A new 837 m long elevated railway has been built at Hitchin in the UK to carry trains between London and Cambridge over the East Coast Main Line. It relieves the main bottleneck at the junction where Cambridge line trains cross the East Coast route on a flat junction. The project involved the design and construction of a 29-span viaduct made of a steel–concrete composite deck, which is effectively composed of 15 jointed structures. Twelve of these are made of two continuous 30 m spans, two structures are made of single, simply supported spans and the remaining length is a continuous three-span structure with a maximum central span of 60 m that crosses the railway at a skew angle of more than 60°. The 425 m curved plan radius alignment was a design challenge as significant torsional effects are caused by the plan curvature, the eccentricity of applied vertical actions, the centrifugal effects and the wind pressures acting on the trains. Steel erection started with the main railway spans, a process that took three overnight possessions due to the curvature, the heavy skew and the geometry of the girders. This paper describes the critical design aspects and the main construction operations of the flyover.

The A82 is a primary route in Scotland that follows the shoreline of Loch Lomond. Following a landslide in the late 1970s, traffic lights were implemented, limiting vehicles to one-way traffic. A widening scheme was required to restore the two-way carriageway over a 400 m length. This scheme included a six-span 168 m viaduct curved in plan to follow the existing road alignment. The form of the structure was highly influenced by the site topography, the adjacent soil/rock slope profile and the loch shoreline. It aimed to provide an economical, buildable solution while minimising intrusion into the sensitive environment and minimising traffic disruption during construction. To meet these criteria, the design was developed to suit a phased construction. A full-length contiguous piled retaining wall supported the existing carriageway and facilitated installation of a trapezoidal steel–concrete composite box-girder viaduct. Partial construction of the deck slab allowed a temporary switch of traffic, enabling the remainder of the bridge works to be completed on the original road alignment. Once constructed, the 3000 t deck was jacked-up to its permanent position using 82 synchronised hydraulic cylinders. This paper summarises how design and construction of the viaduct was achieved in its challenging environment.

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As part of a new University Campus development in the city of Northampton (UK), a new road access bridge was required to span over the river Nene. The client's aspirations and the planning requirements were set to keep the character of the existing landscape while creating an appropriate landmark structure for the new campus. The client's specimen design included a concrete flat arch bridge spanning 49m with a shallow rise of 3.7m above the navigable channel of the river. During the tender design competition, closer reviews of the concrete solution were undertaken by the design and build team, concluding that the imposed loads to the foundations and the construction method required to build the concrete deck would not prove to be economical. Thus, an alternative design was developed using a steelconcrete composite structure. The awarded solution includes 220 tonnes of welded steel plates to form a shallow and flat arch structure with deck cross girders. This bridge case study paper discusses the design aspects from development at planning phase to tender design and detail design as well as the specific construction stages related to the steelwork erection.

In today's environment and client's requirements, solutions for medium span road and rail bridges within the UK tend to result in an overlap between three types of steel-concrete composite decks. From the ban and moratorium imposed on grouted post-tensioned concrete bridges since the early 1990s, braced pairs of plate girders, twin plate girders with cross girders (ladder-deck) as well as steel box sections have dominated the British construction market for spans varying between 30m and 70m. Open box sections (bath tub) are sometimes preferred due to specific geometry requirements or even purely from aesthetic requirements. This paper reviews and compares the options based on the available design guidelines as well as the author's recent experience in delivering detail designs of each alternative arrangement. The aim of this paper is to summarise the main driving factors leading to the choice of a specific arrangement. A discussion on geometrical considerations (skew, curve, span to depth slenderness ratios) as well as a comparison of estimated weight of steel per metre square of deck for each option are included. Each solution is presented from the detail design and fabrication point of views including the transport criteria, the erection consideration and the temporary stability stages. The aesthetic and the whole life cost aspects (such as durability, inspection and maintenance) are also considered in the comparison.