Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 42 9 BRACING 9.1 General Bracing is required to resist longitudinal actions, principally wind actions and provide restraint to members. The bracing must be correctly positioned and have adequate strength and stiffness to justify the assumptions made in the analysis and member checks. 9.2 Vertical bracing 9.2.1 General The primary functions of vertical bracing in the side walls of the frame are:  To transmit the horizontal loads to the ground. The horizontal forces include forces from wind and cranes.  To provide a rigid framework to which side rails may be attached so that they can in turn provide stability to the columns.  To provide temporary stability during erection. According to EN 1993-1-1, the bracing will have to satisfy the requirement of § 5.3.1, 5.3.2 and 5.3.3 for global analysis and imperfections within the bracing system. The bracing system will usually take the form of:  A single diagonal hollow section  Hollow sections in a K pattern  Crossed flats (usually within a cavity wall), considered to act in tension only  Crossed angles. The bracing may be located:  At one or both ends of the building, depending on the length of the structure  At the centre of the building (See Section 9.2.5)  In each portion between expansion joints (where these occur). Where the side wall bracing is not in the same bay as the plan bracing in the roof, an eaves strut is required to transmit the forces from the roof bracing into the wall bracing. 9.2.2 Bracing using circular hollow sections Hollow sections are very efficient in compression, which eliminates the need for cross bracing. Where the height to eaves is approximately equal to the spacing of the frames, a single bracing member at each location is economic (Figure 9.1). Where the eaves height is large in relation to the frame spacing, a K brace is often used (Figure 9.2).