Concrete Society Building Category winner - American Air Museum, Duxford, Cambridgeshire
The museum has been designed to offer a neutral background to the 21 aircraft exhibits, the largest of which is a b-52 bomber with a 16m-high tail fin and a 61m wingspan. It is simple in form, placing its design emphasis on clarity, natural light and an economic system of atmospheric control. Yet despite this, the drama of the broad curve of its concrete roof with the abrupt slice of the 90m, 18.5m high glazed south-eastern facade cannot fail to impress.
The thermal-insulation properties of concrete determined its choice over steel. Concrete would keep the temperature above the dew-point, so that condensation would not occur, minimising the requirement for dehumidification plant. The thermal mass of the concrete insulates against major variations in temperature and so removes the need for both air conditioning and heating.
The double-skin roof spans up to 90m and is made from two precast 100mm concrete shells 900mm apart. The membrane action of the shells allows load sharing in two directions, especially under the weight of the suspended aircraft. Forces from the concrete roof shells are collected into an in-situ curved upper concrete ring beam and then passed across a 'daylight slot' via 34 steel arms spaced every 4m to a lower in-situ ring beam and finally to the abutments and foundations.
The geometry was solved by designing the roof as part of a torus which is defined by only two constant radii. This meant that the 924 precast roof panels could be made from only six sets of standard shell components.
The American Aircraft Museum demonstrates how concrete can be used to realise not only an imaginative design and structural vision but also to actively contribute to the cost-effective operation of the building. The project team, including architect Foster & Partners, engineer Ove Arup & Partners, contractor J Sisk and Sons and concrete contractor O'Rourke Civil & Structural Engineering, is to be congratulated.
CONSTRUCT Award for Innovation and Best Practice
The year the construct award is presented to two projects: Christ Church Court and the Helicon Project, the former for demonstrating the benefits of a fully integrated team approach and the latter for the development of an innovative design solution.
Christ Church Court is a commercial office development in London. Nominated as a Movement for Innovation (m4i) demonstration project, Christ Church Court is a prime example of the benefits to be gained from the input of contractors into the design development.
The structure was designed so that the basement slab would be constructed independently of the pile caps to allow steel erection to proceed only 11 weeks into the project. The site was particularly congested, requiring numerous handovers and return dates between the various trades which were agreed at planning meetings. These meetings, including regular workshops and brainstorming sessions with all members of the project team, were led by the trade contractors in order to demonstrate to the client, Stanhope, the positive interaction within the project team. The meetings proved invaluable, particularly in determining the reinforcement details, temporary works, complex earthworks and retaining-wall support solutions.
The close working relationships that became a strong characteristic of the project team encouraged various patented systems and new developments to be introduced and adopted. A patented method for breaking down pile heads, which received a 1999 Innovation in Construction Award, was used to improve cycle times for pile-cap construction. The adoption of an open- plan site office, the latest it systems including 2d and 3d visualisation, combined with progress monitoring using digital photographs posted on corporate websites, further enhanced the high levels of communications and dialogue.
The construction industry has traditionally been seen as adversarial and lacking in trust. The project team of John Doyle Construction, Bovis and Waterman is to be congratulated on proving the benefits of real teamwork and open communication.
The second winner of this year's construct award was Helicon Court for the approach adopted for the prefabricated slab/column connections for the Helicon Building, a commercial development for London and Manchester Assurance. The construction team comprised Osprey Project Management, John Savage Associates and O'Rourke.
The innovative approach to the connections was facilitated by a number of factors. An external architectural feature of the building is the circular white concrete columns that rise between the silver-grey cladding and the curtain walling. These columns are visible as half cylinders protruding from the face. The connection between the columns and the slabs is therefore only half the cylinder, or a quarter at the corners.
Another factor was the significant pressure to maximise lettable floor space and minimise slab depths. Column sizes were minimised by the use of high-strength concrete. Post-tensioning enabled large span-to-depth ratios to be achieved. Maximum column spacing with large spans resulted in high column loading.
Because of these factors, minimal connections between external columns and floor slabs were required to support the considerable loads. Slab/column connections designed using current British Standard codes of practice had insufficient shear capacity. The construction team developed a prefabricated steel/ slab connector that lengthened the shear path. While the design and form of this connector was simple, it was contained wholly within the initially proposed concrete form and as a result could be repeated throughout the project without affecting the complex junctions of structure, chilled ceiling and cladding at the slab edges.
This innovative solution enabled the lettable floor areas to be maximised while reducing the amount of reinforcement in the shear heads. It demonstrates that concrete design should not be constricted by adherence to the limits set in codes of practice.
Concrete Society/British Precast Concrete Federation Award for Excellence in Precast Concrete
This is a new joint award for a structure that shows best how precast concrete can significantly contribute to its overall success. The award goes to Tarmac Precast Concrete for the grandstand at Lord's Cricket Ground
The new grandstand is part of a total development of Lord's. It provides maximum seating with uninterrupted views, made possible by cutting the number of columns needed to support the upper tier. Right from the start, precast concrete was the main structural option as, with construction restricted to two short closed cricket seasons, work on site had to be limited to final assembly of prefabricated units which had to be of assured high quality.
The lower terrace (Phase I) consists of an interlocking assemblage of precast beams, terraces and slabs supported on in-situ columns on a 7m square grid. Critical to Phase II was the spine truss which enabled the entire grandstand to be free-spanned with two 50m spans. Initially, steel frames were used off this truss to support precast infill floor slabs. However, the design evolved so that the precast floor slabs replaced some steel work to become part of the primary structure. The interlinking required the fabrication of large steel connection nodes to be cast into the concrete to millimetre accuracy. This need for accuracy was further compounded by the fact that most of the elements of the grandstand fitted together with several other elements.
Using precast concrete significantly helped the construction programme. Phase II was constructed in just 15 weeks despite consisting of 108 precast 3.5m x 7m slabs interlinked with 138 precast terrace units, 27 steel raking beam frames, 90 tie rods and the spine truss itself.
The quality of the precast concrete finish is outstanding. It has an almost 'fabric-like' appear- ance and sets a benchmark of visual and textural quality.