Art of understatement.

A finely honed extension to a gallery in the American Midwest revitalises the original building, not only as a place for displaying art, but also as an important civic amenity.

The extension to the Joslyn Art Museum in Omaha, designed by Sir Norman Foster and Partners, epitomises the fine art of British understatement. Opened in November 1994, the extension provides new galleries and support facilities for the museum and succeeds in enhancing the existing building without overpowering it.

The Joslyn Museum is located on Dodge Street, the main east-west thoroughfare of the city. Completed in 1931, the original building is a hybrid of Beaux Arts plan and Prairie-Egyptian Art Deco detailing inspired by Bertram Goodhue. Designed as a concert hall with a fountain-court foyer and adjoining art galleries, the multi-use building has historically been a social centre for the citizens of Omaha, hosting not only theatrical and musical events, but also banquets, bar mitzvahs, wedding receptions, and celebrations of all kinds. Unlike many museums, it has therefore enjoyed the support of a broad constituency in the community. However by the 1980s additional gallery space and storage for the growing permanent collection were badly needed, together with a secure delivery bay, workshops for the preparation of exhibitions, and improved catering and shop facilities. In addition, the entrance portico had become redundant, with nearly everyone entering the building directly from the car park to the north.

A brief for expansion was prepared, funding was secured and submissions were invited from 50 architects. James Freed, Antoine Predock, Christian de Portzamparc, Renzo Piano and Sir Norman Foster were shortlisted for interview by the museum board. Prior to the interview, the Foster practice invested a great deal of time in understanding the problems and needs of the museum and, although none was requested, they took the risk of preparing a scheme to present at the interview. The investment paid off and Sir Norman Foster and Partners were appointed in June 1992.

Located to the north of the museum in the area formerly occupied by a car park, the 58 000 sq ft extension is a simple rectangular pavilion clad with the same marble as the original building and linked to it by a glazed slot. Like the existing building, the new galleries are raised on a piano nobile, with all support spaces such as loading bays, storage, workshops and offices tucked under the podium at ground level. The extension is the height of the flanking wings of the original building, allowing the portico of the old museum to remain the dominant element of the composition. The street along the east entrance facade of the original building, which had been closed for a number of years, has been reinstated and parking relocated. The museum now has two entrances, the original grand portico at piano nobile level and an understated ground level entrance into the glazed link. Placing the new entrance on the same facade as the old and organising vehicular circulation and parking to serve both has brought the portico back into regular use.

While similar in massing and cladding to the original building, the extension differs significantly in detail. The podium is an in-situ concrete frame and flat slab with steel-framed galleries above, and the stone cladding is a veneer on precast concrete panels. In contrast to the monolithic character of the original museum, the stack bonding and dark coloured mastic of the extension clearly articulate both a primary grid of panel joints and a secondary grid of finer stone joints. Vertical panels with mitred joints mark the corners of the building, and the moulded string course and coping of the original building are reinterpreted on the extension as flush details defined by reveals.

The atrium, a long, tall space between stone facades, provides the most delicate of connections between museum and extension and is the focal point for the circulation of both buildings. The new entrance at the east end is a single height transitional space under a bridge which links the existing and new galleries on the cross axis of the original fountain court. Although neutral externally, the emphasis within the atrium is shifted toward the extension. A giant portal cut out of the atrium facade of the new building directs visitors to a grand stair leading directly up to the new galleries and to the bridge connecting back to the existing galleries and fountain court. The restaurant occupies the west end of the atrium and the entire space is used for staging informal concerts, receptions, and large banquets.

The atrium is enclosed by clear, low-emissivity double glazing with no fritting or shading devices. Radiant heating or cooling is provided just below the glass at roof level and on the vertical faces via slotted holes in the aluminium framing of the glazing system. Notwithstanding Nebraska's boiling summers and arctic winters, there have been no apparent difficulties with either solar gain or condensation. The atrium also acts as a buffer to the northern side of the existing museum which had previously been notoriously difficult to heat.

