Keywords: semi-rigid connections, buckling analysis, scaffold structures, finite element analysis.

Tube and fitting access scaffolds are extensively used in the UK and Europe [1]. They make use of semi-rigid connections in the same manner as proprietary systems studied by the authors [2,3,4]. In the UK they are designed using a permissible stress approach set out in BS 5973 [5]. This British Standard contains a great deal of information about best practice in the erection and use of scaffolds, but there is no great detail about the structural design of scaffolds. Crucial to the design method used is an accurate assessment of the elastic buckling behaviour of the scaffold, from which the compressive strength of the compression members is derived. In BS 5973 [5] there is an implicit assumption that the buckling lengths of columns are equal to the lift or storey heights.

This paper is a study of the elastic buckling behaviour of access scaffolds. It offers a simplified buckling analysis of Putlog scaffolds and of independent tied scaffolds using two-dimensional models. The results from these analyses are compared with the output from a finite element analysis, LUSAS [6] with excellent agreement.

The paper describes the primary components of access scaffolds and gives some information about the structural behaviour of the connections, particularly their cruciform stiffness and strength. Two types of scaffold are described, Putlog scaffolds, which are used mainly for small installations, particularly domestic buildings, and independent tied scaffolds which are the more common form of scaffold and are more widely used.

The loads applied to these scaffolds when in service include imposed and wind loads as well as dead load. Unusually for a steel structure self weight is an important component in the loading of a scaffold. Wind loading may be parallel or normal to the façade. When it is parallel to the façade, additional compressive loads may be induced into those columns which are an integral part of the bracing system. This leads to the possibility that isolated columns may buckle independently of the main body of the scaffold. Such behaviour may determine the critical ultimate limit state.

For both types of scaffold the frequency and position of ties to the façade of the building are very important in determining structural behaviour. There are many tie patterns, but for the purpose of determining standard designs only a limited number of such patterns can be dealt with. The role of the tie is to provide restraint to the structure to give it stability and also to resist wind suction.

The buckling modes of scaffold structures are identified. The modes may be parallel or normal to the façade and may be overall buckling modes or ones where only an isolated column is buckled. The analyses show that the buckling length for a scaffold column is strongly influenced by the tie positions, in contrast to current design practice which often assumes that the buckling length is equal to the lift height.

The paper includes details of finite element analyses carried out on three- dimensional models of the scaffolds to verify the results obtained from the simplified two-dimensional models. Some difficulties with these analyses are observed when horizontal as well as vertical was applied to represent the effects of wind. These problems were due to the wide range of stiffnesses in the structure. Good correlation between the finite element analysis and the model analysis was obtained for the buckling lengths

This study indicates that current design practice for tube and fitting scaffolds in the UK may be unconservative.

1

Proverbs, D.G, Holt, G.D, Olomolaiye. "Scaffolding for high-rise concrete constructions: a French, German and UK comparison", Proceedings Institution of Civil Engineers, Structures and Buildings, 128, 59-66 1998. doi:10.1680/istbu.1998.30035

2

Beale, R.G., Godley, M.H.R., "The Analysis of Scaffolding Structures using LUSAS", in "Proceedings LUSAS 1995", Bigwood, D. (Editor), FEA Ltd, Tewkesbury, 10-24, 1995.

3

Godley, M.H.R., Beale, R.G., "Sway Stiffness of Scaffold Structures" Structural Engineer ,75, no.1, 4-12, 1997.

4

Godley, M.H.R., Beale, R.G., "Analysis of Large Proprietary Access Scaffold Structures", Proceedings Institution of Civil Engineers, Structures and Buildings, 146, no.1, 31-40, 2001. doi:10.1680/stbu.146.1.31.40542

5

BS 5973:1993, "Code of practice for access and working scaffolds and special scaffold structures", British standards Institution, London, 1993.

6

LUSAS, "Lusas 13.4 User Manual", FEA Ltd, 2002

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