Portal frame

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Portal frame construction is a method of building and designing simple structures, primarily using steel or steel-reinforced precast concrete although they can also be constructed using laminated timber such as Glulam. The connections between the columns and the rafters are designed to be moment resistant, i.e. they can carry bending forces.

Because of these very strong and rigid joints some of the bending moment in the rafters is transferred to the columns. This means that the size of the rafters can be reduced or the span can be increased for the same size rafters. This makes portal frames a very efficient construction technique to use for wide span buildings.

Portal frame construction is therefore typically seen in warehouses, barns and other places where large, open spaces are required at low cost and a pitched roof is acceptable.

Generally portal frames are used for single storey buildings but they can be used for low rise buildings with several floors where they can be economic if the floors do not span right across the building (in these circumstances a skeleton frame, with internal columns, would be a more economic choice). A typical configuration might be where there is office space built against one wall of a warehouse.

Portal frames can be clad with all sorts of material but the most popular solution, for reasons of economy and speed, is some form of lightweight insulated metal cladding with cavity masonry work to the bottom 2m of the wall to provide security and impact resistance. The lightweight cladding would be carried on sheeting rails spanning between the the columns of the portal frames.

Several companies manufacture software to assist in the efficient design of portal frame structures to design codes such as BS 5950.