In general, the loads and consequent deformations for these frames are in the plane of the structure, and hence these are a 2D (or plane) frame structure. Due to the practical requirement of having a clear space between the supports of a portal frame, providing in-plane bracing is generally not feasible. Consequently, these frames undergo larger deflections and are prone to sway laterally, even under the vertical loads. The concept of sway frames is addressed in more detail in Technical Guidance Note No. 10 (Level 1) Principles of lateral stability. Thus, in spite of the inherent simplicity of portal frames, many aspects of their analysis, design and detailing require careful consideration.
Portal frames can be made from concrete, timber and even glass but the vast majority, in the UK certainly, are constructed from steel. This Technical Guidance Note gives an introduction to steel portal frames and their preliminary analysis. Steel portal frames usually have pinned bases and moment connections at the column/rafter interface and mid-span apex splice in the rafter. Although there are other forms of portal frame (described in Elastic Design of Single- Span Steel Portal Frame Buildings to Eurocode 3), for the sake of brevity and clarity this note will be dedicated to this particular form.
(This article was updated in October 2016 to reflect errata issued since its original publication.)