Case Studies in Structural Design

Before the four buildings, here is the method, because it is the transferable skill. Reading a structure is a disciplined act of looking that any architect can practise, and it goes in three moves. First, name the one idea in a sentence. Resist listing every beam; find the single concept that organises the whole - 'a shell', 'a cantilever', 'an exoskeleton', 'a tube'. If you cannot say it in a sentence, you have not found it yet. The great structures always have one, because they were generated by it.

Second, trace the load path from the sky to the ground. Put your finger on the roof and ask, out loud, where each force goes next: this load enters the shell, flows as compression around its curve to the edge beam, down the perimeter columns, into the raft, into the soil. Every load must reach the ground, and reading a building means following that journey without a gap. Where the path does something clever or difficult - a transfer, a cantilever, a change of material - that is usually where the architecture is most interesting and where the structural idea is most exposed.

Third, ask what the idea costs and forbids. Every structural concept buys something and demands something in return. A shell buys a column-free span but forbids casual openings and demands the outward thrust be tied. A cantilever buys a floating edge but demands a back-span to hold it down and grows expensive fast. An exoskeleton buys open floors but demands the structure survive weather and fire on the outside. Naming the price is what turns admiration into understanding. Apply these three moves - name the idea, trace the load, price the idea - to the four buildings that follow, and to every building you meet afterwards.

The one idea: turn the building inside-out, hanging the structure and services on the outside so the floors inside are completely free. Renzo Piano and Richard Rogers, with the engineer Peter Rice of Arup, designed the Pompidou (1977) as a radical act of expression: the steel frame, the bracing, the ducts, the escalators and the pipes are all on the exterior, colour-coded and celebrated, leaving vast, uninterrupted, column-free floor plates within for a flexible cultural machine.

Trace the load and you meet the detail that makes it work: the gerberette. Each floor beam does not sit on the outer columns directly; instead it bears on the inner end of a short, tapering cast-steel rocker arm - the gerberette - which pivots on a pin at the main column. The floor pushes the inner end down, the gerberette levers about the column like a see-saw, and its outer end is held down by a slender vertical tension tie running the full height of the building to the foundation. This is a beautiful piece of honest structure: the heavy cast pieces are in bending and bearing, the slim ties are in pure tension, and you can read the entire force diagram just by looking. The perimeter columns and diagonal bracing then carry gravity and wind down to the ground, well clear of the glazed facade.

What the idea costs is instructive. Putting steel outdoors demands serious fire protection (the columns are water-filled to resist fire) and lifelong maintenance against weather; the servicing is generous to the point of extravagance; and the whole is only 'honest' because it was engineered to be, at real expense. But the reward is one of the most legible structures ever built - a building that teaches its own load path to anyone who stops to look, and proof that expression, done with an engineer of Rice's calibre, can be both radical architecture and rigorous structure.

The one idea: build a lotus from thin concrete shells, so the structure is the symbol and the symbol is the structure. Fariborz Sahba's Bahai House of Worship (1986), engineered by Flint and Neill, is composed of twenty-seven free-standing marble-clad 'petals' arranged in three layers around a central hall - and each petal is a doubly-curved reinforced-concrete shell. This is form-finding in the tradition of Candela and Torroja: a shell is thin because its double curvature lets it carry load in membrane action, as forces flowing within the surface (largely compression) rather than as bending, so a surface only a few hundred millimetres thick can span dramatically and stand free.

Trace the load and the elegance shows. Wind and gravity on each petal are carried as membrane forces down the curved surface to its base, where the petals meet and lean on one another and on the ring of arches and the podium beneath, which gather everything to the foundations. The doubly-curved geometry is not decoration; it is precisely what keeps the thin concrete in the compression it loves and out of the tension it fears - the same lesson as the masonry arch, scaled up into a modern shell and shaped like a flower. The result is a column-free worship hall seating over a thousand under a canopy that appears to have no visible means of support.

The price of the idea was buildable but severe: doubly-curved shell geometry is fiendishly hard to set out, form and pour, demanding extraordinary formwork, precise reinforcement and construction control - the project is celebrated as a feat of Indian construction as much as design. And a shell forbids the casual: you cannot punch a random opening in a working shell without disturbing the flow of membrane forces. But the Lotus Temple stands as India's finest demonstration that a structural surface, form-found and honestly built, can be a piece of pure symbolism - the structure and the meaning are one curved thing.

