The term Off Site has many meanings; from the manufacture of components such as bricks, lintels and rafters to complete building structures such as the Heathrow control tower.
This five storey structure was constructed off site and then transported 2 kms across the airfield and lifted into position in a three month operation. Set out below are six common techniques with a brief explanation of each.
Light steel framing is generally based on the use of proprietary C or Z shaped steel sections produced by cold rolling from strip steel. The steel strip used in cold formed sections is relatively thin, typically 0.9 to 3.2mm, and is galvanised for corrosion protection. The components are formed in the factory and can either be assembled there or on site.
It is used in a number of ways:
Load bearing multi storey frames (as shown in the attached image of a four storey residential block)
To create individual room modules
As external infill wall panels in traditional or steel framed module construction
Structural Insulated Panels (SIPs) are a structural frame made from a sandwich of two layers of Oriented Strand Board (OSB), either side of expanded polystyrene (EPS).
The nature of the stressed skin panel makes it exceptionally strong, which whilst largely comprising EPS, provides high levels of insulation in thinner wall thicknesses than other forms of construction. With no need for cavity or internal insulation, structural insulated panels can provide a very fast way of constructing very efficient walls with u-values as low as 0.14 .
SIPs can be used as the inner skin of an external wall (in place of timber frame or blocks), or can be used as a pre-insulated roofing structure.
SIPs can also be used as cladding systems on traditional steel or concrete framed buildings.
Timber frame construction uses timber studs and rails, together with a structural sheathing board, to form a structural frame that transmits all vertical and horizontal loads to the foundations.
Platform frame is the most commonly used method in the UK. Each storey is framed with floor-to-ceiling height panels and the floor deck of one floor becomes the erection platform of the next.
A single house in timber frame is shown in the accompanying image.
Cross Laminated Timber (CLT) is engineered timber panels in which the thickness is made up of a number of narrow widths of timber laid together with each layer at right angles to the previous layer.
These panels can be pre-cut in the factory to form wall, roof and floor elements. It can provide fast onsite construction, with good potential for air tightness and a robust wall and floor structure suitable for most finishes internally and externally.
Access permitting, relatively large pre-manufactured panels can be transported to site for fast, factory-quality construction. CLT normally forms the structural floor and wall element of buildings, and has been used successfully to build up to ten storeys.
CLT build up and maximum panel size is shown in the accompanying image.
Precast concrete panels consisting of pre-formed units, including all walls and floors and a flat slab concrete roof are cast in the factory and once cured are delivered to the construction site to be erected on the previously installed foundations.
The main load-bearing structure is completed within weeks and all follow-on trades can be scheduled to commence simultaneously. Roof, brickwork, window, services and floor screed subcontractors all work together to drastically reduce the time required to complete the final construction.
Housing modules are structural boxes made up of walls, floors and ceilings built in the factory. The external elements are constructed to provide the appropriate thermal, sound and fire insulation, in the latter case of up to 120 minutes.
Each module, which comes complete with windows and is fully weather protected, can be finished in the factory to whatever standard is required, including decoration and furniture, thus reducing work on site to the minimum.
The modules are then craned into position before being clad externally, if this has not been done in the factory, and roofed to create the finished building.
Buildings can be detatched, semi-detached or terraces of houses or blocks of flats up to 44 storeys. This method can also be used to add floors to existing buildings.
This technique creates a steel framed building complete with composite external cladding panels, or other cladding, in the factory rather than piece by piece and trade by trade on site. The system can have a variety of floor constructions including concrete floors cast in the factory and comes to site with a high level of internal fitting out completed.
The modules are built in the factory using a combination of hot and cold rolled steel sections to provide clear open span capability in the finished building. Clear spans of up to 15 metres in one direction and effectively unlimited at right angles provides enormous flexibility for design of the finished building.
The only limitation to internal height is that imposed by transportation considerations. Modules are weather protected for delivery and erection so that high levels of fitting out including windows, internal partitioning, mechanical, electrical and plumbing services can be completed in the factory.
These modules can also be supplied with permanent external finishes to further reduce on site construction time. Free standing buildings can be constructed to six storeys and much higher than this where a traditionally lift and stair core is built to provide lateral stability.
These self-contained light steel frames structures, built in the factory, with or without floors, are designed for installation in new build or refurbishment projects. The size, shape and standard of finish is highly flexible from basic shower pods for student flats to luxury bathrooms for top hotels and apartments.
The pods are completed and sealed in the factory for delivery to site where the services are connected up and the external walls incorporated into the building fit out.
These are sections of the complex services installations required in modern buildings, which are designed, planned and assembled in the factory, in a light steel frame, which allows them to be transported to the construction site for installation.
This system has a number of advantages; ensuring that the various systems are fully integrated and eliminating clashes; manufacture in parallel with the building construction thus reducing the overall build programme; semi-skilled labour can be employed in the factory; and the skilled labour needed on site for the installation and commissioning is minimised.
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