The lowest upper bound load corresponds to mechanism 3. This can be confirmed by conducting a moment check to ensure that bending moments anywhere are not violating the plastic moment condition. Additional discussion of plastic analysis of grillages can be found in Baker and Heyman (1969) and Heyman (1971).

2.11.12 Vierendeel Girders

Figure 2.110 shows a simply supported girder in which all members are rigidly joined and have the same plastic moment Mp. It is assumed that axial loads in the members do not cause member instability. Two possible collapse mechanisms are considered as shown in Figure 2.110b and c. The work equation for mechanism 1 is

so that

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