Construction techniques | Power plant design
A concrete history27 October 2011
Westinghouse is looking to use steel concrete composite (SC) construction techniques for its AP1000 reactor building. But while the technique is relatively new to the nuclear industry it has been extensively studied in the UK. By Bassam Burgan
A double skin steel-concrete-steel design (referred to as SC construction) was first proposed in the UK in 1986 by consulting engineers Tomlinson & Partners and Sir Alexander Gibb & Partners for the Conwy River crossing in Wales. A design feasibility study showed that SC construction can eliminate the need for internal reinforcement of the concrete and the result would be a lighter, more economical design compared with any other structure of its type. The SC construction concept that evolved consisted of inner and outer skins of steel separated by a 500 mm thick slab of concrete. The steel and concrete were made to function integrally by closely spaced Nelson stud shear connectors. An example of such construction is shown in Figure 1.
The reduction in overall thickness and the elimination of internal reinforcement cleared the way for a completely different approach to the construction process. These lighter structures could be fabricated in a remote shipyard and transported to the site around the coast, thus eliminating congestion on the roads and avoiding the need for a casting yard, with all the associated local objections. Once in position, the sections could be sunk by placing high density ballast on the tunnel floor. This was thought to lead to speedier construction and better quality control. Furthermore, a reinforced concrete tube tunnel required an outer steel lining to ensure water-tightness; the outer skin in SC structures would account automatically for this requirement.
Due to the innovative nature of the design, a series of tests were carried out at University College, Cardiff, to validate the concept. These included one half-scale and one full-scale specimen in bending and one full-scale corner of a tunnel cross-section. Shear stud pull out tests were also carried out. Results from the tests confirmed that SC structures offer many advantages and can be employed as a viable alternative to conventional reinforced concrete.
Needless to say, the novelty of the concept and conservatism of the construction industry meant that it was not used in the Conwy Tunnel. However, this initiative stimulated much interest and, in 1991, The Steel Construction Institute was funded by the European Commission and British Steel (now Tata Steel) to carry out a major research project to study SC construction (1991-1997). In the first phase of this research, tests on 13 beams (four of which were fatigue loaded), three columns, four corner specimens, six ‘T’-junction specimens and a complete tunnel cross-section were carried out. This work led to a design guide that gave general principles and rules for the design of basic elements such as beams, columns and beam-columns. The second phase of the project provided specific guidance on the use of SC construction in submerged tube tunnels. A final phase entailed fatigue testing of nine SC beams. The results were used to develop fatigue design rules, based on the design philosophy of BS5400 Part 10.
Whilst the above work demonstrated the structural viability of SC construction, buildability remained an issue. This is because of the difficulty associated with handling the large individual steel plates on site and the need to provide support to resist hydrostatic pressures, which would push the plates apart during concreting. For this reason, SCI undertook numerous studies in the mid to late 1990s (funded by British Steel) to look at ways of overcoming the construction difficulties. This led to a solution whereby the shear studs are replaced by a series of bars that connect the plates together. The two joints (between each end of the bar and each of the plates) are formed concurrently by friction welding. This created a robust modular SC structure that is easy to handle and did not require additional support during concreting.
The concept was developed by British Steel (by then Corus) in an extensive programme of analytical work, structural testing, manufacturing trials and full-scale construction demonstrations into a commercial product: Bi-Steel (Figure 2), supported by a comprehensive technical handbook. Bi-Steel has been used in a number of applications, notably core walls for high-rise buildings and hardened structures to resist impact and explosion loading.
Following this, work on SC structures was focused at a few UK universities, including Strathclyde, Cardiff, Imperial College and Southampton. Their work has addressed technical aspects such as the effect of partial interaction, fatigue behaviour, shear strength and finite element modelling of SC structures. However, the new nuclear build programme and the move by reactor manufacturers and energy companies to use modular construction in nuclear power plant to reduce construction time, cost and risk has led to renewed interest in SC structures and their design methodology. A project partnered by SCI, Amec and Caunton Engineering and funded by the UK Technology Strategy Board has looked at the feasibility of developing European design rules fashioned on the Eurocodes. In September 2011, this led to a major multi-partner proposal led by SCI. The proposal has been submitted to the European Commission and several potential industry sponsors. The outcome of this submission will be known in January 2012 and, if successful, will mark the beginning of a three-year research programme to develop European design rules for SC structures in the nuclear sector.
This article originally appeared in the October 2011 issue of Nuclear Engineering International (p19)
Bassam A Burgan, Director, The Steel Construction Institute, Silwood Park, Ascot, Berkshire, SL5 7QNRelated ArticlesIn reactor pre-licencing review, UK regulator prepares for unresolved issues NRC further questions AP1000 design