30 St Mary Axe

30 St Mary Axe (known previously as the Swiss Re Building), informally known as The Gherkin, is a commercial skyscraper in London's primary financial district, the City of London. It was completed in December 2003 and opened in April 2004. With 41 floors, it is 180 metres (591 ft) tall and stands on the former sites of the Baltic Exchange and Chamber of Shipping, which were extensively damaged in 1992 in the Baltic Exchange bombing by a device placed by the Provisional IRA in St Mary Axe, a narrow street leading north from Leadenhall Street.

After plans to build the 92-story Millennium Tower were dropped, 30 St Mary Axe was designed by Norman Foster and Arup Group. It was erected by Skanska; construction started in 2001.

The building has become a recognisable landmark of London, and it is one of the city's most widely recognised examples of contemporary architecture.

Site
The building stands on the former site of the Baltic Exchange (24-28 St Mary Axe), which was the headquarters of a global marketplace for shipping freight contracts and also soft commodities, and the Chamber of Shipping (30-32 St Mary Axe). On 10 April 1992, the Provisional IRA detonated a bomb close to the Exchange, causing extensive damage to the historic building and neighbouring structures.

The United Kingdom government's statutory adviser on the historic environment, English Heritage, and the City of London's governing body, the City of London Corporation, were keen that any redevelopment must restore the Baltic Exchange's old façade onto St Mary Axe. The Exchange Hall was a celebrated fixture of the shipping market.

English Heritage then discovered that the damage was far more severe than initially thought, and they stopped insisting on full restoration, albeit over the objections of architectural conservationists. The Baltic Exchange and the Chamber of Shipping sold the land to Trafalgar House in 1995. Most of the remaining structures on the Baltic Exchange site were then carefully dismantled, and the interior of Exchange Hall and the façade were preserved, hoping for a reconstruction of the building in the future. The salvaged material was eventually sold for £800,000 and moved to Tallinn, Estonia, where it awaits reconstruction as the centrepiece of the city's commercial sector.

In 1996, Trafalgar House submitted plans for the London Millennium Tower, a 386-metre (1,266 ft) building with more than 140,000 m2 (1,500,000 sq ft) of office space, apartments, shops, restaurants and gardens. This plan was dropped after objections that it was totally out-of-scale in the City of London, and anticipated disruption to flight paths for both London City and London Heathrow airports; the revised plan for a lower tower was accepted.

The tower's topmost panoramic dome, known as the "lens", recalls the iconic glass dome that covered part of the ground floor of the Baltic Exchange and much of which is now displayed at the National Maritime Museum.

The Gherkin nickname was applied to the current building at least as long ago as 1999, referring to that plan's highly unorthodox layout and appearance.

Planning process
On 23 August 2000, Deputy Prime Minister John Prescott granted planning permission to construct a building much larger than the old Exchange on the site. The site was special because it needed development, it was not on any of the "sight lines" (planning guidance requires that new buildings do not obstruct or detract from the view of St Paul's Cathedral dome when viewed from a number of locations around London), and it had housed the Baltic Exchange.

The plan for the site was to reconstruct the Baltic Exchange. GMW Architects proposed a new rectangular building surrounding a restored exchange: it would have the type of large floor plan that banks liked. Eventually, the planners realised that the exchange was not recoverable, forcing them to relax their building constraints; they hinted that an "architecturally significant" building might obtain a favourable reception from City authorities. This gave the architect a free hand in the design; it eliminated the restrictive[clarification needed] demands for a large, capital-efficient, money-making building, whose design was per the client's desire.[clarification needed]

The new building's low-level plan satisfied the planning authority's desire to maintain London's traditional streetscape, with its narrow streets. The mass[clarification needed] of the tower was not too imposing. Like Barclays' former city headquarters in Lombard Street, the idea was that the passer-by in neighbouring streets would be nearly oblivious to the tower's existence until directly underneath it.

Design and construction
The building was constructed by Skanska, completed in December 2003 and opened on 28 April 2004. The primary occupant of the building is Swiss Re, a global reinsurance company, which had the building commissioned as the head office for its UK operation. The tower is thus sometimes known as the Swiss Re Building, although this name has never been official and has more recently fallen out of favour, since the company's main headquarters is in Zurich and the Gherkin name has become more popular.

The building uses energy-saving methods which allow it to use only half the power that 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 promote double glazing in residential houses, which avoids the inefficient convection of heat across the relatively narrow gap between the panes, but the 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.

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, its fully triangulated perimeter structure makes the building rigid enough 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 apex.

On the building's top level (the 40th floor), there is a bar for tenants and their guests, with a panoramic view of London. A restaurant operates on the 39th floor, and private dining rooms on the 38th. Most buildings have extensive lift equipment on the roof of the building, but 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 lift only reach the 34th floor, and also a push-from-below lift 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 over long distances: From the north, for instance, it can be seen from the M11 motorway, some 32 kilometres (20 mi) away, while to the west it can be seen from the statue of George III in Windsor Great Park.