Overall stability considerations in the design of steel structures

Abstract

Generally, the analyses used to determine the distribution of bending moments and internal forces in a structure are first-order analyses. Such analyses neglect the so-called P-Δ effects: (the effects produced by the vertical forces acting through the laterally deflected position of the structure). Thus, the stiffness and strength of the structure are overestimated. The present design procedure recognizes this neglect: the structure is first classified as either sway prevented or sway permitted. For either case the forces and moments predicted by the first order analyses are translated into member sizes through the use of ‘interaction equations’. In the sway permitted condition, however, these equations are adjusted to require an increase in column size to attempt to account for the neglect of the P-Δ effects. In the P-Δ method of design, the secondary forces, produced by the vertical loads acting through the laterally displaced shape of the structure, are included directly in the analysis. This approach removes the artificial distinction between a sway prevented and a sway permitted structure, which is implied by the present ‘ K factor’ design approach to columns, and leads to a more rational technique for the design of columns and bracing systems.