2.3 An Urban-Structural Analogy

Just as it is possible to recognize styles of architecture, based on their structural characteristics, it should be possible to recognize and describe different forms of urban structure, using similar characterizations. The point here is to demonstrate the different approaches to expressing urban structure based on the available transport means, which are explained here by analogy with building structure.

By means of the structural analogy suggested in this section, we shall see the influence of the evolution of technologies on forms. Broadly put, the analogy is this: in the case of building structure, the evolution is that of engineering and construction technology, through the use of stronger and more efficient materials. In the settlement case the evolution is that of transport technology, in terms of vehicular modes and their infrastructure, through the use of faster and more efficient means of transport.

Here, four basic characterizations are proposed: （1）Organic; （2）Classical; （3）Modem; and （4）Postmodern. The four types are summarized in this Figure, and discussed in the remainder of this section.

Figure 2.4 Characterizations of urban structure with reference to building structure

2.3.1 Organic Structures

The‘organic'form is characterized by irregularity. There is no inclination to structure the building or settlement with any regularity of form or unity of composition.

The building version of the organic form would be an informal shack, irregular in plan and elevation, with no repetition or regularity of structural members, probably the result of limited pre-planning（if any）and piecemeal accretion.

The urban structural equivalent would be any incrementally evolving, ‘unplanned'settlement which would have very little symmetry or repetition of parts, but which would nevertheless appear to grow outwards from its centre in some logical, if haphazard, manner to retain a relatively compact form.

Although the organic form may superficially appear chaotic or formless, it likely to involve some ‘structural'order, for it to function at all. For the time being, however, the lack of a preconceived order or self-consciously applied structural logic will distinguish this from subsequent forms.

2.3.2 Classical Structures

At t he‘classical'stage it becomes possible to set out whole buildings or settlements in a consistent fashion: with regular building lines, beams and columns now possible for one, and straight streets and a regular city boundary for the other. Just as it takes forethought to conceive and execute a symmetrical building, it requires consistency of planning intentions to create any degree of organization or symmetry in a town plan, and hence arrive at the classical urban structure

The use of standard size and consistency of materials in classical buildings is echoed in the emergence of regular Street specifications in settlements. In‘classical'urban structure the use of faster modes（carriages）allows the straight streets to be exploited for the purposes of movement.

The classical tradition is associated with‘a respect for balance, harmony and order', and networks of‘equalized forces'.

Thus, a symmetrical town plan would theoretically cater for evenly distributed flows of traffic. This balance would normally not prevail very far, due to the inevitability of an asymmetrical hinterland. For example, in Edinburgh's original New Town, one side of the internally symmetrical composition is much better connected to the wider urban structure, which precipitated the emergence of Princes Street as the city's main thoroughfare, ahead of its counterparts George Street and Queen Street.

Classical urban structure was certainly enabled by technology, but was also driven by wider societal aims.

China's typical classical city is usually the center of the country's political dominance. The urban structure of chang'an city in tang dynasty adopts the pattern of symmetry of the central axis, and the whole city layout is neat and clear, which fully reflects the guiding ideology of taking as the center. The urban arterial system has a clear division of labor and USES a regular grid. This planning model was driven by the politics and technology of the time, but also by the widespread social recognition at the time, belonging to the category of classical order and balance.

Figure 2.5 Changan planning and design

2.3.3 Modern Structures

Under the background of globalization, the development of the city will need to adapt to the demand of the global economy, need from the network access to resources in the development of global economy, which requires the uniqueness of the city itself to attract investment, to attract industry and attract tourists, thus creating the uniqueness of city has become the important content of urban planning during this period. This has been fully reflected in various types of urban development planning in the world over the past 20 years, and theories and practices such as urban development strategy planning, urban marketing and site construction have also emerged. In recent years, London's“space development strategic planning”and New York's 2030 plan“greener and larger group agreement”have made full use of the means of strategic planning, with the core of adapting to the needs of global economic development and creating a sustainable society, and made a comprehensive plan for the future development of the city. On the basis of positioning London's urban development, the London strategic planning has established a citywide development policy framework from the four subject areas of residence, employment, transportation and leisure and entertainment, as well as natural resource management, urban design and the Blue Ribbon Network. On this basis, strategic actions have been formulated for various regions in the city. The New York 2030 plan has formulated a strategic plan of action for the future development of the city from six aspects, including land use, water resources, transportation, energy, air and climate change. Other cities have similar plans.

