Suspension Bridges: Vulnerable but vital asset

Long-span suspension bridges

Vanja Samec explains that long-span suspension bridges have an important role in the transportation network therefore their design, construction, and subsequent surveillance and maintenance must be performed very accurately.

Vanja Samec

Vanja Samec

Long-span suspension bridges represent some of the most remarkable, yet most vulnerable, assets in road networks. Due to their important role in the transportation network, the design, construction, and subsequent surveillance and maintenance must be performed very accurately. During the design process, bridge designers must consider and meet many challenges, including the highly non-linear behaviour of the structure, the optimization of the geometry of suspension cables, and the effects of wind.

The continuous change of structural systems is a major reason for non-linear structural analysis. For cable-supported bridges, special optimization procedures are necessary. For long cable-stayed and suspension bridges, bridge designers must consider dynamic wind effect. The extraordinary, ultrathin design of these structures yields significant susceptibility to wind-induced vibrations. Steel bridges, especially, allow for extraordinarily slender main girder cross sections. Sophisticated analysis methods must be applied to determine critical wind velocities for all types of known wind effects. As a result, dynamic wind analyses are increasingly important to bridge engineers. These phenomena include vortex shedding and the lock-in phenomenon, across-wind galloping and wake galloping, torsional divergence, flutter phenomena, and wind buffeting.

Today, computer programs should provide the best possible support for this design process. One such product is Bentley RM Bridge, which has been well-tested and proven on major projects to become a globally recognized, expert system capable of solving virtually any bridge design or analysis problem.

Fig-2

Fig-2

For example, RM Bridge was used successfully by engineering designers on the Hardanger Bridge (Fig. 1), the longest long-span suspension bridge in Norway. This bridge, which opened August 2013, crosses the Hardanger fjord. It has a main span of 1,310 meters and is ranked No. 10 on the list of longest suspension bridges in the world. The Norwegian road authority, Statens Vegvesen, in close collaboration with TDA Norway and Bentley Systems’ Austria team in Graz, performed the design work.

The bridge deck consists of an orthotropic steel box, with a width/depth value of 17.3 meters/3.2 meters. The stiffness of the main girder is relatively small when compared to other bridges of this span type. The distance between the two main cables is only 14.5 meters, which means that the Hardanger Bridge is one of the most slender bridges in the world (Fig. 2).

Among some of the specific challenges on this project were the highly non-linear behaviour of the structure; the need to optimize the geometry of the suspension cables while designing the sag profile; the non-linear behaviour due to the traffic loading; and optimization of the erection procedure, wind loading, and wind-induced vibrations.

Fig-3

Fig-3

Numerical wind investigations of the main girder and pylons were performed with a CFD module that applies the vortex particle method to describe the air flow around the cross-section (Fig. 3).

Future projects in Norway that will require innovative bridge and tunnel technologies, as well as experienced engineers and reliable software applications, include the E39 road between the cities of Kristiansand, Stavanger, Bergen, and Trondheim, which will become a ferry-free highway route.

(Vanja Samec is Global Director RM Bridge – Bentley Systems Austria GmBH.)

 

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