Steel Advisor

Deep Rafter Stability

The New Zealand Steel Structures Standard (SNZ, 1997), in keeping with many international design standards has no minimum stiffness requirement for restraints systems preventing flexural-torsional buckling of steel sections bent about their major axis. To account for restraint system flexibility, NZS3404 categorises these systems as providing full or partial restraint.

6 December 2020

The New Zealand Steel Structures Standard (SNZ, 1997), in keeping with many international design standards has no minimum stiffness requirement for restraints systems preventing flexural-torsional buckling of steel sections bent about their major axis. To account for restraint system flexibility, NZS3404 categorises these systems as providing full or partial restraint. In the commentary to the Steel Structures Standard guidance is provided for roof rafters for the minimum ratio of purlin depth to rafter depth to ensure a lateral restraint system fully restrains the section. No such guidance is provided to allow designers to check that the restraining system is sufficiently rigid to provide at least partial restraint. This is particularly relevant for the bottom critical flanges of deep roof rafters restrained by a flexible tension side system in conjunction with full depth stiffeners (Figure 2). In this paper guidance is provided for structural engineers to help them address this situation.

MEM3601.pdf

Allistair Fussel