This chapter provides an overview of the primary technology that distinguishes
BIM design applications from earlier generation CAD systems. Object-based
parametric modeling was originally developed in the 1980s for manufacturing.
It does not represent objects with fi xed geometry and properties. Rather, it
represents objects by parameters and rules that determine the geometry as well
as some nongeometric properties and features. The parameters and rules can be
expressions that relate to other objects, thus allowing the objects to automatically
update according to user control or changing contexts. Custom parametric
objects allow for the modeling of complex geometries, which were previously
not possible or simply impractical. In other industries, companies use parametric
modeling to develop their own object representations and to refl ect corporate
knowledge and best practices. In architecture, BIM software companies have
predefi ned a set of base building object classes for users, which may be added
to, modifi ed, or extended. An object class allows for the creation of any number
and relationships with other objects. How an object updates itself as its context
changes is called its behavior. The system-provided object classes predefi
ne what is a wall, slab, or roof in terms of how they interact with other objects.
Companies should have the capability of developing user-defi ned
parametric objects—both new ones and extensions of existing ones—and corporate
object libraries for customized features and to establish their own best
practices. Object attributes are needed to interface with analyses, cost estimations,
and other applications, but these attributes must fi rst be defi ned by the
fi rm or user.of object instances, with forms that vary, depending on the current parameters