Delivering a detailed picture of glare effects from a transformative commercial development
Commercial Bay is a $1.5-billion waterfront development planned as an addition to Auckland’s central business district. Anchored by what will be the city’s tallest skyscraper, the PwC Tower, Commercial Bay will also include four other existing and proposed buildings whose business tenants will employ about 10,000 people. Together, the cluster of buildings--linked to transit and nearby shopping and restaurants--will be a lively destination for business, retail and entertainment.
The designers of Commercial Bay planned to use glass facades on the buildings. Buildings with reflective surfaces can have negative effects on their surroundings, creating glare (annoying for neighbours, dangerous for drivers) or even reflecting solar energy in a way that creates dangerous hot zones. Seeking to prevent such problems, many municipalities around the world set requirements for the glazing of buildings.
Auckland’s city codes state that glazing must reflect no more than 20% of incident light. This is a simple parameter designed to serve a worthwhile purpose, but it is not without problems. First, if a building’s cladding reflects less light, that can mean more solar energy is radiated into the building, increasing the building’s cooling needs and therefore its energy consumption.
A second problem is that a material’s surface reflectivity varies according to the angle at which light strikes it. Even glazing that falls well within Auckland’s 20% limit can reflect significantly more than 20% of incident light, if that light is falling on the glass at a glancing angle. The angle at which sun strikes any building changes throughout the day.
Commercial Bay’s façade consultants, RCP, engaged us to perform a detailed, site-specific analysis of the likely performance of the glazing options they were considering, some of which exceeded the official reflectivity limit of 20%. The purpose of this analysis was to help the designers make an informed choice that would meet their objectives of:
- protecting the safety and comfort of their neighbours (complying with the spirit and intention of the 20% rule)
- supporting the efficiency and performance of the new buildings; and
- supporting the architects’ aesthetic vision for Commercial Bay.
After working to understand the context of the project and the relevant aspects of the building code, we embarked on our analysis. We selected three glazing systems, each with a different level of nominal reflectivity: 20% (compliant with Auckland’s rules), 30% and 40%. We then set out to analyze the reflections that would emanate from the proposed development if it were clad in each of these materials. Using Eclipse, a software tool we developed in-house, we ran simulations that computed the intensity and frequency of reflections from the Commercial Bay buildings for each hour in a year. This process gave us insight into how each of the three glazing scenarios would reflect the sun’s rays, as the incident angle of those rays changed across seasons and times of day.
Ultimately, what matters most is not how much light buildings reflect in the abstract but how those reflections affect people and places. So we used our first simulation to inform the selection of 45 receptor points for further study. These receptor points were areas around the development where reflections from the buildings would strike. The points were selected to represent nearby drivers, pedestrians and pilots, as well as occupants of neighbouring towers. We then ran a simulation that computed reflections at the 45 sites (capturing frequency, intensity and duration) at one-minute increments for an entire year. The simulation precisely quantified millions of anticipated reflective events, such as, for example, the glare a commuter driving toward the buildings at 7:45am in late February might encounter, or the reflection a neighbour on a balcony might experience at sunset in mid-July.
Together, these analyses revealed that, at peak times, neither the intensity nor the frequency of Commercial Bay’s reflectivity would differ much between the 20%-reflectivity glazing scenario and the 40% scenario. No scenario presented a risk of severe heat gains. The vast majority of receptor points showed no change in predicted reflection impact at different glazing levels. At the few sites where glazing did make a meaningful difference, the difference did not create a safety risk. As expected, reflections were stronger when the sun was falling at glancing angles. But such angles are determined by the location, orientation and shape of the buildings and glazing has limited power to mitigate these effects. Our analyses confirmed that although some of the glazing options the consultants were exploring technically exceeded the rules, none would have a significant negative impact on Aucklanders’ experiences of their downtown.
Our work equipped Commercial Bay’s design team with a rich understanding of the reflection impacts of their proposed development, and a detailed quantitative analysis to draw on in conversations about code compliance. More broadly, our analysis points to the value of carrying out building-specific studies to understand the implications of glazing choices at particular sites such as nearby roadways and neighbouring buildings instead of relying on simple rules that may or may not achieve their intended results.
Regulating glare is important, but not straightforward. In addition to the properties of the glazing material itself (reflectivity, spectrum, shape, slant), other factors are at play, including building geometry, and the always-shifting angles of the sun’s rays. Because of these complexities, some cities are abandoning simple glass reflectivity limits in favour of site-specific performance measures.