Chapter 1 Discussion Questions
3. A major improvement allows the parameters defi ning one shape to be linked through rules to the parameters of another shape. Because
2.2 PARAMETRIC MODELING OF BUILDINGS
2.2.2 Parametric Modeling for Construction
updated polygon defi ned by the wall intersections with a fl oor slab. The poly- gon is then extruded to the average ceiling height or possibly trimmed to a sloping ceiling surface. The older manual method has all the weaknesses of manual drafting: users must manage the consistency between wall boundaries and spaces, making updates both tedious and error-prone. The new defi nition is not perfect: it works for vertical walls and fl at fl oors, but ignores vertical changes in wall surfaces, and often cannot refl ect nonhorizontal ceilings.
Architects work initially with nominal building element shapes. But engi- neers and fabricators must deal with fabricated shapes and layouts that vary from nominal and must carry fabrication-level information. Also, shapes change due to pre-tensioning (camber and foreshortening), defl ect due to gravity, and expand and contract with temperature. As building models become more widely used for direct fabrication, these aspects of parametric model shape generation and editing will require additional capabilities of BIM design applications.
Parametric modeling is a critical productivity capability, allowing low-level changes to update automatically. 3D modeling would not be productive in building design and production without the automatic update features made possible by parametric capabilities. However, there are hidden effects. Each BIM tool varies with regard to the level of implementation of parametric mod- eling, the parametric object families it provides, the rules embedded within it, and the resulting design behavior. Customizing the behaviors of the object classes provided involves a level of new expertise not widely available in cur- rent architecture, engineering, and fabrication offi ces.
families for embedding different types of expertise (see Table 2–2). They are also related to different specifi c uses, such as materials tracking and ordering, plant management systems, and automated fabrication software. Early examples of such packages were developed for steel fabrication, such as Design Data’s SDS/2®, Tekla Structures®, and AceCad’s StruCad®. Initially, these were simple 3D layout systems with predefi ned parametric object families for connections, editing operations such as for copes that trim members for steel connections.
These capabilities were enhanced to support automatic connection design based on loads and member sizing. With associated CNC cutting and drilling machines, these systems have become an integral part of automated steel fabri- cation. In a similar manner, systems have been developed for precast concrete, reinforced concrete, metal ductwork, piping, and other building systems.
Recent advances have been made in concrete engineering with cast-in- place and precast concrete. Figure 2–10 (see color insert) shows precast rein- forcing embedded to meet structural requirements. The layout can be easily adjusted to the section size and to the layout of columns and beams. Parametric modeling operations can include shape subtraction and addition operations that create reveals, notches, bullnoses, and cutouts defi ned for connections to other parts. A precast fabrication-level architectural façade example is shown
FIGURE 2–10
An automated reinforcing layout and connections for precast concrete in Tekla Structures.
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in Figure 2–11, in terms of the 3D model of the piece and the piece mark (the drawing that describes one or more pieces of the same defi nition). Each build- ing subsystem requires its own set of parametric object families and rules for managing the layout of the system. One set of rules defi nes the default behav- ior of each object within the system; another set defi nes how sections are cut and the layout format for drawing it.
Efforts are now underway within several construction material associa- tions, such as the American Institute of Steel Construction’s Steel Design Guide (AISC 2007), which currently encompasses 21 volumes, and the Precast/
Prestressed Concrete Institute’s PCI Design Handbook (PCI 2004). Members within these organizations have worked together to draft specifi cations for defi ning the layout and behaviors of objects in precast and steel design. Use of
FIGURE 2–11
A parametric model of an architectural precast panel and the piece mark drawing derived from it.
Image provided courtesy of High Concrete Structures.
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these tools by fabricators is discussed in more detail in Chapter 7. It should be noted that despite the fact that fabricators have had a direct hand in defi ning these base object families and default behaviors, they often need to be further customized so that detailing embedded in the software refl ects a company’s specifi c engineering practices.
Two steel fabrication applications and three mechanical/electrical system BIM layout systems are summarized in Table 2–2. They show the relative cov- erage and embedded knowledge of these building system applications.
In fabrication modeling, detailers refi ne their parametric objects for well- understood reasons: to minimize labor, to achieve a particular visual appear- ance, to reduce the mixing of different types of work crews, or to minimize the types or sizes of materials. Standard design-guide implementations typically address one of multiple acceptable approaches for detailing. In some cases, various objectives can be realized using standard detailing practices. In other circumstances, these detailing practices need to be overridden. A company’s best practices or standard interfacing for a particular piece of fabrication equip- ment may require further customization. In future decades, design handbooks will be supplemented in this way, as a set of parametric models and rules.
Several fabrication-level CAD systems in widespread use today are not general-purpose parametric object modeling BIM design applications. Rather, they are traditional B-rep modelers, possibly with a CSG-based construction tree and a given library of object classes. For many purposes, these are fi ne products. AutoCAD Architecture is a common platform for construction- level modeling tools such as CADPipe and CADDUCT, which are examples of such tools. We review AutoCAD MEP in Table 2–2 as one example. Some Bentley and Vectorworks products are also of this type, with fi xed vocabular- ies of object classes. Within these more traditional CAD system platforms, users can select, parametrically size, and lay out 3D objects with associated attributes. These object instances and attributes can be exported and used in other applications, such as for bills of material, work orders, and fabrication.
These systems work well when there is a fi xed set of object classes to be com- posed using fi xed rules. Appropriate applications include: piping, ductwork, and cable tray systems. Architectural Desktop was being developed in this way by Autodesk, incrementally extending the object classes it could model to cover those most commonly encountered in building, before it acquired Revit.
New object classes can be added to these systems through the ARX or MDL programming language interfaces.
A critical difference between these earlier systems and BIM is that users can defi ne much more complex structures of object families and relations among them than is possible with 3D CAD, without undertaking programming-level
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software development. With BIM, a curtain wall system attached to columns and fl oor slabs can be defi ned from scratch by a knowledgeable nonprogram- mer. Such an endeavor would require the development of a major application extension in 3D CAD. See for example the custom objects in Figure 2–6.