If post-construction mitigation is required to protect the public from dangerous conditions, it will always be significantly more expensive and unsightly than a sensitive, integrated design
Over the last 20 years in the UK, there has been a dramatic increase in city-centre living and working. High-rise buildings give a new skyline to cities like London, Leeds and Manchester. Most recently, Fosters have submitted plans for its new Tulip tourist tower next to The Gherkin in the City, which will add to the record 510 towers over 20-storeys planned or under construction in London over the coming years.
But these new buildings can cause complications - some of which have been documented across the UK in the last decade - where skyscrapers are blamed for creating strong winds which can be unpleasant or even dangerous.
There are two main ways to assess the wind impact of a skyscraper. One is by placing a scale-model in a wind tunnel, such as those used by Formula One teams. The other (also used in Formula One) is Computational Fluid Dynamics (CFD), which uses a virtual model to simulate the wind. Both methods are well validated, and can have similar costs and times to test a single design. However, when multiple iterations are required, CFD starts to show real benefits.
During design development, the choice of method can depend on the project approach. A wind tunnel best suits a design workshop, where members of the design team attend on the day of testing to discuss strategy. However, a project can experience frequent design changes over the following weeks. Subsequently reorganising this workshop each time can be costly and cause delays. CFD best suits an iterative approach, in which a new design can be retested quickly and cheaply as the project evolves.
When testing the final design, each method produces extra data that the other might miss. Wind tunnels produce extra data about turbulent gusts that CFD can only provide at greater expense. CFD will tell you how much cumulative turbulence there is, but not whether it is made of one big gust, or many little ones. CFD results show wind conditions from 100,000 points around a development. Wind tunnel results are limited by the number of physical probes (typically only 100 or so locations) and could miss some important features. Either way, neither is a one stop shop.
So why are these crucial tools not always implemented? Cost could be a factor; but for a major tower, an extra analysis at the end of the design is marginal compared to the total costs of the building.
Bad press associated with wind issues around buildings often sees the problem as just one of comfort, which of course at times it can be. However, high winds can be dangerous and cause injury to pedestrians. Solutions are best identified earlier in the design process. For example, Hilson Moran undertook early-stage modelling on 100 Bishopsgate in the City of London, which was both cost-effective and meant that solutions could be integrated more sensitively into the design.
If necessary, there are ways to mitigate the impacts of wind, even near the end of the design process. Trees, particularly evergreen species, have been shown to significantly decrease wind effects. The use of porous structures such as fences, trellises, banners, or even public art can also be effective.
If post-construction mitigation is required to protect the public from dangerous conditions, it will always be significantly more expensive and unsightly than a sensitive, integrated design. Large baffles to deflect the winds, significant changes to traffic design, even restricting access at certain times. But, even at this stage, successful analysis and design can help to reduce the impacts.
The largest clients and developers who are experienced in building these structures understand the need for wind analysis as early in the design process as possible. For the UK, the building of these new towers is of huge importance. They create jobs, commercial and residential space and add to the city’s attractive and dynamic nature – benefits which could be undermined by a poor wind environment. Designing and testing from the start to work with the wind supports the benefits these new towers will bring.
For the tallest towers, combining both CFD and wind tunnels will give you the complete picture. Early intervention will always be favoured over retrofitting.