In contrast to wood-stud construction, the thin-walled C-, U-, or Z-sections permit a range of connection options, can be sheathed with a number of different materials, and are not flammable. In Germany this construction method has not yet spread: only ca. 1 % of residential projects utilise light steel sections. In Japan and in Sweden the percentage is considerably larger at approximately 15 %.
Above all, in comparison to wood panel construction, LSF-construction exhibits the following characteristics:
• extremely lightweight (max. 0.075 kN/m)
• exceptional ratio of stiffness to own weight
• dimensional stability of the sections
• stable with respect to corrosion
• favourable acoustic characteristics
• shape of sections unaffected by moisture
• quick assembly on site
• potential for recycling and reusing all materials in system
• certain board materials fire-resistant
• high standard of joint and connection technology
• conducive to prefabrication
Structural behaviour of stud-construction
The structural system consists of vertical studs which are connected at the top and the bottom by channel tracks. The upper track distributes the vertical loads to the studs. Floor decks are attached either with consoles or are laid between the vertical members. From the structural-engineering perspective, LSF-construction differs considerably from frame construction. The transferral of loads does not function via a skeleton which stands independent of the building envelope. Through the sheathing a composite building component results: a panel which transfers loads both within its plane as a slab, and also perpendicular to it as a plate. The stiffness of the sheathing is so great that, with sufficient fastening, buckling of the sections can be ruled out.
Construction principles
As in wood frame construction, in LSF-construction there are platform and balloon-frame systems. In the balloon-frame system the ceilings are supported laterally or in front of the studs; in the platform system the wall elements rest directly on the floor deck. This offers the advantage of successive floor construction for building progress.
In addition to the pure forms of this construction type, hybrids are also in use: larger distances can be spanned with hot-milled sections, or the structural system can be executed as frame construction. For buildings of more than four storeys steel frame construction typically comes into play.
The grid dimension of the sections varies, but should be oriented to the sheet formats. An economical standard construction is based upon 62.5 cm, which, when the loads are greater, can be reduced to 31.25 cm. Doors and windows should adhere to the grid in order to avoid disrupting the vertical members. Large openings in the load-bearing walls and floor decks can be spanned with reinforced or doubled-up cold-formed sections or with hot-rolled steel profiles. Type and thickness of the sheathing must adhere to building physics, structural, fire-protection and acoustic requirements.
Characteristics of cold-formed sections
Cold-formed sections are up to 12 m long and consist of coated or uncoated cold-milled steel sheets. The transverse section is attained by extracting, bevelling or rolling in a cool state. The wall thickness is only 1–2.5 mm, yet the sections meet structural requirements in multi-storey buildings.
Fabrication, Prefabrication and Assembly
Construction in LSF involves addressing altered design criteria. Assembling light-steel-section stud framing is possible both on the building site and in the factory. The two types of fabrication differ in detailing, execution, logistics, whereas the advantages and disadvantages must be weighed for each project. The framing may be constructed of individual sections at the building site, as uncut raw material, as sections delivered to site in the desired length, or as completely separate building components (e.g. roof framework or wall framing) and set up on the floor deck or on the basement deck.
With prefabrication the construction time can also be significantly reduced, since the construction site can be hooked up to utilities and possibly also the landscaping can be completed, while at the same time the construction of the building is progressing. The planning is more time-consuming than for on-site framing and must be completed earlier (prior to shop fabrication).
An assembly or a building component typically must adhere to different building physics standards. In LSF-construction these can be met with respect to acoustics, fire protection, and thermal insulation. However, for a solution attuned to the system, the early integration of detailing addressing building physics is crucial in the overall design process.
Sound protection and acoustics
For residential projects with the acoustic insulation and the sound-impact insulation in LSF-construction, the same operative acoustic forces are valid as in other light construction types. Acoustic insulation is not attained through mass, but with consistent discontinuous construction (of the two faces) and acoustic uncoupling.
The most important factors influencing acoustic insulation are:
• the flexural strength of the connection of the two faces
• the distance between the two faces, determined by the choice of sections – the larger the better
• the low bending rigidity of the individual faces
• the ratio of mass to area of the individual faces – the larger the better
• type, characteristics (e.g. flow resistance specific to length) and filling ratio of the insulation
The impact-sound-proofing requirement is more difficult to fulfil in lightweight floor deck systems than is the airborne sound-proofing. The latter is usually automatically fulfilled when the impact-sound insulation is sufficient. In order to attain a good value the sound immission on the upper side of the floor deck should be uncoupled from the lower side. This can be achieved with floating screed above, separated from the load-bearing structure.
