Daylighting is always useful in the daytime occupancy of this building type. If internal loads were low, then solar gain would also be useful most of the year in this cool, gray climate. Client needs in this program were straightforward but complicated by the site context. Interior requirements were dominated by criteria of flexibility. With computer networks in place and the prospect of subleasing parts of the building designed in, space allocations, workstations, and all the attendant servicing would have to be responsive to change.
Farnborough is renowned for its air shows, and these cause periodic noise problems. The Royal Aircraft Establishment is less than a mile from Briarcliff House, and normal flight paths carry low-flying aircraft across the site daily. Another persistent source of noise at the site is automobile traffic. The A325 Farnborough Road to the M3 motorway is a major thoroughfare to the east of the building. A main artery, Meudon Avenue, runs across the south end of the site. Large park-and-ride asphalt lots cover the ground between the building and Meudon. A major bus stop is located immediately in front of the building, where waiting passengers stand under Briarcliff’s glass canopy.
The office functions of Briarcliff are physically connected to a series of shopping malls that make up part of the city center. A feature of the Briarcliff program was to connect the parking and bus areas to the enclosed mall with an independent entry. The physical connection between the two structures was also problematic during construction; when piles foundations were being driven, bottles rattled precariously on the store shelves next door.
The design team’s approach to the problems of noise and sun dictated an introverted plan and a buffer to the noisy aspects of the east and south orientations. South sun was desired though, so these aspects had to be reconciled. Contextually, the new building was envisioned as a key link between the parking, transit centers, pedestrian routes, and the adjoining town center office and retail spaces. Spatially, the design called for a building that would “contribute an element of order and visual interest to the undisciplined and rather drab character of the neighborhood.”
Appropriate Systems
There are too many similarities between this project and the Willis Faber Dumas (WFD) headquarters to ignore (see case study #7). Suspended and mullionless glass facades are the most obvious parallel to Foster’s design. The grass sod terrace at Briarcliff and its greenhouse pavilion cafe are also interestingly similar to those at WFD. No explicit reference or statement by the design team has ever connected the two buildings, but the progression of technologies from one office headquarters to the other is clear enough to provide linkage. One identifiable but tenuous tie between the two projects is the prior association of their mechanical engineers—Arup’s Peter Warburton and Foster’s Loren Butts previously worked together for the U. K. engineering firm Haden Young.
The 1 acre site is configured as an introverted arrangement whereby the building wraps around and protects an interior terrace. Connections from the building’s ground floor to its immediate exterior surroundings are made via a generous glass-canopied walkway with separate entrances to the retail mall and the insurance offices. The south edge of the building and its covered walkway provide shelter for bus passengers and access to outlying public parking lots. Along the east side of the building is a service dock and a gated entrance to covered parking for 20 cars off Farnborough Road. To the west are a service access and then a pedestrian mall connecting to the retail shops of Queensmead.
The building footprint takes up its entire site but gives back some important elements. The first of these is the continuous glass canopy around the south and east sides, which keeps off the weather without darkening the wide walkway. Another public feature is the commercial space and mall entrance next to the transit stop. The designers also provided a generous skylight at the northwest corner of the first floor roof where the extension of the retail area connects to the existing mall.
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Figure 6.19 The garden terrace. |
Internally, the building remakes its own site, with the garden terrace captured inside the plan at the second level. The ground level is a solid mass dedicated to retail, park
ing, service cores, and a computer center as well as a double-height lobby entry. The three levels of office space above look out to the captured terrace and across to the restaurant and coffee bar.
Construction is cast-in-place posttensioned concrete. The supporting soil is acidic sandy loam (pH about 3.5) with a meter of peaty clay topsoil. The water table is 5.9 ft (1.5 m) below grade, so the building rests on driven pilings. Arup Acoustics was brought in to monitor the ground vibrations for fear of damage to the connecting retail shops. A supermarket kept its wine and spirits bottles against a common wall, which had to be monitored visually during piling operations. Houses less than 328 ft (100 m) away were also surveyed, but the only damage revealed was restricted to an aquarium in one of the houses. It seems the resident observed that vibrations during construction caused wave motions in the aquarium and made his fish seasick.
The U-shape form of the plan generated a radial organization to the main structure. It is entirely of poured-in-place concrete. Round columns at the exterior wall carry a perimeter beam supporting one end of the posttensioned floor beam ribs. A second column line 10 ft (3.0 m) inside the interior wall carries another beam. The floor beams cantilever over the inside beam and leave the captured terrace view unobstructed at the perimeter. The space between the inside column line and the solid glass wall to the terrace is used for circulation.
The rib beams were designed to complement the underfloor air supply plenum by incorporating the return air path. Ridges at the center of each beam are left for a fluorescent light strip, which feeds the return air duct in the floor plenum above. This required careful design for the placement of reinforcement cable in the beams and resulted in a slightly deeper profile.
