Further Aspects of Design by P-Δ Model

In the P? 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 braced and an unbraced 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. The PΔ technique does imply, however, that the maximum strength of a beam-column can be reached regardless of the magnitude of the sway rotation of the member. For beam-columns in flexible structures, or those subjected to high axial loads, the usable strength is less than the maximum strength of the member in the sway prevented condition. A limitation on the drift will avoid structural arrangements for which this strength reduction is significant.