In Australia, most sheds up to clear span of 24 metres are fully made from roll formed materials plus some custom made brackets to bolt the portal frame together. The roll formed frames come straight off the roll forming machine, cut to the exact length required with holes punched in the exact positions specified. The frame materials are already galvanised and no further painting or coatings are required (in the standard non-corrosive setting).
This is much more efficient than old way of having the portal frames fabricated. The fabrication method method, where hot rolled steel frames are cut to size and welded, has the extra costs of wastage, a high labour component, and then it has to make an extra trip for galvanising and painting factories if the frame is not to be treated and painted on the building site. This increases the lead time and the cost.
These days there is really no "standard shed size" because all of the materials are order and cut to the precise length for your specific shed.
There is also no such thing as a standard site. Each building site needs to be individually assessed to determine the wind speed that the shed or garage needs to be designed for. Factors such as the wind region, the surrounding terrain and other buildings, position on a slope and gradient of the slope, and if applicable, the potential snow coverage; all need to be taken into account. The shed frames are specifically engineered for a specific wind load and snow load. See windspeedcalculator.com.au.
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| Fig 1 Shed frame depicting columns, rafters, purlins and girts |
For a standard gable roof shed, each portal frame consists of two columns and two rafters. See Fig 1 where there a three portal frames.
The portal frames are connected together with purlins on the roof and girts on the walls. The portal frames in Fig 1 are very simple. They do not use knee braces or an apex brace.
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| Fig 2 Portal frame with knee brace |
Sometimes, for larger spans, it becomes cost effective to use knee braces. This can be beneficial in certain cases where the knee brace may strengthen a weak point. Because the brackets for knee braces are quite expensive, with smaller span sheds it is usually less expensive to use a bigger frame size to fix a frame that fails. With the larger span sheds, where the increment in cost to the next frame size is larger, it may be less expensive to add knee braces to strengthen.
The effect of a knee brace on the portal frame is quite complex. The length and angle affects its performance.
It takes an experienced shed designer, with good flexible shed engineering software tools to work out the best design for your portal frames.
The first step to getting the most cost effective shed frames is to determine the size of the shed you need, and the wind load and snow load for your site where you intend to build your shed. You can use this online shed designer to prepare your shed design. You will need to provide a valid email address and login to use most of the features to customise your design, but it is worth it.
