Sunday, October 4, 2009

Vancouver's Bridges: Alex Fraser Bridge

The Fraser River begins in the Rocky Mountains and flows for 1375 km (870 mi) before reaching the City of Vancouver. Then it splits into several arms and forms large islands before flowing through a delta into the Strait of Georgia.

The Alex Fraser Bridge crosses the Fraser River about 20 km (16 miles) east of the Strait of Georgia. Although the bridge and river share the same name, the river was named after the early 19th century explorer Simon Fraser while the bridge was named after a Canadian politician named Alex Fraser.

The Metropolitan Vancouver region seems to have an insatiable need for bridges. In the couple of years since I last visited Vancouver, they've built the Golden Ears Bridge and the North Arm Bridge (two extradosed bridges) over the Fraser River. Both were designed by Buckland and Taylor, who have become experts in cable-stayed and extradosed bridges. Last year, I took a two day class on cable-stayed bridges, which was just some engineers from Buckland and Taylor discussing their past projects, including the Alex Fraser Bridge.

When the Alex Fraser Bridge was completed in 1986, it was the longest cable-stayed bridge in the world with a main span of 465 m (1526 ft). Twenty-three years later, it has dropped to the 32nd longest cable-stayed bridge (the Sutong Bridge in China is currently the longest cable-stayed bridge with a main span length of 1088 m (3570 ft)).

The Alex Fraser Bridge has two reinforced concrete H-shaped towers that are 154.3 m (506 ft) tall. Usually, there are obstructions like trees, buildings, or mountains that make it hard to photograph a bridge, but the Alex Fraser Bridge is hard to miss. Its by far the biggest thing in the landscape. Both the tower legs are on the river banks, which makes this part of the river (south of Annacis Island) about the same width as the Huangpu River in Shanghai and much narrower than the Hudson River in New York. Although the approach spans look normal in depth, the cable spans are very thin. The precast deck panels are 220 mm or 8.7 inches thick and are supported on two-meter deep steel girders. The bridge was built as two balanced cantilevers with each 9 m (30 ft) deck segment built on both sides of each tower before being attached to the tower with cables. The side spans are only 183 m (600 ft) long so they must have anchored the side spans to the back piers before they finished putting down the deck for the center of the main span.

We'll take a closer look at the Alex Fraser Bridge tomorrow.
Creative Commons License
Vancouver's Bridges: Alex Fraser Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

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