Tensile structures are extremely rare in traditional architecture, because the most common traditional materials of that architecture - stone, brick, and mud - have almost no tensile strength. Timber is the exception, but even timber is relatively weak in tension, and has traditionally been used either under pure compression (posts and columns) or under bending (beams, lintels, joists, and rafters), where the tensile strength is a component of bending strength. Timber trusses, which contain members in pure tension, are a relatively recent invention, and rely on quality metal fixings at the nodes. Large-scale tensile structures didn’t become feasible in architecture until the appearance of quality steel, in the form of truss members and cables.
Traditional and even contemporary buildings are almost all compressive, and all extant ancient buildings are too, because only compressive structures endure. The pyramids of Egypt are the standout example, and owe their extreme longevity to the maximal stability of their form- a pyramid is really just an organised pile of rocks, tapering from base to apex, at or near the angle of repose.
In contrast to the inherent stability of compression structures, tensile structures are inherently unstable; or rather they have a dynamic stability, which is part of their aesthetic appeal. They are less suited to fixed buildings than they are to portable or ‘velocity’ structures where lightness is important- think sailboats, early aeroplanes, bows, and bicycles.
There is one traditional architectural tensile structure, however, that shares the same attributes as these non-architectural tensile structures: the tent. Indeed, tension and tent (and many other English words like tendon and tendril) come to us from the same root: the proto-Indo-European ten-, meaning ‘to stretch’. Tents make use of the few ‘contrary’ traditional building materials that excel in tension but have no compressive strength - fabric, hide, cord, and rope - in combination with economical use of compression elements such as poles.
Perhaps because of their ephemeral nature, tents get far less attention in the architectural world than the more solid and long-lasting building typologies constructed in stone or brick or even timber. Probably the best-known vernacular tent type is the ‘black tent’, whose traditional distribution stretches from North Africa to Central Asia; tents of this type are still in common use by the nomadic groups of these areas. The more famous Mongolian ger and Central Asian yurt are arguably not true tents, at least not in the structural sense, since the ‘skin’ is just a veneer hung over timber lattice walls and a timber rafter roof.
Black tents are perfectly suited to the hot, dry, windy desert climates in which they are found. Black may seem an ill-advised choice of colour, but it gives the fabric greater longevity against UV radiation, and also serves to create a vertical temperature differential in the tent, drawing air through it from the bottom to the ridge. The density of the fabric weave means that under normal conditions, air (but not sand) can pass through the fabric itself; on the rare occasion that it rains, the threads swell up to become more watertight. The walls of the tent can be opened or closed depending on the conditions, and the low-slung form allows the tent to stand against ferocious wind and sand storms.
The traditional method of repairing the fabric of the black tent is somewhat poetic and almost biological: when the strips of fabric closest to the ground are frayed beyond repair by wind and sand, they are removed, but not directly replaced; instead, the two sides of the tent are unstitched at the ridge, the two new strips are inserted there, and the two sides are stitched back together, so that each strip, newer than the strip below it, moves down one position in the wall, until it eventually reaches the bottom and is removed in its turn.
