gifFace Strut Tetragon Simple Mitre

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The strut width has been deliberatelty increased to show the geometry

Mitred Struts have been designed to facilate modular construction in wood. For example, cut a face panel from marine ply, attach the half struts, then slot the planel into place. The half-struts are bolted to their adjoining partners in the adjacent faces.

Each top half of a mitred strut lies flush against the inner face, and along the dihedral edge. The angle Sigma provides the cutting angle to ensure that the top plane of the strut sits flat against the face whilst both sides of the strut remain parallel with the dihedral bisector. All strut faces are guaranteed coplanar.

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1. Exterior view: half a mitred strut on the inside edge of a face.

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2. Interior profile: half a mitred strut, looking down a face edge.

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3. Angle sigma is the cutting angle.

Note: the width dimension entered in the Struts window is the total width of two half struts. Hence the width of each member U is half the actual width W.

The mitre area is shown in translucent green:

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You can adjust the strut profile and interior appearance of mitres by clicking the Options button [...] at the right of the Type combo. This brings up the ‘Strut Options’ window. The contents of this window will vary depending on the strut type. Here are the options for mitred and simple box struts:

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Click the counter to change the section profile. The possible sections are:

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Strut profiles with inner dihedral planes (e.g. type 2), allow for the attachment of face panels both inside and outside the dome; the void can then be filled with insulation behind a vapour control layer.

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Profile types 3 and 4 can be used for the manufacture of PPT moulds; the struts are fixed to the outside of the subdivided PPT; the interior panels are then treated with a release agent (such as vaseline or talc) and coated in fibre-glass. When the mould is removed, the fibre-glass panels can be used to build the skin of a dome, with the joins forming along the PPT edges. Profile types 3 and 4 build mitres on the exterior of the dome.

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Above: a single PPT mould utilizing strut profile 3. Note the PPT edges: only half-struts are required (the other halves belong to the adjacent PPT’s). The fibre-glass is applied to the reverse (smooth) side. Another interior mould can be constructed for the fibre-glass to wrap around the inner PPT edge; this provides a bolting suface to join PPT’s.

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A view inside a dome showing mitred struts fixed to the interior face panels. The vertex marked A can be moved towards the hub normal by changing the radius of the hub normal cylinder; this is an imaginary cylinder whose central axis is aligned along the hub normal. Moving vertex A towards the hub normal creates a star shape; whilst this offers no structural advantage, it has an asthetic value. Vertex A cannot lie on the hub normal itself or the mitre planes will intersect each other.

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Cylinder intersect OFF

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Cylinder intersect ON; radius = 25

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Cylinder intersect ON; radius = 10

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Cylinder intersect ON; radius = 5

Profile type 2: the interior strut width is guaranteed to be the same as the exterior strut width, ie the sides of the strut remain parallel.

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The strut width has been deliberatelty increased to show the geometry.

The strut construction method accounts for non-coplanar cells. With such cells, if a strut is simply built from one edge end to another, the strut planes will also be non-coplanar. The solution is to build a mid-section of the strut and extrude it to both ends; intersection points with adjacent extrusion planes are then computed. This ensures that the strut geometry remains true, even with non-coplanar cells:

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The strut width has been deliberatelty increased to show the geometry.

Strut generation will halt with a “plane intersection error” if your struts are too wide or too deep for the given subdivision frequency. In this case, change the strut width or depth and try again.