Urban structure appears to be one of the leading issues in the poll about the most important facets of urban design. But what is urban structure anyway? This post aims to open the discussion about urban structure…
In general, people have tried to identify the modes of how cities grow into the current state and why certain areas are wealthier than others. This is also referred to as urban structure. According to two articles I found here  and here , urban structure generally speaks to the arrangement of land use in urban areas and more precisely urban public and private space. The arrangement of urban public space has impact on accessibility, environmental sustainability, safety, social aspects, cultural creativity and economics. Further, urban structure also concerns the degree of connectivity and accessibility – and is hence in some way interrelated to other factors of the current poll.
One way of trying to understand urban structure and how cities grow, is to look at cities and develop certain models out of the findings.

There are three basic models described, explaining where different types of people and firms tend to exist within urban space.

1. The concentric zone model (see here and here): In this model, a city tends to grow outwards from a central reference point in a series of rings. These five concentric rings represent different urban areas of different use and different socio-economic clusters of its inhabitants. Moving out from the city centre, land use changes as distance from the centre increases.
   • The inner ring = Central Business District.
   • Second ring = Wholesale light manufacturing; zone of transition; industry and poorer-quality housing
   • Third ring = Low class housing; zone of independent workers’ homes; housing for working-class
   • Forth ring = Medium class housing; zone of better residences
   • Outer ring = High class housing; commuters’ zone; residential suburbs

   Please find the assumptions and some limitations of this model here.

2. The sector model: This model proposes that a city develops in sectors rather than in rings. Hence, this model may be seen as a refinement of the concentric model. It argues that different areas are of different attractiveness for certain activities – this may be by chance, or geographic and environmental causes. With the growth of the city, these activities prosper and expand – eventually they become sectors.
3. The multiple nuclei model: This  model argues that a city has more than one centre (nucleus) around which activities emerge. Here, different centres attract different entities. Hence, land use may be different in the nuclei.

To sum up, cities appear to have structure, which can be defined by various linkages between elements of urban activity. The structure has order and in turn this order has spatial qualities (click here for more ).
However, each city is different. See here  for a brief comparison of North American cities and Third-world cities. Also, one has to consider the dynamics of growth and urban structure. In their paper, ROSSI-HANSBERG, E. and WRIGHT, M.L.J. (2006) propose a general equilibrium theory of economic growth in an urban environment to address the tension between local increasing returns and aggregate constant returns. Here, it is suggested that “variation in the urban structure through the growth, birth, and death of cities is the margin that eliminates local increasing returns to yield constant returns to scale in the aggregate” (p.1).

What is your understanding of urban structure? Does it resonate with the models above, or do you propose a different view? And how do you think urban structure is affected by historical and geographic circumstances of different cities?

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