Building representation in BIM
This chapter approaches BIM as a symbolic building representation and explains its key differences from analogue representations and their facsimiles in CAD. It analyses how a model is built out of symbols that may have an uneasy correspondence with real-world objects and how abstraction applies to these symbols. It concludes with a view of models as graphs that reveals what is still missing in BIM.
discrete symbols but these referred to implementation mechanisms: the geometric primitives that comprised a symbol in analogue representations. In BIM, the symbols explicitly describe discrete building elements or spaces — not their drawings. BIM symbols usually come in “libraries” of elements, i.e. predefined symbols of various types. The types can be specific, such as windows of a particular kind by a certain manufacturer or abstract, e.g. single-hung sash windows, or even just generic windows. The hierarchical relations between types enable specificity and abstraction in the representation, e.g. deferring the choice of a precise window type to a later design stage, without missing information that is essential for the present stage, as all currently relevant properties of the window, e.g. its size and position, exist in a generic window symbol.
Entering an instance of a symbol in a model normally follows the next procedure:
• The user selects the symbol type from a library menu or palette
• The user positions and dimensions the instance in a geometric view like a floor plan, usually interactively by:
◦ Clicking on an insertion point for the location of the instance, e.g. on the part of a wall where a window should be
◦ Clicking on other points to indicate the window width and height relative to the insertion point (this only if the window does not have a fixed size)
Modifications of the instance are performed in three complementary ways:
• Changes of essential properties such as the materials of a component amount to change of type. This is done by selecting a different symbol type from the library menu or palette and linking it to the instance.
• Changes in the geometry of an instance involve either repositioning the reference points or numerically changing the relevant values in any of the ways allowed by the program interface: in dialogue boxes that pop up by right-clicking on the instance, in properties palettes, through dimension lines or schedules.
• Changes in additional properties that do not conflict with the type, e.g. the occupancy of a space or the stage where a wall should be demolished, are entered through similar facilities in the interface, like a properties palette. Some of these properties are built in the symbols, while others can be defined by the user.
BIM symbols make all properties explicit, whether geometric or alphanumeric. The materials of a building element are not inferred from its graphic appearance but are clearly stated among its properties, indicated either specifically or abstractly, e.g. “oak” or “wood”. Most properties in an instance are inherited from the type. This concerns not just materials but also any fixed dimensions: each wall type typically has a fixed cross section. This ensures consistency in the representation by keeping all similar elements and components truly similar in all critical respects.
Consistency is essential for many tasks, such as cost estimation or procurement.
Many of the relations between symbols are present in BIM, even if they are not always obvious or directly accessible. Openings like doors and windows, for example, are hosted by a wall. They are normally entered in a model after the wall has been placed and in strict connection to it: moving
a window out of the hosting wall is not allowed. Connected walls may also have a specific relation, e.g. co-termination: if one is moved, the others follow suit, staying connected in the same manner.
Similarly, spaces know their bounding elements (which also precede them in the representation) and if any of these is modified, they automatically adapt themselves. Through such relations, many links between symbols are hidden in BIM. A door schedule, for example, (Figure 1) reveals that, in addition to its hosting wall, a door knows which two spaces it connects (or separates when closed).
Figure 1. A door schedule in BIM reveals that each door is aware of the spaces it connects
Quite important is the explicit symbolic representation of both the ‘solids’ out of which a building is constructed (building elements like walls, floors, doors and windows) and the ‘voids’ of the building (the spaces bounded by the building elements). In analogue representations, the spaces are normally implicit, i.e. inferred by the reader. Having them explicit in BIM means that we can manipulate them directly and, quite significantly from the perspective of this book, attach to them information that cannot be linked to building elements. Similarly to specifying that a window is made of sustainable wood, one can specify that a space is intended for a particular use, e.g. “office”
or for specific activities like “small group meeting” or “CEO’s meeting room”. Such characterizations
relate to various requirements (usually found in the brief), such as floor area and performance specifications, e.g. acoustics or daylighting, which can also be attached to the space and used to guide and evaluate the design. Making spaces explicit in the representation therefore allows for full integration of building information in BIM and, through that, higher specificity and certainty.
Spaces, after all, are the main reason and purpose of buildings, and most aspects are judged by how well spaces accommodate user activities.