Tall buildings can have major impacts on the wind conditions in their surrounding context, especially when a building is considerably taller than surrounding buildings. Tall buildings tend to intercept the stronger winds that exist at high elevations and redirect them downwards towards the ground level. Winds around the base of such buildings can be accelerated up to several times the values that existed prior to the tall buildings, thus creating uncomfortable and sometimes dangerous conditions for pedestrians.
Wind Comfort and Safety Studies, introduction
The City of Mississauga’s Planning and Building Department recently published a compact guide called Wind Comfort and Safety Studies. Applied to Halifax, the projects proposed for 6009/6017 Quinpool Road and 2032/2050 Robie Street would be required to undergo a full Quantitative Wind Tunnel Study. This study would consider both developments together, as well as the larger surroundings that could be affected by them: 400 meters in all directions (as shown below). This would involve constructing a physical scale model of the existing buildings, proposed buildings, significant features, developments under construction, and any future buildings with a potential impact on wind in the area. This model would be placed in a wind tunnel and tested to obtain data for comparison to municipal wind speed standards.
As well as considering the safety of passers-by, wind comfort and safety studies could both enhance and forestall damage to surrounding buildings, streets, sidewalks, and public amenities such as Welsford Park and the North Common. They could also add to the enjoyment of proposed developments by residents and patrons. Indeed, the developers and designers of both developments have invested heavily in open balconies, green roofs, and terraces for their residents’ enjoyment. It would be unfortunate if these amenities were too dark or too windy to be used.
Wind roses for the Halifax area (such as the annual summary above) show wind direction throughout the year. As far as we are aware, the projects proposed at Quinpool and Robie have not been tested with a wind study. How is that possible, given that both wind and shadow were mentioned as concerns in the City Staff’s Report to HRM Council of 2 June 2014? Even now, the wind at the Quinpool/Robie intersection can be threatening. If additional tall buildings are built there, waiting for the light to change would be even more of a thrill. The city’s ambitions for the North Common and the Oval also would be at risk.
As shown in the annual wind data for the Halifax area, we often experience sustained winds of up to 50 km/hr, with dangerous gusts of up to 91 km/hr. Sustained winds of 30, 40, or 50 km/hr, accelerated by towers at Robie and Quinpool, would seriously compromise the use of the North Common in summer and winter. Kids on skates would rocket downwind but need a tow on the way back. Gusts of 91 km/hr are awesome in their own right, but just imagine the impact on Quinpool Road, Robie Street, and the North Common after those gusts have been accelerated by a factor of 100%  by the three new towers proposed by Westwood and APL Properties Ltd. Giving these proposals a free pass, without proper wind testing and compliance with municipal standards, would be irresponsible. If we lived in Mississauga, we would have no reason to fear for the physical safety of our citizens due to wind, nor would we have to fear wind damage to our most cherished civic assets and amenities.
- As noted in Wind Design Guide, on a flat site with no building to divert it, a sustained wind of 50 km/hr at 10 meters above ground level would be roughly 37.5 km/hr at head height. That would earn it a wind ranking between B5 (fresh breeze) and B6 (strong breeze). With a 25-storey building (without mitigation) on the site, the sustained wind speed at ground level would increase 100%: from B5 (37.5 km/hr) to B8 (75.5 km/hr). B8 is considered dangerous for most human activity. Not to mention the gusts …
J. Grant Wanzel / 14 May 2015