30 St Mary Axe (Gherkin) Case Study: Diagrid, Plan and Light Wells (Foster + Partners)

30 St Mary Axe, the London tower everyone calls the Gherkin, is a 40-storey office building by Foster + Partners with Arup as structural engineer, built from 2001 to 2003 for the insurer Swiss Re. It teaches three things a studio jury will ask about: how a perimeter diagrid lets the structure and the facade work as one, how a round plan cut by spiralling light wells brings daylight deep into the floor, and how far its natural ventilation and energy claims held up once tenants moved in. Below: brief, site, plan, structure, facade, the record on energy, and lessons for India.

The brief and the client

Swiss Re began developing proposals in 1998. Arup's Dominic Munro, writing in New Steel Construction, says the client wanted an environmentally progressive building with a high standard of working environment, on a site in the heart of the City's insurance district. The building was to be Swiss Re's London headquarters, with a public plaza and shops at its foot. The City of London describes it as "London's first ecological tall building", the phrase the building's own site uses, and lists 46,400 m² of net office space plus an arcade of shops and cafés off a new piazza.

The building services engineer, now Tyréns Hilson Moran, calls it Britain's first mixed-mode high-rise office: a tower meant to switch between opening windows and mechanical air. The firm says it designed the shell and core and the fit-out of Swiss Re's own floors, using computational fluid dynamics and dynamic thermal modelling to size the systems.

Site and context: the Baltic Exchange

The site carries a recent history. On the night of 10 April 1992 an IRA bomb exploded outside the Baltic Exchange, the shipping exchange at 30 St Mary Axe. Royal Museums Greenwich records that three people were killed and 91 injured. London Remembers says the blast partly demolished the 1903 exchange building by Smith and Wimble, and that it was later taken down, with much of it salvaged. The exchange's First World War memorial glass was restored over more than ten years and has been at the National Maritime Museum since 2005 (RMG).

Swiss Re was given the go-ahead for its tower in August 2000 (Post, 24 August 2000), and basement works began that December (Munro). Digging the site also turned up a Roman grave: Ian Mansfield reports a girl's skeleton found in 1995, with pottery dated to AD 350 to 400, reburied under a simple stone marker beside the tower.

The tower sits back from the street in its own plaza, among older, lower City blocks and the church of St Andrew Undershaft. Munro notes that because the convex surface curves away from the eye, the full height is not apparent from street level. For setting out a site study like this, see our guide to documenting a building for a case study.

Concept: one form for structure, light, air and wind

The design reads as one idea worked through several systems. The circular plan swells to its widest at level 17 and tapers to a glass dome. The ArchDaily project text, provided by the architects, says the aerodynamic shape was chosen to make the most of natural light and ventilation and so reduce energy use (ArchDaily, 2019). Munro, for the engineers, adds that the streamlined shape protects against a windy environment at street level. Neither source publishes wind-tunnel results, so treat the ground-level benefit as the design team's claim.

The City of London notes that the building has only one piece of curved glass, the lens at the very top. Everything else is flat panels arranged on the curve, which is what gives the facade its faceted, diamond look.

The Gherkin floor plan: six spokes and spiralling light wells

A typical floor, as Munro describes it, is a ring of office space in six "spokes" laid out on a 1.5 m grid around a circular core of lifts, stairs and services. Between the spokes, six triangular zones at the edge are left open as light wells. The key move is that each floor's light wells are offset by 5 degrees from the floor below, so the voids twist up the tower in spirals. That twist also makes a small balcony at each level and opens views up and down through the building.

The plan dimensions are tight by office standards. The deepest distance from core to glass is 14 m, and no point of office floor is more than 8.5 m from a light well (Munro). The plan trades area for light: Martin Spring, writing in Building (April 2008), calls it "not very spatially efficient", with only 63% of the gross area usable. Floors are 4.15 m from floor to floor.

Spring notes that the six light wells split each floor into six sections, which suited cellular offices, and that one law firm ran a private staircase up through a light well. The original drawings belong to Foster + Partners; for the plan and structure in the engineers' own words, read Munro's 2003 article.

The Gherkin structural system: the diagrid

The structure is a steel diagrid at the perimeter. Munro explains it this way: the perimeter columns lean to follow the helical path of the floors, and columns spiralling in both directions cross at nodes every two storeys, forming a balanced grid. Between nodes each member is straight, which is why the glass can be flat. Horizontal steel hoops at every node level tie the grid together. Near the top the hoops are in compression; in the middle and lower levels they are in significant tension, with a design tension of 7,116 kN at level 2.

