Progressive collapse of system scaffolds with accidental removal of bracing members

Steel scaffolds are widely used as temporary supports in construction. A single or a series of braces in a system scaffold is occasionally removed by workers for ease of access or by incidental damage, thus undermining the load capacity and robustness of the overall structure. Based on the Cubic Quadruple Function (CQF) element proposed by the authors, a reliable and safe practical design is suggested to allow for plastic hinges formation with plastic moment redistribution in transoms, braces and ledgers but the failure load is assumed once a plastic hinge is formed at standards and posts which are predominantly under axial compressions, leading to buckling with small moment redistribution capability. This research paper studies the reduced load capacities of scaffolds under a common incident of removal of a group of bracing members, by workers in other trades during construction. Findings in this paper are useful for code development, engineers for erection of system scaffolds against possible progressive collapse and site inspectors. We further propose that an unfactored working load design approach should be adopted to check scaffold stability using the global collapse load factor concept, rather than using the limit state factored loads in some codes such as the Eurocode EN12811 which is more appropriate for permanent structures with much longer design life.