1. The Bosporus is 30 km long, with a maximum width of 3.7 km and a minimum width of 750 m. Its depth varies from 36.5 to 124 m in midstream. There are two bridges built across the strait. The first, the Bosporus Bridge, completed in 1973, has the main span of 1,074 m and its side spans are 231 and 255 m long. To avoid (столкновение судов и опор) the builders constructed a three span suspension bridge. The width of the (проезжая часть) is 33.4 m, and it allows six (полоса движения автомобильного транспорта) and two (пешеходные дорожки). The stiffness girder of the tubular cross section is 3.0 m high. It has good (обтекаемость) for increasing the aerodynamic stability of the bridge. The experimental scaled down version, representing 1/50th the bridge actual size, endured (испытания) in a wind tunnel. The bridge (был рассчитан) to resist wind forces and could withstand (порыв ветра) up to 162 km/h. One (пилон) stands on the European side and another one on the Asian side in order not to interfere with the transit of commercial and naval vessels through the strait.
The second bridge, the Fatih Sultan Mehmed, completed in 1988, has the main span of 1,090 m (fig. 12.1b).
2. Currently, civil engineers consider the preliminary study of the (подводный) tunnel construction through the Bering Straits as technologically feasible. A bridge crossing could not (выдерживать) strong sea currents and stay in serviceable condition over a long period due to low temperatures and severe ice conditions. The drift miners will have to dig through unpredictable (вечная мерзлота). Nevertheless tunnelling under constant temperatures is much more advantageous in comparison with concrete (укладка), metal (сварка), bolt (установка) at 40˚C below zero. The wide building practice within the Polar circle areas shows that humankind can develop and master new suitable technologies.
3. For water crossings, a tunnel construction is generally more costly than a bridge. Navigational considerations may limit the use of high bridges or (подъемный мост) spans intersecting with shipping channels. In addition, this is a strong factor in a tunnel favour in areas with expensive real estate, such as Manhattan and urban Hong Kong. Boston’s Big Dig project replaced elevated motorways with a tunnel system to increase traffic capacity and hide (поток транспорта).