Structural Design Steel Shed or Shelter

Price: The fee quoted is maximum. Contact us for a quote to match your specific needs. The fee is a maximum so there are limitations on what we supply. It is most suitable for agricultural and industrial buildings which are empty shells. Larger projects are possible, but those projects need to be broken into self-contained stages, no more than $5500 in cost, and each stage needs to be paid, before progressing to the next.

Structural design of steel shed or shelter, includes structural drawings and calculations (commonly referred to as engineering).

A shed has four enclosed walls, whilst a shelter can have one or more of the four walls open, including all walls open and becoming a canopy.

Some Design Issues to Consider

There is no span limit however as structures get taller and wider, they start to exceed the capabilities of a 610UB125, at which point either need to use castellated sections, use welded plate girders, use trusses, or space frames. For example the upper limit for cold-formed steel c-sections is typically 18m, using rigid moment frames, higher spans may be possible using knee braced frames.

It is also to be noted that at spans less than UB’s are suitable for, it is typically more economical to adopt trussed roof construction. The web of a UB is largely waste steel, its just there to keep the flanges apart. Basically for bending we want a pair of flanges, for which the section modulus is approximately Z=DBT, where D=depth between flanges, B=breadth of flange and T=thickness of flange. Castellated sections cut the section depth in half, and then offset and weld back together to create a section approximately twice the depth, with hexagonal holes in the web. Modern cellform sections achieve the same objective but with circular holes in the web. We get roughly double the strength without increase in weight of steel, but there is more fabrication involved. A truss fabricated from SHS, RHS, CHS, EA, UA or c-section is likely to be lighter.

Note whilst for some agricultural and industrial buildings UC, SHS columns maybe preferred, this shouldn’t preclude the use of light weight c-sections for the roof. If the issue is bumping into the posts then should have wider spaced columns, and properly protect with crash barriers and buffers. The crash barriers are likely cold-formed steel. Also can use crash barriers to constrain turning until vehicles are clear of the building, so as to minimise the width of door opening required.

Note the ADR limit vehicles to 2.5m wide and 4.3 m high. Roller doors are likely to add an extra 600mm of height, and bridge cranes another 1.8m for corbel supports and the bridge beam and travelling crab. Industrial racking needs to fit between columns, though preferable for fire protection to keep storage away from the walls and place circulation aisle at the walls. Depending on height racking also needs to fit between the roof trusses.

With multi-span buildings the internal columns can be simple props, and be spaced further apart. For example with cold-formed steel double span sheds we have had frames at 3m centres, but internal posts at 6m centres, creating greater internal space, better suited for sheds used for small boat manufacture for example.

Cold-formed steel sheds make economical building envelopes. Other loads can be supported on additional framing. Its not necessarily economical to support crane loads on building structures, so even if designed from the start for cranes, the crane runway beams may be supported on their own columns, but braced against the building columns.

For cold-formed steel sheds its common practice to remove columns to create larger doorways, this requires provision of eaves beams and ridge beams. Putting cross-bracing in the roof plane works in theory, but experience typically shows that it doesn’t work in practice. The ridge beams also make the steel erectors working on the structure feel safer.

It is generally preferable to set the frames to match the door openings. So if want 5m wide door openings, then around 5.5m frame centres is preferable to 3m centres. Large openings may require deep beams or trusses.

Fixed base portals require large footings, whilst pinned base portals require larger steel frames.

Uniform sections tend to be wasteful of steel. So optimum design of portals involves tapered steel sections. These are typically only available from specialist steel building fabricators: who fabricate the sections from welded steel flat bar. These are typically pinned based portals.

Another option is to use haunches. The haunches are fabricated by cutting a length of section used for the rafters in half using a diagonal cut producing a triangular section equal to the depth of the section. This is typically welded to the ends of the rafters, at the knee and possibly the ridge. This typically also done by specialist steel fabricators.

It is preferable that the knee connection has a depth of around twice the depth of the rafter, this is traditionally achieved by haunches. If haunches cannot be provided then the next alternative is an extended flange plate with a central gusset to stiffen the plate.

In Australia cold-formed steel typically has a strength of fy=450MPa whilst hotrolled steel (UB’s etc) have a strength of fy=300MPa. However whilst the c-sections extra material strength can provide for lighter structures, this is only the case where strength is the critical factor for design. If stiffness or deflection is the controlling factor then both cold-formed and hotrolled steel have the same Youngs modulus or elastic modulus (E=200GPa). Where deflection is an issue, selection is controlled by E.Ixx rather than fy.Zxx, to reduce deflection need to increase Ixx and to do that need more material or need to increase depth between flanges or truss chords.

Skillion roof sheds have a greater tendency to sag than the equivalent span gable roof. Thus they typically require deeper structural sections or trusses than required by a gable roof. Avoid skillion roofs if looking for a visibly flat ceiling, or at least make sure identify the concern.

Notes on Buying a Shed

Even if buying a cold-formed steel shed from one of the many suppliers, it is still advisable to get building design drawings drawn up first. These drawings should include site plan, and the floor plan and elevations, a typical section and clear identification of the dimensions wanted.

The widths of sheds, as I have typically designed for cold-formed manufacturers and based on the designs they’ve had since 1985, have measured from the outside face of girts. Heights are also measured ignoring the thickness of cladding. So dimensions are at the envelope formed at the outer face of girts and purlins. The manufacturers designs may also have been originally in feet, so for example a 20ft shed which is approximately 6.1m, will be classed as a 6m shed. Fine if putting in the middle of no where, not fine if got limited space. Whilst others will have a 6m shed, and declare it to be 20ft, in which case loose internal space. So need to know cladding thickness and actual dimensions of building.

Just because the shed supplier is using a computer and got flashy 3D images, doesn’t mean they are actually carrying out the structural design for your shed, or determining actual cutting dimensions for your specific shed. The software may be just looking up standard designs. So at  the minimum draw up some freehand sketches of what you want to be clearer about what you want. Don’t rely on verbal description and carefully read any drawings you get back.

If shed supplier won’t provide documentation before you buy, due to their fears you will use to get a cheaper quote elsewhere, then there you have a good reason to get drawn up first. Your drawings define the building you want, if their computer generated drawings don’t match, then they are not supplying the building you asked for, and they need to make modifications.