Sunday, 8 June 2014

Sample Spreadsheet Calculations for Portal Frame Shed

Example spreadsheet calculations for determination of wind loads on a building with  a doubly pitched roof to the criteria of AS1170.2. Once reference wind pressure been found, then pressure coefficients on the external surfaces are found for directions theta=0 (transverse wind load) and theta=90 (longitudinal wind load). The moments in an assumed single span doubly pitched portal frame (or gable frame), are then calculated using Kleinlogel rigid frame formula: the frame is assumed to have fixed bases (eg. moment connections). Then based solely on sectional strength a minimum size steel section is selected. This section may not be suitable if cannot provide adequate lateral torsional restraint, and pass the detailed member capacity checks.

These calculations can be carried out using the following spreadsheet:

Additional structural calculations are required to design a full building, this is just the calculations for the action-effects of the primary frame, and the design actions on the building. Ignoring the rigid frame, the spreadsheet simply provides the wind actions for the surfaces of the building which can be used to assess/design other components. Other components would include the following:

  1. Roof X-Bracing and struts
  2. Wall X-Bracing and struts
  3. Roof Purlins (cladding rails)
  4. Wall Girts (cladding rails)
  5. End Wall Mullions


[08/06/2014] : Original
[14/09/2015] : Added some notes.

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

Height Span Chart Cold-Formed Steel Sheds TC2

Height Span Chart Cold-Formed Steel Sheds TC3