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Low decks expose Florida bridge to Ivan surge

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US STRUCTURAL engineers have concluded that a major motorway bridge in Florida was unable to stand up to a massive storm surge induced by Hurricane Ivan because the deck was too close to the water level.

A Florida Transportation Department engineer told NCE that most of the 36 year old Escambia Bay Bridge's deck is only 4m above water making it vulnerable to powerful storm surges.

As a result around 1km of the 4.1km long bridge's deck was swept off its supports and into the sea when Ivan hit the southern Florida city of Pensacola on Thursday last week.

A lorry driver was killed when the cab of his articulated lorry plunged into the water as one of the 20m span deck sections was engulfed.

Other deck sections were shoved sideways by the force of the surge and were still balanced precariously on their supports this week.

Florida is seeking federal funds for a new $200M (£125M) bridge with greater capacity and which will also have a higher deck to comply with design code changes introduced since the existing bridge was built in 1968.

'I expect that in the near future there will be a replacement bridge which will be higher above the water', said Florida Transportation Department district structures and facilities engineer John Locke.

The two lane bridge carries eastbound traffic on the I10 east of Pensacola to Milton. A second parallel bridge, 20m to the north carried westbound vehicles. Both structures suffered serious damage, although the southern bridge bore the brunt of the storm.

About 75% of both structures are made up from a series of 20m long concrete deck sections bolted to cross beams. These beams are supported by three circular piers across the bridge's width (see picture below).

Each bridge has 221, 20m spans plus three 86m central spans supported on longitudinal steel beams. These central spans are 18m above sea level at the point where the bridge crosses a shipping lane.

Most of the spans lost were at the eastern end of the eastbound bridge. 'On the eastbound bridge, we lost 46 spans, 29 at the eastern end, and 12 on the westbound, ' said Locke. The sections were swept into the 5m deep water after they were torn from their anchor bolts.

Some of the cross beams were also pulled from their piers, and the force of the storm surge snapped some piers from their piled 3.3m diameter hollow concrete foundations.

'Nineteen piers were damaged on the eastbound bridge and seven on the westbound, ' said Locke.

He believed the storm surge was powerful enough to have overtopped the low lying parts of the two bridges.

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