Tuesday, October 15, 2013

Feasibility of Cold-formed Steel Shed (Simple Structural Software Application)

The simplest program I have written to check the frames size of a cold-formed steel shed is EngineLT.exe. It is a cut down version of a larger program I wrote in Delphi 3, the problem with the larger program is I never really got round to working out how to get  the Borland Database Engine (BDE) onto another machine, and I had more important things to do. So I ripped the database out, and simplified the interface. To give the following:

It is based on AS1170.2:1989, so its results are potentially no longer valid. But it provides a quick check on whether a given size of c-section is suitable for a given gable frame shed. A gable frame comprising of two rafters and two columns. The moments in the frame are calculated using Kleinlogel formula for a fixed base rigid moment frame.

The minimum sectional moment capacities of the available c-sections here in Australia are hard coded into the program. The minimum yield strength of the steel is 450MPa. Low internal pressure coefficients are adopted to reflect the status of the industry with respect to the standard calculations held by most suppliers. Remove one or both columns and get a different rigid frames, which are not within the scope of the program. Simply providing carry beams is not acceptable.

Roughly removing one column makes the roof a propped cantilever and the moment at the remaining knee is approximately 1.5 times the maximum knee moment in the full frame. Remove both columns and the maximum moment moves to the ridge and is approximately 3 times greater than it was in the full frame. Typically the maximum moment in the full frame is at the knee, but not always. Any case the program doesn't provide the details, nor can it handle these variations. The point is, the structural form is a specific rigid frame, any variation which changes this structural form requires a different mathematical model to check stresses in the frame.

It is common practice in the cold-formed shed industry to standardise frames at 3m centres and rip columns out for larger doors in the side: this changes the structural form and is beyond the scope of the standard calculations held by most shed suppliers. Merely providing carry beams is not adequate, in fact if remove too many columns, and want really large openings, an entirely different structural form may be required for the entire building: such as large parallel chord trusses span the entire length of the building, with two columns.

So before messing around with carry beams, the first check should be with changing the frame spacing. If want 5m wide doors then see what frame size is required for frames at 6m centres. Sure adopting 6m centres or larger means the C7510 girts/purlins typically used will no longer be valid. By adopting larger spaces, potentially reduce the amount of fabrication and onsite labour: though may increase the need for cranes.

Any case those are other design issues. When I wrote the program, my interest was providing a tool which would allow experimentation with something other than the standard 3m spacing. The application has no data files, and doesn't produce any result files or print any reports. It simply allows playing around with the basic input parameters, and seeing how they affect frame size. The frame size may not be possible when detail design is carried out due to lack of lateral torsional restraint to the frames: provided by combination of girts/purlins and flybracing. However if the program indicates a frame is not possible its highly unlikely that detailed design can make it possible.

EngineLt can be downloaded here as  a zip file.

The frame size determined by the programme should be comparable to the frame size selected from the height/span design charts I have produced. The prime difference between the two is that the charts are to wind loading from AS1170.2:2002, whilst this program is to AS1170.2:1989.

Design Chart for C-Section Frames AS1170.2 Terrain Category 3 (TC3) 
Design Chart for C-Section Frames AS1170.2 Terrain Category 2 (TC2)

Users of EngineLT must accept this disclaimer of warranty :
EngineLT  is supplied as is. The author disclaims all warranties, expressed or implied, including without limitation, the warranties of merchantability and of fitness for any purpose. The author assumes no liability for damages, direct or consequential, which may result from the use of EngineLT .