Sunday, 8 June 2014

Member Selection Charts for Portal Frame Sheds (cold-formed Steel)

10° Doubly Pitched Frames

26 Design Charts, basically iso-moment contours in a span versus height space. The iso-moment curves reflect the AS4600 section moment capacities of readily available cold-formed c-sections.

 Depth/Thickness 10 12 15 19 24 30
100 X X X X
150 X X X X
200 X X X
250 X X
300 X X
350 X
Typical Sizes of C-Sections Available in Australia.

Each chart is for a different spacing of the portal frames, so once a spacing has been decided on, and the appropriate chart chosen, it is then possible to identify which c-section to use for a given height and span for AS1170 wind region A1 and Terrain Category 3.

Since the charts are based on a linear elastic analysis and are only iso-moment curves, it is possible to calculate magnification and/or reduction factors for different loading conditions, and so select the appropriate c-section for say TC2. Further more it is not necessary to restrict selection to c-sections, once the moment capacity of the c-section has been identified other structural sections with compatible structural capacities can be substituted.

It should be noted that the design basis behind the charts is purely bending moment and moment section capacity. The charts are therefore only suitable for estimating purposes and a Chartered Structural Engineer (CPEng. NPER(structural)) should be consulted to determine if the charts are suitable for a particular building project.

To put it another way use the charts for conceptual design to assess the viability of a building proposal, then obtain the services of a structural engineer to complete the detail design: number and location of fly bracing, connections, footing piers etc... . When costing allow for the possibility that the section size may have to be increased when the building is fully engineered.

If the charts say not feasible, then not feasible, if charts indicate is feasible, then detail design may require a larger section to accommodate performance issues not considered in the charts. Such as high gravity loads which may buckle the columns, or deflection constraints. The feasibility of forming a moment connection may also indicate the use of thicker material or a larger section. The charts therefore only consider one issue: the minimum strength frame for the ultimate strength live loading and wind loading to Australian loading code AS1170.


[08/06/2014] : Original
[14/09/2015] : Editing of Notes