Steel is one of the most versatile materials used in building and construction. Light gauge strip steels in particular are used extensively in the UK building and construction industries. In the industrial sector, the main growth area for strip steel has been for the cladding of walls and roofs, where the market share is now approaching 90%. The major use in domestic housing is for steel lintels which have enjoyed increasing success at the expense of pre-cast concrete lintels. Strip steels are also used for a large number of other applications.
Profiled steel cladding provides a means by which the weatherproof envelope of industrial buildings can be rapidly erected. The profiles are roll formed from organic coated steels (OCS) produced on continuous strip lines which incorporate cleaning and chemical pretreatment with paint application. The paint systems are normally applied to a hot-dipped zinc or zinc alloy coated steel base. For a hot dipped coated steel, a standard G275 coating is used (i.e. the coating mass of zinc is 275 g.m-2, including both surfaces). British Steel's organic coated steel systems for building cladding are based on an external weathering system plus primer, following a suitable pretreatment. A variety of weathering or top coats is available, of which the most widely used is plastisol, a thick, tough, leathergrain PVC coating with high resistance to damage during site handling.
Cladding manufactured from organic coated steel is strong enough to give adequate structural performance, durable and aesthetically acceptable. The profiled sheets are often used as parts of systems incorporating insulants and internal linings which may, for example, be steel based or plasterboard. Profiled steel cladding is available in two basic shapes, sinusoidal or trapezoidal. Within these there is great scope for variations on design: there are currently more than 80 different designs of profiled sheet on the market.
The primary consideration in designing a profiled sheet is the required structural performance. Other considerations such as appearance and weather tightness can be addressed once the basic structural requirements have been satisfied.
During the life of a building, the external cladding will be required to withstand a range of different forces. The wall cladding will be subject to wind pressure and possible impact loads from vehicles, crane loads etc. Roof cladding will be subject to wind loads (both direct and suction), snow loads and foot traffic during construction and maintenance. Both wall and roof cladding sheets may also be subject to membrane forces generated by wind loading on the whole building.
The load bearing capacity of a given profile is determined by the precise shape and frequency of the ribs: in general, the deeper the profile, the greater the span for a given load. The selection of a suitable profile for a given application is assisted by the provision of comprehensive load/span tables from the profile manufacturer. Much of British Steel Technical Welsh Laboratories' R&D effort on profiled cladding is devoted to providing assistance to customers in the development of components and systems, through product design evaluation with structural testing and theoretical calculations.
Current and recent R&D work includes the development of a test to assess the ‘walkability’ of a profiled cladding for roofs to ensure that the designs have sufficient strength to resist the dynamic loads imposed by foot traffic during erection, the evaluation and development of concealed fix systems and evaluation of the effect of solar gain and thermal cycling on cladding systems. This type of work has enabled continuous product development of cladding and systems for both existing and new applications.
The major opportunity for increasing the use of profiled cladding is in overcladding for the refurbishment of existing buildings. The main problem in this development will be to ensure that the supporting framework is provided with adequate corrosion protection.
In traditional domestic housing, steel lintels are lighter than their concrete equivalents and can be handled on site without the use of special lifting tackle. They also leave the appearance of the face brickwork unimpaired. Steel lintels are produced by a number of manufacturers from hot or cold rolled medium gauge strip. Each manufacturer offers a wide range of designs to suit a variety of applications. Corrosion protection is provided by fabricating from pregalvanised sheet, post-galvanising or the application of fusion bonded epoxy powder coatings. Strip steel components and systems are also used for a wide range of internal applications in the domestic and fight industrial/commercial sectors. Examples of these include doors, door frames, partitioning and ceiling tiles. However, the full potential of strip steel in these applications has not yet been realized. Using tools such as finite element analysis component and system design can be optimized for both thermal and structural performance.
