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Article updated on 02/03/20 by Ben Pilkington
Steel is one of the most versatile materials used in construction. Light gauge strip steels, in particular, are used extensively in various construction industries. In the industrial sector, the main growth area for strip steel has been for the cladding of walls and roofs. Comparatively, its major use in domestic housing is for steel lintels, which have experienced increasing success at the expense of pre-cast concrete lintels. Strip steels are also used for a large number of other applications, and steel consumption is closely tied to investment in the construction industry.
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 the 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, which means that the coating mass of zinc is 275 g.m.-2 on both surfaces. 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, leather-grain polyvinyl chloride (PVC) coating with high resistance to damage during site handling.
Cladding manufactured from OCS is strong enough to give adequate structural performance, all the while maintaining excellent durability and aesthetically acceptable conditions. The profiled sheets are often used as parts of systems incorporating insulants and internal linings which may, for example, be steel-based or drywall.
Profiled steel cladding is available in two basic shapes, of which include sinusoidal or trapezoidal. Within these, there is great scope for variations on the design. n fact, there are currently over 80 different designs of profiled sheet that are available 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 and crane loads. Roof cladding will be subject to wind loads, both direct and suction, as well as 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 entire 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.
Research and development (R&D) effort on profiled cladding is devoted to providing assistance to customers in the development of components and systems. This type of work is achieved by product design evaluation that encompasses both 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. These walkability tests ensure that the designs have sufficient strength to resist the dynamic loads imposed by foot traffic during erection. During a typical test, workers will evaluate the integrity of concealed fix systems and 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 a special lifting tackle. Steel lintels also leave the appearance of the face brickwork unimpaired. These materials 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 of steel lintels is provided by fabricating from pre-galvanized sheet, post-galvanizing 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 light industrial construction sectors. Examples of these applications 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 components 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 include trusses, secondary beams and load-bearing walls, as well as in prefabricated buildings.
The main advantage associated with steel frames as compared to traditional block or brickwork is that the prefabricated frames can be erected quickly and made weatherproof to allow internal work to be started earlier in the production sequence. This significantly reduces construction time and labor costs for the builder. Compared to timber framing, a steel frame provides very accurate internal dimensions, while also eliminating the effects that can occur from absorbed moisture, such as swelling, shrinkage and cracking, as well as ensuring 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 and galvanized steel U channels. Modular frames, which are typically 5 by 2.4 meters high, are prefabricated at the factory and bolted together on-site over traditional brick or concrete foundations. Internal room wall partitioning is constructed and assembled in the same way, 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 tons of light gauge galvanized steel sections may be used in a three-bedroom house.
The thickness of the sections ranges from under 1 to 2.5 millimeters (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, such as by deadweight floor or snow, and sideways loading, such as wind, must be applied to the test frame, including an appropriate safety factor, according to building standards. Frame stiffness is determined at up to one and a half times overload and frame strength, which determines the frame’s resistance to collapse, at two times overload. By refining design through validation test programs, optimized systems have been produced and demonstrate their capability of meeting stringent regulatory 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. Buildings’ lifecycles are also an important factor to consider when developing more environmentally-friendly projects and minimizing requirements for finite resources. For the new generation of steel-framed systems, a G275 hot-dipped galvanized product (Galvatite) is employed. The heavy-duty zinc coating on this product provides galvanic protection of the steel base, particularly at cut edges, in addition to maintaining a low corrosion rate in many environments.
In internal environments, adherent corrosion product films form on the zinc material, 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 minimize 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, thereby reducing the risk of surface condensation. Careful selection of materials, with regard to thermal and water vapor resistance, can also reduce the risk of interstitial condensation by ensuring that dewpoint conditions only occur outside the mainframe.
Major work is currently being done to increase the use of cold-formed sections within the building industry. The greatest restriction to its wider usage is the lack of information and design guidance on connections between, and holes in, cold-formed sections. Development and evaluation work are being undertaken with 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, like frames, manufactured from strip steel. One major aim of current research is to optimize 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 strip steel 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 programs of continual process and product development are designed to further develop the role of strip steel products. Wider applications are continuously being sought to promote strip 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 domestic (DIY) applications and roadway and acoustic screening, for example, is being actively pursued.
Primary author: John Godwin
Sources: Materials World Vol. 1 no. 2 pp. 92-94 February 2003. https://www.oecd.org/industry/ind/steel-market-developments-2017Q2.pdf