Portal Frame Steel Buildings For Maintaining Strength Over Wide Spans
If you are looking for a big, open-plan building – something like a warehouse, workshop or barn, for example – then a portal frame building might be just the answer to your needs.
When it comes to design, the starting point is usage. It may well be that the purpose of the building – the activity that will take place and the machinery that will be used – dictates the height and width of the structure.
If your main need is for an unobstructed floor area, allowing machinery and/or goods to move around unimpeded, then a portal frame building can allow for that. They offer incredible flexibility of space.
If you’re using it for manufacturing purposes, for example, then the headspace can allow for tall machinery to be strategically positioned. It’s even possible to incorporate a travelling crane; the rails can be attached to the columns and span the space at the height of the walls.
The skeleton of a portal frame building is a combination of vertical supports (columns) and horizontal beams (rafters). The strength comes from a combination of those elements and the connections between the two. These can be enhanced by use of haunches or deeper rafters. Additional bracing, secondary steelwork and cladding help spread the load and restrain the primary steelwork. It’s an efficient design for wide-span buildings and ensures a robust structure.
Portal frame buildings are most commonly single-storey, although low-rise versions – perhaps incorporating an internal or external mezzanine, maybe for office space – are also popular.
It almost goes without saying that you can have whatever doors or windows you need, where you need them – that sort of flexibility is a pretty standard benefit of a steel building.
Roofs are most commonly pitched, although other styles, including hipped, can be fitted. Mono-pitched roofs are generally only added if a smaller span is required or the new portal frame building is to be erected near to an existing building.
If the span is especially wide (in excess of 30 metres, say) and there’s no requirement to have an uninterrupted space below, then a propped portal frame can be used. With this, a series of braces, or props, are inserted below the beams at the highest point of the pitch, to offer additional support.
A tied portal frame is one where braces are fitted between the columns at the point where they connect with the rafters, to add strength. It might also be necessary to add vertical hangers that connect the braces to the beams, for additional support. The downside of this is the impact it has on headroom.
Rafters may also be curved, and cellular beams may be utilised either to reduce weight across large spans or purely for aesthetic reasons. Whatever the style of the roof, it needs to be sufficiently robust to withstand the effects of snow loads, high wind speeds and gusts.
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