Gap Office Building, 901 Cherry Street
General Information
Location San Bruno, CA
Owner Gap Inc.
Architect William McDonough + Partners; Gensler
Engineer Arup
Completed 1997
Building Use Offices
Size 195,000 SF
Stories Two
Cost Unknown
Relevant codes California Building Code, Title 24
Mixed-Mode System
Mixed-Mode Strategy Concurrent system. The location's mild climate allowed the design team to integrate the operable windows with an underfloor air distribution (UFAD) system. Both systems are allowed to operate at the same time in complimentary ways. The HVAC system takes advantage of the thermal mass of the concrete slabs by using nighttime ventilation to reduce the need for chiller cooling. The operable windows then provide additional comfort and control. Air from both the HVAC and windows is brought in at a low level, and allowed to exhaust at a higher level, reducing the conflict between the systems. The occupied office spaces in the building were limited to two stories, and each of the three main office sections is organized around a 30' x 90' atrium topped by a roof monitor which provides daylighting and exhausts heated air.
Natural Ventilation Details From a thermal comfort standpoint, the building was designed to potentially work as a naturally ventilated office during much of the year. Air enters by means of operable windows around the perimeter, drawn in by warm air rising due to the buoyancy effects and exiting through vents at the narrow ends of the roof monitors.

User groups expressed concern regarding potential noise and dust from the freeway, so the building was planned so it could operate in either natural ventilation mode, or in mechanically-conditioned mode if the windows were closed. If noise and dust became a problem during the primary business hours, it was assumed that one operating mode would be that windows might be opened only during evening hours or weekends when less people may be working. The building was also zoned so that different areas could be operated differently. For example, perimeter offices can open their windows without interfering with the supply to the remainder of a floor. If windows were opened in the individual office pods, other modules could still be operated using strictly mechanical ventilation or cooling.
HVAC System Details The building has a raised floor with underfloor air distribution (UFAD), a system which provides multiple functions. The underfloor plenum enables flexible cable management and an energy efficient air supply, and also allows for an uncluttered ceiling with only lighting and sprinklers visible. Eliminating the suspended ceiling allows for a taller ceiling height for better daylight distribution. The building's integrated mechanical rooms are organized along the east side of the office bays in a partial basement level. Supply air is delivered through ducts buried below the lower level slab, which then rise to the second floor level at the perimeter of each atrium. Air is supplied through passive swirl diffusers in the raised floor, and through linear slot diffusers along the perimeter.
Configuration & Control A UFAD system offers many benefits in terms of energy efficiency and indoor air quality, but the control strategy is different from a conventional ceiling supply system. One reason is due to the fact that cooled air is being delivered at the floor, closer to the occupied zone, and purposely takes advantage of stratification and a high exhaust. The other reason is that the supply air in the underfloor plenum is coupled to the thermal mass of the concrete floor deck, thus creating a thermal flywheel effect between the air handling unit and the occupied space, but also creating an opportunity to utilize nighttime ventilation.

Night time ventilation is a key strategy in providing a low energy system. Analysis of local weather data in San Bruno revealed that, for the majority of hours between midnight and 8:00 am during the summer months, the ambient temperature is between 55° and 65°F which is the ideal temperature range for precooling.

A fundamental benefit of nighttime ventilation is for minimizing peak loads and associated costs, by operating on off-peak electricity. The control strategy is designed so that system blow airs through the underfloor plenum between midnight and 8:00 am, whenever the outside temperature is less than the inside temperature, and above 53°F (assuming a 2°F pick up across the supply fan). The control system will then turn off the fans if either the room temperature reaches 68°F, or the floor slab surface drops to 60° F. By cooling the slab at night, higher supply air temperatures can be used during the day while still keeping the indoor temperature below 76° F by the end of the day. Higher supply temperatures translate to lower energy use, by reducing the runtime and load of the chillers. By precooling the room and thermal mass before occupancy in the morning, a proportion of the subsequent internal heat gains are absorbed into the exposed structure during the day to be removed at night, further reducing the load on the chiller.

