Chapter 1 Discussion Questions
2.1 THE EVOLUTION TO OBJECT-BASED PARAMETRIC MODELING
2.1.2 Object-Based Parametric Modeling of Buildings
instances and other properties defi ned and controlled according to a hierarchy of parameters at the assembly and at an individual object level. The shapes could be 2D or 3D.
In parametric design, instead of designing an instance of a building element like a particular wall or door, a designer fi rst defi nes an element class or family which defi nes some mixture of fi xed and parametric geometry, a set of relations and rules to control the parameters by which element instances can be generated.
The shape from a model family will vary according to its context. Objects and their faces can be defi ned using relations involving distances, angles, and rules like attached to, parallel to, and offset from. These relations allow each instance of an element class to vary according to its own parameter settings and the con- textual conditions of related objects (such as the walls a given element butts into). Alternatively, the rules can be defi ned as requirements that the design must satisfy, such as the minimum thickness of a wall or concrete covering of rebar, allowing the designer to make changes while the rules check and update details to keep the design element satisfying the rules and warning the user if the rules can- not be met. Object-based parametric modeling supports both interpretations.
While in traditional 3D CAD every aspect of an element’s geometry must be edited manually by users, the shape and assembly geometry in a parametric modeler automatically adjusts to changes in context and to high-level user controls. In this sense, it edits itself, based on the rules used to defi ne it. An example wall class, including its shape attributes and relations, is shown in Figure 2–7. Arrows represent relations with adjoining objects. Figure 2–7 defi nes a wall family or class, because it is capable of generating many instances of its
FIGURE 2–7
Conceptual structure of a wall-object family, with vari- ous edges associated with bounding surfaces.
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class in different locations and with varied parameters. Wall families can vary greatly in terms of the geometry they can support, to their internal composi- tional structure, and how the wall can be connected to other parts of the build- ing. These are determined by how the wall class designers set up the wall’s parameters and the parameters assigned and objects related to a wall instance.
Some BIM design applications incorporate different wall classes to allow more of these distinctions to be addressed (but don’t try to convert one type of wall to another—it cannot be done).
For most walls, the thickness is defi ned explicitly as two offsets from the wall control line, based on a nominal thickness or the type of construction.
The offsets may be derived from an ordered list of layers that show the core, insulation, cladding, interior fi nish, and other signifi cant properties of the wall object. Some systems support tapered walls, applying a vertical profi le to the section. The wall’s elevation shape is defi ned by (usually) one or more base fl oor planes; its top face may be an explicit height or related to a speci- fi ed set of adjacent planes (as shown in Figure 2–7). The wall ends are defi ned by the wall’s intersection, having either a fi xed endpoint (freestanding) or associations with other walls or columns. Special operations are required if some layers protrude beyond the fl oor level, such as for covering the founda- tion with the wall fi nish. The control line of the wall (shown along the bot- tom in Figure 2–7) has a start and end point, so the wall does too. A wall is associated with all the object instances that bound it and the multiple spaces it separates.
Wall construction such as stud layouts can be assigned to one or more lay- ers in the wall (multiple when providing acoustical or thermal breaks). Door or window openings have placement points defi ned by a length along the wall from one of its endpoints to a side or to the center of the opening with its required parameters. The construction and openings are located in the coordi- nate system of the wall, so they all move as a unit. A wall will adjust its ends by moving, growing, or shrinking as the fl oor-plan layout changes, with win- dows and doors also moving and updating. Any time one or more surfaces of the bounding wall changes, the wall automatically updates to retain the intent of its original layout. Constructions should, but may not, update themselves when the length of a wall changes.
Walls are ubiquitous and complex. A well-crafted defi nition of a paramet- ric wall must address a range of special conditions. These might include:
The door and window locations must not overlap each other or extend beyond the wall boundaries or where a wall tee intersection blocks an opening. Typically, a warning is displayed if these conditions arise.
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A wall control line may be straight or curved, allowing the wall to take varied shapes in plan.
A wall may intersect fl oor, ceiling, other walls, stairs, ramps, columns, beams, and other building elements, any of which are made up of mul- tiple surfaces and result in a more complex wall shape.
Walls made up of mixed types of construction and fi nishes may change within segments of a wall.
As these conditions suggest, signifi cant care must be taken to defi ne even a generic wall. It is common for a parametric building element class to have over 100 low-level rules for its defi nition and an extensible set of properties.
These conditions show how architectural or building design is a collaboration between the BIM object class modeler, who defi nes the system of behaviors of BIM elements, and the architectural or building user, who generates designs within the products’ rule set (or building semantics). It also explains why users may encounter problems with unusual wall layouts—because they are not cov- ered by the built-in rules. For example, a clerestory wall and the windows set within it are shown in Figure 2–8. In this case, the wall must be placed on a nonhorizontal fl oor plane. Also, the walls that trim the clerestory wall ends are not on the same base-plane as the wall being trimmed. BIM modeling tools have trouble dealing with such combinations of conditions.
In Figure 2–9, we present a sequence of editing operations for the sche- matic design of a theater. The designer explicitly defi nes the bounding relations of walls, including end-wall butting and fl oor connections, in order to facilitate later easy editing. When set up appropriately, changes such as the ones shown in Figures 2–9a to 9g become simple and it is possible to make quick edits and updates. Notice that these parametric modeling capabilities go far beyond those offered in previous CSG-based CAD systems. They support automatic updating of a layout and the preservation of relations set by the designer. These tools can be extremely productive.
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FIGURE 2–8
A clerestory wall in a ceiling that has different paramet- ric modeling requirements than most walls.
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the sloped house fl oor; (d) these are aligned to the sloped fl oor; (e) rules are added to align the sloping wall with the lobby fl oor;
(f) the areas of the house are used for quick estimates of seating; (g) the lobby depth is increased to provide more space, automati- cally changing the slope of the house fl oor and the bottom of the side walls; (h) the house space area is reviewed to consider seating implications.
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