Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 52 the absence of other methods, the stiffness check may be based on the work of Horne and Ajmani [4] . Thus, the support member (the purlin or sheeting rail) should have I y,s such that:   1 2 2 2 3 y y,f y,s 190 10 L L f L L L I I    where: f y is the yield strength of the frame member I y,s is the second moment of area of the supporting member (purlin or sheeting rail) about the axis parallel to the longitudinal axis of the frame member (i.e. the purlin major axis in normal practice) I y,f is the second moment of area of the frame member about the major axis L is the span of the purlin or sheeting rail L 1 and L 2 are the distances either side of the plastic hinge to the eaves (or valley) or points of contraflexure, whichever are the nearest to the hinge (see Figure 9.18). Hinges that form, rotate then cease, or even unload and rotate in reverse, must be fully braced. However, hinges that occur in the collapse mechanism but rotate only above ULS need not be considered as plastic hinges for ULS checks. These hinges are easily identified by elastic-plastic or graphical analysis. Analysis cannot account for all of the section tolerances, residual stresses and material tolerances. Care should be taken to restrain points where these effects could affect the hinge positions, e.g. the shallow end of the haunch instead of the top of the column. Wherever the bending moments come close to the plastic moment capacity, the possibility of a hinge should be considered. 9.6 Design summary Bracing must be provided with adequate strength and stiffness to act in conjunction with the purlins, side rails and eaves beams to resist horizontal actions, including wind, to provide overall stability to the building and to provide local stability to the columns and rafters. Bracing must be provided:  To side walls, in a vertical plane; see Section 9.2  On plan at or near the roof of the building; see Section 9.3  Stays are required to stabilise inner flanges of the columns and rafters where they are in compression and potentially unstable; see Section 9.4  At, or near, plastic hinge positions to provide torsional restraint; see Section 9.5.