Because the diagrid is a stiff triangulated shell, it carries all of the wind. The core only carries floor loads, so it could be an open steel frame rather than a heavy concrete shaft. Sway was limited to 50 mm over the full height, and member sizes were governed by strength. Column tubes range from 508 mm diameter and 40 mm thick at the base to 273 mm and 12.5 mm at levels 36 to 38 (Munro).

• Nodes: made as separate pieces with three intersecting planes. Compression passes through milled bearing faces (0.1 mm tolerance) and tension through bolted splices.
• Floors: radial steel beams on 10 degree lines carry a 160 mm composite slab on metal decking that spans up to 4.75 m, built without temporary props.
• Dome: 30 m across and rising 22 m above the top of the diagrid, a fully welded lattice of triangular sections. Its top piece, the "spider", went up in one lift in March 2003. The upper three levels from level 38 hold corporate rooms.
• Foundations: 333 straight-shafted piles, 750 mm in diameter, in London Clay.
• Steel: 8,358 tonnes in the 2003 account, of which 29% is diagrid, 24% core columns and 47% beams.

Construction

Steel fabrication began in July 2001 and steel reached site in October 2001. Munro describes a repeating cycle, worked in two-storey bands at about one band a fortnight: core steel, core decking, diagrid A-frames, radial beams and hoops, floor framing and decking, then concrete. The last diagrid frame, to level 38, went up in October 2002, plumb to within 10 mm over 160 m. Skanska says it delivered the building in under three years, on time and to budget.

The Gherkin double skin facade and materials used

The materials are few: a steel frame, composite concrete floors and glass. On the office floors the facade is a ventilated double skin. Spring describes three layers of glazing with a ventilated cavity. A 2005 report in Building explains that blinds in the cavity catch solar heat before it reaches the office, and that the heat can then be reclaimed or rejected depending on whether the floor needs heating or cooling. Munro adds that supply and exhaust air is ducted through plenum boxes connected to slotted facade transoms; beam webs are pierced for air only at the perimeter.

The Gherkin sustainability claims: light, air and energy

The environmental strategy was mixed-mode. Spring reports that windows were designed to open up to the 32nd floor when wind speed was below 10 mph and the outside temperature was within a set band, which the design team expected for up to 40% of the year. Fresh air would move through the spiralling light wells. The light wells, Spring writes, were designed to cut energy use by half.

That figure needs care. Foster + Partners presented the tower as using 50% less energy than a traditional prestige office building (quoted by Spring, 2008). This was a design-stage claim. In the same article, the owner's property services director, Richard Stead, said "40-50% energy savings are a bit over-ambitious", and Spring reported that the building's energy use had not yet been audited. We found no published, measured energy figure for the building.

Reception: awards, windows and tenants

The building won the RIBA Stirling Prize in 2004 (City of London). Its later record is more mixed, and is worth knowing for any viva.

• A falling panel, 2005. Building (2 June 2005) reported that a triangular piece of glazed metalwork fell from the 28th floor, and that about half of the vents in the double skin were sealed as a temporary measure. Insurance Times (12 May 2005) said about 350 of 700 windows had been permanently closed, while a spokesman said over half could still be opened.
• Tenants and natural ventilation, 2008. Spring found that once the building was let to 18 companies, most did not use the opening windows. Law, finance and insurance firms put up glass partitions for confidentiality, leaving fresh air nowhere to go, and open light wells carried sound between floors. Even Swiss Re limited opening after staff found the air stuffy.
• A refurbishment, 2024 onwards. Building Design (13 May 2024) reported that Foster + Partners was working with the owner, Bury Street Properties, on upgrading the offices to current energy expectations. Time Out (1 May 2024) listed new ventilation, LED lighting, treated glass, a reglazed dome and new curtain walling at the base.

What designers in India can take from it

This is our reading, not a claim from the sources.

1. Let the structure and the envelope share one geometry. The diagrid works because the plan, the floor beams, the glass panels and the nodes all follow the same two-storey module. Where the facade is designed after the frame, the two rarely meet cleanly. Even on a smaller Indian project, setting out the structural grid and the facade module together, at concept stage, saves the awkward junctions that appear when they are drawn separately.
2. Daylight depth is a planning number. The 8.5 m rule here is a useful check for any deep Indian office or institute plan. Courtyards and cut-outs bring light in, but in a hot climate a glazed void also collects heat, so shade it and decide early how it will be ventilated.
3. Natural ventilation depends on the fit-out. The Gherkin's mixed-mode idea was undone partly by glass cabins. If an Indian office relies on cross-ventilation or opening windows, write the rules for partitions and air paths into the tenant fit-out guide, not only the base build drawings.
4. Separate predictions from metered results. A design-stage energy saving is a target. When you cite performance in a portfolio, a rating submission or a thesis, say which it is, and ask for a year of metered data where it exists. Our guide to IGBC, GRIHA, LEED and the energy code explains what Indian ratings measure.

