What is a steel-framed building? The main members of the load-bearing system

A steel-framed building is one whose load-carrying frame is built from structural steel sections. The regular frames you see when you walk into an industrial building are that frame. This article explains, in plain language, the members that make it up and what each one does.

The main frame: column and rafter

Columns carry vertical loads down to the foundation. Wide-flange H sections (HEA, HEB) are common because they resist bending well in both directions.

The rafter or roof beam spans between two columns and carries the roof load to them. For medium spans, solid-web I sections (IPE) are enough. As the span grows a solid-web beam becomes heavy; at that point a truss made of individual bars does the same job with less material.

The frame formed by column and rafter repeats at regular intervals along the length of the building. That interval is the bay spacing.

Secondary members: purlins and girts

Roof cladding does not sit directly on the rafters. Purlins run along the building on top of the rafters, and the cladding is fixed to them. On the walls, girts do the same job. These members are usually cold-formed or hot-rolled U / C / Z sections.

Stability: bracing

Frames are strong in their own plane, but a system is needed to tie them together along the length of the building. Roof and wall bracing carries horizontal loads such as wind and earthquake down to the foundation and keeps the structure stable during erection. Tubes, angles or rods can be used as bracing.

Connections

Members are joined with bolted or welded connections. Common practice is to weld in the workshop under controlled conditions and to bolt on site. At the column foot, a base plate and anchor bolts transfer the load to the concrete foundation.

The safety of a steel structure often depends more on correct connection detailing than on section size.

Cladding

Roof and facade cladding goes over the frame: trapezoidal sheet, sandwich panels or other systems depending on the project. Cladding is not load-bearing, but it determines the thermal, water and fire performance of the building.

Summary

• Columns and rafters form the main frame
• Purlins and girts carry the cladding
• Bracing provides stability against horizontal loads
• Connections are the most critical points of the system

This article is for general information. The load-bearing system of every building is determined by the calculations of a licensed engineer.