Being symbolic representations, models in BIM can be described by graphs that express their structure in terms of symbols and their relations. These graphs are similar but not the same as the building, adjacency or access graphs discussed in a previous chapter: those are graphs that describe the design rather than the representation. In the graphs that describe a BIM model, symbols are usually represented by vertices and relations between symbols by edges (Figure 6 &
7). The edges often make explicit what is implicit in the model, for example, that each window or door is hosted by a particular wall and that walls connect to each other in a specific manner, e.g.
co-terminate at the ends.
Figure 6. Floor plan of a model in BIM, comprising four walls, a door and a window
Figure 7. Graph of symbols in Figure 6
The graph summarizes the basic structure of the model: the entities it comprises and their basic relations, including dependencies, e.g. between the shape and size of the room and the configuration of walls that bound it. These relations and their constraints underlie many behaviours in BIM, for example, that doors and windows tend to stick to the hosting walls and that walls try to retain their end-connections.
From an information perspective, this becomes even more interesting if we zoom in on the properties of the symbols and see how they are affected by relations and constraints, as in the case of a window and the wall that hosts it (Figure 8). Both elements are represented by
discrete symbols, each with its own set of properties. The hosting relation means that some of the properties of the wall are inherited by the window. For example, the orientation of the window is by definition the same as the orientation of the wall. This constrains the behaviour of the window symbol when the user positions it in the model or modifies either the window or the wall.
Figure 8. Graph of a window and the wall that hosts it in a model
Describing such dependencies is especially important for relations that may be missing from the orthographic views and modelling workflows of BIM software, for example between walls and floors (Figure 9). The vertical dimensions of walls are usually determined by a combination of wall symbol properties and constraints, model setup (levels) and the presence of symbols like floors to which the top or base of a wall can attach. Users have to manipulate the wall and floor dimensions
and relations in multiple views, which can be summarized in this rather simple graph that affords the overview necessary for IM.
Figure 9. Graph of a wall and its relations to floors
Such graphs also reveal other relations that determine the compatibility of symbols in a relation (a type of parameterization that remains neglected). Wall and window width, for example, must be such that there is a technical solution for inserting the wall in the window: the width of a wall constraints the acceptability of window types. The same applies to length and height: assuming that the window comes in a standard size, the wall should be longer and higher (or at least equal) in order to accommodate it. The example seems trivial but dimensional incompatibilities of this kind are common in walls that combine multiple openings and different components.
Understanding the building representation in terms of symbols, relations and constraints is key to both parameterization and decision making. Therefore, it becomes a main task for IM. In addition to using graphs to describe the structure of a model, the model should be set up in a way that transparently expresses all dependencies and safeguards them effectively and consistently in all workflows. This concerns relations between symbols in the representation, as well as with external constraints, such as planning regulations, building codes and briefs. For example, modellers should make explicit the maximum height allowed by the planning regulations for a particular design and connect it to the relevant symbols and properties, e.g. the position of the roof. By doing this at the onset of a design project, they ensure that designers are aware of the constraints within which they work, e.g. that they are not allowed to place roofs or other elements higher than permitted. As will be discussed in following chapters, such feedforward that guides design is
preferable to feedback, i.e. allowing designers to generate solutions that are then tested according to regulations they may have not taken into account.
Key Takeaways
• BIM is a truly symbolic building representation that employs discrete symbols to describe building elements and spaces
• Symbols in BIM integrate all properties of the symbolized entities, which determine their pictorial appearance
• BIM symbols are largely independent of graphic implementation mechanisms and immune to most geometric biases
• The correspondence between BIM symbols and some building elements is problematic in certain respects due to the structure of these elements, persisting geometric biases and human perception
• Abstraction in BIM is both typological (as symbols are at various abstraction levels) and mnemonic (based on similarity of properties and relations like proximity and hosting between symbols)
• Models in BIM can be described by graphs of symbols and relations; these graphs afford the overview and transparency missing from BIM software interfaces
Exercises
1. In a BIM editor of your choice (e.g. Revit), make an inventory of all wall types (Families in Revit) in the supplied library. Classify these types in terms of abstraction, clearly specifying your criteria.
2. In a BIM editor of your choice, make a simple design of a space with four walls and two floors around it. Identify properties of the building elements and space symbols that connect them (e.g. dimensions) and overlapping properties (e.g. space properties that refer to finishings of the building elements).
1. Make schedules and graphs that illustrate your findings.
2. Compare the schedules and graphs.
3. Expand your design with another space and a door that connects them. Make a schedule and a graph that illustrate the key relations between the spaces.
4. In the expanded design, describe step by step how a change in the size of one room is propagated to other symbols in the model.
Notes
1. A comprehensive general introduction to BIM is: Eastman, C., Teicholz, P.M., Sacks, R., & Lee, G., 2018. BIM
handbook (3rd ed.). Hoboken NJ: Wiley.