Overview
It should be recognized that there are few significant differences between how we build today, compared to how our ancestors built several thousand years ago. While modern buildings may be bigger, taller, stronger and go up faster, they are not fundamentally different. Based on the physiological, psychological and social needs of humankind, it may be vigorously argued that the principles of building enclosure have not changed at all. But the cultural context of architecture has continued to evolve, and it is interesting to expose the common tectonic thread which binds past, present and future.

Traditionally, enclosures are either monolithic or composite assemblies. In monolithic enclosures, such as load-bearing masonry, a single material may act as the structure, the cladding (skin) and the interior finish. The control of heat, air and moisture flow that resulted was unintentional, or incidental. Composite assemblies generally assign critical control functions such as the control of heat transfer or air leakage, to separate materials, or combinations of materials.

The triumph of modern architectural science is the rejection of the incidental quality of environmental separation provided by traditional building materials and assemblies, in favour of a deliberate selection and arrangement of materials, according to their intended function in response to physical phenomena. However, it is important to appreciate that our scientific sophistication has not appreciably displaced enclosure tectonics.

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Enclosure Tectonics
Building enclosures may be simply classified by examining the tectonics related to their structure, cladding and interior finishes. Enclosures consist of: 0) natural formations, which are human adaptations rather than creations; 1) stacked units; 2) frames; 3) shells and plates; and 4) air-supported fabrics. Claddings and interior finishes involve: 1) wet coatings, 2) discrete units or panels; 3) fabrics, films, sheets, rolls; or 4) the incidental outcome of monolithic construction.

Stacked Units
It is difficult to ascertain whether or not stacked units preceded frame structures. If observations of children are any indication, it appears that stacked units are a more intuitive means of constructing a building enclosure. However, a roof structure using a frame assembly is often the only practical means of completing the enclosure.

Stacked units account for most of the classical and monumental architecture of ancient times, and many of the technical issues relating to Western traditions are well documented.

The Roman arch represents a triumph in the use of stacked units building technology.

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Frames
Frames, unlike stacked units, do not depend on the self-weight of the materials to provide structural integrity. Instead, the strength of the framing materials, and in particular their connections, provide the required strength and rigidity. Frame structures require some form of cladding, unlike stacked units which usually serve as the interior and exterior finish.

Examples of frame building enclosures.

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Shells and Folded Plates
Shells and folded plates are intrinsically different than stacked units and frames, as may be inferred from looking at the natural shell structures of sea life. The relatively thin diaphragm forming the shell is created (cultivated) uniformly rather than being assembled in discrete pieces - a luxury modern construction technology has not yet afforded architects. Most shell and plate building enclosures are constructed from reinforced concrete, however, moulded plastics and metals may be used for smaller applications. New materials may further advance the possibilities for this type of enclosure.

Examples of various shell and folded plate enclosures.

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Air-Supported
Air supported enclosures owe much to their aviation predecessors, the hot air balloon and the dirigible. These represent relatively recent innovations in building enclosures, ranging in scale from sporting facilities (e.g., tennis courts) to the roof structures of stadiums.

From optimistic delight to horror and tragedy, the introduction of hydrogen virtually halted the development of air-supported enclosures until recently.

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Hybrids
Most buildings are hybrid enclosures, utilizing several of the typical approaches previously presented. There are many reasons for this mixing of typologies, the most common being fitness for intended purpose, buildability, appearance and cost. Historically, the availability of materials and skilled labour also factored into the decision-making criteria. When dealing with hybrids it is advisable to review the critical considerations pertaining to each basic enclosure typology as outlined in the table below.

Attached to each typology are a host of materials and a tradition of techniques, many of which remain practically the same as their original predecessors. Advances in materials and techniques accelerated after the Scientific Revolution when design (architecture) began separating from craft (construction), however, many forms of construction remain historically intact (e.g., stone masonry, plastering, heavy timber, etc.), and are merely applied to different forms and styles. The process of transition from generalist to specialist, once scientific thinking is introduced, has been witnessed in other technologies, albeit in an accelerated time frame. Aviation since the time of the Wright brothers, and microcomputers from the development of personal computers are two notable examples. The fundamental architecture of these technologies remain largely unchanged, but the complexity and sophistication have demanded specialization, and a fractionation of the disciplinary knowledge base.

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Enclosure Finishes
Implicit in both pure enclosure typologies and hybrids are interior and exterior finishes. The application of interior and exterior finishes varies depending on the type of building enclosure which is selected. Aside from the skins, furs, grass mats and mud used by primitive man to make these abodes comfortable, natural formations have their own integral finishes. But all of the artificial building enclosures imply some type of interior and exterior finishes, as outlined in the table below.

Based on previous discussions, it may be concluded that frame structures demand the most careful consideration of interior and exterior finishes, since the voids between structural members must be appropriately infilled to provide critical environmental control functions, or preferably clad over to avoid performance problems. For all of the other types of enclosures, the finishes are either monolithic, or layered over the entire surface of stacked units, plates or shells. Perhaps the one anomaly in this discussion involves cast-in-place concrete, which can be used to create frames, plates and shells. It appears to be the most ambivalent material and, not surprisingly, requires special design, detailing and construction techniques.

It is reasonable to assume that few fundamental innovations in building enclosures are likely to be developed in the future, at least in terms of the techniques of construction. This should not be misinterpreted as implying that one person can learn all there is to know about building enclosures.

Most modern buildings involve hybrid arrangements of basic building techniques that have not fundamentally changed since the time we abandoned trees and caves to fashion artificial shelter. The next challenge is to encode new science within inherited tectonics, so that having mastered the compulsory elements, we are free to explore artistic expression within a broader canvas of appropriate ecological adaptation. The section which follows on Limit States Design deals with some of the latest thinking on the scientific design of building enclosures.

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