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TransPro Sept qxd


TransPro Sept qxd

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Design of UK highway bridges continues to evolve as efficiency of construction and maintenance exerts greater influence on choice of structural form and material. Jon Masters reports.

Steel composites meet bridge cost challenge
01/02.2005 pg. 022 assage along the route of the M6 Toll, and several other major road projects either recently completed or under construction, reveals some shift in the highway bridge market. All 46 overbridges on the tolled motorway are steel composite structures and many of them are semi-integral – a design philosophy borne from several factors. Consideration of whole life cost is an important influence, as is a number of technical developments and the shift to Design & Build, ECI (Early Contractor Involvement) road contracts. Private finance has promoted measurement of whole life


Overbridges at the A34/M4 Chieveley junction were built as composite with weathering steel after evaluations of whole life and construction costs

cost and the Highways Agency is now routinely asking its ECI teams to investigate the most cost effective designs for bridges’ lifetimes. Interserve and Atkins is the ECI team for the grade separation of six roundabouts on the A1 between Peterborough and Blythe. Its whole life cost report recently presented to the HA favours steel beams for each of seven major bridges with spans up to 27m. Construction costs were included in the calculations and overall it seems the economics are pointing to steel composite structures. It’s a similar story on the A1 between Darrington and Dishforth, another privately financed D&B scheme where 27 of 36 primary structures are in steel (see box). The biggest change is in the market for spans of 25m and less. Steel now takes around 20% of this sector according to recent research. “The cost considerations are influenced by the fact that there is often a shortage of skilled labour for concrete bridges, whereas much of the skilled work of a steel deck is carried out in the factory,” says Faber Maunsell technical director Charles Cocksedge. “Design is being driven by whole life cost and the steel fabricators have helped to make the economics stand up.” Another influencing factor favouring composite structures is the reduction of minimum headroom for weathering steel beams over roads from 7.5m to 5.3m. This has brought weathering steel into the highways market. The height reduction makes weathering steel commensurable in

A steel composite and semi integral bridge design philosophy was selected for the M6 Toll

terms of vertical alignment and its cost is similar to painted steel, but with advantages of lower maintenance and whole life cost. This was the conclusion drawn by Costain and its designer Mott MacDonald when tendering for the Chievely A34/M4 Junction 13 improvement project. Construction of the A34/M4 scheme was completed in 2004, with five steel overbridges built in weathering steel – the selection of which contributed to a winning tender. The HA assigned a monetary value related to speed of construction and added cost to bids offering contract periods in excess of the shortest offer. “We had to come up with the lowest capital cost, whole life cost and the solution quickest to build,” says Mott MacDonald’s bridges department project engineer David Place. “The illustrative design included concrete overbridges, which we changed to composite construction with weathering steel for advantages



Ladder decks form horizontally and vertically curved alignments at the Holmfield junction

Multi spans favour steel
01/02.2005 pg. 024 of whole life cost and programming.” “We knew from previous contracts, such as the A2/M2 and the A43 Silverstone Bypass, that composite would allow a shortened programme and a more attractive tender,” says Costain project manager Darren James. Greater consideration of bridge maintenance costs has also led to a drive for integral and semi integral bridges. There are limits to the spans and amount of bridge skew to which both can be applied. But where possible, greater integration between deck and supports is being pursued to reduce mechanical bearings and movement joints – the parts that have tended to go wrong through wear and tear. A standard form of bridge design on the M6 Toll features fairly simple bearings on piles, which are independent of a surrounding reinforced earth bank seat or abutment in terms of vertical load. The deck interacts horizontally with the abutment, so there is no need for movement joints. “This semi integral arrangement represents a half way house in the analysis of risk and cost,” says Cocksedge. “Fully integral bridges are further free of maintenance in theory, but the calculations become very complex for what are essentially big portal frames with external earth and water pressures. Soil behaviour and structural design are different sciences and even with semi integral bridges, it will be perhaps 20 years before we can evaluate whether we have got it right.”

Upgrade of the A1 between Darrington and Dishforth includes two improvement schemes, of which the £183M southerly Ferrybridge to Hook Moor section is the most complex. It includes a large new bridge over the river Aire at Lagentium and three viaducts forming part of a new intersection with the M62 at Holmfield. The 33 year Darrington to Dishforth Design, Build, Finance & Operate concession is held by the AMEC, Alfred McAlpine, Dragados and Kellogg Brown & Root consortium Road Management Services (RMS). For most of the bridges in the southern section, RMS has adopted a standard steel ladder deck design, with the exception of the Aire crossing. This is a three span structure with a central span of 85m, side spans of 35m and eight plate girders supported on elliptical columns. The girders are up to 4.5m deep at the piers, close to the maximum that steelwork fabricator Fairfield Mabey could handle.
The A1(M) will cross the Aire at Lagentium on a three span steel composite bridge.

Each of the multi-span viaducts forming the grade separated interchange at Holmfield has two main edge girders linked by cross members to form a ladder deck. The viaducts are curved both horizontally and vertically. “That does not lend itself to concrete, even using trapezoidal segments,” says RMS engineering manager Mike Gettings. Other bridges on the project are typically of two 25m spans. Mr Gettings says the aim was to keep them as simple as possible, to make a standard overbridge for side roads – effectively setting up a production line of similar bridges. “There is a general trend to steel composite and ladder deck construction,” he says. “The weight and structural depth of concrete beams becomes significant at this sort of span and the structural steelwork approach is a relatively easy method and cuts down craneage costs. Above a single span, steel lends itself better to the requirements of the job.”

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