Steel Buildings in Europe

Part 4: Detailed Design 4 – 4 A ‘sway-sensitive frame’ is a frame where horizontal flexibility is such that there needs to be some allowance for the effects of deformed geometry. A ‘sway-insensitive frame’ is a frame with sufficient horizontal stiffness that second order effects may be ignored. It should be noted that horizontal stiffness needs to be considered separately in two, usually orthogonal, directions. Thus, a frame may be:  A sway-sensitive frame in both directions  A sway-sensitive frame in one direction and a sway-insensitive frame in the other  A sway-insensitive frame in both directions. 2.3.2 Distinction between sway/non-sway and unbraced/braced concepts Both sway-sensitivity and braced/unbraced concepts relate to horizontal structure. However, they are essentially different. Sway-sensitivity definitions entirely relate to horizontal stiffness and behaviour . Braced/unbraced are descriptions of structural arrangement. It follows that, in each of the two orthogonal planes, a frame may be:  Braced and sway-insensitive  Braced and sway-sensitive  Unbraced and sway-sensitive  Unbraced but sway-insensitive (unusual, but possible). 2.4 Second order effects 2.4.1 Basic principles The sensitivity of any frame to second order effects may be illustrated simply by considering one ‘bay’ of a multi-storey building in simple construction (i.e. with pinned connections between beams and columns); the bay is restrained laterally by a spring representing the bracing system. First and second order displacements are illustrated in Figure 2.2.