Usually, the floor system will be sufficient to act as a diaphragm without the need for additional steel bracing. There are two types of horizontal bracing system that are used in multi-storey braced frames: Horizontal bracing (in the roof) in a single storey buildingĪ horizontal bracing system is needed at each floor level, to transfer horizontal forces (chiefly the forces transferred from the perimeter columns) to the planes of vertical bracing that provide resistance to horizontal forces. Ratio of maximum deflection (compared to bracing at 34°) The table below gives an indication of how maximum deflection varies with bracing layout, for a constant size of bracing cross section. Wide bracing systems will result in more stable structures. Narrow bracing systems with steeply inclined internal members will increase the sway sensitivity of the structure. This provides an efficient system with relatively modest member forces compared to other arrangements, and means that the connection details where the bracing meets the beam/column junctions are compact. Where possible, bracing members inclined at approximately 45° are recommended. For bracing members, design forces at ULS due to the combination where wind load is the leading action are likely to be the most onerous. Guidance on the determination of equivalent horizontal forces and on the consideration of second order effects in discussed in the sections below, and a Frame stability design tool is also available.įorces in the individual members of the bracing system must be determined for the appropriate combinations of actions. Second order effects due to sway (if the frame is sensitive to second order effects).Equivalent horizontal forces, representing the effect of initial imperfections.The vertical bracing must be designed to resist the forces due to the following: Note that when crossed diagonals are used and it is assumed that only the tensile diagonals provide resistance, the floor beams participate as part of the bracing system (in effect a vertical Pratt truss is created, with diagonals in tension and posts - the floor beams - in compression).
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