Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 9 Designers less familiar with steel design may be surprised by the use of plastic moment of resistance and redistribution of moment in combination with elastic analysis. However, it should be noted that, in practice:  Because of residual stresses, member imperfections, real inertias that differ from those assumed, real connection stiffness that differs from that assumed and lack of fit at connections, the true distribution of moments in any frame is likely to differ substantially from that predicted by elastic analysis.  Class 1 and 2 sections are capable of some plastic rotation before there is any significant reduction in capacity due to local buckling. This justifies a redistribution of 15% of moments from the nominal moments determined from the elastic analysis. The results of elastic analysis should therefore be regarded as no more than a reasonably realistic system of internal forces that are in equilibrium with the applied loads. In a haunched portal rafter, up to 15% of the bending moment at the sharp end of the haunch can be redistributed, if the bending moment exceeded the plastic resistance of the rafter and the moments and forces resulting from redistribution can be carried by the rest of the frame. Alternatively, if the moment at the midspan of the portal exceeded the plastic resistance of the rafter, this moment can be reduced by up to 15% by redistribution, provided that the remainder of the structure can carry the moments and forces resulting from the redistribution. If an elastic analysis reveals that the bending moment at a particular location exceeds the plastic moment of resistance, the minimum moment at that point after redistribution should be the plastic moment of resistance. This is to recognise that a plastic hinge may form at that point. To allow reduction below the plastic resistance would be illogical and could result in dangerous assumptions in the calculation of member buckling resistance. 3.2.3 Plastic analysis Plastic analysis is not used extensively in continental Europe, even though it is a well-proven method of analysis. However, plastic analysis is used for more than 90% of portal structures in the UK and has been in use for 40 years. Traditionally, manual calculation methods were used for a plastic analysis (the so-called graphical method, or the virtual work method, etc.). These manual methods are not discussed in this publication, because plastic analysis is usually undertaken with software, most of the time using the elastic-perfectly-plastic method. The principle of this method is illustrated in Figure 3.4 and Figure 3.5.