force-deformation behavior. The stiffening elements give the plate a dependent of the direction of loading force-deformation behavior so that their properties are not isotropic, which is called anisotropic in the jargon. Since the two stiffeners perpendicular - in the jargon orthogonal - to each other, the plate has an orthogonally - anisotropic behavior, or if the two terms are contracted, an orthotropic behavior.
History
First constructions date back to the 1920s. America steel plates were used for the construction of the movable bridges have been stiffened with riveted steel beams. 1938 published by the American Institute for Steel Construction ( AISC ), the first reports of a designated as Battle deck floor construction that is built similar to a deck on battleships. It consists of a cover plate made of steel, which is supported by double T- girders.
After the Second World War, was awarded in 1948 the German engineer Wilhelm Cornelis, who worked at MAN, a patent for a road bridge with a flat sheet, which contained the description of the orthotropic plate. The technique was first opened in 1948 Deutz bridge applied, the superstructure consisted of a hollow box in steel with an orthotropic deck plate. In the Mülheim Bridge opened in 1951 for the first time an orthotropic plate beam was applied. This was followed in 1954, the river bridge in the Porta Westfalica as steel box girder bridge with orthotropic deck.
The orthotropic plate was easier compared to the structures already known. This allowed both a building material and weight saving, as well as ultra-slim bridges are built with large spans.
End of the 60s and beginning of 70s rushed four Stahlholhlkastenbrücken with orthotropic plates during the construction phase: the south bridge, the bridge Prater, the Cleddau Bridge and the West Gate Bridge. The apparent connection between these collapses led to an overreaction in the professional world and to a highly critical assessment of these designs, the orthotropic plate concerned not to blame for the collapse and two of the four collapses were caused by improper procedures during construction. However, it was found that the construction of bridges with orthotropic decks is challenging and is particularly sensitive to errors in construction details. Early constructions often used too thin a cover plate.
For general applications, construction was replaced with orthotropic deck slabs of concrete beam bridges that can be created more cost-effective because the expensive welding work is required. In addition, concrete bridges can be created on the spot when needed without prefabricated parts. Orthotropic deck carriers are only used in bridges where weight plays a crucial role, such as for movable bridges or long cable-stayed and suspension bridges.
Older bridges with orthotropic decks are often no longer cope with the increased traffic loads and must be extensively renovated. One way to clean up is the application of new pavement, the better distributes the loads on orthotropic plate and this stiffens zusätzulich.
Structure and function of the structure
The orthotropic plate consists of the cover plate, including the welded longitudinal ribs and the transverse beams.
Cover plate
The cover sheet can be 10 to 15 mm thick and is covered with a pavement of mastic asphalt, which is usually about 60 mm thick. The mastic asphalt protects the cover plate from corrosion and distributes the contact forces of vehicle wheels better on the construction. The cover plate acts both to the longitudinal ribs as for the transverse webs as a flange. When using an orthotropic plate in a plate girder bridge from steel, the cover plate is also a top flange for the side rails.
Longitudinal ribs
The longitudinal ribs have a generally trapezoidal cross -section and are welded at two welded seams on the underside of the cover plate. They are usually 6 to 10 mm thick and enclose with the cover plate a cavity whose strength behavior similar to a hollow box and makes the orthotropic plate Rigidity. The longitudinal ribs often have openings on the underside, that allow the access to the screws, which hold together the individual bridge segments.
In place of trapezoidal fins have other uses less suitable profiles. Mostly to handle the target patents. The cross sections used were U- profiles, V-profile or champagne flute profiles. The latter caused over the other profiles a four times higher welding effort.
Originally I-, L- and T- profiles have been used, in which the cover plate and the longitudinal rib did not form a closed cavity. These structures are compared to the design with closed longitudinal ribs torsionally, but could be easily calculated. In addition, the underside of the cover plate is less protected from corrosion, since a larger surface area is exposed to air compared to ortothropen plates closed profiles.
Transverse webs
The cross-members are arranged at orthotropic plates open ribs at a distance of 1.5 to 2.5 m, in such closed ribs at a distance of 3 to 5 m. The cross-members are provided with recesses for passing the ribs. The ribs are welded to the cross members, only the bottom of the rib, the welding is often omitted and the recess therein such that the rib will not be affected, and thereby squeezing stresses between rib and cross member during bending of the plate in the longitudinal direction can be avoided.
The cross-members supported on the one hand, the cover plate in the transverse direction, together with the longitudinal ribs and a grating, which increases the strength of the panel.
Applications
Orthotropic deck plates come in bridges for use where weight plays a very important role, such as for movable bridges or long cable-stayed and suspension bridges. Compared with pure concrete structures can be saved with orthotropic plates with such bridges up to a quarter of the weight, because not only the road carrier easily fails, but also the parts that wear this.
Orthotropic plates can also be used to strengthen existing bridges by the existing concrete deck slab is replaced by an orthotropic plate.
The disadvantage is the often premature formation of smoothness for road bridges in the winter.