Fabrication is the process used to manufacture steelwork components that will, when assembled and joined, form a complete frame. The frame generally uses readily available standard sections that are purchased from the steelmaker or steel stockholder, together with such items as protective coatings and bolts from other specialist suppliers.
Although a wide range of section shapes and sizes are produced, the designer may find that the required section size is not available. In this case, built-up girders may be fabricated from plate. Sections and plate girders may also be strengthened by stiffening the web or flanges depending upon the load to be carried.
Most modern steelwork fabrication factories have computer aided design and detailing (CAD) which is linked directly to factory floor computer numerically controlled (CNC) machinery creating a genuine CAD/CAM environment. The accuracy of the computer generated details being transmitted directly to the computer aided manufacturing (CAM) machinery increases the quality standards of production
The fact that machinery has taken over from the tape measure means that the frame is produced to high quality standards which are reflected in the speed and accuracy of steel erection on site. This results in significant benefits both to the client and main contractor.
(Image courtesy of Paddy Wall Sons)
[top ] Design for economic fabrication
There are a number of general “technical” factors to consider in terms of design for economic fabrication that apply to most building projects. Some relate to the complexity of the particular project, whilst others are specific to the type of project, e.g. multi-storey. portals. trusses.
[top ] Specification
Conformance with the National Structural Steelwork Specification for Building Construction (NSSS) will reduce uncertainty. Additional project specific clauses, more demanding tolerances, and additional testing will increase costs.
[top ] Bay size
Diagonal steel bracing – Silverstone
(Image courtesy of Kingspan Structural Products)
Structural steelwork prices are influenced by the size of each individual piece (i.e. number of pieces per tonne), which is largely dependent on bay size. Consider larger bay sizes, where the extra weight due to longer spans may be totally offset by the reduced price per tonne and the saving in number of columns and related workmanship. Also the resultant column-free space generally adds value to a project.
Structural steel frames must have the required degree of lateral resistance to wind loads. This can be provided by a stiff core or by the frame itself – diagonal steel bracing is usually a less expensive solution than moment frames.
[top ] Complexity
Modern CNC fabrication equipment can cope with complex individualistic designs but, in general, the more complex the fabrication the greater the cost. Fabrication is more economic with:
- Single square end cuts
- One hole diameter on any one piece avoids drill bit changes
- Holes in flanges and webs aligned where possible
- Web holes having adequate flange clearance
- Rationalised range of fittings for connections
- The use of a small range of standard bolts. e.g. M20 grade 8.8 for shear connections
Wherever possible, leave the choice of the connection detail to the steelwork contractor as the type and design of connections directly influences the total frame cost. Rationalise the range and tonnages of section sizes used.
Complex individualistic designs are going to cost more per tonne, even with modern CNC equipment. If you wish to keep costs down – keep it simple!
[top ] Materials
Avoid mixing steel grades where possible and rationalise the range of section sizes / tonnages used in order to minimise cost, lead times and factory handling. In general steel grade S355 offers greater economy unless stiffness / deflection governs the design, in which case S275 should be used.
Tubular steelwork at the National Football Centre
(Image courtesy of Billington Structures Ltd.)
Ensure that the most appropriate sections are specified for the particular application. “I” sections are usually most economical in conventional framing, while tubular sections are a possible alternative for columns and long span trusses. While tubular sections have a higher first cost than “I” sections, their lower overall gross weight required to perform the same function can largely offset this, often with an “aesthetic bonus”. Asymmetric beams are often used to reduce the construction depth of floor systems. Steel plate is used to fabricate connections. stiffeners and base plates and can be used to fabricate economic ‘I’ sections for longer spans.
[top ] Architectural influence
Ensure that unnecessary finishing is not specified and that any applied corrosion resistant coating is appropriate for the environmental conditions to be encountered. Grinding of welds to improve the appearance of the steel is usually only required for exposed steel in close proximity to a building’s occupants.
[top ] Quality of engineering and documentation
Completeness and accuracy of information are vital for a steelwork contractor to be able to properly assess the work involved. Where the steelwork is pre-designed, ensure that all member sizes are shown and that the connection forces are shown or are available. Guidance on identifying the requirements for the production and exchange of accurate, timely and detailed information for projects involving steelwork is available in BCSA publication no. 45/07.
Further guidance on the design of buildings for economic fabrication is available in:
Guidance on the design for economic fabrication of bridges is available in BCSA publication Steel Bridges. Chapter 7 – Costs.
[top ] Materials and components
Steel sections and plate. structural bolts. welding consumables, light gauge decking and corrosion protection and fire protection systems are all manufactured to European standards and in many cases are CE marked. In addition to these materials steel fabrication also needs fabrication equipment (e.g. cutting, sawing, drilling, welding machines etc.) and software (e.g. for computer controlled machinery).
[top ] Sections and plates
A stockholder’s external facility
(Image courtesy of Rainham Steel Company Ltd.)
Steel sections and plates are generally manufactured to BS EN 10025-2  for open sections, BS EN 10210-1  for hot-finished tubes. and BS EN 10219-1  for cold-formed hollow sections. All sections are accompanied by Type 3.1, product specific CE Inspection documents.
Large orders can be supplied direct from the steel mills, but Steelwork Contractors normally obtain their steel sections and plates from a steel stockholder. Steel stockholders play a vital part in the steel construction supply chain, ensuring that the market is supplied with what it needs when it is needed. The industry has an extensive network of depots serving all parts of the country. The stockholders provide a range of products to the construction industry including heavy structural sections. plates. light sections. cladding materials, flats, angles. Total stocks held represent about 40 days supply. All BCSA stockholder members have quality management systems such as BS EN ISO 9001  in place supplemented by individual identification systems to ensure full traceability from the steel manufacturer to the steelwork contractor. The specification and quality controls placed on stockholders by the BCSA ‘Model Specification for the Purchase of Steel Sections and Plates’ ensures the quality of steel sections and plate placed on the UK market by BCSA stockholders.
[top ] Bolts
Although the majority of structural bolts are manufactured outside of the European community they are all CE marked and manufactured to European standards – BS EN 14399  for preloaded bolts and BS EN 15048-1  for non-preloaded bolts. The exacting quality control requirements placed on European manufacturers and distributors by the BCSA ‘Model Specification for the Purchase of Structural Bolting Assemblies and Holding Down Bolts’ ensures the quality of structural bolts placed on the UK market by BCSA bolt distributors.
[top ] Proprietary products
Proprietary products such as some cellular beams. fasteners, purlins. cladding and roofing systems are also available and can provide novel and cost effective alternative solutions. Many of these are manufactured to either European product standard or European Technical Approvals.
Cellular beam manufacturing