Inelastic incremental analysis of an industrial Pratt truss

In a refurbished steel mill, the crane girder itself is used as the top chord of a supporting Pratt truss. Subsequent to rehabilitation the crane loads are increased and, to assess the capacity of the truss, non-linear incremental finite element analyses are carried out. These show that all members are satisfactory, attaining safety ratios greater than 1.00. The safety ratio is the load in a member, at any step of the analysis, multiplied by the resistance factor for steel members and divided by the factored load the member must sustain. The inelastic analysis shows that the truss action dominates and that the only bending moments to be considered are those introduced by joint and out-of-plane eccentricities. At the limit point, a particular member, without eccentric moments, can be taken to be axially loaded with pin ends. Failure of a shear diagonal in a panel does not lead to collapse of the truss because the surrounding panel members carry shear by vierendeel action. Code analyses are quite conservative.