Keywords: elastic support, column base connection, soil-structure interaction, second-order effect, buckling length, semi-rigid, diaphragm action, non-linear.

Steel buildings consisting of planar frame units and corrugated steel roof working as a horizontal bracing have been studied in this work. In practice those steel frames have often been calculated as planar frames using simplified boundary conditions. Most of the existing studies on frames with elastic joints assess the effect of beam-column connections on the global frame behaviour, while the base connection has been modelled either as rigid or pinned. Although a number of existing publications deal with column base stiffness, only very limited information is available on possible soil-structure interaction effects on the distribution of forces and the stability of the frame.

This paper evaluates the inaccuracy, included by the simplifying assumptions, applied in the design calculations of common steel frameworks. For this purpose a three dimensional model including soil-structure interaction, column base connection rigidity and the diaphragm effect of the corrugated roof sheeting has been investigated. Second-order effects were included by using a non-linear analysis.

The roof diaphragm provides lateral support to the frame. Classification of frames as sway or non-sway depends strongly on this lateral support, which is treated as an elastic spring at the column top. The stiffness of the lateral support depends on the location of the frame in the building, the stiffness of the diaphragm itself and the end frame as indicated by Davies [1]. Lateral displacements of the end frame increase deformability of the diaphragm support and consequently the stiffness of the diaphragm is modified.

The buckling length factor for columns was determined using several methods for comparison. First, the buckling length was calculated using the Wood method [2], accepted by the Eurocodes. Second, the approach proposed by Lui [3] was used for the buckling length factor determination. Finally, the buckling factor was determined using an eigenvalue analysis based on the finite element model.

It has been shown that using planar frames with simplified support conditions might cause deviations from the real structural behaviour and in some cases the results from simplified methods were non-conservative.

1

J.M. Davies, "Developments in stressed skin design", Thin-Walled Structures, 44(12), 1250-1260, 2006.

2

R.H. Wood, "Effective Lengths of Columns in Multi-Storey Buildings", The Structural Engineer, 52(7), 341-346, 1974.

3

E. Lui, "A novel approach for K factor determination", Engineering Journal, 29(4), 150, 1992.

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