Second-order analysis and design of steel scaffold using multiple eigen-imperfection modes

Abstract

Casualties due to scaffold collapse are common in many countries. However, current design of scaffolds still mainly relies on experimental results and judgment, which are non-scientific, uncertain and unreliable. From experience, lack of fit at joints between scaffold members greatly affects their buckling strength, implying that initial imperfection is an important factor influencing the overall buckling strength of scaffolds. This chapter proposes a design approach that uses multiple eigenbuckling modes as initial imperfections in order to simulate imperfection due to lack of fit or crooked members. There are two main types of scaffolding, namely access scaffolds and the support scaffolds. The former are subjected to moderate loads and are therefore aimed for light to medium duty; whereas, the latter take much higher loads from, for instance, concrete weight and workers. Even though collapse frequently occurs in both types of scaffolding systems, this chapter addresses the analysis and design of support scaffolds for taking heavy vertical loads. A second-order analysis method using the eigenbuckling mode as imperfection is proposed for design of practical scaffolds on site. The magnitude of the imperfection is taken as 1/500 of the scaffold height. It is therefore possible for engineers to assess more directly the condition of a new or used scaffold in-situ or in the laboratory by assuming the length of the global imperfection rather than a reduced effective length for an adverse condition. This method can be used in the design of scaffolds under various conditions.