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www.elsevier.com/locate/autcon
Automation in Construction 13 (2004) 291–312
Parametric 3D modeling in building construction with examples
from precast concrete
Rafael Sacks*, Charles M. Eastman, Ghang Lee
College of Architecture, Georgia Institute of Technology, Atlanta, GA 30332-0155, USA
Accepted 12 May 2003
Abstract
Buildings are complex products containing relatively large numbers of distinct parts that are collected in multiple assemblies
for different design, analysis and production purposes. Modeling buildings in fully parametric 3D computer-aided design
(CAD) systems offers numerous benefits in terms of productivity, the ability to rapidly generate design alternatives at different
levels and elimination of errors that result from the disparity between different drawings in current practice. However, full
realization of these benefits requires specialized functionality, including top-down modeling, objects with functional behavior,
the ability to embed contextual design intent, automation of layout and detailing and appropriate management of similar objects.
An effective system must provide such functionality while maintaining adequate response times. The requirements, features and
performance have been examined as part of specification of a new 3D parametric CAD platform for the North American Precast
Concrete Software Consortium (PCSC). They are described and discussed after a review of solid and parametric modeling, with
examples from the domain of precast concrete construction.
D 2004 Published by Elsevier B.V.
Keywords: Parametric building modeling; Computer-integrated construction; Design automation; Precast concrete
1. Introduction
The theoretical benefits of computer-integrated
construction—lower engineering costs due to auto-
mated analysis and drafting, lower production costs
due to reduced error rates and data integration sup-
porting management functions and automated fabri-
0926-5805/03/$ - see front matter D 2004 Published by Elsevier B.V.
doi:10.1016/S0926-5805(03)00043-8
* Corresponding author. Department of Civil and Environ-
mental Engineering, The Technion-IIT, Room 840 Rabin Building,
Technion City, Haifa 3200, Israel. Tel.: +972-4-8293190; fax: +972-
4-8323433.
E-mail addresses: [email protected],
[email protected] (R. Sacks).
cation—have been widely stated [6,34]. Practice,
however, lags far behind theory. Computer-aided
design and drafting (CADD)1 has been widely adop-
ted by precast concrete design and fabrication com-
panies, as it generally has by the rest of the
construction industry. A survey of North American
precast concrete producers found that 96.3% use
CADD in-house; the remainder outsource their design
work to consultants, whom the survey found to be
100% CADD users [23]. However, electronic drafting
has not resulted in any change in the process work-
1 Note: The term ‘CADD’ (computer-aided design and
drafting) is deliberately used rather than ‘CAD’ (computer-aided
design) to emphasize the drafting aspect of the technology.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312292
flows. In CADD, computers are used to generate
drawings, which are the medium of communication
during the production and erection stages of the
precast process. CADD drawings are only readable
as graphics, so that information transfers for process
activities such as structural analysis, bills of material,
coordination between building systems, quality con-
trol, rebar fabrication and piece production, must be
done by people. For many of these activities, the labor
cost of translating data from CADD to some automa-
tion application negates the economic viability of the
automation; manual data entry is also prone to human
error. In practice, little of the design and production
automation potential inherent in information technol-
ogy is exploited with CADD; in terms of the business,
design and production process, CADD has simply
replaced physical drawing boards with electronic
ones.
In many manufacturing industries, on the other
hand, drafting has been replaced by computer-aided
design based on 3D solid modeling. Solid modeling
supports a wide range of automation and quality
control applications that utilize the information gen-
erated. In the field of building design and construc-
tion, the parallel potential benefits include the use of
knowledge-based design tools, automated detailing
and drawing production, automated interfaces to
structural, thermal, vibration and other analyses and
quality improvements. The impacts are not limited to
design and engineering—automatic fabrication and
assembly is also possible (e.g., Refs. [20,22]).
