Cable, Tensile & Membrane Structures

A cable is the simplest structure imaginable and, for pure spanning, the most efficient. Because it has no stiffness, it cannot carry bending or compression at all - it can only be pulled - and so it always takes the exact funicular shape of whatever load hangs from it, just as the chain did in the last lesson. Loading it evenly gives a parabola; loading it with its own weight gives a catenary; hang a point load and it makes a sharp V. The cable finds its shape automatically, and in that shape every particle is in pure tension.

This is why tension is so material-efficient. A compression member must be stout enough not to buckle - a slender column fails by bowing sideways long before its material is crushed - so compression always carries a size penalty. Tension has no such penalty: pulling on something only makes it straighter and more stable, so a tension member can be worked right up to the full strength of its material and made as slender as that strength allows. A high-strength steel cable a few centimetres thick can carry a load that would need a compression strut many times its bulk. Pound for pound, tension is the cheapest force to resist, which is why the longest spans humanity builds - the great suspension bridges - hang their decks from cables rather than propping them on arches.

The catch, and the theme of this whole lesson, is that a cable is only a tension member and utterly floppy in every other respect. It cannot resist a load that tries to push it the other way, and it changes shape the instant the load pattern changes. On its own a single hanging cable is unstable under anything but the one load it was shaped for - a gust from below will simply flip it. Everything clever about tensile structures is a way of dealing with that floppiness: giving the tension form enough stiffness to hold its shape under changing and reversing loads. There are two great strategies for doing so - weight, and pre-stress - and the rest of the lesson is really about them.

The most direct tensile building is the suspension roof, which does to a building what a suspension bridge does to a road: it hangs the roof from cables slung between masts or high supports. The main cables take the load in pure tension and sag to their funicular curve; the roof deck hangs from them on secondary hangers. Because the structure spanning the space is only cables, a suspension roof can cross vast distances - arenas, stadiums, hangars - with a fraction of the material a truss or beam would need. To keep such a light roof from lifting and fluttering in the wind, designers either weight it down (a heavy concrete deck), tie it down with a second set of opposing cables, or pre-stress it, as the next section describes.

The cable-stayed system is the suspension roof's stiffer cousin. Instead of hanging the deck from a draped main cable, straight cables (stays) run directly from the top of a mast down to points on the roof or deck, each stay carrying its portion of load in a straight line of tension back to the mast. Cable-stayed structures are stiffer and easier to erect than suspension ones for medium spans, which is why so many modern bridges and mast-supported roofs are cable-stayed; the tell-tale fan or harp of straight cables from a single pylon is their signature. In both systems the mast is the crucial compression member - it is pushed down hard by the sum of all the cable pulls - and it stands as the one part of a tension structure that is emphatically in compression.

Both systems share the same defining consequence: the cables pull inward and down on their supports with a huge horizontal force, and that pull has to go somewhere. In a suspension roof the main cables continue past the masts as backstays and are anchored into the ground; in a cable-stayed roof the mast leans against balancing stays. The masts, the backstays and above all the ground anchorage are not accessories - they are where the enormous tension of the roof is finally caught and held, and they frequently dominate the engineering and the cost of the whole structure.

A single hanging cable is stable only under the one load that matches its shape; change the load and it moves. The elegant cure is pre-stress: build tension into the structure before any external load arrives, so that it is already taut and holds its shape, and so that varying loads only increase or decrease that built-in tension rather than trying to reverse it. Pre-stress is the single idea that turns floppy tension elements into stiff, usable structures, and it appears in two main geometric forms.

The first is the double-cable or opposing-cable system: pair a downward-sagging load-bearing cable with an upward-arching stabilising cable and tie the two together with spreaders or a taut web, then tension them against each other. Now the assembly resists both downward load (taken by the sagging cable) and upward wind uplift (taken by the arching cable), and the mutual pre-tension keeps the whole thing rigid. This is the principle of the cable-truss and the bicycle-wheel roof, where an outer compression ring and an inner tension ring are laced together with pre-stressed spokes exactly like a bicycle wheel - one of the most elegant long-span roofs there is.