Figure 2.6 London Urban Design

Figure 2.7 New York Urban Design

From the above urban design concept, we can see the importance of transportation in urban planning and design. In the planning of the transportation problem as the focus on the element in the urban design, the current from the solution in terms of design, what we see in“modern”phase is to increase the number of available modes（similar to the new building materials）, speed and capacity of these patterns and artery（similar to the used in buildings） intensity with the increase of the members.

The increase in speed and capacity allows the generation of new settlement forms, simultaneously allowing bigger, more dispersed settlements, as well as allowing new levels of concentration in dense central business districts. There is also simultaneously a distinction between different modes, and between arteries of different capacity within modes, and a separation of urban functions.

Slouch describes how the creation of the interstate system had replaced‘the meandering back

roads of America with the no-nonsense, high-velocity interstates...'and in doing so had effectively ‘expanded the nation's bandwidth; the new roads carried more traffic, more efficiently, at greater speeds'.

The high capacity infrastructure of the fast modes（highways and railways）become the main structural elements at the scale of the settlement as a whole, analogous to the steel or reinforced concrete skeletons of modern buildings. Slower modes would also co-exist, as with a matrix of smaller vehicular and pedestrian streets, which are analogous to the brick infill or the curtain wall in building structures. These smaller roads are more local, only carrying their‘self weight'in traffic terms. They are also qualitatively different in that their functions become increasingly less to do with movement and more to do with access: a multi-purpose Street might include market stalls, public seating and frontage access to buildings, in addition to serving as a thoroughfare-analogous to a brick wall providing texture, insulation and privacy as well as structural strength.

Regarding the economic imperative in structural design and the functionality of structural elements, it has been pointed out that“It is usually more economical to‘gather the forces together'and conduct them along discrete members of high strength and stiffness”. This is analogous to the gathering of traffic flows together into high capacity highways forming the strategic road network; or gathering many passengers together in high occupancy, high capacity public transport vehicles.

Of course, Holgate also points out that the structural engineer's concern for structural efficiency may be just as irrationally moralistic'as any other architectural guiding principle. This is echoed in urban structure, where the pursuit of a narrow definition of functionalism saw traffic efficiency become such a high priority in settlement design in the 1960s. Here, proposals for free-flowing urban motorways and the comprehensive segregation of vehicular and pedestrian routes such as those in Traffic in Towns demonstrated the logical conclusion of full accommodation of the motor vehicle in urban areas.

And so, although the main structural elements in the modem stage are now emphasized -skeletal members used expressively in modern architecture; highways the most dominant feature of modern settlement layout-we must not forget that the fundamental（raison d'etre） of the structure is a supporting role. A bare skeleton is not much use as a‘building'if the functions of enclosure that a wall conventionally provides（partition and insulation）are not also present. Similarly, a settlement based on movement alone would have no urban function if there were no buildings to inhabit or public spaces to occupy, as conventionally built into the form of the street.

Modern functionalism is also equated with‘articulation', or in the architectural context, the‘separation of the parts of the building that perform different functions'. Articulation is clearly another manifestation of modern urban structure, in the sense that the transport arteries are for the most part separated from the buildings and other spaces occupied by people, often in conjunction with separation of land uses. The buildings themselves tend to be stand-alone‘pavilions'or‘isolated monuments'set back from the roads as well as being separate from each other, forming‘isolated pods of development', or‘still life'set-pieces. This tendency towards articulation has also had implications for the central functions of settlements, as the‘town centre'could now be conceived of as a single entity, and sometimes built as a single building.

