A-frame

An A-frame is a simple structure which resembles the capital letter A, where two lengths lie at an angle meeting at the top and a central brace ties them together. Where self standing the frame may have three angle lengths meeting at the top, so the sides form a triangular pyramid or tetrahedron with equilateral triangles on each side, or four length forming a pyramid, with a tie between each.

The structure can be seen at many different scales, including early tipi or tent shaped shelters covered in hides, structures over wells, or simple sawhorses with two A-frames with rigid connections to a horizontal beam. Most modern day timber roof structures have a tie beam and two rafters that form an A-frame which is tied to other A-frames via a ridge beam and purlins forming the roof structure.

At a larger scale, the Eifell tower although with curved angle lengths to resist wind loads, represents in its simplest for an A-frame, whilst the more contemporary London eye is supported by a single frame base, with a series of wires acting as the ties.

A similar structure might be referred to in English as a trestle, which in its simplest form is a similar framework consisting of a horizontal beam supported by two pairs of sloping legs, used to support a flat surface such as a table top. In the case of trestle tables the two pairs of sloping legs may make up a flat frame joined together at the top, often with a hinge and with a tie at the bottom stoping the supports from spreading and collapsing, in combination this makes up an A frame.

[edit] Related articles on Designing Buildings

• A-frame house.
• A history of timber framing
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• Span.
• Structure.
• Structural principles.
• Superstructure.
• The history of timber construction in the UK
• Timber shell structure.
• Types of beam.
• Types of construction.
• Types of structural load.
• Types of structure.