Steel Buildings in Europe

Part 4: Detailed Design 4 – 8 If  cr < 3,0 the recommended approach is to increase the stiffness of the structure. 2.6 Design of steel bracing systems to achieve  cr  10 for all combinations of actions 2.6.1 Introduction Vertical bracing is designed to resist wind load plus equivalent horizontal forces given by EN 1993-1-1, § 5.3. First order frame analysis can be used for braced frames, provided that the vertical bracing provides sufficient stiffness. For first order analysis to be applicable, EN 1993-1-1, § 5.2.1 requires that  cr  10 for the whole frame and therefore for each storey of a multi-storey building. Simple guidance is given below for the selection of bracing members so that sufficient stiffness is provided for such analysis to be valid. This allows the designer to avoid either the complexities of second order analysis, or of allowing for second order effects by amplification of first order effects. The method also permits the design of the frame to be undertaken without any recourse to computer analysis (such analysis is normally necessary in order to determine horizontal displacements and hence,  cr ). The parametric study that led to these design recommendations is presented in Access Steel document SN028a-EN-EU [4] . The bracing arrangements considered by this study are presented in Figure 2.3. 2.6.2 Scope The design procedure presented below was derived for buildings with the following limitations:  Height not exceeding 30 m  Angle of bracing members between 15° and 50° to the horizontal  The bracing arrangements are as shown in Figure 2.3 Note that the procedure does not depend on the steel grade.