Cold formed sections are used in light industrial, domestic housing and secondary frame applications. There is strong evidence that designers wish to use cold formed sections more widely in building and construction, to replace both timber and the lighter hot rolled sections. Examples of the use of cold formed sections are in trusses, secondary beams, and load bearing walls as well as in prefabricated buildings.
The main advantage of steel frames over traditional block or brickwork is that the prefabricated frames can be erected quickly and made weatherproof so as to allow internal work to be started earlier in the production sequence. This significantly reduces construction time and labour costs for the builder. Compared to timber framing, a steel frame provides very accurate internal dimensions, no problems from absorbed moisture (e.g. swelling, shrinkage, cracking) and immunity from attack by insects and vermin.
Components and Construction
For domestic housing, the load bearing components of a steel frame system are typically a series of horizontal, vertical and diagonal rolled formed, galvanised steel U channels. Modular frames, typically 5 by 2.4 m high, are prefabricated at the factory and bolted together on site on traditional brick or concrete foundations. Internal room wall partitioning is constructed in the same way and assembled, followed by the second storey and roof. At the first storey level, steel floor joists are used to support the floor loadings from the second storey. In all, up to two tonnes of light gauge galvanised steel sections may be used in a three-bedroomed house, figure 1.
Figure 1. Construction of a steel framed house.
The thickness of the sections used ranges from under 1 to 2.5 mm or greater, depending on function. The basic frame is designed to withstand all internal and external forces acting on the house, since no contribution to strength and stiffness can be assumed from the inner plasterboard lining or external brick skin. The combined action of vertical loading (dead weight, floor, snow) and sideways loading (wind) must be applied to the test frame, including an appropriate safety factor, according to British Standards. Frame stiffness is determined at up to one and a half times overload and frame strength (resistance to collapse) at two times overload. By refining design through validation test programmes carried out by British Steel, optimised systems have been produced which are demonstrably capable of meeting stringent Building Regulation structural performance requirements.
One of the major issues with steel frame systems is the question of durability. For example the structural components in domestic housing are required to have a nominal life of 60 years, or in some cases 100 years. For the new generation of steel framed systems, a G275 hot dipped galvanised product (Galvatite) is employed. The heavy duty zinc coating on this product provides galvanic protection of the steel base (e.g. at cut edges) and has a low corrosion rate in many environments.
In internal environments, adherent corrosion product films form on the zinc which act as barriers to prevent further attack. To further enhance the durability of steel frame systems, the ‘warm frame’ concept has been developed to minimise the risk of both interstitial and surface condensation. This design places the insulation on the outside of the steel frame to prevent cold bridging of the frame, and reduces the risk of surface condensation. Careful selection of materials with regard to thermal and water vapour resistance can also reduce the risk of interstitial condensation by ensuring that dewpoint conditions only occur outside the main frame.
Major work is currently being done to increase the use of cold formed sections within the building industry. The greatest restriction to the wider usage is the lack of information and design guidance on connections between, and holes in, cold formed sections. Development and evaluation work is being undertaken with the Steel Construction Institute using theoretical analysis techniques, including finite element analysis and structural testing. Finite element analysis is also used in the evaluation of the effect of design modification on the structural performance of components and systems (e.g. frames) manufactured from strip steel. One major aim of current research is to optimise manufacturing and assembly techniques for light gauge steel frames. The greatest opportunity for the increased use of cold formed sections is in domestic housing for both new houses and extensions to existing houses.
Light gauge steel strip has proved to be a strong, durable and economic material for many applications within the building and construction industries. However, within the steel industry active programmes of continual process and product development are designed to develop the role of strip steel products still further. Wider applications are continually being sought to promote steel as a viable alternative to traditional building materials. For example, in many countries throughout the world, strip steel products are used to provide attractive, lightweight, easy to erect and durable fencing systems. In the UK this potential market had not been tapped and the development of fencing systems, for example for domestic (DIY) applications and roadway light and acoustic screening is being actively pursued.