Abstract
Codes of practice rely on the effective length method to assess the stability of multi-storey frames. The effective length method involves isolating a critical column within a frame and evaluating the rotational and translational stiffness of its end restraints, so that the critical buckling load may be obtained.The non-contradictory complementary information (NCCI) document SN008a (Oppe et al., 2005) to BS EN 1993-1 (BSI, 2005) provides erroneous results in certain situations because it omits the contribution made to the rotational stiffness of the end restraints by columns above and below, and to the translational stiffness of end restraints by other columns in the same storey.Two improvements to the method are proposed in this paper. First, the axial load in adjoining columns is incorporated into the calculation of the effective length. Second, a modification to the effective length ratio is proposed that allows the buckling load of adjacent columns to be considered. The improvements are shown to be effective and consistently provide results within 2% of that computed by structural analysis software, as opposed to the up to 80% discrepancies seen using the NCCI (Oppe et al., 2005).
Highlights
Many codes of practice rely on the effective length method to assess the stability of frames
When column BC buckles the axial load in column CD is: (3846 Â 1.5 = 5,769 kN < 11,913 kN) BC is critical
As a result the non-contradictory complementary information (NCCI) approach has been shown to provide unreliable results in certain cases, by incorrectly evaluating the contribution made by adjoining columns to the rotational stiffness of end restraints
Summary
Many codes of practice rely on the effective length method to assess the stability of frames. If the method is applied to unbraced frames where columns of varying stiffness exist in the same storey or columns have different applied loads, significant errors will be encountered that are potentially unconservative, as seen in Section 3.2.1 below To address this issue, a modification factor is adopted which is applied to the effective length ratio obtained using the sway design chart, and accounts for columns that will have end restraints with translational stiffnesses between zero (sway case) and infinity (non-sway case) and even negative translational stiffnesses. A modification factor is adopted which is applied to the effective length ratio obtained using the sway design chart, and accounts for columns that will have end restraints with translational stiffnesses between zero (sway case) and infinity (non-sway case) and even negative translational stiffnesses These are often called partial sway frames.
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