The basic procedure for shed design involves assessing the loads imposed by the environment, determining the effects of those loads on the frame and its connections and then specifying such so that have adequate resistance. {what follows is a rough outline and subject to revision.}

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Environmental Assessment

Environmental considerations include, rain, flood waters, hail, snow, wind, sun, heat, and earthquake. For structural design and Adelaide, and otherwise around South Australia the main consideration is wind loading.

To make the assessment typically require the site address and need to make reference to maps, such as:

1) Adelaide UBD street directory
2) Atlas of SA {Now Location SA Map Viewer}
3) Adelaide Metropolitan Area Wind speed map {highlight and search, its location keeps changing}
4) Google Earth

These maps are so that can assess the terrain and topography and the resultant influence on the magnitude of wind loading.

If the building is a residential shed then the above maps are probably adequate, however if the building is a light industrial shed, then it is also preferable to also get the following:

1) Site survey and contour plan (spot heights to Australian Height Datum (AHD)
2) Soil bore logs

Loading Requirements
Loads are typically taken from the Australian loading code, which comprises the AS1170 series of Australian Standards. The main codes of concern are:

1) AS 1170.0 Structural Design Actions Part 0: General Principles
2) AS 1170.1 Structural Design Actions Part 1: Permanent, Imposed and other Actions
3) AS 1170.2 Structural Design Actions Part 2: Wind Actions

A spreadsheet for basic wind load calculations can be found here. Whilst a sample printout of the report  can be found here.

Steel structures are typically considered ductile, and earthquake loads typically low, so that wind loading is typically the more critical loading and magnitude of earthquake loads can be ignored. As for snow loading that is generally not an issue in Australia, however if building located in an area where it does snow then it’s important to advise.

From the wind loading code a site reference design pressure is determined traditionally referred to by the variable qzu, though this is no longer explicitly used in the code. This reference pressure is then used in combination with pressure coefficients to determine the wind pressures on each surface of the building.

Through calculation the loads given in the codes need transforming into the loads applied to each structural member. The loads on a given structural element may not always follow a typical action and reaction scenario. This is because some loads are magnified locally for small support areas, and the reactions from these loads do not have to be applied to the member which supports a larger area.

To save some calculation time these loads can be calculated using a spreadsheet, as for example:

Wind loads on gable frame to Australian wind code as1170.2 and Sample printout.

Design Action-Effects

Once the loads (actions) on each element of the frame have been determined then the action-effects of these loads on the frame have to be determined.

For a simple rigid frame this can be done using rigid frame formula which can be found in publications like:

  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
  3. Kleinlogel (1958), Rigid Frame Formulas, Frederick Ungar Publishing

The newest version of  the steel designers manual can be found at Construction Steel Institute website, or amazon.com.

Whilst extracts of what can be found in Kleinlogel are available here.

If the frame isn’t simple as may be the case if columns are removed to provide larger openings, then frame analysis may be required. Here we use either:

1) MicroStran
2) Multiframe

Member Design

The design or specification of the members is dependent on selecting structural sections with resistance greater than the design action-effects. The maximum resistance which can be permitted for a given material is controlled by Australian Standard materials codes. Shed frames can be made from hot-rolled steel, cold-formed steel, timber or reinforced concrete. The materials typically used in South Australia are hot-rolled steel and cold-formed steel for which the codes are:

1) AS4100 Steel Structures.
2) AS4600 Cold-Formed Steel Structures Code

Some simple calculators for assessment are available from ExcelCalcs:

  1.  Hot Rolled Steel to AS4100
  2.  Cold-formed Steel to As4600

Theses spreadsheets are also available from the current website: Steel design, and cold-formed steel design.