With few exceptions (most notably Frank Gehry’s
use of CATIA and Xsteel for the Walt Disney Concert
Hall [16]), parametric solid modeling software has not
been applied in the architecture, engineering and
construction (AEC) industry. The primary reason is
the additional human effort required to build a geo-
metrically and topologically accurate solid model of a
building and the corresponding absence of economic
incentive for building designers to undertake that
effort [9]. However, a new generation of 3D paramet-
ric modeling tools is now becoming available (e.g.,
Autodesk Revit, Graphisoft ArchiCAD, Bentley Tri-
forma, Design Data SDS/2, Tekla Xsteel). These hold
the potential to make modeling of buildings and their
subsystems, such as precast concrete structures, cost
effective, thus opening the door to many additional
design, production and erection benefits.
This paper surveys technical issues associated with
the use of parametric solid modeling to design build-
ings at construction levels of detail. It begins with a
review of solid and parametric modeling concepts and
software, describing their essential features as they
apply to the building industry. Several examples are
reviewed. We then detail a range of issues that must
be considered in any implementation of parametric
modeling software for building design and construc-
tion. The issues focus on the specific characteristics of
building design. The first set is applicable to computer
modeling of buildings in general; the second set
relates specifically to parametric modeling. An exam-
ple of a parametric model of a small precast concrete
assembly is presented to illustrate some of the issues
discussed. The work is based upon the authors’
involvement with the North American Precast Con-
crete Software Consortium’s (PCSC) specification of
a software system for integration and automation of its
engineering design procedures [8]. Although the
examples are drawn from the realm of precast con-
crete, the discussion applies in principle to other
sectors of the construction industry.
2. Solid and parametric modeling
2.1. Early solid modeling
The manufacturing and aerospace industries began
using three-dimensional computer-aided design
(CAD) systems based on surface modeling in the
early 1970s. These industries recognized that accurate
representation of a part’s geometry could lead to
automatic analysis of the part’s behavior (structural,
thermal, acoustic, etc.) and support its automated
fabrication. However, defining the 3D shape of a
mechanical part was very complicated, tedious and
error prone, requiring cutting and trimming of each
individual surface to match with the others it inter-
sected. In the mid-1970s, the work of Braid [3],
Requicha and Voelcker [26] and others led to the
development of solid modeling. Solid modeling pro-
vided for the representation of volume-enclosing sets
of surfaces and powerful editing operations that
allowed single operations for gluing shapes together
or subtracting one shape from another, while preserv-
ing volume-enclosing properties [1,18]. Solid model-
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 293
ing operations combined with curved surface editing
of the solid’s faces (their bounded surfaces) allowed
easy definition of any 3D shape on a computer and
enabled assembly of multiple shapes. Solid modeling
allowed many of the original goals of 3D CAD to be
realized: accurate representation of a three-dimension-
al shape; automatic derivation of any shape measure-
ment, including volume and surface areas; cutting of
sections, including automatic derivation of section
properties; automatic drawing generation of parts or
assemblies, with automatic dimensioning. Solid mod-
eling also allowed automatic checking of spatial
interferences between assemblies, and with fully de-
fined shapes, sufficient information was available to
support numerical control machining or other fabrica-
tion processes.
Early solid modeling CAD systems were brought
to the AEC market in the 1980s (RUCAPS, Calma,
TriCad, PDMS) but were generally unsuccessful.
They were highly complex, required an approach to
design quite different to that to which designers were
accustomed, were unreliable and required expensive
hardware. They suffered a number of drawbacks from
the point of view of building design:
– While solid modeling intuitively supported defi-
nition of an accurate geometric model of a part, it
was not obvious how it should be used to design
large assemblies, as required in buildings.
– Revising a solid model could be tedious; moving
an opening in a wall required filling in the existing
opening (with an addition operation) and subtract-
ing a new opening in a second operation.
– Defining 3D solid shapes required significantly
more effort than defining equivalent 2D drawing
representations using simpler and cheaper systems.
As a result, even though 3D modeling systems
were developed during the same period as early
CADD systems, CADD was the technology adopted
for building design and construction.
2.2. Development of parametric modeling systems
One of the approaches to solid modeling was based
on the representation of shapes as algebraic formula-
tions of solid primitives (e.g., cylinder, box, wedge).
The primitives could be combined using the set
operations of union, intersection and subtraction in
an algebraic expression. Evaluating the expression
generated a temporary solid model in memory, but it
was not stored. This representation was very compact
and easily edited [25], but could not support some
operations, such as taking any arbitrary measurement.
