Choosing between options
Each arch, vault and cupola has specific properties and technical and architectural capabilities. Therefore, it is important to have a good understanding of the various "families" of building systems and the architectural possibilities which follow on from these, in order to be able to make a relevant choice between options. Such choices, indeed, have a very great influence on the acceptance and the appropriation of the technology. Therefore, attention has to be taken in order to meet all the different facets of the needs and constraints of a construction.
The various forms of arches, vaults and cupolas are closely linked to one or more building methods. They are more or less inexpensive, depending on how much specialized equipment is needed and how difficult the construction work is. The issue of scaffolding is also an important factor and can determine the preferred dimensions, notably for the height of the arch, vault or cupola.
The curved forms generated by arches, vaults and cupolas define free variable heights which increase with the span and the rise. Depending on requirements, one should first determine the curvature of the element and the height of its spring-point. The ability to move between one space and another also makes it important to take account of the ability to open out between the interior and the exterior of a vault or of a cupola. One can distinguish between open, semi-open and closed types.
Building systems using arch, vault and cupola structures can range from the simplest to the most complex.
The main distinction to be made is between modules which are independent from one another (whether connected or not) and sophisticated systems which respect very precise geometrical traces.
The sophisticated systems enable to get an optimum use of the material and perform very well from the point of view of costs. On the other hand, their replicability can be tricky as a high degree of precision at the laying out stage is required.
As a general rule, the more the shapes of the vaults or the cupolas are visible, the cheaper the building. Certain models permit a simple and inexpensive transition to net or sloped roofs.
Building design: from the simplest to the most complex
Arches, vaults and cupolas offer a very great number of architectural possibilities with a more or less typical and attractive character. These depend on the materials and models used, the methods of utilization, and finally the building techniques which arc linked to them. The architectural aspect is one of the principle vectors of cultural and social acceptability of a technology.
Section of house in Sfax, Tunisia
Section of church, Valladolid, Spain
The rounded shapes, especially of cupolas, can result in a significant degree of reverberation which is an asset or a disadvantage depending on the intended use of the space. Simple measures, such as corbelled elements or suspended cloth, can be used to reduce reverberation.
Stability of arches, vaults and cupolas
To ensure stability, the form should be as close as possible to the line of force. In the case of vaults, for evenly distributed loads, the best form resembles that of an inverted suspended chain (catenary). This applies both to flatter and to slender forms.
The corresponding ideal cross-section of a cupola is somewhat different and can only be worked out graphically or mathematically.
Combining vaults and cupolas results in more complex issues of stability and allows much more flexibility in shape.
Stability of supports
Arches, vaults and cupolas are heavy roofing elements and they exert thrusts which increase with the degree of flatness, the span and the weight of the structure. These thrusts may be concentrated or dispersed. They have a tendency to push out the supporting bases (walls, piers and foundations) and must be taken into consideration. The most common solutions are: cancelling them out by juxtaposition, widening the wall, buttresses, and tie-beams or ring-beams.
Project for a 2 classroom block, Senegal
Details and finishings
Details and finishings can influence the choice of structure. Attention must be paid to:
- the bonding pattern(s), particularly for visible masonry,
- channelling and removal of rainwater,
- window frames.
Forms of "in versed chain "
Segmental arches exert greater thrusts than slender arches