Engineering Software Structural Analysis A380 Haldon Chalet Overbridge

Leading analysis and design software

	A380 Haldon Chalet Overbridge
	Devon County Council : SuperSTRESS/H-LOAD

By working with Integer Software's SuperSTRESS, Devon County Council can handle routine bridge assessment tasks as well as demanding new construction projects. For example, when the council decided to build an overbridge across the A380 to prevent accidents, traffic disruptions during construction were not an option. It has now designed an aesthetically pleasing single span bridge that will need minimal road closures.

Devon County Council's Bridge Office is part of the council's Engineering Design Group and employs 30 structural engineers and technicians. Its responsibilities include a programme of bridge assessments and strengthening schemes as well as routine maintenance and inspection work. There are over 3,500 bridges in the county.

Integer's SuperSTRESS frame analysis suite has played an important role for the council over several years. Balancing power, facilities and ease of use, the PC-based graphical analysis software has been particularly useful for the county's bridge assessment programme. Computer models of existing structures can be created and loadcases applied to check bridge capacity. It's then used to design any strengthening works, for bridges failing to meet the current load carrying capacity.

The proposed new 40-metre span bridge across the A380 south of Exeter has been the most recent challenge. A busy dual carriageway is crossed by a minor road at Haldon Chalet where drivers emerge into fast-moving traffic. According to Senior Professional Bridge Engineer Andy Matthews, the difficulty of the crossing denies vulnerable users, such as cyclists, pedestrians and horse riders, access to the forest walks in the area.

Artist's Impression

"The junction has a poor accident record, including several fatal accidents," said Matthews. "Even non-serious accidents can cause extensive delays especially during the holiday period."

Devon County Council decided on a £3.5 million scheme to realign the minor road and build an overbridge. Supported by four main steel beams, a concrete bridge deck between two abutments would carry a 5.5 metre carriageway, 1.5 metre footway and a 3 metre bridleway.

"With a design span of 40 metres, the concrete deck slab couldn't be cast in one hit. It was the consideration of how the bridge would be constructed over a live carriageway that governed the design as opposed to the final in-service state," said Matthews.

H-LOAD Model

SuperSTRESS and H-LOAD, the highway loading pre-processor, quickly allowed engineers to check the design against required traffic usage using HA (normal) and HB (abnormal) loadings. However, at a combined weight of 650 tonnes, beams and deck could not be placed in one lift.

"The design of the steel beams to take highway loading is relatively easy on this type of bridge. I knew from good design practice that bridges like this are cast in stages," said Matthews. "It was how the bridge would be built which was the technical challenge. SuperSTRESS was my first port of call as it's flexible and easy to use."

The initial step was to create a SuperSTRESS 3D model for the whole structure. Matthews made full use of the intuitive drawing input, paying close attention to the cantilever falseworks hung from the edge beams. However, this put a sideways force into the steel beams in addition to the normal vertical design movement from self weight.

"The cantilever falsework introduces a twist which you have to consider quite carefully. This was the beauty of SuperSTRESS. I very quickly set up a 3D model to see how the cantilever would twist the beams," said Matthews.

So what happens if there's a technical query about using SuperSTRESS for such detailed investigations? Matthews says that he finds Integer's support staff - all of whom are engineers - very friendly, responsive and knowledgeable. They're also keen to listen to suggestions for program improvements.

Having completed the falsework calculations, it was time to determine the safe extent of successive wet concrete loading. Once the deck was constructed, the beams under highway usage would be at 97 percent of capacity. Pouring too great an area of concrete at once risked failure.

The special loadcases for successive pours of concrete - working simultaneously from the bridge ends - were input using the graphical interface. As each SuperSTRESS analysis was run, the results were pasted into a spreadsheet for final checking. Adjustments were then made to the limits of the wet concrete loading to ensure the beams were not overstressed.

"It was very quick and easy to edit a loadcase, changing the way that the wet concrete was poured. It was the first time I'd used the new Windows version of H-LOAD and I found it very good. With SuperSTRESS you can call up a loadcase, see it on the screen, check it's in the right place, then hit the analyse button. An alternative concreting sequence could be checked in an afternoon," said Matthews.

The eight-month bridge construction and road alignment project will start in April 2005. An overnight road closure is planned for steel beam lifting with possible closures during the concreting work. Over 550 tonnes of concrete will form the 250 mm thick deck in three successive operations. As users of the new Haldon Chalet bridge are safely crossing the A380 in 2006, Devon County Council's Bridge Office will continue to work with Integer Software on other challenging projects.

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