Prefabricated Steel-Frame Buildings: From the Factory to Erection on Site

In a prefabricated steel-frame building, much of the frame is made in a factory rather than on site, and the site mainly lifts and connects. This guide covers the main types of offsite construction, where hot-rolled beams, hollow sections and plate fit in, the steps from factory to site, and the limits a project should plan for from day one.

What offsite construction means for a steel frame

The Australian Steel Institute (ASI) describes modular construction broadly as any form of construction in which prefabricated parts or modules are assembled off site and then erected into the final structure. Steel is not new to this. A CPD module published by Building magazine calls steel "the original offsite framing material" and says most of the value-add in steelwork happens in the factory, in some cases up to 90%.

Four common approaches

The Off-site Construction Research Centre (OCRC) at the University of New Brunswick groups offsite construction into four types:

The American Institute of Steel Construction (AISC) adds the kit-of-parts approach, in which the design standardises as many components as possible so they are interchangeable. AISC also describes framing the floor of an entire bay, including decking, off site or on the ground, then erecting it as a single piece.

Where beams, hollow sections and plate fit

The sections are the familiar ones; what changes is where they are fabricated and how far they are standardised. SCI P430, Offsite Modular Steelwork: Design Advice (The Steel Construction Institute and BCSA, 2020), makes three points:

• Single-storey, standardised columns. The guide says the approach is helped by single-storey standardised steel sections with simple connections. NSC, announcing the guide, adds that single-storey members are easier to handle than longer elements.
• Standard connections. Fin plate details are typical for beam connections and can be taken from tables of standardised details.
• Factory fire protection. Columns are generally protected with an intumescent coating applied off site; the Building module notes that factory application reduces the risk of delay to following trades.

In the Ahan Asa catalog, IPE beams to EN 10365 are listed in 12 m lengths, square hollow sections (SHS) to EN 10219-2 in 6 m lengths, and S235JR plate for connection and base plates from 8 to 60 mm thick with EN 10029 tolerances. Whether a column is a hollow section or a beam section is the designer's decision; we compare the two in Box column or double IPE?. Offsite systems in thin cold-formed sections follow a different logic, covered in our guide to light steel framing.

From factory to site

1. Design and connection detailing. According to the Building module, the steelwork contractor details all connections from the forces specified by the consulting engineer.
2. Supply and fabrication. Rolled steel comes from mills or distributors and is cut, drilled, welded and coated in the factory. The same source gives a typical lead time from design through fabrication to delivery that "may be 10 working weeks".
3. Changes before dispatch. Drawing changes are best handled in the factory, before the steel reaches site.
4. Transport. P430 states that panel size is limited by transport restrictions, and gives a 12 m × 2.4 m floor panel weighing approximately 9.5 tonnes as an example.
5. Erection. Panel size and type may also be limited by the capacity of the site craneage.

Benefits and limits

The sources cite precise tolerances for factory-made floor panels (P430), factory work that is "effectively right first time" and inherently safer than typical site conditions, a programme less affected by bad weather, and less noise and dust for neighbours (Building). SCI P431, the companion client guide, lists higher quality, faster construction and reduced safety risks, as reported by NSC.

The limits follow from the same logic:

• Dimensions, openings and connection details must be fixed earlier, since a change after fabrication costs more than a change on a drawing.
• Transport and crane capacity set the size of panels and modules, along with site access.
• ASI notes that there are many forms of modular system on the market, a number of them proprietary (typically in the connection details between modules), so the designer, fabricator and erector need to be aligned early.
• The building still has to meet local building regulations in design and execution.

Sourcing tips

• Take the section list from the fabrication drawings (section, size, grade, length) and estimate weight from the nominal kg/m; steel is invoiced on actual weight.
• Compare stock lengths (12 m for IPE, 6 m for SHS) with member lengths to reduce offcuts.
• Ask for a mill test certificate (MTC) with every delivery.

Summary

A prefabricated steel-frame building uses the same beams, hollow sections and plate as a conventional frame, but cutting, welding, coating and sometimes whole panels or volumetric modules are completed in the factory. Specialist sources point to quality, accuracy, safety and speed as the gains, and to transport, cranage and early decisions as the constraints. To request a quotation for your section list, contact us.