The one idea: wrap the tower in a triangulated diagonal cage so the skin itself is the structure, and shape it aerodynamically so it barely disturbs the wind. Foster and Partners with Arup completed the Gherkin in 2004, and it is the building that made the diagrid famous. Instead of vertical perimeter columns plus separate bracing, the exterior is a lattice of steel diagonals meeting at nodes; because triangulation is inherently stiff, this single system carries both the gravity load and the wind load, and it does so with markedly less steel than a conventional framed tube of the same height. The floors span from this perimeter diagrid to a central core, leaving column-free office space in between.

Trace the load and the diagrid's logic is clear: gravity and lateral forces both resolve into axial tension and compression along the diagonals, spiralling down the triangulated tube to the base - the perimeter is doing the work of both column and brace at once, which is why the interior can be so open. The rounded, tapering form is not styling either: its aerodynamic profile reduces the wind loads and, importantly, calms the swirling downdraughts that tall flat facades throw onto the street, while the tapering top and base suited the site and the views. The lens-shaped light wells spiralling up the plan give natural ventilation and light - architecture and structure and environment resolved together.

The idea's cost lives in the nodes. Every point where diagonals cross is a complex, heavily loaded steel connection that must be fabricated and assembled with precision, and the diagrid geometry disciplines the whole facade - you accept the diamond grid as the building's face. But the Gherkin proves the diagrid's promise: a stiffer tall building using less material, with the structure become the unmistakable image of the building. It is the direct built embodiment of the tall-building ladder from the previous lesson.

The one idea: cut deep double-height garden terraces into a simple concrete tower by cantilevering the corners, so every flat gets shade, a view and Mumbai's sea breeze. Charles Correa designed Kanchanjunga (completed 1983) with the great structural engineer Mahendra Raj, and it is a masterclass in structure serving a climatic and spatial idea. The tower is, in essence, a reinforced-concrete structure with a central core and columns, but its signature is the way the corners are carved away into recessed, double-height, planted verandahs - a modern reinterpretation of the old bungalow verandah that protected colonial houses from sun and rain, lifted twenty-eight storeys into the air.

Trace the load and the discipline behind the drama appears. Those open corner terraces mean the enclosing structure must cantilever to create the sheltered voids without a column standing in the view, and cantilevers are demanding: a projecting slab or beam must be held down by an adequate back-span and carefully reinforced for the tension on its top face, all resolved back into the core and the column grid that runs to the foundation. Mahendra Raj's engineering made Correa's carved-away corners stand up cleanly, which is exactly the architect-engineer partnership this module has argued for - the poetic idea (verandahs in the sky, orientated to catch the breeze and turn away from the harsh west sun) made real by rigorous structure.

The price of the idea is the price of every cantilever: it costs more material and more care than a load path that simply runs straight down, and it fixes where the heavy structure must be. But Kanchanjunga shows that expressed structure need not mean a shell or a diagrid shouting for attention; here the structural idea is quiet, in service of comfort and dwelling, and no less inseparable from the architecture for that. Remove the cantilevers and you lose the building. It stands, with the Lotus Temple, as proof that India's finest architecture has always been co-authored with its finest engineers - a lineage worth carrying forward.

Line the four up and the whole course is visible in them. The Pompidou expresses a steel frame turned inside-out; the Lotus Temple is a form-found concrete shell that carries load in its curved surface as the arch does; the Gherkin is a diagrid tube, a direct rung of the tall-building ladder; and Kanchanjunga is a cantilever in service of climate and dwelling. Two are unmistakably global, two unmistakably Indian, and all four share the one quality that defines great structural architecture: you could not remove the structure without destroying the building, because the structural idea is the architectural idea.

Notice, too, that every one of them was co-authored. Piano and Rogers had Peter Rice; Sahba had Flint and Neill; Foster had Arup; Correa had Mahendra Raj. This is the deepest lesson of the module and of the whole course: expressed, inseparable structure is never the work of an architect alone or an engineer alone, but of the two thinking together from the first idea. The buildings that read as inevitable were found, jointly, not styled and then propped.

So carry the three-move method with you - name the one idea, trace the load from sky to ground, price what the idea costs and forbids - and apply it relentlessly, to famous buildings and to the ordinary ones you pass every day. It will make you a sharper critic, a better collaborator with your engineer, and, above all, a designer who can make structure carry the idea rather than merely carry the loads. That is what it means to design as an architect in command of structure, which is where this course set out to take you.