Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 12 (a) First hinge forms 1 (b) Load increases – rafter approaches yield 1 (c) Load increases, second hinge forms and a mechanism leads to collapse 1 1 (d) 1 Plastic resistance moment Figure 3.6 Elastic-perfectly-plastic method of analysis, showing state of frame as horizontal and vertical loads are increased proportionally a) Elastic throughout; (b) Plastic hinge at eaves;(c) Rafters approaching plasticity; (d) Plastic hinge in rafter It is recognised that some redistribution of moments is possible, even with the use of elastic design. EN 1993-1-1 § 5.4.1.4(B) allows 15% redistribution, as discussed in Section 3.2.2, although this is uncommon in practice. Where haunch lengths of around 15% of the span are acceptable and the lateral loading is small, the elastic bending moment diagram will be almost the same as the plastic collapse bending moment diagram. As illustrated in Figure 3.3, the maximum hogging moment at the end of the haunch is similar to the maximum sagging moment in the rafter. In such cases, an elastic analysis may provide an equivalent solution to a plastically analysed frame.