Connection Design

Connection design is not fully covered by Australian Standards. Whilst connection design can be carried out by calculation using some basic theories and by reference to overseas standards (UK, USA, Europe), it is not always practical or possible to design and/or assess a connection by calculation and so physical testing is required. For one-off construction physical testing is not generally practical and therefore connections need to take standard forms which have been tested by industry associations for the benefit of the whole industry, rather than connections which are proprietary and tested for the sole use of a single fabricator. It is the design of the connections which may make a DIY shed impractical.

We typically design the connections with reference to the CSI steel designers manual and the old permissible stress version of Australian connections manual.

New versions of the Australian connections manual to limit state codes are available here:

Structural steel connections series – Rigid connections suite – Set of 5 books

The methods in these references need to be adapted for use with cold-formed steel. For a bolted end plate connection at the knee, it is important to check the bending of the column flange, and the compression and shear buckling of the column web and strengthen appropriately. Having strengthened the column head, it is apparent from testing that the flanges of the rafter also need to be strengthened for compression.

Many of the current sheds in the marketplace have large gusset plate connections, similar to those found in timber portal frame construction.

Footing Design

Cold-formed steel sheds are typically designed to have rigid base connections: that is connections which resist rotation. Achieving zero rotation required for full rigidity is impossible, as all materials deflect to some extent under load. However for practical purposes and small buildings and/or transient loads such as wind, the base connections may be considered fully fixed. To achieve this fixity deep concrete piers are bored, and the columns are embedded directly into these piers, or otherwise connected using anchor bolts.

Girts and Purlins

Girts go around the girth of the building and support the wall cladding. Purlins are placed on the roof and support the roof cladding. Girts and purlins are typically cold-formed sections and selected from manufacturers load capacity tables rather than direct calculation to AS4600.

Common suppliers in South Australia are:

1) Lysaght 
2) Stramit 
3) Fielders
4) Stratco

NB: These tables are typically not used for sizing the portal frame, as they only consider bending of the structural sections. AS4600 needs to be used to check the capacity of the sections for combined bending and axial forces.

Gable End Walls

End walls can be made from a pinned and braced frame rather than a rigid portal frame. From a production viewpoint doing so is not entirely sensible. If the end walls are made from different framing then, that is 2 frames in the entire structure which vary from all other frames. That is 2 frames which have the potential to be mismatched with all the other frames. Making different end wall frames represents unwarranted variety in the fabrication of the frames and the construction of the building. The end frames represent additional workshop details which need to be produced to fabricate a different frame.The potential for error and delays caused by 2 frames is not worth the potential cost savings in material. Further whilst there may be cost savings in material, there are additional costs in detailing, and erection of such frames.

Additionally differing end frames also complicate any future extensions. It is therefore typically better to make the end wall frames the same as the main portal frames in the building. This being the case then the end wall columns can be assumed not to be carrying any axial load and can be designed as beams. These end wall mullions, can be designed either as simply supported beams, or propped cantilevers. For both cases the maximum moment in the mullion is the same, however, for the propped cantilever a smaller load is transferred to the roof structure.

Since these are beams they can potentially be designed using the purlin/girt load capacity tables, however here we typically check direct to AS4600.

Longitudinal Bracing

Wind blowing across the width of the building is typically resisted by the rigid connections in the portal frames. Wind blowing down the length of the building, is resisted by cross bracing placed in the walls and roof of the structure. It is preferable that end wall mullions align with struts in the roof of the building. But this is not always practical.

The bracing system primarily comprises of tension elements and compression struts, either checked against AS4100 or AS4600 depending on the materials used.

It is also to be noted that the bracing induces additional axial loads in the the portal frames, and also induce additional loads on the footings.

Checklist

The following is a simple checklist used in our original Quattro Pro spreadsheets for shed design.

checklist

 


Notes:

  1. Example Reports and Calculations
  2. Simple Application for Checking Frame Size
  3. Bundle of spreadsheets to aid with structural design

Revisions:

[16/09/2013] : Original
[15/05/2016] : Added link to spreadsheet bundle. Modified some links, and corrected some typing errors.
[09/04/2017]: Added headings, and reference to example reports.

 


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