Checklist

Advantages to using steel

Steel has been the material of choice for prefabrication for many years. Unlike other framing or carcassing materials, it has the advantage of strength and reliability combined with relative low weight. Even thin walled steel components can achieve structural performance levels well in excess of other systems. Its strength makes it an ideal choice for larger projects with several floors and potentially larger modules.

Fixings and fixing methods are also well established – welding, riveting and bolting technologies are advanced and even bonded joints are available. Assembly and fixing can be easily configured in the factory and automated assembly is a real possibility on large-scale projects.

Another advantage of steel is that it is readily available from stock in many shapes and sizes, making it easier for designers to find the ideal profile. If not, steel is so versatile it can be formed into almost any shape to suit the designer’s requirements as steel fabrication machines are sophisticated and relatively commonplace, making the customising of sections straightforward.

Steel is also compatible with a large number of materials freeing designers to use insulation and finishes from a wide range of sources. Repair is also relatively easy as replacement sections are easily sourced and fixed in.

Off-site framing options

• The traditional balloon frame consists of relatively closely spaced vertical members with a degree of cross bracing. Some designs use the inner or outer skins and joints to provide the stiffness and wind bracing. The balloon frame has relatively good spacing for insulation and services. However, careful detailing is required to provide an uninterrupted surface for application of both internal and external skins. Some manufacturers use specially made sections and fixings to get around this.
• Some manufacturers are using small and accurately made frame members to create a truss type frame. This has the potential to create either stronger or lighter structures whether through panel or volumetric units. However, this frame has yet to stand the test of time, as failure of one joint will substantially weaken the whole unit.
• Some structures are built using a panel or cassette system. Wall and roofs are constructed with framed panels that are fixed together. These are typically used for commercial and other large-scale projects.
• Frames can be dispensed with completely by using monocoque construction. This is where the skin of the unit doubles up as the structure. Sheet steel is used in combination with a frame at high stress points to provide complete volumetric units. Although cars have been made as monocoques since the 1950s these are yet to be used widely in construction.

The ability of steel to flex may reduce the risk of premature breakage, but has to be designed out of a structure. Flexing of any kind can be disconcerting for the users and can cause acoustic problems, particularly if a harmonic effect is set up. Flexing can be prevented by using larger sections and thicker steel, but this not always the best solution as it will increase weight considerably. Clever use of geometric shapes can stiffen the framing considerably without adding excessive weight but this will increase manufacturing complexity. A subtle balance between the thickness of the steel and layout of elements needs to be found. The acoustic properties should be improved using a stiffer frame, but check for any undesirable harmonic effects. This should be possible using the designer’s software.

Thermal properties

Steel possesses no thermal mass and is a good conductor of heat. This potentially has two effects: first, the building has no ability to even out the high temperatures increasingly experienced in summer; and second, the steel can act as an effective thermal bridge making insulation difficult. Care must therefore be taken to provide an effective thermal break between inside and outside. It is preferable to put the insulation outside the steel frame. However, this can cause fixing problems, so use specialised fixings to ensure good performance and durability. Check the details of these components and the relationship with the vapour control system. Specifiers should ask for a thermal swing calculation that estimates how quickly the building will heat up and cool down.

Acoustics

Poor acoustic performance is a concern with steel. Steel will not deaden any sounds and may actually contribute to sound transmission. Extreme care must be exercised to avoid resonant frequencies and transmission paths that will amplify and direct sound across the construction. Any framing system should be thoroughly tested prior to manufacture and checked for resonance.

Corrosion

Corrosion is the other Achilles heel of steel – there have been instances of steel constructions and assemblies corroding to the point of collapse. Specify a belt-and-braces corrosion-proofing system. This will normally consist of good-quality rust proofing such as a zinc coating. This should include all brackets and holes, and cut ends should be treated too. Austenitic steel should be used for fixings.

Great care should be exercised to ensure the steel and its surrounding components are not exposed to either condensation or moisture from outside. The former can be controlled with the placement of a good quality vapour control layer (see “Checklist” on sports buildings, 17 June, page 73) and a breathable membrane or adequately robust external cladding. This is especially important if the steel is embedded in the construction build up. Also, bear in mind the dew point analysis, which determines the risk of condensation. Remember that the dew point analysis carried out under BS 5250:2002 Code of Practice for the Control of Condensation in Buildings is for steady state conditions, but in reality relative humidity fluctuates constantly.