30 St Mary Axe

30 St Mary Axe is a building in London's main financial district, the City of London. It is widely known by the nickname "The Gherkin", and occasionally as The Swiss Re Tower, Swiss Re Building, Swiss Re Centre, or just Swiss Re, after its previous owner but principal occupier. It is 180 m (590 ft) tall, making it the second-tallest building in the City of London, after Tower 42, and the sixth-tallest in London as a whole. The design is by Pritzker Prize-winner Lord Foster and ex-partner Ken Shuttleworth and Arup engineers. It was constructed by Skanska of Sweden between 2001 and 2004.

The building is on the former site of the Baltic Exchange building, the headquarters of a global marketplace for ship sales and shipping information. On 10 April 1992 the Provisional IRA detonated a bomb close to the Exchange, severely damaging the historic Exchange building and neighbouring structures.

The UK government's statutory adviser on the historic environment, English Heritage, and the City of London governing body, the Corporation of London, insisted that any redevelopment must restore the building's old facade onto St Mary Axe. The Exchange Hall was a celebrated fixture of the ship trading company.

Baltic Exchange, unable to afford such an undertaking, sold the land to Trafalgar House in 1995. Most of the remaining structures on the site were then carefully dismantled; the interior of Exchange Hall and the facade were preserved and sealed from the elements.

After English Heritage later discovered the damage was far more severe than previously thought, they stopped insisting on full restoration, albeit over the objections of the architectural conservationists who favoured reconstruction.

The architects, Foster and Partners, crafted a distinctive cone-like shape to reduce the wind turbulence around the Gherkin. Its design won the prestigious RIBA Stirling Prize for the best new building by an RIBA architect in 2004. It was the first time that the prize jury was unanimous in their decision. The building also won the 2003 Emporis Skyscraper Award for the best skyscraper in the world completed that year. It was constructed by Skanska, completed in 2004 and opened on 28 April 2004.

The building uses energy-saving methods which allow it to use half the power a similar tower would typically consume. Gaps in each floor create six shafts that serve as a natural ventilation system for the entire building even though required firebreaks on every sixth floor interrupt the "chimney." The shafts create a giant double glazing effect; air is sandwiched between two layers of glazing and insulates the office space inside.

Architects limit double glazing in residential houses to avoid the inefficient convection of heat, but the Swiss Re tower exploits this effect. The shafts pull warm air out of the building during the summer and warm the building in the winter using passive solar heating. The shafts also allow sunlight to pass through the building, making the work environment more pleasing, and keeping the lighting costs down.

Most tall buildings get their lateral stability from either a core column or by an unbraced perimeter tube without diagonals — or some combination of the two. This normally means that if they are designed to be just strong enough to resist wind load, they are still too flexible for occupant comfort. The primary methods for controlling wind-excited sways are to increase the stiffness, or increase damping with tuned/active mass dampers. To a design by Arup, Swiss Re's fully triangulated perimeter structure makes the building sufficiently stiff without any extra reinforcements.

Despite its overall curved glass shape, there is only one piece of curved glass on the building — the lens-shaped cap at the very top.

The primary occupant of the building is Swiss Re, a global reinsurance company, who had the building commissioned as the head office for their UK operation. As owners, their company name lends itself to another nickname for the building variants on Swiss Re Tower, although this has never been an official title.

On the building's top level (the 40th floor), there is a bar for tenants and their guests featuring a 360° view of London. A restaurant operates on the 39th floor, and private dining rooms on the 38th.

Whereas most buildings have extensive elevator equipment on the roof of the building, this was not possible for the Gherkin since a bar had been planned for the 40th floor. The architects dealt with this by having the main elevator only reach the 34th floor, and then having a push-from-below elevator to the 39th floor. There is a marble stairwell and a disabled persons' lift which leads the visitor up to the bar in the dome.

The building is visible from a long distance: from the north for instance, it can be seen from the M11 motorway some 20 miles away while to the west it can be seen from the statue of George III in Windsor Great Park.