If you are choosing a tall-building or workplace thesis, our B.Arch thesis topics by typology lists commercial and mixed-use briefs where these questions come up.

Can you go inside the Gherkin? Visiting

The tower is an office building and is not open to the public. The top-floor restaurant and bar, run by Searcys, closed on 1 January 2026 because of building renovations, and its notice gives no reopening date. The building took part in London's Open House Festival from completion until it changed owners (Dezeen, 2022), so check that year's programme. The plaza and the outside can be seen from the street. For tenant enquiries, the building's own site is thegherkin.com.

Looking for an architect who thinks about structure, light and air together? Find an architect on MOA.

FAQ

When was the Gherkin built?

Planning consent came in August 2000, steel went up from October 2001, and the dome was completed in 2003. Sources give completion as 2003 or 2004; a tenant's release says the building opened in May 2004.

Who designed the Gherkin?

Foster + Partners designed it for Swiss Re, with Arup as structural engineer, Hilson Moran for building services and Skanska as main contractor.

How tall is the Gherkin and how many floors does it have?

It is 179.8 m (590 ft) to the top of the dome, with the highest occupied floor at 167.1 m. Arup and the Skyscraper Center give 40 floors above ground plus a basement; the City of London says 41 storeys.

What materials was the Gherkin built from?

A steel diagrid and steel core, composite concrete floors on metal decking, and flat glass panels. The only curved glass is the lens at the top. The office facade has three layers of glass with a ventilated cavity holding blinds.

Does the Gherkin really use half the energy of a normal office?

That was the design team's claim. The owner's property services director called 40 to 50% savings "a bit over-ambitious" in 2008, and we found no published measured figure.

Sources

• Dominic Munro (Arup), New Steel Construction: New building for Swiss Re, July 2003 (accessed 3 Nov 2026)
• CTBUH Skyscraper Center: 30 St Mary Axe, building data sheet (accessed 3 Nov 2026)
• City of London: 30 St Mary Axe (The Gherkin) (accessed 3 Nov 2026)
• Tyréns Hilson Moran: 30 St Mary Axe (accessed 3 Nov 2026)
• Skanska UK: 30 St Mary Axe, London (accessed 3 Nov 2026)
• ArchDaily: 30 St Mary Axe Tower / Foster + Partners, 12 November 2019 (accessed 3 Nov 2026)
• Martin Spring, Building: 30 St Mary Axe: a gherkin to suit all tastes, 25 April 2008 (accessed 3 Nov 2026)
• Building: Half of the Gherkin's windows sealed shut, 2 June 2005 (accessed 3 Nov 2026)
• Insurance Times: Gherkin's hot spot, 12 May 2005 (accessed 3 Nov 2026)
• Cameron Andrews, Post: Gherkin gets green light, 24 August 2000 (accessed 3 Nov 2026)
• Royal Museums Greenwich: Baltic Exchange Memorial Glass (accessed 3 Nov 2026)
• London Remembers: Event, 10 April 1992 (accessed 3 Nov 2026)
• Ian Mansfield, IanVisits: Tomb of a Roman girl at the Gherkin, 10 December 2018 (accessed 3 Nov 2026)
• Tom Lowe, Building Design: Fosters working on plans to refurbish the Gherkin, 13 May 2024 (accessed 3 Nov 2026)
• Sabah Osman, Time Out London: The Gherkin is getting its first ever makeover, 1 May 2024 (accessed 3 Nov 2026)
• Searcys: Searcys at The Gherkin closure notice (accessed 3 Nov 2026)
• Tom Ravenscroft, Dezeen: London's most significant buildings at Open House, 8 September 2022 (accessed 3 Nov 2026)
• Kirkland & Ellis: Kirkland moves into new London office, 3 April 2006 (accessed 3 Nov 2026)
• The Gherkin: thegherkin.com (accessed 3 Nov 2026)
• Wikimedia Commons: 30 St Mary Axe (Swiss Re Building) and St Andrew Undershaft church, Carlos Delgado (accessed 3 Nov 2026)
• Wikimedia Commons: Freedom of panorama (accessed 3 Nov 2026)