It was called constructive solid geometry (CSG), as
distinguished from classical solid modeling of the
bounded surfaces, which was called boundary repre-
sentation or B-rep.
Developers of B-rep systems realized that they too
could benefit by recording the operations carried out
to create a solid model. Instead of editing the model
directly, some operations were more easily imple-
mented by editing the operations on a history log
and then recreating the object. The CSG expression
was called the unevaluated model, and the boundary
representation generated from it was called the eval-
uated model [26]. Later, the ability to explicitly
reference parameters that defined earlier operations
from within subsequent operations was added, leading
to the development of what are now called parametric
solid modeling systems [14].
The concept of parametric modeling was not new;
some of these capabilities were embedded in the very
first CAD system, Sketchpad [33]. However, exploi-
tation of the power of this capability has evolved
slowly, as knowledge of desired CAD system behav-
ior has grown, and as the significant computing power
required for the complex computations necessary to
derive assemblies of shapes automatically has become
more readily available. In order to define assemblies
of shapes, people utilized additional capabilities: in
addition to solid shapes, they relied on 2-D shapes for
grid lines, dimensions and other construction elements
[7]. Grid lines allowed definition of the location of
solids relative to them (on or offset from grid inter-
sections). Dimensions provided controls for varying
the shapes of the primitives that that were added and
subtracted to construct a solid model.
Parametrics greatly expanded the ease of use of
solid modeling. Rules that could act as design con-
straints were added to early parametric solid modelers
to guide the regeneration of a design in light of
different local conditions. For example, a rule could
be defined and applied to insure that a hole intended
for a bolt would be resized correctly if the diameter of
the bolt itself was changed. For building design, this
R. Sacks et al. / Automation in Construction 13 (2004) 291–312294
means that instead of composing a building assembly
as a collection of instances of typical parts with fixed
geometry, the geometry of each part can be derived
from the spatial relationships between it, its neighbors
and the building grid. In this way, changes to any of
the parameters defining grids or spacing can be
propagated to all the parts automatically.
Fig. 1a illustrates the principle—the beam is auto-
matically sized to fit between the columns, and the
corbel supports are automatically sized to fit the beam
span. Any change made to any of the independent
dimensions (lAB, w1, w2 or c) will result in propaga-
tion of the change to the beam and to the supports. In
addition, if the beam is removed, the supports are
automatically removed (recognition of connections
between pieces as a separate logical entity is crucial
to enabling this behavior).
Fig. 1. Parametric model of a be
Note that in parametric CAD systems, shapes are
not only initially generated based on operations and
constraints; the constraints are also maintained as an
integral part of the model geometry during editing.
This allows expression of the intended dynamic
design behavior of the objects. In this context, we
define ‘‘behavior’’ as ‘‘automatic system actions that
maintain the topological and geometrical consistency
to the relationships within and between model
objects.’’ In Fig. 1b, if the column is rotated (a user
action), then the system will automatically reposition,
reshape and resize the beam (a system reaction) to
restore conformance to constraints 1 and 2. In non-
parametric systems, rules may be imposed when
shapes are generated, but are not applied once the
shape is inserted. (For example, in AutoCADR [Auto-
desk], a nonparametric modeler, a line can be drawn
am between two columns.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 295
perpendicular to another line, but the relationship
between the two lines is not maintained by the system
in the event that either line’s orientation is changed.)
Parametric modeling enables modelers to build and
refine complex model assemblies with ease. The
operations on solids typically apply to two shapes at
a time (‘‘intersect shape A and shape B’’), and these
are considered topologically related. Other operations
are unary (‘‘chamfer this edge’’). Rules between
variables and shapes define additional topological
relations; for example, the equations in Fig. 1 relate
the columns to the grid, the beam to the column and
the grid and the corbels (beam supports) to the beam,
the column and the grid. An important issue is that
when certain solid modeling operations are applied
between shapes, the relations among the surfaces
making up the shapes may change. Shah and Mantyla
[32] describe the evaluation procedure, from the point
of view of implementation, including definition of
topology, definition and application of constraints and
evaluation of the model. Evaluating a model can be
complex in situations where constraints conflict with
one another (the model is termed ‘over-defined’), or
where they do not define a unique result (the model is
termed ‘under-defined’). Most systems alert the user
to such conditions, requesting resolution before the
model is fully evaluated (e.g., SolidworksR).Parametric modeling makes a significant contribu-
tion to design in that, along with solid modeling, it
allows modelers to generate computer representations
of physical objects not only as they look, but also to
define semantic relationships between the objects’
representations, allowing them to be easily created
and edited. The relationships define necessary topo-
logical relations among the objects making up a
system, defining a graph of relations expressing how
the different parts of an assembly ought to be derived.
