A Hierarchic structure of the building model

made.

The third feature of a complex system is its ability for adaptation and perhaps this one is the most important for the generative design system. The adaptation consists of a change in the behaviour of a system that improves its chances of survival. An analogy to the adaptation would be the gradual transformation of a building model, in order to satisfy the design objectives in the best possible way.

3. A HIERARCHIC STRUCTURE OF THE BUILDING MODEL

In the subsequent paragraphs I argue for a hierarchic structure of a building model. I refer to selected, general approached to hierarchic structure of a system and apply them to the building model specifically. The point for structuring a building model in a hierarchic way is to make the generation process more efficient.

An important property of complex systems related to its hierarchic structure is near-decomposability, a notion proposed by Simon (1962). It posits that the number of relations is higher within subsystems than between subsystems. Near-decomposability is a property that facilitate a species’

development, because the evolution process does not ‘need’ to decompose and reorganise entire structures of individuals, but it keeps the integrity of groups (cells, tissues or organs for example). Following Ashby (1960: 192), the hierarchic composition of systems significantly increased the efficiency of their adaptation: “No complex adaptive system will succeed in adapting in a reasonable amount of time unless the adaptation can proceed subsystem by subsystem, each subsystem relatively independent of the others.”

21 More generally speaking external signaling would be the information exchange between a developing building model and a building environment, where a designer functions as a ‘mediator’.

Thus, forming groups of elements and operating on them rather than on basic building elements would make the generative process much easier.

Such approach corresponds to the actual design practice, where a design process involves more often reconfigurations of groups of elements, such as staircases, rooms, certain fragments of walls and facades, than individual building elements. In this manner, instead of decomposing the model into its smallest elements and working on them individually every time there is a need for redesign, an architect can keep the successfully designed parts of a model only reorganizing them.

A reasonable arrangement of a building model might enhance efficiency of the generative design system. First step in such an arrangement would be recognition of distributions of connections between building elements. In the second step, strongly connected building elements should be grouped. For example, adjacent stair units together with handrails, balustrades and the like, should become a group. In this manner, similar types of elements (a certain type of wall unit) or cooperating types of units (e.g. frame of a window, glass units) that are close to each other should make groups. Examples of such groups might include:

6. an entrance (doors and windows combined);

7. a bearing system component (columns and beams combined);

8. a ramp system (ramps and floors combined), and so on;

McShea extends the idea of near-decomposability by the notion of nestedness: a higher-level entity consists of ‘lower’ entities. McShea illustrates nestedness with the following biological example:

Level 1: Prokaryotic cells (the simplest cells, such as bacteria);

Level 2: Aggregates of level 1 organisms, such as eukaryotic cells (more complex cells whose evolutionary ancestors originated from the fusion of prokaryotic cells);

Level 3: Aggregates of level 2 organisms, namely all multicellular organisms;

Level 4: Aggregates of level 3 organisms, such as insect colonies and ‘colonial organisms’ such as the

Portuguese man o’war (Mitchell, 2009: 110).

When it comes to the building model, its organization does not have to be

‘two-dimensional’ (building elements and groups of elements). It seems beneficial, that the building model is hierarchic on more than two levels, in a manner of nestedness. Small groups of elements can make up bigger groups.

Example of a structure that has more than two levels of hierarchy could be as follows. Level 1 of organisation would contain very basic components that represent elements manufactured and quality-controlled in a factory, like screws, beams, cladding plates, bricks, insulation fragments, window panes etc. Level 2 would contain assemblies of these elements: e.g. units of certain type of a wall, balustrades, doors, windows etc. Finally, level 3 would contain assemblies of elements of level 2, that refer to functional parts of a building, such as: staircases, entrance areas, communication areas, or structurally coherent parts of a building, like for example a glazed façade.

In the automatic adaptation process, only certain level of the building model would be decomposed. The depth of such decomposition should be a parameter of adaptation that is adjusted by the generative design system dynamically. Usually, large alterations of a building model occur at the outset of a design process and then the generative design system could operate on very basic building elements. Later phases of the adaptation could involve redistribution of only groups of elements. This approach can be applied very concretely in the context of Evolutionary Computing, using strategies such as a parameter adjustment or a parallel-terraced scan, where the extensiveness of building model’s decomposition can be an adapting parameter (ref. section VIII.3.)

Finally, Christopher Alexander (1970) applies the concept of near-decomposability to architectural design. Novelty of Alexander’s approach consists in the fact that he focuses on connections among the properties of a building model instead of connections among the building elements. He starts with the concept of misfit. A misfit occurs when a certain property of design does not fit the context of its use (for example a kettle that is too small, or hard to pick up when it is hot, or hard to store in the kitchen etc.) The role of the architect is to ‘solve’ these misfits, i.e., to adjust the properties of design to the design context.

Here comes the reference to the near-decomposability. Because the number of design properties is large, and the cognitive capacity of a designer is limited, a standard design practice is to ‘decompose’ the design problems

into sub-problems. This decomposition is usually based on verbal categories, such as aesthetic, functional, economic, acoustic, structural etc. It is easier to work and solve design problems within each category separately, because this limits the number of interdependent qualities that the designer needs to take into account. Eventually, the designers try to put together the separate solutions and apply them to the design.

However, according to Alexander, such a conventional approach is problematic, because the distinction into verbal categories does not reflect the actual distribution of connections among the design aspects and the design properties from the different verbal categories might strongly depend on each other. For example, a functional quality (the shape of a home theatre room) is connected to the acoustic quality (quality of music) and thus, the solutions worked out separately for the functional aspects (overall layout and shape of rooms) could not match the acoustic qualities. Therefore, the conventional division of a design problem into sub-problems based on verbal categories is deficient.

Alexander proposes an alternative division, based on the actual

dependencies amongst the design qualities. For example, the shape of a home theatre room would be in the same group as the acoustic quality of this room, because one property affects another. In this method it is essential to identify all the relevant design qualities and the links between them. Because the distribution of the links is not uniform (the design structure is not

homogenous), it is possible to mark out groups of qualities which are more strongly interconnected (figure 4).

Figure 4. Two ways of dividing design properties. The first way (left) is the conventional one, where qualities are divided according to the verbal categories (functional, structural, economic etc.) The second way (right) is proposed by Alexander - here the qualities are grouped in terms of their actual dependencies.

Alexander’s approach can be applied to the generative design system in a very interesting way, using EC techniques for multi-objective problems. Here I only shortly refer to the discussion in section VIII. Design involves multi-objective problems, because it considers many different and interconnected properties of a building model; and these properties have to meet respective design objectives. One approach to multi-objective problems is evolution of individuals in ‘environmental niches’. Grouping qualities of design that are interconnected can be compared to grouping individuals in separate environmental niches.

PRINCIPLES FOR THE GENERATIVE DESIGN PROCESS

Three following principles, that address the building model, conclude this section:

1. A generic structure for building models should be flexible. The generative design system can represent only those building models that are anticipated in a predefined generic structure (it cannot represent all conceivable building models).

2. A building model should consist of elements which resemble real architectural components. That is, building elements should consist of geometric objects and auxiliary information about which architectural component is represented by the object. The

information can include physical properties of a certain element, its function or how it relates to other objects.

3. A building model should have a hierarchic structure. Certain groups of building elements should be strongly interconnected making groups of elements. This would be beneficial in the adaptation process, because each redesigning of a building model would not entail a total decomposition of the model, but the groups of strongly interconnected elements would be kept.

SECTION VI – BUILDING