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Why "a building is not a tree"

Updated: Apr 14

Or, how Christopher Alexander’s landmark 1965 paper can still guide us in thinking about the coronavirus, and other urban challenges


The UK Urban Task Force demonstrated in this figure that the same population density can take very different forms, with very different connective properties (red lines added), and different impacts on health and vitality -- including disease transmission.


By Michael Mehaffy


The coronavirus pandemic has forced a sober reassessment of a number of urban characteristics, and none has been more maligned – inaccurately so, I fear – than urban density. A closer look reveals that clustering of people in certain environments (like nursing homes) is far more likely to spread infections than in other places with similar population densities - notably public spaces. As I have written elsewhere, it’s quite possible to maintain “sociable distancing” in many kinds of public and semi-public spaces.

What are the lessons for urban density in private places? One of the topics that needs careful assessment in the wake of the pandemic is the impact of tall buildings, which also tend to bring many people into close contact -- notably in their elevators, lobbies, and other spaces.

Epidemiologist Shai Linn has observed that the incidence of infectious spread can be high in tall buildings. He has drawn an analogy to the spread of coronavirus and other diseases in cruise ships: in both environments, people tend to crowd into elevators, stairs and other common areas. In both environments, infections (of all kinds) can spread rapidly.

There is an important point to be drawn from Linn’s work and others’. The issue is not merely that many people are in spatial proximity, but that they must pass through "choke points" of centralized spaces, where airborne transmission is much more efficient. (Successive touching of "fomites" like door handles and buttons is also part of the problem, but can be controlled more easily.)

What is the deeper problem with these centralized spaces? One can think of the structure of a tree, where all the branches, twigs and leaves are connected only through the trunk. Similarly, in a tall building or a cruise ship, all the parts are connected through central elevators, stairways and common areas.

By contrast, a web-network doesn’t have to concentrate everyone into central spaces – even when a given unit of space has the same number of people, that is, the same “density.”


The drawing at the start of this post makes this point. We can contrast a tall building with a street lined with tightly packed rowhouses, or a series of small apartment buildings, each with its own entry on the street. Such a web-network allows people to be in social proximity – able to practice what I have called “sociable distancing” – without being forced into the kind of adjacency that allows transmission of pathogens.

As it happens, the urban and architectural theorist Christopher Alexander described these two kinds of structures in a famous 1965 paper. Alexander, who is better known as the author of the classic book A Pattern Language, wrote in his paper that “A city is not a tree” – or at least, a good city is not. That is, the best cities are not dominated by centralized tree-like structures, but rather, they have many web-like sets of connections that he referred to as “semi-lattices.”

An obvious example of a tree-like structure can be seen urban street patterns. Many sprawling suburban communities show a tree-like pattern that is easy to differentiate from, say, the web-like grids of many older cities (as in the figure below). The trouble with tree-like patterns is that they force traffic into limited “choke points” where it becomes congested and hostile to pedestrians. This pattern doesn't allow vehicles or pedestrians to connect through other shorter trips between the branches, as is the case with the web-network. That usually means neighborhoods with tree-like structures are not walkable, are not very well suited to transit, and are prone to traffic congestion.


The sprawling, "tree-like" pattern at the top of this drawing makes it much more difficult to travel to the different destinations by transit, or especially, by foot. The pattern at the lower part of the drawing is much more inter-connected, offering many more ways to move and connect. Drawing by The Prince's Foundation.

For Alexander, there is an even more fundamental problem for cities organized as “trees.” Cities get their vitality and their dynamism from these inter-connections -- from the diversity of people who come into mutual contact, from the mixing of different activities and movements, and from the “overlaps” that happen when things are not neatly segregated into tree-like schemes.


He concluded:

It must be emphasised, lest the orderly mind shrink in horror from anything that is not clearly articulated and categorised in tree form, that the ideas of overlap, ambiguity, multiplicity of aspect, and the semilattice, are not less orderly than the rigid tree, but more so. They represent a thicker, tougher, more subtle and more complex view of structure.

If good vibrant cities are not “trees,” what about buildings? It seems the same logic applies: at the scale of buildings too, and especially as they connect to the public realm, we should seek overlap, multiplicity of aspect, and the other characteristics that Alexander celebrates. We should seek buildings that are more fine-grained, with redundant connections to the street, rather than one centralized “tree trunk,” as tall buildings typically feature.


In structural terms, we can compare a tall building to a kind of “vertical cul de sac” – or a kind of vertical gated community, with all the same potential problems of that problematic structural form.

And as we can now see, for similar structural reasons, such structures are also more resilient in the face of a pandemic.

The diagram at the top of the post, developed by the UK Urban Task Force in 1999, shows three schemes, each with exactly the same relatively high population density (75 Units/Hectare or 30 Units/Acre), but with very different network structures. The scheme of small flats to the lower right offers many different connections to the street, and it avoids centralized “choke points” where everyone must come into close proximity.


It's often assumed -- wrongly, as research has shown -- that tall buildings are necessary to achieve higher population densities. Yet these three schemes all have exactly the same density. They only differ in the way that those populations can connect -- as "trees," or as "web-networks." The tall building is clearly a tree, with all its structural vulnerabilities.

Unfortunately, at this moment in urban history, the growth of tall buildings around the world is nothing short of explosive. As research is showing, the factors that propel their growth seem to have less to do with best practice knowledge, and apparently more to do with the dynamics of short-term capital, images, branding, and even the egos of their promoters. This is not the path to sustainable or resilient cities. It may in fact be the path to catastrophe.


Let us hope that, as this pandemic prompts a reassessment of recent urban orthodoxy, the tall building, along with other mega-structures, will be part of a much-needed critical re-assessment.

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