Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 32 7 RAFTER DESIGN 7.1 Introduction Portal frame design is usually governed by the verification of members at ULS. Although SLS checks are important, orthodox frames are generally sufficiently stiff to satisfy the SLS deflection limits. Economy in the overall frame can usually be achieved by the use of plastic analysis; this requires Class 1 or 2 sections throughout and Class 1 where there is a hinge which is predicted to rotate. 1 2 1 Bottom flange in compression 2 Top flange in compression Figure 7.1 Portal frame bending moments, gravity actions As shown in Figure 7.1, rafters are subject to high bending moments in the plane of the frame, that vary from a maximum ‘hogging’ moment at the junction with the column to a minimum sagging moment close to the apex. They are also subject to overall compression from the frame action. They are not subject to any minor axis moments. Although member resistance is important, stiffness of the frame is also necessary to limit the effects of deformed geometry and to limit the SLS deflections. For these reasons, high strength members are generally not used in portal frames, but lower steel grades with higher inertias. Optimum design of portal frame rafters is generally achieved by use of:  A cross-section with a high ratio of I yy to I zz that complies with the requirements of Class 1 or Class 2 under combined major axis bending and axial compression.  A haunch that extends from the column for approximately 10% of the frame span. This will generally mean that the maximum hogging and sagging moments in the plain rafter length are similar. 7.2 Rafter strength The resistances of all critical cross-sections of the rafter must be verified in accordance with Section 6 of EN 1993-1-1.