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March 2020

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Luma Bendini

Toronto City Hall – a pedestrian comfort case

In the late 1950’s, the City of Toronto put in place an international competition to decide on the design of  the new  City Hall, a structure to replace the house of municipal government first built in 1899. 

The initiative created a buzz around the architectural world! Some said that the city’s symbol should be designed by local talent. Others said that an international competition would make sure the very best design was to be built. The second argument won and 500 design proposals later (from 42 different countries!), the chosen project was designed by Finnish architect Viljo Revell.  

Here is a Wind Comfort analysis of how the 27-storey building performed in a virtual wind tunnel test.  

The location: 

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The southeast financial district changed significantly since the City Hall was constructed in 1965. The current simulation was conducted with a modern representation of the area in shape of a CAD model. Therefore, this case study is mainly illustrative, with only 4 wind directions performed around the model. The goal is to show how a wind study is conducted and how in times of complex urban developments, it shouldn’t be ignored.  

Simulation Summary: 

4 Wind Directions  

Weather data considered from 3 years preceding the current year 

Implicit Large Eddy Simulations CFD Model applied 

Urban Wind Profile used with a visualisation at 1.75 metres high  

Full simulation report: https://app.ingridcloud.com/sim/fb2c44d3fb72433183c3ad343944bc73 

Wind Rose: strong westerly winds 

Strong winds are registered coming from the west during the first and the last quarters of the year. That combined with Toronto’s winter temperatures can create a very unpleasant experience for pedestrians around the main square, especially considering that its location promotes outdoor winter activities, with a famous ice skating rink in the central area of the square. 

Just by looking at the weather data we can expect that pedestrian comfort will be compromised! So, let’s look at the Lawson-based criteria evaluation in the next section. 

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Pedestrian Wind Comfort 

The previously analysed weather data was the input for a computational simulation that predicts wind in the given areas. Combining real-life weather data with fluid flow simulations are powerful tools for assessing pedestrian comfort! 

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As it can be seen in the Lawson-based visualisation, the main area of the square is mostly not suitable for long-term activities. With the exception of the northwest part that is sheltered, the predominant area is not suitable for longer sitting in all quarters. 

 The months of January, February, March, October, November and December (Q1 and Q4) display a potentially dangerous area in the entrance of the round chamber, on top of the ramp that leads to the towers.  

Although this evaluation is a very conservative analysis, the red areas should not be ignored. And if analysing this scenario is conducted in an early design proposal, a more comprehensive evaluation would be required.  

 The overall result is that the square is recommended for short-term activities, such as strolling, cycling, standing or occasional sitting.  

Next step: validating the design 

It’s clear that a more comprehensive wind assessment of the area is required to fully understand the effects of high-speed westerly winds in the area. Also, considering a larger surrounding area for wind analysis would be needed, since the denser the surrounding, the more sheltered from the wind the area of interest can be.

Ingrid Cloud can be used for validation, running transient simulations with 36 wind directions. 

If this were to be an early design scheme, important design optimisations could be considered for enhancing  pedestrian comfort in the main square. However, since the architectural choices are already made, all we can say is: be prepared for a bad hair day when visiting Toronto’s City Hall Plaza!   

Physical Wind Tunnel Testing 

The original competition and early design scheme had intended for the two towers’ structural systems to resolve independently to the foundation, but the wind tunnel performed by The Institute of Aerophysics at the University of Toronto confirmed excessive horizontal deflection at the top of the towers. They were then modified to be connected at the podium roof or third-floor slab level – which reduced the effective unsupported length of each vertical shell. 

Microclimate considerations  

In 1990, an extensive microclimate analysis was conducted with the purpose of analysing how future developments in Toronto’s downtown area would affect street level sun, wind and pedestrian comfort conditions. The study considered the effects of buildings on wind conditions at street level, understanding that many of the upcoming developments would subject pedestrians to strong mechanical forces of wind.

In general, such conditions can be avoided if the height and placement of future building volumes are modified.

- Sun, wind, and pedestrian comfort: a study of Toronto's Central Area, UC Berkeley.