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
2.2.1 Parametric Design
Conceptually, building information modeling tools are different fl avors of object-based parametric modeling systems. They are different because they have their own predefi ned set of object classes, each having possibly different behav- iors programmed within them, as outlined above. A fairly complete listing of the predefi ned object families provided by major BIM architectural design tools is given in Table 2–1 (as of mid-2010). These sets of predefi ned object families are those that can be readily applied to building designs in each system.
In addition to vendor-provided object families, a number of Web sites make additional object families available for downloading and use. These are the modern equivalent of drafting block libraries that were available for 2D drafting systems—but, of course, they are much more useful and power- ful. They include, for example, furniture, plumbing and electrical equipment, and proprietary fasteners for concrete fabrication. They are available both as generic objects and as models of specifi c products. They are discussed in Chapter 5, Section 5.4.2, where some of the sites are listed.
The built-in behaviors of BIM objects identify how they can be linked into assemblies and automatically adjust their own design when their context with
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other objects change. Examples are walls and their updates when other walls or ceilings change, as shown in Figure 2–9. Another is how spaces update in most systems when their bounding walls change. These object classes also defi ne what features can be associated with building objects. A connection is a basic feature in a fabrication-level BIM application. Can a connection be made in the face of a wall (a feature often encountered in precast concrete)?
Because of such possible limitations, it is important that users can extend the given base object classes or create new ones to address issues not originally anticipated by the BIM software developers.
The base objects that are built into the most popular BIM design software are shown in Table 2–1. The parametric objects supported in BIM construction tools are listed in Table 2–2. These tables only list objects that come with the BIM
Table 2–1 Built-In Base Object Families in Major BIM Architectural Design Applications
BIM DESIGN Tool
Base Objects ArchiCAD v14
Bentley Architecture v8.i
Revit Architecture
v2011
Vectorworks 2010
Digital Project V1, R4, SP 7 Site model Mesh tool, site
objects
(Contoured model) (Topo surface)
& site objects
In Landmark product
Surface model
Space defi nition (manual)
(manual)
(automatic)
(manual)
(automatic)
Wall
Column
Roof
Stair
Slab
Zone Zone Zone Area Area
Beam
Unique Objects for Each Platform
Cast-in-place, precast concrete, steel, masonry, thermal & mois- ture, furnish- ings, equipment, conveying systems, plumbing, HVAC, electrical, site
Curtain walls, truss, plumbing, toilet accessories, handrails, shelving, shaft
Area, component, ceiling curtain system, curtain grid, mullion, truss, beam system foundation items, ramp, railing
Window wall, mech. equipment, kitchen cabinet, railing, elevator, escalator, rail, pipe fi ttings, duct fi ttings, mechani- cal equipment
Pipe, duct, mech.
equipment, railings, opening, opening profi le construction equipment
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Tool
Base Objects Tekla v16.1
Design Data SDS/2
Revit MEP v9.1 (Objects)
AutoCAD MEP (Objects & Blocks)
Bentley Mechanical and Electrical v8.i Base Objects Part
Beam Polybeam Contour plate Welds Weld Logical weld Polygonal Weld Loads Load line Load area Load point Bolts Bolt array Bolt circle Bolt list Reinforcing Rebar strand Rebar mesh Single rebar Rebar group Rebar splice Task type
Grid lines Member Material Connection Bolts Holes Welds Loads Moments
Air terminals Communication devices Cable tray Connectors Conduit Connectors Duct fi ttings Duct accessories Duct connectors Electrical devices Elect equipment Elect. fi xtures Fire alarm dev.
Flex duct Flex pipes HVAC zones Lighting devices Lighting fi xtures Mech. equipment Nurse call devices Pipe accessories Pipe connectors Plumbing fi xtures Space
Cable tray Cable tray fi tting Conduit Conduit fi tting Device Duct
Duct custom fi tting Duct fi tting Duct fl ex Engr. space Hanger Multiview part Panel Pipe
Pipe custom fi tting Pipe fi tting Plumbing line Schematic line Pipe fl ex Plumbing fi tting Wire
Space
Mechanical:
Ducts Pipes Connectors Valves
Grills & Diffusers Dampers Filters Silencers Electrical:
Cable trays Power distribution – Lighting – Fire alarm – Emergency Lighting
Telecommunications – Information
technologies – Security – Public address – Lighting protection – Video
– EIB Spaces
Engineering zones Knowledge
Functionality
Clash detection 4D simulation Work packet
coordination Quantity
take-offs Supports
automated fabrication Interfaces to multiple structural analysis tools
Automatic connection design Erectability
checks Quantity
take-offs Supports
automated fabrication Interfaces to multiple structural analysis tools
Synchronized schedules Duct and pipe
sizing/pressure calculations HVAC and
electrical system design
Conduit and ca- ble tray modeling (gbXML)
interface for use with Autodesk® Ecotect® Analysis software and Autodesk® Green
Building Studio® Web-based analysis and IES
Synchronized schedules Interfaces for
fabrication Automatic duct siz-
ing based on space demands
Electrical circuit manager Interference
checking
Radiator sizing and number
Plumbing pipe sizing
Exchange data with energy analysis programs such as EDSL/TAS,
ECOTECT, Trace 700, Carrier HAP,
Green Building Studio, etc.
Feeder and branch circuiting
Automated circuiting and labeling
Online design checks for circuit load, length, and number of devices Automated fi xture
arrangement Bidirectional links
to third-party lighting analysis programs:
– Lumen Designer – DIALux – Relux
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application, not the externally available objects available from other sources.
Some companies have tried to include as broad a range of desired objects as pos- sible. Others have limited their built-in objects to those with specifi c parametric behavior that is related to other objects in the addressed market sector.
Each of the BIM design applications also includes other objects that are used to modify primary building shell objects. They include openings and joints in walls and slabs, openings for skylights and dormers in roofs, connectors for beams, columns, and other structural objects.
A distinction exists between those objects that interact with other objects, such as walls, beams, slabs, columns—that have complex behavior that are the core of a BIM design tool, and other objects that do not need to have para- metric behaviors, such as bathroom fi xtures, door and window products with fi xed sizes, and other objects that do not vary with their context. This second class, sometimes called building object models, are more easily created and made available in external libraries because they do not depend heavily on the dynamic parameters of other objects. This second class is widely available on building object Web sites and the libraries supporting this architecture are reviewed in Chapter 5; fabrication-level building objects are also discussed in Chapter 7. The third class of objects is the commercial products that are cus- tom-made to their context. These include curtain wall systems, complex ceil- ing systems, cabinetry, railings, and other architectural metalwork. These are simple or complex parametric objects whose defi nition requires the same care in defi ning their behavior as the base objects in a BIM design tool. Only a few new object classes have been defi ned for this class of building products (see Chapter 5, Section 5.4.2). Architects and fabricators sometimes build their own object classes for this use (see Figure 2–6 for an example) or rely on sim- pler nonparametric objects that users must continuously update and manage.
A functional difference in building modeling tools from that of other industries is the need to explicitly represent the space enclosed by building elements. Environmentally conditioned building space is a primary function of a building. The shape, volume, surfaces, environmental quality, lighting, and other properties of an interior space are critical aspects to be represented and assessed in a design.
Until recently architectural CAD systems were not able to represent building spaces explicitly; objects were approximated using a drafting system approach, as user-defi ned polygons with an associated space name. Credit is due to the General Services Administration (GSA) for demanding that BIM design applications be capable of automatically deriving and updating space volumes, beginning in 2007. Today, as shown in Table 2–1, most BIM design applications represent a building space as an automatically generated and
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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.