Moreover, in contrast to organic and classical（and especially cosmic）urban structures, the‘town centre'of a modernist settlement need no longer be at the geographical centre of the town. In the case of Harlow, the town centre was placed near the railway, towards the northern periphery of the town; in Livingston, the town centre'hangs off the central spine route, just like any other neighborhood branch（and in this case remote from any railway station）.

It should also be pointed out that although the liberation from historical urban structures generally resulted in expression of‘efficient', flowing movement in a functionalist manner, it did not preclude the creation of new symbolism, now possible at an ever vaster scale.

2.3.4 Postmodern Structures

What then of postmodernism? Here we are on more uncertain ground, even when limiting the discussion of postmodernism to its architectural interpretation. This is because postmodernism's very existence as a distinct genre, and the possibility of its supplanting modernism, are both in doubt-not least due to the plurality of interpretation that postmodernism itself affords. Notwithstanding this uncertainty, it is at least possible to identify that which is equated with postmodern architecture, and by extension urban structure, whatever the significance of its labeling.

For the‘postmodern'stage of architecture, forms are characterized by traditional or neo-traditional styles and to some extent traditional materials, at least for facing purposes. Postmodern buildings will take advantage of modern structural technology, even if this is not expressed explicitly but is hidden by a facade made out of traditional materials. Whatever label is attached to this contemporary period, it certainly encompasses plurality, in the sense that not only is there a variety of‘postmodern'forms, but these have sympathetic coexistence with genuine‘traditional'buildings and with distinctly non-traditional hi-tech buildings.

As for any‘postmodern'stage of settlement design, this is similarly open to a variety of interpretations. For example, a postmodern town plan might be not a plan at all - or not a town at all. It could be an acentric urban accretion, an agglomeration of past growth, manifesting itself as a collage - the‘joint existence of the overtly planned and the genuinely unplanned'- or a town pian deliberately contrived to look like any of these.

Postmodern urban design has been be equated with‘no zoning'or‘mixed use zoning'; it has also been suggested that urban design itself is the planning equivalent of postmodernism. Postmodern urbanism could also be a distinct style or trend, as with the neo-traditional forms of the New Urbanist and Urban Villages movements.

In neo-traditional urbanism, the traditional neighborhood development is typically based on‘grids of straight streets and boulevards（instead of highways）which are lined by buildings'. A conclusion here might be that there may not be any single, distinct manifestation of postmodernism in urban structure. This stance also happens to match the‘collage'model. Certainly, it ought to embrace a plurality of forms, as neo-traditionalism mingles with the regeneration of genuinely traditional areas and continued pursuit of‘conventional suburban' layouts.

If there is a direct parallel, in particular, between postmodern architecture and neo-traditional urban design, then it would be the use of contemporary technology, structurally, upon which a‘traditional'facade is appended or hung. In a postmodern settlement, then, while traditional urban blocks and streets may be recreated at the scale of the individual building, the whole system will still be underpinned by the use of fast modes, particularly the car. Thus, ‘pods'of neo-traditionalism may still be appended to a distinctly modern, segregated highway system.

While a modernist town design might have a‘town centre'plugged in rather arbitrarily to an asymmetric urban structure, as we have seen, the postmodern town might not have a centre at all. Rather it could have a multiplicity of foci spread out in the system, so the system become in effect a distributed‘network'. Here, movement is no longer simplistically‘gravitational'towards the centre of the settlement. The settlement is no longer‘centripetal'. While modernist settlement design‘liberated'urban structure from traditional forms of streets and squares, replacing them with free -flowing highways and plugged -in pods of development, then, postmodern settlement design might be said to have‘liberated'urban structure from the need to be so expressive about the technological possibilities of fast movement and segregation. Instead, it simultaneously allows the traditional street grids, which unite slow traffic with building frontages, to co-exist with the highways created to cater for faster moving traffic. Of course, regardless of conscious intentions towards collage, some of these highways are inevitably inherited. Large infrastructural objects like urban motorways tend to endure, being resistant to the vagaries of architectural fashion.