The plan of the original museum is a three-bay composition with a main central bay containing the auditorium, fountain court and foyers flanked on the north and south by secondary bays of gallery spaces. Likewise, the plan of the extension is a tripartite composition. In lieu of being symmetrical, the new building is inflected toward the old with a circulation and services spine forming the southernmost bay, and the new galleries occupying the central and northern bays. Mechanical plant is located above the single-height circulation zone, ideally positioned to serve both the atrium to the south and the galleries to the north. Overlooking the atrium at podium level, the circulation spine leads from this bright, busy and noisy space through transitional zones with reduced light levels and buffered acoustics, and finally into the galleries themselves.

Museum Director Graham Beal, formerly Curator at the Sainsbury Centre for the Visual Arts, played a key role in formulating the gallery brief which determined that the galleries of the original building were designated for permanent displays of the museum's own collection, while new daylit galleries were to be designed for visiting exhibitions and for changing exhibitions of the permanent collection. Maximum wall space was required together with a high degree of flexibility in gallery configuration and lighting. Because the budget did not allow for the sophisticated mechanised systems of the Sainsbury, flexibility had to be provided in other ways.

The Foster practice has risen to the challenge, providing a high degree of flexibility with an ingenious fixed plan. The new gallery can be used as a single 14 000 sq ft space or as a series of up to six smaller exhibition spaces. North-south storage walls create a 6000 sq ft central gallery with 4000 sq ft galleries at either end. Each of these galleries is further subdivided by an east-west services wall from which air is supplied at high level and extracted at floor level.

The restraint of the exterior of the building makes the dramatic section of the galleries a surprise. Half vaults spring from the central services wall up to north-facing clerestory lights above the circulation zones along the outer walls. The gallery walls are 14ft and 20ft high respectively, with an additional 4 ft on the high side between the cornice revel and the light deflectors. The original proposal for the gallery was a shallow full vault with central clerestory similar to Foster's Sackler Galleries at the Royal Academy (AR December 1991) and the Kimbell Art Museum by Louis Kahn. The asymmetrical section - a late development - provides appropriately scaled walls for hanging both large and small works of art and uses daylight both to define circulation and to unify the galleries. This is not an architecture of discrete rooms, but of interconnected space.

There are no light sensors or automatic shading devices. On a bright day, the daylight level in the galleries is 55 lux. For dull days, back-up, off-the-shelf fluorescent lighting is concealed above the deflector fins. If required, the galleries can be blacked out by manually operated clerestory blinds. Individual works are lighted by four rows of adjustable spotlights running the length of the half-vaulted ceilings.

To expedite design and construction, the Omaha office of Henningson, Durham & Richardson (HDR) was selected by Fosters to be the collaborating architects and to act as structural and services consultants for the project. Kiewit Construction, an international contractor based in Omaha, was appointed by the museum shortly after Foster's appointment and actively participated in the pre-contract stages of design. A guaranteed maximum price for the extension was negotiated at the outset, and the results achieved for $140 per sq ft are impressive. The close collaboration between the design team, contractor and client made it possible to achieve savings that enabled renovation work to be carried out within the original building.

There is no doubt that the Joslyn Art Museum has been revitalised by the new extension. Old and new are woven together into a coherent, unified whole. Not only do the new galleries achieve a high degree of built-in flexibility, the scheme enables the museum and extension to be used jointly or separately, and the atrium to be used when the museum and the extension are closed. While the Joslyn takes high art seriously in terms of exhibitions, theatre and concerts, the atrium has enabled many informal events to be added to the diary. The importance the practice attaches to architecture as a social art is clearly manifest.

The new wing of the Joslyn is deceptive in its simplicity and compelling in its modesty. Unlike other extensions to American museums, such as the addition to the Allen Art Museum at Oberlin College by Venturi and the Toledo Art School designed by Frank Gehry (AR August 1993), the Joslyn extension does not call out for attention. Deferring to the original building externally with the utmost civility, Fosters have reserved the drama for inside, providing fine new galleries and a spectacular atrium which is both the social heart of the museum and an important new civic amenity for the city.