A double skin assembly wraps the exposed perimeter of the building. Its exposure takes in the east side to Farnborough Road, the broad south exposure toward the parking areas, and the west orientation facing a pedestrian mall. The interior layer of the double envelope consists of alternating clear single glazing and solid metal panels. The exterior rain skin is 0.4 in. (10 mm) reflective bronze glass in panels 11.9 ft high by 5.9 ft wide (3.6 m X 1.8 m). Each of the rain skin glass panels is suspended from its aluminum substructure by six 0.25 in. (6 mm) stainless steel bolts through gasketed connections countersunk in the glass. Joints in the glass panels are silicone sealed, giving the exterior a smooth face. Behind that is a 3.9 ft (1.2 m) cavity airspace where supply air ducts and grated metal service walkways are located within a thin tubular steel
frame. The outside bronze glass skin continues upward past the fourth floor to enclose a penthouse containing all the heavy mechanical equipment. The same exterior skin turns outward at the base of the building to form a canopy over the entire length of the street side. A gutter is tucked into the corner transition from vertical wall to projecting canopy so that rainwater is diverted from the sidewalk.
On the courtyard side of the offices is clear singlepane glass. Toward both northern corners of the terrace the office windows are protected by automatically controlled horizontal blinds on the outside surface. These blinds are activated by solar cells. The cladding of the restaurant and coffee bar is also single-pane clear glass.
The base of the south side double envelope is left open where the canopy glass turns outward. Winter air is drawn up the air cavity and preheated to provide tempered makeup air for ventilation. During warmer months the rising air is exhausted by stack effect on the north leeward side of the penthouse, taking with it absorbed solar heat gain. In all seasons the air cavity serves as a thermal buffer to the interior and circumvents uncomfortable glass surface temperatures to the interior office space.
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Figure 6.20 Construction at Briarcliff. (Photograph courtesy of Arup Associates.) |
A few vented-skin buildings were built prior to Briarcliff: Cannon Design Group’s office for the Hooker Chemical Company, 1981, GEW Colonge by Kraemer and Sieverts, 1982, and British Sugar Corporation by Arup Associates, 1975.
All heavy HVAC equipment is located in the glass-covered penthouse that runs the length of the building. There are four sets of fans serving equal portions of the offices. Return air is brought up through the ducted light fixtures and into a duct in the floor cavity of the interior circulation corridor above. Outdoor air is taken from the solar- heated south side air cavity and either mixed with return air when the space is underheated or exhausted from grilles on the north side of the penthouse. The three-story air cavity produces a stack effect, acting as a chimney to enhance the movement of air and heat in both seasons. Summer ventilation air is taken directly through the mechanical plant room walls. Air balance is maintained throughout the year by exhausting an appropriate portion of the return air. A computerized monitoring system switches to 100 percent ventilation air or evaporative cooling whenever mechanical cooling can be averted. Supplemental heat is provided by perimeter radiators that are controlled by thermostatic valves.
Cooling and outside air requirements are provided by air handling units in the penthouse. These recirculate a mixture of return air and outside air, conditioning it and then supplying it back to the offices via vertical ducts in the air cavity south wall. Interior raised floor plenums are pressurized with supply air without the use of horizontal ducts or terminal units. Floor-mounted twist pattern dif
fusers can be located wherever needed and closed when not in use.
Heat transfer simulations were developed to account for the large thermal storage mass provided by the exposed concrete floor ribs. This thermal inertia was predicted to smooth temperature fluxuations during a daily cycle by absorbing peak heat gains from the offices during the day and slowly radiating it back during unoccupied night hours. The resulting design prescribed a supply air temperature of 65°F (18.5°C) with up to six air changes. The anticipated peak return air temperature was 82°F (28.0°C).
Energy monitoring at Briarcliff validated the design strategy. The sophisticated combination of vent skin envelope, solar chimney, internal mass, and underfloor supply air plenum proved to be comfortable, flexible, and efficient.
The environment at Briarcliff integrates several aspects of what would become the prototypical highly serviced workplace. Flexibility is provided by a raised floor of pedestals at 2.0 ft (0.6 m) on center, supporting particle – board panels finished with carpeting. Below this are organized routes of cabling for electrical, data, and telephone systems. The raised floor plenum also provides supply air to any spot in the building without the need for ducts. Despite fears of drafty air from low-level diffusers causing discomfort, the system drew no complaints, perhaps because supplying from the floor allows warmer supply air temperatures and lower flow volumes to be used.
The air-handling light fixtures in the ceiling ribs are fitted with T8 warm white fluorescent lamps with an average lighting power density of 3.67 W/ft2 (39.5 W/m2) to provide 60 fc (600 lux). Custom-made fixtures run down the center of the circulation path. These are indirect fluorescent sources with curved housings painted white to match the ceiling. Lighting in the double-height reception entry is provided by 250 W tungsten halogen downlights and wall washers.
Organizationally, the three floors of office space above the ground level are identical. A central elevator and stair shaft at the main entry is complemented by fire stairs at both ends of the plan. Circulation around the open office plan happens at the courtyard side of the plan along a continuous open corridor cantilevered outside the column line.