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Travelling shutters explained

THE ALMUÑÉCAR viaduct's twin concrete box girder decks use post-tensioned in situ construction with internal tendons. A travelling shutter system was used for deck box pours. This method is well established, but not the favoured option for UK concrete box girder bridges, say UK bridge experts.

Travelling shutter systems are often designed and produced by specialist falsework companies, and in principle are similar.

Column heads (or arch crowns) are fitted with temporary platforms that support rollers or other forms of bearing each side of the permanent bridge bearings. Steel trusses capable of spanning between at least three support points are then launched across the gaps.

The trusses also carry bearings on their upper surfaces to support a travelling shutter system, which can be relatively complex if the deck cross section varies across the span.

Each section of deck can be poured as one, or as a series of pours. During pours, the shutter is locked to the trusses.

When a span is complete and the concrete has gained enough strength, the shutter is unlocked from the trusses and locked to the deck section. The trusses beneath it are then unlocked from their supports and pulled forward to reach the next support point.

Support trusses are normally fitted with some form of nose to help them overcome the inevitable droop as the end of the cantlilevered truss is shifted on to the top of the next pier.

These are not usually designed to carry more than dead loads from the truss itself.

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