Record-Breaking Suspension Span to Link Two Continents

With backing from a Middle Eastern developer, an American firm is moving ahead with plans to erect an intercontinental bridge across the Red Sea. The proposed crossing, which has been estimated to cost at least $12 billion, would for the first time create a direct ground transportation link between the Arabian Peninsula and the Horn of Africa. It would also surpass by more than 700 m the current record for the world’s longest suspended span.

The effort gained momentum in April, when Middle East Development LLC, of Dubayy (Dubai), issued a notice to proceed to Noor City Development Corporation of Napa, California, authorizing it to manage the project. Since then Noor City has been working to assemble an international design and construction team, with hopes of selecting a design firm and a contractor by mid-August, says Tariq Ayyad, the firm’s president. Once the team is in place, making rapid progress toward construction will be a high priority. “We are not going to spend three years for design and engineering,” Ayyad says. “It’s got to be done in six months.”

A preliminary feasibility study by COWI, which is based in Lyngby, Denmark, has settled upon two possible alignments across the Bab al Mandab, the strait that joins the Red Sea to the Gulf of Aden. East alignment consists of two segments. An initial, 3.5 km long segment would connect mainland Yemen to the Yemeni island of Barim (Perim) which divides the straight into channels. At that point, the bridge would either continue straight across the island or bend southward toward the strait’s western channel at a different angle. A second, 21.5 km segment would then link Barim to the African nation of Djibouti. The total length of the crossing, including the overland structure on Barim, would be approximately 29 km.

The most remarkable feature of the proposed bridge, however, is not its overall length but the length of its main spans, which would carry both rail and highway traffic over some of the world’s busiest shipping lanes. One possible configuration calls for a pair of main suspension spans that would each be 2,700 m long and would be separated by an intermediate span of 1,800 m, says Lars Hauge, who is the director of international bridge projects for COWI. By comparison, the main span of Japan’s Akashi-Kaikyo Bridge, which currently has the longest suspension span in the world, is 1,991 m.

In the feasibility study, COWI engineers drew on their previous work on such large suspension bridges as the eastern part of Denmark’s Great Belt Bridge and Sweden’s High Coast Bridge, as well as two mega projects that so far have not been built: a bridge across the Strait of Gibraltar and another across Italy’s Strait of Messina. In the 1980s and 1990s, the firm performed feasibility studies and a preliminary design for the Gibraltar project, which envisioned a 27 km long structure featuring three 3,500m main suspension spans. For the bridge that would cross the Strait of Messina, COWI completed a prebid design two years ago that included a 3,300 m main suspension span.

Just what form the foundation for the Bab al Mandab crossing would take is not yet know, notes Hauge, since more information is needed regarding the geotechnical conditions at the bottom of the strait. It is clear, however, that the bridge would require foundations in places where the strait is more that 100 m deep. For that reason, he says, the substructure design is likely to draw as much on offshore oil platform technologies as on traditional bridge engineering. The bridge concept for the Strait of Gibraltar, for example, where water depths can exceed 300 m, incorporated concrete gravity platforms similar to those typically used by the oil and gas industries.

As for the superstructure, the bridge design would also obviously need to take wind into account, especially its effects on the main suspension spans. An aerodynamic box girder of the type envisioned for the bridge over the Strait of Messina may be required, notes Hauge. Nevertheless, he says, the concepts developed for that structure demonstrate that a span of such length can be designed with current technology.

The bridge’s location, near the juncture of two tectonic plates, is another concern, although a suspension bridge would tend to fare better in an earthquake than a shorter, less flexible span, observes Hauge. “Seismicity is an issue, and it has to be treated carefully, but it is not deemed to be a showstopper,” he says. “So far, we are confident that the project is feasible.”