Bridge Blogging

I am fortunate to live near an engineering marvel in progress: the new Tacoma Narrows bridge. Most folks have heard of the Tacoma Narrows bridge – at least the first one, “Galloping Gertie”, which catastrophically failed during a windstorm in November 1940.

Built at the cost of $6.6 million dollars, designed by world-famous bridge architect Leon S. Moisseiff (who also designed the Golden Gate bridge), it embraced the light, elegant design principles in vogue at the time – and was designed with complete ignorance of the aerodynamic effects of high winds on bridges. Moisseiff had inadvertently created a mile-wide airplane wing, with its light-weight narrow deck and plate-girder sides. It survived only 4 months after completion. In a strong-but-typical November windstorm, the wave-like undulations were severe enough to unseat a cable from its saddle on the West tower, creating a corkscrew torsional motion which ripped the bridge to shreds. The only casualty, surprisingly, was Tubby the three-legged dog.

May he rest in peace.

The fallen span of the bridge remains at the bottom of the Tacoma Narrows, and has been designated a National Register of Historic Places to prevent salvage. It is one of the world’s largest artificial reefs, and home to a plethora of marine life, as well as the world’s largest octopuses. The remainder was disassembled and sold for scrap during WWII. The caissons and anchors (for the cables, on either bank) were used, largely unmodified, to support the towers and cables of the second Narrows Bridge.

The second – and current – Narrows bridge was begun in 1948, and completed October 14, 1950, 29 months after construction began, at a cost of $14 million. It was one of the most highly researched bridge engineering projects in history, and greatly advanced the understanding of aerodynamics in suspension bridge construction. A 1/72 replica of both the original and the new bridge were built in a wind tunnel and thoroughly tested for several years prior to design completion.

Designed to carry 60,000 cars per day, the current bridge ferries over 90,000, and has become a major chokepoint for traffic in the rapidly growing South Puget Sound area. These transportation pressures have given rise to the new Tacoma Narrows Bridge project.

The Tacoma Narrows is a formidable natural barrier. Carved out by ancient glaciers, over a mile wide and 260 feet deep, with steep, unstable banks on either side, it is a hostile environment for a suspension bridge. Wild tidal currents rip through the Narrows twice daily, through the sole portal between the Pacific Ocean and the entire South Puget Sound. High winds and fog are common. The Puget Sound area is also prone to major earthquakes.

The new Narrows Bridge project is the largest engineering endeavor in the U.S. in the last 30 years. Construction began in late 2002, after approval of an $800 million public-private financing package. The new caissons underwent initial construction in the Port of Tacoma, and were towed to their location in the Narrows, where they were secured in place on the surface with a series of anchor cables radiating circumferentially. These cables, and flotation tanks in the caissons, where used to control the descent of these floating concrete islands, as layers of concrete were added to the top.

The task was akin to building a 25-story building from the roof down, all the while holding its precise position in the alternating rip tides of the Narrows. The bottom of the caissons is configured to be a mammoth cookie cutter, with a sharp steel knife edge to cut through sediments to reach bedrock 50 feet beneath the bottom.

On the east and west banks are the anchors – enormous concrete fortresses designed to secure the cables with their huge tractive forces to the sandy glacial till on either side of the Narrows.

Over the past several months, the towers have begun to rise from the caissons, and now are nearly at the deck level of the current bridge.

The new bridge is scheduled to be completed in 2006, and renovations to the existing bridge made after this. The final project is scheduled for completion in 2007. You, my faithful readers, will see history unfold on this very blog, with regular updates until then.