A different control strategy is implemented at the perimeter of the building. In the climate of San Bruno, spaces adjacent to the glazing can require heating first thing in the morning and cooling in the afternoon. The thermal lag characteristics of the underfloor system is therefore incompatible with this load profile. To address this, a separate system is provided which can provide individually controlled heating or cooling in the perimeter offices. The thermostat utlizes a wider dead band (no heat or cool between 70 and 76°F) to avoid competing with the floor supply system.
Building Design Process
Time Line 1994-1997
Design Tools Physical daylighting models and heliodon studies were utilized by consultants Loisos+Ubbelohde to evaluate daylight distribution and direct solar penetration.
Energy Analysis Energy analysis was conducted using the DOE-2 simulation software
Commissioning Prior to occupancy, the building was commissioned by a third party agent. The schedule for final occupancy presented some challenges due to some decisions by the building owner which required design changes to the TI layout only a few months before occupancy. Fortunately, the raised floor system was able to significantly reduce the time required for such reconfiguration. After occupancy, retro-commissioning was done for controls of lighting controls and automatic rolling shades.
Code Conflicts Code review was performed by independent code officials hired by the City of San Bruno. Officials were concerned about potential fire hazards from the grass roof, which was addressed by fire-hose cabinets located on the roof.
Building Performance
Outdoor Air/Noise The site's proximity to the San Francisco Airport was a contributing factor in the decision to use a grass roof. The mass of the soil acts as an acoustic barrier to airplane noise. Also to reduce noise, windows on the north façade facing a major freeway are not operable, and were glazed with laminated glazing.
Occupant Satisfaction Although evidence is mostly anecdotal, occupant satisfaction with the building has been very positive. Gap Inc. moved its corporate recruiting department to the building, as it showcases the Gap's commitment to providing employees with healthy and attractive workplaces.
Actual Energy Data The building was not equipped with energy monitoring equipment. The owner's review of energy bills indicates that the building uses approximately 30% less energy than other buildings occupied by the Gap, matching the design expectation.
Additional Building Features
Sustainable Sites A central feature of the site design was to retain a grove of oak trees on the site. The condition of the trees was continuously monitored by arborists throughout construction. The grass roof reduces rainwater runoff from the roof during peak rainfall events, and provides natural treatment of runoff.
Energy and Atmosphere • Energy efficiency is achieved primarily by the fact that a significant proportion of convective gains from lights, equipment and people rise to high level and are exhausted so that the ventilation system 'sees' a smaller cooling load.
• The UFAD system contributes to chiller load reduction by way of its higher supply air temperature, utilization of stratification, and by taking advantage of nighttime ventilation coupled with the floor slab thermal mass.
• Comfort temperatures are maintained in the bottom 6 feet of the space only, temperatures near the ceiling are typically 4-5°F higher.
• High performance Viaracon glazing was used throughout, allowing for 70% daylight penetration with only 30% solar infrared penetration.
Materials and Resources • The two story configuration allowed fireproofing to be eliminated from most of the steel structure.
• Material choices included FSC-certified wood flooring, veneers and redwood benches in the pool area.
• Other interior finishes included concrete café counters with recycled content, low VOC paints, and table tops and benches made from eucalyptus trees removed from the site before construction.
Indoor Environmental Quality • It is believed that UFAD systems provide better indoor air quality by supplying air closer to occupants.
• In this building the HVAC system is designed to maximize the use of economizer cycles, which further increases air quality.
• All workspaces are located within 25' of windows or daylighting from the atriums.
Project Team
Design Architect William McDonough + Partners
700 East Jefferson Street
Charlottesville, Virginia 22902
434-979-1111
http://www.mcdonoughpartners.com
Executive and Interior Architect Gensler
2 Harrison Street, Suite 400
San Francisco, CA 94105
415-433-3700
http://www.gensler.com
MEP and Structural Engineer Arup
901 Market Street, Suite 260
San Francisco, CA 94103
415-957-9445
http://www.arup.com/americas/
Civil Engineer BKF Engineers
255 Shoreline Drive Suite 200
Redwood City, CA 94065
650-482-6300
http://www.bkf.com
Landscape Architect Hargreaves Associates
398 Kansas Street
San Francisco, CA 94103
415-865-1811
http://www.hargreaves.com
Grasslands Consultant Rana Creek
35351 East Carmel Valley Road
Carmel Valley, CA
93924 831-659-4851
http://www.ranacreek.com
Building Contractor Swinerton Builders
260 Townsend Street
San Francisco, CA 94107
415-421-2980
http://www.swinerton.com
Interiors Contractor Webcor Builders
951 Mariners Island Blvd., 7th Floor
San Mateo, CA 94404
650-349-2727
http://www.webcor.com
Additional Information
Awards • Savings by Design Integration Award 2000
Business Week Magazine Design Award 1998
• Green Roofs Award of Excellence 2003
Sources • Pride of Place, Business Week, Nov 1998, http://www.businessweek.com/1998/44/b3602015.htm
• A Whole New World, Time, June 1999 http://www.time.com/time/archive/preview/0,10987,990301,00.html
• William McDonough + Partners project description http://www.mcdonoughpartners.com/projects/gap/default.asp?projID=gap
Contact Primary Contact
William McDonough + Partners
700 East Jefferson Street
Charlottesville, Virginia 22902
434-979-1111
http://www.mcdonoughpartners.com

 
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