Rigid Frame Analysis for Shed

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Description

A zip file containing a collection of MS Excel workbooks.

  1. schShedDesignerR02.xls (primary workbook)
  2. schTechLIB.xla (Technical Library Add-in)
  3. as4055.xls (Simplified Wind Classification for Manufactured Structural Products (MSP’s))
  4. schStruMtrl.xls (Materials Library)

Purpose

Assist those involved with structural design of manufactured structural products (MSP’s), in this case the analysis of the main portal frame of steel sheds. Though it can be used for any kind of building employing portal frames (rigid moment frames) for the main structure: shacks, houses, warehouses, factories, offices, schools, hospitals, aircraft hangars.

Spreadsheet analysis of framed shed using Kleinlogel formula for frame with doubly pitched roof and fixed bases ((eg. gable frame). Wind actions are calculated for each surface of the frame, for transverse (theta=0) and longitudinal (theta=90) loading, then design action-effects are calculated. From the maximum calculated bending moment the smallest steel section with suitable sectional moment capacity (Φ.Ms) is then looked up. The selected section could be C-section, UB, UC, PFC, RHS, SHS. The results can be printed out as an 8 page report (A4 sheets).

Sample report here: schShedDesignerR02

NB: After selection of section based on sectional moment capacity additional calculations are required to check member capacity for combined actions. These additional calculations are not included in the workbook.  Typically for cold-formed steel sheds the moment due to wind loading is approximately twice that from live loading conditions, such that combined wind loading and axial tension is the critical condition. Consequently non-linear analysis is not overly critical for cold-formed steel. Another issue to consider is the feasibility of designing a connection which can transfer the moment capacity of the section. Since over all frame design is dependent on the member capacities and the feasibility of connections, such additional calculations have been excluded, that way designers can add the calculations specific to their particular frame. For example in-house we have a version of the workbook for all the components of a cold-formed steel c-section shed, using either gusset plate connections or welded and bolted end plate connections. For other situations we only print out that part of the spreadsheet relevant to the project and have other stand alone spreadsheets for member and connection design. Compromise has to be made between rapidly designing and documenting a whole building of a specific structural form versus the flexibility of designing any building structure: along with the hassle of keeping similar spreadsheets consistent and up to date.

So for example a shed fabricator wants the speed of a shed design spreadsheet whilst a structural designer wants the flexibility of structural design spreadsheets.  Also the one calculation report can have different input forms to suit the needs of specific users.

Setting up Spreadsheet Environment

The primary workbook (schShedDesignerR02) is linked to several other workbooks, and is also dependent on an excel add-in. Guidance on setting up MS Excel to use these multiple files is provided by the following:

  1. SpreadsheetEnvironmentIssue1(pdf file)
  2. Setting Up Spreadsheet Environment (online)

These documents are typically based on setting up for use of AS600 or AS4100 quick calculator workbooks, but the same principles apply: install add-in, update links.

User Required Skills

Skills: Design & Analysis

The spreadsheet purpose is to assist with structural design, therefore basic requirement is knowledge of structural design, such as engineering mechanics (statics), mechanics of the strength and stability of materials, and knowledge of Australian standards. This would typically be provided by minimum formal education awarding either an Associate Diploma in Mechanical Engineering or Civil Engineering, or under the modern Australian  Qualification Framework (AQF), an Advanced Diploma in Structural/Mechanical Design. Occupational titles may include: Associate Technologist, Engineering Associate, Engineering Officer.

Skills: MS Excel

Experience with: named ranges, linked workbooks, data validation and add-in’s. Knowledge of VBA useful but only required if wish to modify the technical library.

Product Configurators for Structural Products (SP’s)

The workbook, can be used as a building block to create a product configurator used by salespeople and customers. For assistance in developing such tools please enquire.

South Australia has a minister’s specification for software used by persons who are not “engineers”. Similar requirements are now imposed nationally via the Australian Building Codes Board (ABCB) Protocol for Structural Software. Part of the requirement involves training in use of such software: clearly such requirement is incompatible for use of such software by the public. Though training could be implemented online as staged access to the software. That is the software can be made easier to use, and training can be built into the software.

Also helpful to clarify that the software is not structural design software, it is a product configurator, using knowledge-based configuration to assess the suitability of the selected options. The suitability being based on a mathematical model of the structure: the model doesn’t change but the numerical inputs can.

The limitations of standard calculations is that they represent one structural model and one set of input parameters. A series of standard designs can provide enveloping cases, which can be used as described in TechNote#2 , this can be extended further to produce height vs span charts illustrating the limitations of given structural sections for a given structural form. The method of TechNote#2 hits a snag when the shed wanted is narrower span but greater height than covered by the standard designs. The height/span charts were created to resolve this issue.

However, changing the set out of end walls changes the structural model for the shed and the end-frames. Similarly removing side columns changes the structural model for the shed and the main portal frames. This can be addressed by providing multiple models of suitable plane frames. It is to be noted that even if opt for using 3D frame analysis, it is still necessary to limit final design to a series of plane frames.

Typical general purpose 3D structural analysis/design software sizes each member in isolation without considering the practicalities of fabrication and construction. Using such software the designer has to make decisions regarding the practicality of member sizes. Another issue when using 3D software is that loads are not applied to every single member, even though all members need designing for a given load. The reason is that the loads are not expected to be applied to every member at the same time, therefore require one load case for each load and for each member. Doing this would produce hundreds of load cases, which is not necessary if the structure is divided into assemblies and members of common types (eg. typical frame, end frame, which are further divided into rafters, columns). Such group technology families are not only important to manufacturing but also important to automating the process of  structural design.

Standard sheds and canopies typically adopt a 3m frame spacing, and such spacing is not compatible with roller doors approaching 6m width, hence columns are removed. By increasing the frame spacing, the removal of columns can be avoided along with the consequent need for another structural model. To accommodate higher loads on such frames, c-sections can be placed back-to-back to create I-sections. Where an I-section with a 6m load width is near equivalent to a c-section with a 3m load width.

The spreadsheet provides a single structural model with a large number of parameters: by suitable adjustment of parameters it is possible to avoid the need for more structural models. In anycase the design of a product configurator requires determining the variety of structural models required: this can be investigated by looking at product structure trees: if only adjusting quantities then a single structural model is suitable, if adding or deleting branches then more than one structural model is required.

References

The source of the Kleinlogel formulae used for the frame analysis are the following two books:

  1. British Iron and Steel Federation(1967),The Steel Designers Manual (3rd ED), Crosby Lockwood and Son Ltd
  2. Owens, G.W and Knowles P.R(1996), Steel Designers Manual , 5th edition, The Steel Construction Institute, Blackwell Science

Other useful books for structural design can be found in the Bibliography


Revisions:

  1. [02/02/2016] : Original
  2. [27/01/2016] : Expanded Information