Marian Kubisz of Adept Consulting (UK) looks at the increased specification of cold rolled steel frames in the current push for offsite construction solutions, and why system design considerations need to be brought into projects at the earliest stage
Hot rolled steel frames are generally being considered as the original offsite framing solution, but the advent of cold rolled and formed steel products has seen them play a much bigger part in delivering panelised and volumetric modular construction solutions.
Steel Framing Systems (SFS) are now commonly used supporting cladding facade materials, as infills or oversailing the hot rolled steel or concrete frame. They are replacing blockwork solutions, being much quicker and cleaner and eliminating wet trades.
The systems are generally designed at the last stage of the project – either by the manufacturer or in most cases by the subcontractor. This process can be lengthy and painful however, as too many people are involved, notwithstanding the client team’s expectations that it will be short and straightforward.
By involving a specialist structural engineer/detailer in the early stages as part of the client side design team this problem would be eliminated. Another advantage is that by fully developing the design as early as RIBA Stage 2 or 3, an early dialogue with the supply chain can be established that will ensure lead times are adhered to. Delays on site can occur when the preferred supplier has reduced time to order materials or allocate labour, and these can be extremely costly.
By discussing and co-ordinating the process based on the end goal (the handover date), all parties – architect, structural engineer, detailer and principal contractor – can ensure the SFS system is 3D modelled and detailed before the manufacturer and or subcontractor is involved. The more complicated details and structures that require hot rolled steel in hybrid form (parapets, large openings, wind posts etc) can be identified and detailed prior to the tender process.
The tender, based on a generic design, can then be priced very accurately – so the cost difference is in the subcontractor ‘prelims,’ rather than a design variation. This eliminates misunderstandings and boosts post-supply chain selection, hence giving client and contractor cost certainty.
We have in recent years seen cold rolled replacing hot rolled steel in areas, such as small to medium portal-framed and beam and column structures within the retail, commercial, agricultural and leisure industries. The increase in panelised framing means better quality factory assembled units that can include sheathing boards and insulation, speeding up the programme onsite.
Light Gauge Steel Framed (LGSF) structures are made up of cold rolled or pre-formed ‘C’ and ‘U’ sections bolted together in factory jigs to ensure accuracy. Onsite assembly often utilises rolled formed metal decks within the structure – load bearing solutions that can be used up to 15 storeys. These are increasingly common in private and social housing as well as student accommodation. We are also seeing an acceleration in the use of LGSF across the new schools programme therefore supporting the Government’s push for greater use of offsite, as well as the “Build Back Better” initiative.
LGSF challenges traditional methods of construction in low rise residential as well, with more and more builders looking to use this system for two or three storey individual houses on schemes across the country. Major firms who have had the foresight to adopt this method are seeing the benefit in terms of reduced remedial work post-completion, thanks to the lack of shrinkage and movement (preventing cracking at plasterboard seams and fixings).
LGSF is a regular contributor to the volumetric modular sector, with components used as floor and roof joists, as well as wall infills supporting the inner and outer leaf treatments. Either loose or panelised, these bring a lot of advantages, starting with the component itself being rolled in different depths and using different gauges – allowing the system to be precisely sized to fit.
The growth in the UK has been exponential, and so have the engineering and detailing support resources. Standard methods of connection are Tec screw, bolt or rivet depending on the manufacturer’s preferred solution, but all are accepted within the industry.
Co-ordination within the engineers and detailers can be seamless with the deployment of BIM throughout the design process; production of a 3D model shared easily in IFC format. This allows the consultants to check for clashes and co-ordination, reducing or eliminating issues onsite. This also applies when there is a requirement to combine with hot rolled steel (either as individual components, steel frames or cages).
When considering offsite construction methods, it is crucial there is early discussion between the architect and structural engineer about the system to be adopted, even prior to the decision on choice of material. LGSF systems are incredibly flexible, but not universal; all have their advantages and disadvantages. Involving a specialist engineer early on provides the project team with various key advantages:
- Structural issues can be identified and discussed, avoiding later costs of any changes required
- Accurate early ‘loading down’ information cuts delays in getting the information to the foundation engineer (if going the subcontractor route you will need to wait for the information until after appointment, they will then have to appoint an engineer to produce details)
- Accurate structural information can then be shared with all subcontractors
- Changes during the design process can be discussed with other parties in the design team, saving on cost.
There are specific advantages for architects of this approach:
- Identifying right system for the project (not all are fit for the same purpose)
- Understanding of the system from the outset – being able to discuss the benefits and limitations should help define the structure
- Identifying any potential issues before final decisions are made.
System limitations & further considerations
LGSF is a very flexible system suitable for different structure types, from small extensions, SFS infill or oversailing wall systems, and open plan portalised frames, to multi-storey structures. With LGSF structures being required to go higher, hybrid solutions need to be considered to overcome systems’ individual limitations. The higher you go, the greater the requirement for racking (X-braces, K-braces or portalised frames), therefore walls that can accommodate those are needed within the floor plan.
Large open areas within a building (such as hotel entrance lobbies, kitchens, restaurant dining rooms) require a deeper supporting structure, affecting overall floor depth and overall floor to floor distance. LGSF systems also require secondary fire protection in the form of fire rated board.
Despite these considerations, systems have compelling benefits, such as their exceptional light weight, greatly reducing load on foundations. LGSF structures are made using cutting edge technology with material rolled and cut to in a bespoke 3D modelled way, reducing wastage significantly. The technology also minimises any occurrence of human error.
LGSF elements have been refined over time to maximise their structural capacity, again meaning wastage is minimised, and projects with the highest strength to weight ratio possible. Lastly, section profiles can be recycled meaning the systems can help meet green goals.
Marian Kubisz is managing director and owner of Adept Consulting (UK)