There are several distinct types of drawing involved in a structural steel project, each serving a different purpose and produced at a different stage of the design and construction process.
General Arrangement Drawings
General arrangement drawings, often referred to as GA drawings, are produced by the structural engineer as part of the overall building design. They show the layout and geometry of the structural steel frame in plan, elevation and section, giving an overall picture of how the structure is arranged within the building.
GA drawings typically show the positions of columns, beams and other structural members, the overall dimensions of the frame, floor levels and structural grid references, primary connection points and the relationship between the steel structure and other building elements such as foundations, floors and cladding.
GA drawings are the starting point for the fabrication process. The steel fabricator uses them to understand the overall scope and geometry of the steelwork package before producing more detailed fabrication drawings.
Structural Calculations and the Engineer's Design
Alongside the GA drawings, the structural engineer produces a set of calculations that define the sizes, grades and properties of each steel member required to carry the loads acting on the structure. While the calculations themselves are not drawings, they inform the member sizes shown on the GA drawings and are an essential part of the design package.
The fabricator will typically receive the GA drawings and member schedule rather than the full calculations, but it is worth being aware that every member size and connection detail shown on the drawings has been derived from a set of structural calculations that define its minimum requirements.
Fabrication Drawings
Fabrication drawings are produced by the steel fabricator, based on the structural engineer's GA drawings and specification. They are the detailed working drawings from which the steel components are actually manufactured in the workshop.
Fabrication drawings are far more detailed than GA drawings and contain all of the information needed to fabricate each individual component accurately. This includes the precise dimensions of every member, the exact positions of all holes for bolted connections, the size, type and length of all welds, the grade of steel to be used, the surface preparation and coating requirements and any special fabrication instructions relevant to particular components.
Each fabricated component is typically assigned a unique piece mark, which is a reference number or code used to identify that component throughout the fabrication, delivery and erection process. Piece marks appear on both the fabrication drawings and on the physical steelwork itself, usually marked in paint or chalk, making it straightforward to match components to drawings on site.
Fabrication drawings must be checked and approved by the structural engineer before fabrication begins. This approval process is an important quality assurance step that confirms the fabricator has correctly interpreted the engineer's design intent.
Erection Drawings
Erection drawings are produced alongside or as part of the fabrication drawing package and show how the fabricated components are to be assembled on site. They are the primary reference document for the steelwork erector during the construction phase.
Erection drawings typically show the positions of all columns on the structural grid, the erection sequence for the frame, the location of each fabricated component using its piece mark reference, the positions and details of all on-site connections, and any temporary works or propping required during erection.
A well-prepared set of erection drawings makes the site erection process significantly more efficient and reduces the risk of components being installed in the wrong position or orientation.
Connection Details
Connection details are drawings that show the specific design of the joints between structural members. In some cases these are incorporated into the fabrication drawings, and in others they are produced as separate detail sheets.
Connection details typically show the size and arrangement of bolts, the dimensions and thickness of end plates, cleats and gusset plates, the size and type of welds, and any specific requirements for high-strength or preloaded bolt assemblies.
Connection design is an area of structural engineering that requires particular care, as connections must transfer loads safely between members while also being practical to fabricate and erect. In the UK, connection design is often carried out by the fabricator's own engineering team working within parameters established by the structural engineer, a process known as contractor-designed connections.
