SC is the name of a generic steel-concrete composite construction system using planar components comprising two steel plates connected by a grid of tie bars with structural concrete between the plates. The plates act as load bearing formwork during the placement of the concrete (core) and, in the completed condition, they provide the reinforcement to the concrete. The tie bars, apart from holding the two plates together during transportation, erection and concreting, act as transverse shear reinforcement. Composite action between the steel plates and the concrete core is achieved through the use of headed shear studs welded to the steel plates.
Several manufacturers of third generation nuclear reactors have made composite steel-concrete (SC) modular construction an integral part of new nuclear power plant (NPP). Modularization using SC structural elements speeds up construction, simplifies the attachment of equipment support points and reduces costs. However, whist design codes were developed in Japan, South Korea and more recently the USA, there has been no European guidance for the design of SC structures.
A European funded four year project investigating the behaviour of SC structures was recently completed (SCIENCE, 2017). Structural member tests were conducted to study bending and out-of-plane shear failure. Connection tests studied wall-to-wall, floor‑to‑wall and foundation‑to‑wall behaviour. LOCA tests were performed at small scale with highly controlled boundary conditions under thermal loading and at large scale (including comparative reinforced concrete specimen tests) under combinations of thermal and mechanical loading. Fire tests were conducted on floor, floor‑to‑wall and wall large scale specimens. This was supported by FE parametric studies. Design guidance following the format of the Eurocodes was developed using the results from this project and other studies available in the literature. A reliability analysis to Annex D of EN 1990 was performed to derive partial safety factors. The guidance has been trialled by applying it to the design of the “Diesel Ultime Secours (DUS)” building (Diesel Generator Building), which is being used as a post Fukushima safety measure on all EDF nuclear reactor sites in France.
The final project report was presented to the European Commission’s Expert Monitoring Group (TGS8) and was recommended for showcasing as a success story.
This work was funded by the European Union’s Research Fund for Coal and Steel (RFCS) under grant agreement no. RFSR-CT-2013-00017 and Electricité De France (EDF). The technical work was performed by The Steel Construction Institute – SCI (UK), University of Surrey (UK), Teknologian tutkimuskeskus VTT (Finland), VTT Expert Services Ltd (Finland), Karlsruhe Institute of Technology (Germany), SMP Ingenieure im Bauwesen GmbH (Germany), EGIS Industries SA (France), Electricité De France (France), Ecole Normale Superieure de Cachan (France), Ecole Speciale des Travaux Publics (France), Centre Technique Industriel de la Construction Métallique (France), Efectis (France). The project was coordinated by SCI.
AN INNOVATIVE FORM OF STEEL-CONCRETE (SC) STRUCTURES FOR NUCLEAR POWER PLANT
New generation NPP manufacturers have been developing modular construction techniques that make greater use of pre-fabrication and factory assembly and reduce reliance on site activity. Most have adopted a technique that uses composite steel/concrete (SC) modules made by pouring concrete between two steel plates. The plates serve the dual purpose of reinforcement and permanent formwork to the concrete. Shear studs welded to the inside surfaces of the plates ensure composite action between the steel and concrete and can be spaced such that the compression steel plates do not buckle under load. They also provide the means for easy attachment of support points for equipment, eliminating the need for embedded steel plates or concrete expansion anchors.
To support the surface plates during assembly, lifting, transportation, concrete pouring and curing, the two plates must be connected together to form a self-supporting unit. In this way, parts of modules may be fabricated off-site and transported as units to be assembled on-site and connected together. Many attempts have been made over the past 25 years to devise practical, economic and safe methods of connecting the plates. SCI has been working with SCI Member Caunton Engineering in a project funded by Innovate UK to design and construct a new generation of SC modular construction (‘Steel Bricks’) that overcomes the manufacturing problems of earlier SC systems. A Steel Brick is made by cutting a pattern into a flat steel plate and then folding the plate to form an ‘L’ shape. Shear studs are welded to the sides of the ‘L’ section and then two ‘L’ shaped plates are welded to form a ‘U’ shape. The circular holes in the base of the ‘U’ allow concrete to flow between bricks joined side-by-side and Steel Bricks can be welded together to create large structural modules. The base of the ‘U’, which is integral to the Steel Brick, provides the means of holding the two sides of the panel together and acts as out-of-plane shear reinforcement to the concrete.
Following completion of several workshop and field trials, a full scale structure (part of a diesel generator building on a nuclear power plant site) has been constructed to demonstrate the buildability of the system.
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