Positive influences on the acoustic insulation of floor constructions are:
• low bending rigidity and high unit weight of screed
• low rigidity of impact-sound insulation at large depth (up to 50 mm)
• avoidance of acoustic bridge at walls and ceiling
• careful execution of the connections at walls (edge insulation-strips)
• weighting down with rubble, mats or stone plates
• resilient flooring, e.g. carpeting
Dry-assembly sub-flooring systems improve sound-proofing, weigh less than screed and can support loads immediately following installation. No moisture – which would require time to dry out – is introduced.
In order to attain the targeted acoustic characteristics attention must be paid to the execution of the connections:
• acoustic separation from flanking building segments
• acoustic separation from adjoining building components
• connections via insulating strips and insulation
• special sections with integrated sealing strips
Thermal protection in winter
For exterior building components the primary insulation is integratively arranged, i.e. it is in the same plane as the studs. The cavity must be completely filled with insulation in order to avoid thermal bridging and convectional current. The integrative insulation plane is typically supplemented with an additive layer. This is placed on the outside or, in some cases, inside to mitigate the thermal-bridging effect of the metal studs.
LSF-construction almost always meets energy-efficiency standards. With an additional composite thermal insulation system or exterior insulation it can fulfil passive-solar standards, making it possible to do without an additional heating system.
Thermal protection in summer
For thermal protection of LSF buildings in the summer, thermal mass is not the determining factor. Other priorities play a role in the planning. They are listed in order of their importance:
• reducing amount of potential irradiation intensity (through glass surfaces) in the space (design, orientation, layout, shading, glass characteristics)
• sufficient thermal insulation and wind- and airtight exterior building components
• optimisation of dimensioning of the space’s heating and cooling sources
• adjust air exchange rate in combination with exterior air to the interior-exterior climate conditions (enable cross-ventilation, particularly at night)
• optimisation of dimensioning of thermal transmission coefficient and the storage capacity in combination with the radiation temperature through the correct building component layering
Fire protection
The load-bearing capacity of steel decreases nearly linearly at temperatures above 500°C. In LSF-construction, the steel sections are in space-enclosing components, e.g. are integrated in walls with resistance characteristics. The following board materials can be used for fire-protective claddings for walls and ceilings:
• plasterboard in accordance with DIN 18180
• plaster-fibreboard
• glass-fibre-reinforced plasterboard
• calcium-silicate boards
With the appropriate insulation Category A construction is possible.
A non-load-bearing wall must guarantee the room enclosure for the duration fire-resistance. A load-bearing or stiffening wall must, in addition, retain its structural integrity, which is why all load-bearing steel studs must be encapsulated.
The insulating layer also has an effect on behaviour: in a non-load-bearing component the insulation can increase the fire-resistance if mineral fibres with a melting point >1000 °C are used. In a load-bearing member, the insulation prevents convection in the cavity, resulting in conditions in which the critical temperature 500 °C is more quickly reached. According to DIN 4102-4, neither vapour barriers nor wind- and airproofing membranes inhibit fire protection.
Fire can spread in the cavity of the LSF-construction, forcing noxious fumes into parts of the building unaffected by fire. In order to prevent fire from spreading from floor to floor, and to secure that they are smoke-tight, the joints must be sealed with mineral-wool sealing strips, putty or foam sealants and carefully infilled.
Corrosion Protection
Cold-formed steel sections consist of hot-dip galvanised strip steel, its zinc coating 20 μm thick, which corresponds to 275 g zinc per square metre. The hot-dipping constitutes good corrosion-resistance for the lifespan of a building. Assaults on the protective coating can occur during transport and storage. Thus, transport packaging should rule out mechanical damage. While in storage the sections should not come in contact with dirt or moisture. Due to the cathodic protection of zinc, which can “heal” damaged parts, the ends do not require additional galvanisation.
Potential developments
With steel, structural solutions are possible which have great potential with respect to industrialised fabrication, variability of fitting out, and the flexibility in design. The science and art of optimal use of resources in steel construction are in the initial stages. By realising additional exemplary projects in LSF-construction, the path to limiting material excesses will bring about new, innovative and aesthetic buildings.
