Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 38 8 COLUMN DESIGN 8.1 Introduction As shown in Figure 8.1, the most highly loaded region of the rafter is reinforced by the haunch. By contrast, the column is subject to a similar bending moment at the underside of the haunch. The column will therefore need to be a significantly larger section than the rafter – typically proportioned to be 150% of the rafter size. Figure 8.1 Typical bending moment diagram for frame with pinned base columns subject to gravity loading The optimum design for most columns is usually achieved by the use of:  A cross-section with a high ratio of I yy to I zz that complies with Class 1 or Class 2 under combined major axis bending and axial compression  A plastic section modulus that is approximately 50% greater than that of the rafter. The column size will generally be determined at the preliminary design stage on the basis of the required bending and compression resistances. 8.2 Web resistance The column web is subject to high compression at the level of the bottom flange of the haunch. In addition, EN 1993-1-1 § 5.6(2) requires that web stiffeners are provided at plastic hinge locations, if the applied transverse force exceeds 10% of the member’s shear resistance. For these reasons, full depth stiffeners are usually required to strengthen the web. 8.3 Column stability 8.3.1 Column stability under maximum gravity combinations Whether the frame is designed plastically or elastically, a torsional restraint should always be provided at the underside of the haunch. Additional torsional restraints may be required within the length of the column because the side rails are attached to the (outer) tension flange rather than to the compression flange. As noted in Section 6.3, a side rail that is not continuous (for example, interrupted by industrial doors) cannot be relied upon to provide adequate