Necessity of tailormade models

The decision-making models relevant to Open Design are 'glass box' models. In these models the control unit, in which the decision variables are represented, is open and transparent. An open control unit enables the decision maker to work out what the best solution is. The goals and criteria on the basis of which he decides are known.

In current practice, decision making models for design problems are more of a 'black box' type: the control unit is closed and often fixed. Model specialists consider design problems to be too complicated to be incorporated into the model in an open and transparent form. In order to simplify matters, they first devise a standard behaviour for the decision maker, which they then incorporate into the model.

The usefulness of black box models can only be tested by evaluating the outcomes of the model, not the behaviour of the model itself. The best known black box models are the System Dynamics model (Forrester, 1969), and the Gravity model (Lee, 1973).

In Open Design, it is necessary for stakeholders to be able to change their decisions during the decision making process. It has been shown (Van Loon, 1998, p. 239) that the 'glass box' models facilitate this very well, and that they can be constructed during the process itself.

If a model of a system with an open control unit is required, the goals and criteria of the decision-makers have to be incorporated. This must take place in such a way that the decision-makers can reach agreement. This lays the foundation for a departure from the fixed solution space. In goal-oriented models this space is in fact 'free' since it can be discussed, negotiated and changed during use. Moreover, the modelling process for the 'free' solution space progresses quite differently from what is set out in operations research modelling. The modelling itself becomes part of the design process, because

Figure 1.4 Mathematical modelling in Open Design

the modelling runs parallel to the designing.

At the outset of the design process, the solution space will be an unstructured collection of possibilities, such as a stock of materials, a plot of land, an assortment of building elements (doors, windows, etc.), prototypes, etc. The goals are vague to begin with: a good living environment, an attractive building, enough space, efficient use of energy and so on. As the process progresses, the collection of elements will become structured. During this process, the vague initial goals are gradually expanded, allowing for explicit requirements and limitations on the use of the available resources.

In Open Design, this whole process is based on mathematical modelling and computer processing of the mathematical model (Fig. 1.4). As becomes apparent from the figure, modelling and model application are interwoven, which produces many advantages in a multi-party design situation. The members of the team can see from the various sub-solutions whether the related parts of the model are acceptable to them. Usually, they will not all agree. Each person will judge the situation from the point of view of his own interests and will try to influence the model accordingly. The modelling thereby also becomes a multi-party process. It is, therefore, necessary to combine modelling and the use of the model in one integrated process.

The nature of this intertwined process of mathematical modelling and (com puter) processing the results, brings along the necessity to build a tailor-made model for each individual case. Attempts to build models with a wide validity, for instance certain categories of office buildings, are bound to fail. The architect must be able to 'play' with his model. When new stakeholders are identified, their preferences must be incorporated by adding new constraints. Creating such tailor-made models can only be learned by doing so. We have seen students at first wrestling for weeks with a fairly simple model, yet later being able to set up a completely new model in just a few hours.

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