When Neal Crankshaw, one of our regulars on the forum, asked for advice about creating a groin vault we figured that he had something a bit special in mind, and here it is, his Art Deco Mausoleum.
Also sometimes called a double barrel vault or cross vault, a groin vault is what you get at the intersection of two arched or rounded roofs, although the term is probably more commonly used to refer to the shape you see inside as opposed to the shape on the outside. What Neal needed to know was how to mark this out onto a flat sheet of card such that the pieces would fit together correctly when bent and fitted onto his model.
A number of approaches were proposed which ranged from trial and error to the purely mathematical. In the end however wwizzard and Kishkumen described a geometrical approach that has the added benefit of being able to cope with gothic arches as well as semi-circles, and any other shape you might like too.
The three images below were posted on the forum by Kishkumen to support an explanation given by wwizard.
In the first image we have a plan of the proposed roof (drawn to scale) with a circle below it to describe the semi-circular end section. Note that this could just as easily be an arch. The end view is then marked off at intervals. In this case the circle has been marked at 15� intervals but in the case of an arch it would probably be easier to just measure around the edge of the arch with a ruler. These points on the edge of the end view are then extended upwards to allow corresponding lines to be drawn onto the plan view.
The second image has been generated from information measured from the first. We start with the base line, whose length is half the diameter of the circle in the first image. If we were using an arch then it would represent the perimeter of the arch where it is to meet the roof. This baseline is then marked at the same measured intervals as the circle/arch in the first diagram. We can then draw the perpendicular lines by measuring the lengths of the equivalent lines on the plan view.
All that remains is to join up the ends of these lines with a curve. If you are in any doubts about the path of this curve, simple draw more lines on the first diagrams and transfer more measurements to the second until it becomes clear.
Note that Kishkumen has also added tabs to aid construction and in the he final image we see a 'mock-up' that he created to demonstrate that it really does work!
Armed with this information Neal set to work drawing the main pieces of his building onto foamcore and cutting them out as shown in the image above.
In the image to the left, the pieces have been assembled with cardboard strips on the roof to serve as decoration as well as covering up those rather tricky joints at the intersections.
An additional layer of card with slightly different cutouts has also been applied to the faces of the foamcore in order to create panels however the lower area of the building is still to be enclosed (see final image below) with panelled corner pieces.
Prior to painting, Neal also added steps at the base and a handrail around the upper level. Well we wouldn't want any of the snipers falling off and hurting themselves would we?
