by Angus J. MacDonald
When is a span a long span? One answer is: when, as a consequence of the size of the span, technical considerations are placed so high on the list of architectural priorities that they significantly affect the aesthetic treatment of the building.
The technical problem posed by the long span is that of maintaining a reasonable balance between the load carried and the self-weight of the structure. The forms of longest-span structures are therefore those of the most efficient structure types. In the pre-industrial age, the structural form that was used for the widest spans was the masonry vault or dome.
The development of reinforced concrete in the late 19th century allowed the extension of the maximum span possible with the compressive form-active* type of structure. Reinforced concrete has a number of advantages over masonry, the principal one being its capability to resist tension as well as compression and its consequent ability to resist bending.
Because of this ability, compressive form-active structures in reinforced concrete can be made very much thinner than those in masonry. This allows greater efficiency, and therefore greater spans, to be achieved because the principal load on a dome or vault is the weight of the structure itself.
Another advantage of reinforced concrete is that it makes easier the adoption of "improved" cross-sections. This technique has been used with masonry domes, however, the twin skins of the dome by Filippo Brunelleschi for Florence Cathedral (1420-36) being an example. >>>
* "Form-active" refers to a structural element, such as a column or arch, in which the shape of the longitudinal axis, in relation to the pattern of applied load, is such that the internal force is axial. "Non-form-active" elements, like beams, are subjected to bending stresses only. "Semi-form-active" refers to elements that combine bending and axial stress.
This article is excerpted from Structure and Architecture, Second Edition by Angus J. MacDonald, with permission of the publisher, Architectural Press, Inc.
Airship hangars at Orly Airport, France (1921) by Eugène Freyssinet, structural engineer. The skin has a corrugated cross section that allows efficient resistance to secondary bending moment.
Photo: Courtesy Architectural Press
The dome by Filippo Brunelleschi for the Florence Cathedral is a semi-form-active structure.
Image: R. J. Mainstone
Click on thumbnail images
to view full-size pictures.