BUILDING STUDY: GOING NATURAL
March 1, 2002
By Canadian Consulting Engineer
Engineers on Canada's temperate west coast have been using natural ventilation to cool modern buildings for several years. In central and eastern Canada, though, engineers have not been so eager to ve...
Engineers on Canada’s temperate west coast have been using natural ventilation to cool modern buildings for several years. In central and eastern Canada, though, engineers have not been so eager to venture down the same road because it’s much more difficult to maintain comfortable indoor air environments in climates that can be anything from glacially cold to tropical and steamy.
Now, however, the engineers of a large campus building in Toronto are also taking the plunge and are using natural air currents to cool the building as part of a holistic design approach. The Computer Science Building at York University in northwest Toronto is designed by a team that includes Keen Engineering (mechanical), Carinci Burt Rogers Engineering (electrical), Yolles (structural). The architects are a consortium of Van Nostrand Di Castri and Busby + Associates — the latter having worked previously with Keen Engineering’s Vancouver office on green buildings on the west coast.
The 102,000-s.f./9,475-m2 Computer Centre is so outstanding in its environmental design it was one of the three Canadian projects exhibited at the international Green Building Challenge Symposium held in the Netherlands in October 2000. Built for a modest $168 per square foot, the facilility has been occupied since September and was officially opened in March.
The building’s form and structure help to minimize its need for mechanical cooling and reduce its energy consumption to 40% of an equivalent building designed to the ASHRAE 90.1 standard.
The south facade, for example, is fully glazed and open, but has an elaborate system of aluminum shades, roof overhangs and a large canopy to reduce solar gain. Similarly, the south and east walls are saw-toothed in form to minimize sun glare. The glazing is high efficiency with thermally broken frames (U+0.32, SC=0.45) and no thermal bridging.
The structural framing is generally cast-in place concrete with the slabs constructed as 300mm thick flat plates. The mass of these slabs serves as a “heat sink” to reduce the temperature swings and reduce the heating and cooling loads in the building. The cement content in the concrete has been reduced by using a 50% fly ash replacement, which substantially reduces the “embodied energy” (energy required in manufacture) of the concrete.
Structural engineer David Gray, P.Eng, of Yolles notes that the 300-mm slabs used in the building are also sufficient to support the additional loading associated with the planted roof. A series of 22-m-long steel trusses were used to accommodate the planted roof system over the large lecture theatre at the south end of the building. This vegetative roof system was developed with manufacturer Soprema, incorporating a membrane, rigid insulation and a special lightweight growing medium that can support short grasses in a 6-inch as opposed to the normal 9-inch layer (see detail p.25). The roof not only serves as a grassy recreation space for students during the summer, but also functions as an insulating layer for the building. It is also a rain-water collector, storing stormwater runoff in rooftop reservoirs.
The building uses a mixed mode approach to condition its indoor environment, with atria as the vital points of operation. One atrium is a long, double-height circulation spine running north-south through the block. Another is a four-storey “tree atrium” courtyard that lets light deep into the core of laboratories and offices in the building’s northwest corner.
In the spring and fall when temperatures permit, windows around the building’s perimeter open automatically and air flows inside the classrooms and offices. From there the air goes through high level openings in the partition walls into corridors, which in turn link into the atria. The stack effect in the courtyard atria is strong enough to draw in the perimeter air thanks to openings at the upper levels of the space which expel the air. The openings are automatically controlled, and have wind-detection sensors that close them to eliminate down draughts.
To further boost the stack effect and movement of air, massive smoke exhaust fans, which are required by the building code for fire protection, do double duty and can be operated at a slow speed to exhaust ventilation air from the atrium when necessary. The systems are all controlled by a Johnson Controls Metasys building automation system.
In the winter and summer, when outside temperatures are extreme, the building is heated and cooled by perimeter fan coils fed from the York campus’s chilled water and steam lines.
Ventilation during winter and summer is by two air-handlers, with CO2 detectors to control the outdoor air flow. One air-handler delivers outdoor air at the basement and main floor levels to supply the lecture theatres. The theatres have underfloor air delivery to encourage “stratification,” i.e. while the occupants at the lower level are kept comfortable, temperatures in the empty upper spaces are allowed to be higher. During mid-season mode the theatres draw outdoor air through an intake grate at grade. The large theatre relieves air through the roof by gravity relief hoods with motorized dampers.
The other air handler serves the rest of the building. Its outdoor air mixes with return air in the corridor and atrium spaces, and is then drawn into the closet fan coil units for distributing into the classrooms and offices. Each office has damper control over its own supply air grille, and the laboratories and classrooms have independent thermostats.
Conveying the electrical conduit and substantial communications cabling to the various workstations was a challenge to do discreetly in an interior that is mostly exposed concrete and open ceilings. Kevin Hydes, P.Eng. of Keen Engineering suggested integrating the cables and power raceways into the air handling passageway around the building’s perimeter. The resulting cabinetry even does triple duty as a work surface for the students.
As part of the energy-saving strategy, users have to tolerate a broader range of temperature and humidity conditions than they might in a standard building. The temperature in the lecture theatres, for example, will be as warm as 78F in the stratified zone, although only 75F in the occupied zone. In winter the atria will be maintained at a cool 65-68F, while the classrooms and offices will have a set point of 70-72F. These stepped temperatures may actually benefit the users as they will have a transition zone to help them adjust when entering the building from an extremely cold or hot day outside.
The lighting load is 0.9 watts per square foot — much less than the ASHRAE 90.1 standard of 1.84 watts per square foot. Corrie Burt, P.Eng. of Carinci Burt Rogers says they managed this saving with high-efficiency sources and equipment, and by “placing light where it was needed,” often by combining direct with indirect light. In the court atrium, for example, they combined floodlighting with indirect light on the roof trusses from large bi-ax compact fluorescent lamps. Occupancy sensors and photocells dim lights when they’re not needed. Fire protection involved full sprinklering, addressable smoke detectors, and a fire alarm system that triggers the exit doors to open and fans to remove smoke from the building immediately.
The building’s mechanical equipment size is reduced by half thanks to the various energy saving strategies. It it is expected to consume 1,940 MJ/m2 a year for prime energy, producing 67 kilograms per square metre of CO2. Over its 75-year life it is predicted to save 85,700 tonnes of greenhouse gas emissions.
As the green building movement ambles slowly along in Canada, it is the universities that are taking the lead. York University and the University of British Columbia are at least two that have decided to take the bull by the proverbial green horns and have adopted formal policies on sustainable design. Administrators at those institutions are now dictating that new construction on their campuses has to conform to strict environmental standards. Hopefully more client groups
will soon pick up the trail set by academe.– Bronwen LedgerCCE
Client: York University
Mechanical engineering: Keen Engineering (Mike Godawa, P.Eng., Kevin Hydes, P.Eng., Wilson Cheng, P.Eng.)
Electrical engineering: Carinci Burt Rogers Engineering (Corrie Burt, P.Eng.)
Structural engineering: Yolles (David Gray, P.Eng.)
Architects: Busby + Assoc./Van Nostri Di Castri/Architects Alliance
Images: Courtesy Architects Alliance
Photography: Steven Evans