From the structural behaviour, it was observed that one of the braces is in tension, while the other is in compression (depending on the direction of the load). However, this usually comes at the expense of extra cost, and increased bending of the horizontal beams. The result shows that x-braced frame is more efficient than single diagonal cross bracing in reducing the lateral displacement of the structure. Structures with extensive bracing will be more stable.įigure 9: Deflection of framed structure with cross bracing (x-bracing) The sway sensitivity of the structure will be increased by narrow bracing systems with steeply inclined interior elements. This results in an efficient system with low member forces compared to other configurations, as well as compact connection details where the bracing meets the beam/column joints. Bracing members that are inclined at about 45 degrees are preferred whenever possible. Design forces at ULS are anticipated to be the most onerous for bracing members due to the combination where wind load is the dominating action. Sway-induced second-order effects (if the frame is flexible).įor the right combinations of actions, forces in specific members of the bracing system must be determined.Equivalent horizontal forces, which illustrate the effects of initial imperfections.Vertical bracings must be designed to withstand the following forces: The floor beams participate as part of the bracing system when crossing diagonals are used, and it is considered that only the tensile diagonals produce resistance (in effect a vertical Pratt truss is created, with diagonals in tension and posts in compression). Single diagonals, as shown, must be designed for either tension or compression, however crossing diagonals can be utilised with narrow bracing components that do not resist compressive stresses (then only the tensile diagonals provide the resistance). Two orthogonal bracing systems provide horizontal force resistance in a braced frame building: The beams and columns in a multi-story building are typically placed in an orthogonal pattern in both elevation and plan. The global analysis model may therefore assume pinned connections between the columns and the beams for braced frames constructed in accordance with BS EN, provided that the columns are designed for bending moments due to eccentric reactions from the beams. The beam reactions are applied eccentrically to the columns in the regularly used joint configurations in the UK, which assume a pinned connection but also assume that the beam reactions are applied eccentrically to the columns. If the joint is categorised as nominally pinned according to BS EN, and this classification is based on previous satisfactory performance in similar instances, a simple joint model, in which the joint is assumed not to transfer bending moments, may be utilised. The Eurocodes take the simple construction design approach into account.
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