Scaffold Design, Loading, and Structural Principles
Design understanding separates advanced from basic candidates. You must know how loads are classified: dead (self-weight), live/imposed (persons, materials, equipment), and environmental (wind, snow). EN 12811 defines load classes (e.g. class 2 = 1.5 kN/m², class 3 = 2.0 kN/m²) for working platforms, and these drive bay size, standards spacing, and ledger configuration. Understand load paths: platform to transoms, transoms to ledgers, ledgers to standards, standards to base plates and ground. Stability comes from bracing - facade bracing, ledger bracing, and plan bracing - plus ties to the structure at defined frequencies and pull-out resistance.
Traps to expect: assuming a 'heavy duty' scaffold is unlimited (it is a defined class); forgetting that wind loading can govern design and that netting/sheets increase wind area; confusing 'independent' and 'putlog' scaffolds (putlogs bear into the wall and are not independent); and missing that a design is required above certain heights or for special conditions per TG20.
Worked example: A class 3 platform (2.0 kN/m²) over a 2.4 m × 1.8 m bay carries a design imposed load of 2.0 × 2.4 × 1.8 = 8.64 kN per bay. Compare with the scaffold's rated capacity and the tie capacity - if the load exceeds the class, reduce bay size or upgrade the class; never simply 'add more boards'.