ABSTRACT
The plastic theory makes the assumption that the ultimate load is reached before secondary effects, such as member instability, cause failure. At collapse load the zone below the point load is fully plastic, and is transformed into a ‘plastic hinge’. The beam each side of the plastic zone remains elastic. If shear occurs across a beam section where the bending moment is high, the plastic moment value at the hinge position may be reduced. Axial force tends to reduce the value of the plastic moment. For a frame to transform into a mechanism and collapse, it is necessary for a number of hinges to develop. A single bay portal frame with ‘pinned’ column bases has one redundant restraint, and therefore two hinges must form to cause a collapse. The plastic design method has been extended to include a wide variety of steel structures, such as grillages, vierendeel girders, and multi-storey frames.