The second form is the anticlastic or saddle surface, and it is the key to fabric structures. An anticlastic surface curves in two opposite directions at once - like a saddle or a Pringle - sagging one way and arching the other. The two opposing curvatures pull against each other, so tensioning the surface pre-stresses it into a taut, stable membrane that resists both downward load (carried by the sagging curve) and uplift (carried by the arching curve) without ever going slack. This is why tensile fabric roofs are never simple domes or flat sheets - a synclastic (dome-like) or flat membrane would flap and invert - but always saddle shapes drawn between high points (masts) and low points (anchors). Frei Otto, the great pioneer of tensile architecture, found these minimal, doubly curved forms with soap-film and stocking models, letting surface tension compute the perfect pre-stressed shape - the tension world's exact equivalent of the hanging models that found compression shells.

When the tension surface is a continuous sheet rather than a net of cables, you have a membrane structure - a fabric roof. The classic materials are woven glass-fibre coated with PTFE (Teflon), a durable, self-cleaning, fire-resistant fabric good for permanent roofs, and polyester coated with PVC, cheaper and more flexible for shorter-life or demountable structures; both are typically translucent, giving a soft daylight glow. A third material, ETFE, is not a woven fabric but a thin transparent polymer film, used either as a single stretched layer or, more often, as inflated cushions of two or three layers kept plump by a trickle of air. ETFE cushions are extraordinarily light and highly transparent - the shimmering skins of the Beijing Water Cube and the Eden Project biomes - and being air-supported they resist load partly by their internal pressure. All these membranes are, structurally, pure tension surfaces: they must be pre-stressed into anticlastic shapes to stay stable, and they can carry no compression or bending whatsoever.

The great inversion of all tension structures is that wind uplift, not gravity, governs. A fabric roof may weigh only a couple of kilograms per square metre - so little that its own weight is almost irrelevant - which means the design is driven entirely by the wind trying to suck it upward and shake it. This is why pre-stress and double curvature are not optional but essential, why the fabric must be detailed to never go slack (a slack membrane flaps itself to destruction), and why IS 875 Part 3 (wind loads) and careful dynamic analysis sit at the very centre of tensile design. The featherweight that makes these roofs possible is also their chief vulnerability.

Above all, tension structures are ruled by the anchorage. Every cable and every membrane edge pulls, hard and permanently, and that pull must be caught and held against for the life of the building. The masts push their compression into foundations; the backstays and edge cables drag their tension into massive concrete anchor blocks, ground anchors or rock anchors that resist by sheer weight or by gripping the earth. It is a standing truth of tensile architecture that the visible roof is often the easy part and the invisible anchorage is the real structure - a graceful fabric canopy may sit on foundations far larger than anything a conventional building of the same size would need. Draw a beautiful tension form and you have, in the same stroke, committed to catching an enormous pull in the ground.

Tensile structures win decisively in one situation: large, permanent-enough, celebrated long spans where lightness is a virtue and a sculptural, translucent roof is wanted. Stadiums, arenas, airport forecourts, transport interchanges, exhibition pavilions and shade canopies are their natural home, because at large spans the material savings are enormous, the translucency gives free soft daylight, and the swooping doubly curved forms are genuinely beautiful - an honest expression of pure tension, the mirror image of the compression shells of the last lesson. For temporary and demountable structures - event tents, deployable shades - fabric is almost unrivalled, since it packs small, erects fast and weighs little.

The honest limits are equally clear. Tension structures are poor at resisting concentrated and reversing loads: you cannot casually hang heavy plant, walk on a membrane, or apply a big point load without local stiffening, and heavy snow or ponding water is a real hazard for flatter forms. They demand specialist design and fabrication - form-finding, patterning the fabric into flat cutting shapes that tension into the curved surface, and dynamic wind analysis are all specialist skills - and the membrane materials, though improving, have a finite life (PVC-polyester perhaps 15-20 years, PTFE-glass and ETFE much longer) and offer limited thermal insulation and acoustic control. They are rarely the right answer for a small, cellular, or heavily serviced building, where a conventional frame is simpler and cheaper.

The design attitude, then, mirrors the whole module. A tension structure is a form-active system that must be shaped to its forces - pre-stressed, doubly curved, and anchored - from the very first sketch, in close partnership with a specialist engineer; it cannot be bolted on to a form conceived as anything else. Chosen for the right span and occasion and designed with respect for wind uplift and the tyranny of the anchor, it gives architecture its lightest, most graceful, most efficient long spans - the natural conclusion to a module that began with the humble frame and ends with a roof that seems to float.