Keywords: temporary structures, adjustable telescopic props, steel props, finite element analysis, axial strength, sensitivity analysis.

The work reported here concerns the structural behaviour and ultimate strength of adjustable telescopic steel props used in temporary structures for civil engineering applications. They comprise two slender circular hollow tubes with different diameters and connected through a pin and collar nut, making it possible to extend or shorten the prop length. The prop bottom endplate is often not mechanically connected to the foundation and the connection stems from friction and bearing action.

Adjustable telescopic steel props typically contain different types of initial geometric imperfections, namely those stemming from (i) the interplay between the tubes, (ii) tubes and/or endplate imperfections, and (iii) load eccentricities. The behaviour and ultimate strength of these props is governed by plasticity or second-order effects: it depends on their extension length, initial imperfections and on the strength of its tube connection. Props are often re-used several times without proper care during their life cycle. One often sees re-used props with very significant initial imperfections. When performing realistic numerical analyses of these structural elements, properly modelling the initial imperfections always constitutes a formidable challenge.

The paper presents results of a numerical investigation on adjustable telescopic steel props under compression. In particular, one describes the procedures adopted to (i) model the initial imperfections and (ii) estimate the prop structural response and ultimate strength. The advantages and disadvantages of each approach are assessed by comparing the numerical results with experimental values obtained from prop specimens tested at LNEC [1]. After providing background information, the paper describes the numerical models employed, addressing the (i) finite element selection, (ii) mesh configuration and density; (iii) contact algorithm adopted and (iv) numerical techniques utilised. Then, the merits of these numerical models are assessed and the differences between them are discussed. Particular attention is paid to the numerical procedure proposed in EN 1065 [2] to estimate the prop ultimate strength. Then, one presents a sensitivity analysis concerning the variation of the prop resistance with the (i) inner and outer tube thickness values, (ii) hole dimensions and spacing, and (iii) endplate dimensions. Finally, the paper closes by presenting the main conclusions drawn from this work, outlining its key contributions: (i) development of a reliable numerical model to investigate the structural response and ultimate strength of adjustable telescopic steel props and (ii) a proposal to improve the performance of the structural model prescribed in EN 1065.

1

J. André, "Behaviour and Quality of Steel Telescopic Props. Analytical, Numerical and Experimental Investigation", M.A.Sc. Thesis (Structural Engineering), Instituto Superior Técnico, Technical University of Lisbon, 2008. (Portuguese)

2

Comité Européan de Normalization (CEN), "Adjustable Telescopic Steel Props", EN 1065, Brussels, 1998.

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