Wednesday, 7 October 2015

Julie Futcher - What makes a tall building good?

‘Does London need Tall Buildings?’ is the title of an upcoming Council of Tall Buildings and Urban Habitats (CTBUH) event; it poses disarmingly simple questions such as ‘what is the practical contribution of tall buildings to the urban realm? and; Are tall buildings good or bad for us? Not surprisingly the answer to these questions is not straightforward and depends on the perspective (whether indoor or outdoor), the purpose (for commercial or residential use), the context (as an isolated tall building or as part of a cluster) and the metric (energy, density, profit, etc.). However, to some extent the questions posed are moot; London already has an eclectic mix of tall buildings and many more are planned. Here, I look at the effects of some tall buildings on the distribution of natural energy (sun and wind) in the surrounding urban landscape.  
Section through the Eastern Cluster City of London

In an urban setting, buildings interact with each other and the intervening streets, parks and plazas and it becomes impossible to examine buildings in isolation from their urban context. Tall buildings will cast shadows, reflect light, divert wind and modify all aspects of climate at the ground. Where air quality is poor an isolated tall building may aid in the ventilation of near-surface however, it may also result in gustiness that makes walking difficult. While taller buildings may be a more efficient use of space and offer the potential to reduce carbon emission, they can also make outdoor spaces less pleasant and increase the energy demands of buildings cast in shadow. In fact, a marked feature of the new tall buildings in London is that they trumpet their green credentials as making best use of natural resources to achieve energy efficiency while ignoring their impact on the surrounding area.

The potential impact of tall buildings on the urban realm is best demonstrated by 20 Fenchurch Street which has a BREEAM rating of excellent and an unusual ‘form’ that earned it the 2015 Carbuncle Cup. This ‘form’ caused the surface to behave like a parabolic mirror focusing the Sun’s energy onto a nearby street; since 2013 the envelope has been modified to prevent this from happening again. However, what is not so well known is that the buildings height interfered with the ‘rights to light’ of neighbouring buildings. Nonetheless, the buildings sustainable credentials (including the Sky Garden) were considered so significant that the Corporation of London made a compulsory purchase on these rights, effectively reassigning ownership of shared resource.

Another example is 100 Bishopsgate, yet to come out the ground, but once built the bulk and height will shade the south facing façade of the neighbouring Heron Tower for much of the day. In normal circumstances, this might be beneficial. Most commercial office functions occur during the daytime when the combination of internal and external loads generates a significant cooling demand; in these circumstances shading of a glazed surface reduces this demand. However, in the case of the Heron Tower, the soon-to-be shaded façade is where photovoltaic array have been embedded in the fabric to generate electricity. 100 Bishopsgate will simply block access to its energy source.

‎At 22 Bishopsgate, a planned 278m-high tower sits among other tall buildings. While the mutual shadowing effects may benefit the buildings and their energy use, they also create an outdoor space that is almost always in shade; while this might be a desirable outcome in a hot climate, it does not make for a pleasant space in mid-latitude London. There is little to be gained if tall building achieve efficiencies for indoor private spaces at the expense of the outdoor public spaces.
22 Bishopsgate (Red)
In general The Eastern Cluster the City of London is a good example of how tall buildings can be mutually beneficial by providing shade. However, what is best for office buildings may not be best for residential buildings, which are occupied in the morning and evening hours.  

The proposed residential Bishopsgate Goodsyard (BGY) Development on the boundary of the City of London occupies a wide site oriented east-west. BGY will cast a shadow on its neighbours which includes the Boundary Estate, a historic social housing scheme that was designed to maximize ventilation and access to sunlight. BGY will make best use of the natural resources available to it but because of its height it effectively captures these from its neighbours. These two neighbouring residential schemes highlight the growing conflict between the right to a passive resource and those of high density.

These examples demonstrate that energy management strategies require a spatial approach that accounts for the wider impacts of buildings on their surroundings. But currently we have no way of evaluating the influence of the ‘form’ a building takes outside the envelope of the building itself – rather buildings are seen as isolated entities that neglect their dynamic effect on the neighbouring buildings. 

There are many examples of good tall buildings throughout the city and the world that demonstrate that tall buildings have a role to play in resilient cities futures. However addressing their strategic role in their urban setting is only part of the picture, we need a resilient city wide plan that considers current and future needs of urban occupation that ensures all new buildings, streets, parks and plazas are ‘designed in an integrated way’ that addresses many of the issues that organisations like CIBSE Resilient Cities and CTBUH are starting to address. But what is for sure London is about to embark on a massive change to its infrastructure, to be successful we need to be vigilant and take our time.

Dr Julie Futcher is an architect and freelance consultant at Urban Generation.

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