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UK modellers aid new Tacoma Narrows bridge

ICE news

HR WALLINGFORD has been carrying out hydraulic modelling simulations for the new Tacoma Narrows Bridge Crossing in the state of Washington, US, a conference at the ICE was told last week.

'This has been a massive study for us, ' said HR Wallingford principal engineer Dr Richard May, speaking at the International Association of Hydraulic Engineering & Research (IAHR) conference.

The Tacoma Narrows hit the headlines in 1940 when the bridge failed spectacularly due to wind induced loads causing elastic excitation.

A new bridge has been open since 1950 and was originally designed to carry 60,000 vehicles per day. But studies show that as many as 90,000 vehicles use the corridor. This is expected to rise to 120,000 by 2020.

For continuity the new bridge will be an exact copy of the existing dual pier threelane suspension bridge. On completion in 2007 it will carry three lanes of eastbound traffic. Westbound vehicles will use the existing bridge.

Contractor Bechtel wants to float out the two new octagonal piers into the 1220m wide Puget Sound. These will then be constructed to full height in the water and using the increasing mass would sink down to the bed of the channel.

However with in a 70m deep channel and a flow velocity of 4m/s, careful design has been needed to calculate how to keep the concrete caisson piers in place.

This is where HR Wallingford came in. The team constructed a 1:100 scale model of the project that simulated the tidal conditions in the channel.

It was discovered that the piers must be secured on the bank with temporary cables.

As the structure grows more cables will have to be added to take the increasing mass until eventually 32 cables will hold it.

Construction of both piers recently began on site.

Also presenting at the conference were Dr Peter Mason of Black & Veatch, Professor Peter Mantz of JBA Consulting, Professor Alan Irvine of the University of Glasgow, Dr David Rhodes of Cranfield University and Professor Roger Bettess of HR Wallingford.

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