In parametric modeling, the graph is a depiction of
design intent. For example, the simple model in Fig. 1
specifies the intent regarding the size of the haunches
the beam sits on, and that the spacing between column
and beam is to be generated by shortening the length
of the beam, rather than notching the column.
In these ways, parametric modeling software appli-
cations dedicated to specific design domains define
relations and constraints to express the logic of design
conditions. Using this technology, assembly relations
can be defined, which enable rapid definition of
complex assemblies and their quick and effective
revision. This structure releases the designer from
the onerous task of maintaining the logic of the design
intent and the resulting integrity of the model. Para-
metric modelers therefore contribute greatly to mak-
ing entry and maintenance of solid models of complex
assemblies, such as buildings, a realistic endeavor.
Today, a growing number of parametric modeling
systems exist in the architectural market that capitalize
on these benefits (among them, Bentley Triforma,
Graphisoft ArchiCad and Autodesk Revit).
3. General considerations for computer modeling
of buildings
The following characteristics apply not only to
parametric modeling; they are desirable for any prac-
tical building modeling system. In the case of para-
metric modelers, however, they are essential condi-
tions for realization of the benefits that can be achieved
by implementing the functionality outlined in the
previous section.
3.1. Maintaining a ‘single-source’ model
The driving principle behind modeling buildings
rather than representing them in drawings is the
opportunity to represent all design and fabrication
information in a single integrated source. Irrespective
of the view in which a user edits the model, the
changes are propagated to the single source and are
accurately reflected in all other possible views. In this
sense, assembly and detail drawings, bills of material
and other production documents are relegated to the
status of reports. These reports are all guaranteed to be
consistent, and cross-checking across all data is easy
and can potentially be automated. They are not stores
of the information as they are in traditional CADD.
This principle is maintained in most parametric solid
modeling systems and is considered essential for
eliminating coordination errors.
Insuring that a single source will be maintained
implies that the system must provide automated rou-
tines to generate the production documents, including
plans, elevations, sections, details and reports. If prep-
aration of documents is not automated and remains a
time-consuming task, users will prefer to make changes
R. Sacks et al. / Automation in Construction 13 (2004) 291–312296
directly on drawings rather than editing the model and
redrawing all of the documents. This invalidates the
model, as it is not updated. A common issue, in this
regard, is the current ease of revising a drawing or any
other report locally without adding the change to the
central model. Some systems support such capabilities,
but the effect is that the changes are made without any
form of auditing or permanent record.
3.2. Scope
For any construction domain, the goal of replacing
paper drawings with digital models requires that the
application used to instantiate and modify the design
information must completely cover the scope of
assemblies and components used in the building
being modeled within a particular organization. If
any element cannot be modeled, users are forced to
add the missing data on paper printouts, thus violat-
ing the principle of maintaining one repository for the
data. On the other hand, no single application can be
expected to provide all of the diverse design, analy-
sis, detailing and other functionality that might be
needed to develop meaningful models of real build-
ings for all types of companies, in regions with
different climatic and code requirements. In the
precast development, this is one of the reasons for
the concept of a precast parametric platform with
independently provided plug-in applications, as op-
posed to a comprehensive precast parametric appli-
cation. The platform architecture proposed for the
Fig. 2. Software system architecture for prec
PCSC (see Fig. 2) is designed to support plug-in
software modules to be developed by independent
software providers. The basic requirement for the
parametric platform is that its data structures and
types and its geometric operations must support the
full scope of real-life objects that are incorporated in
a precast concrete building.
4. Special considerations for parametric modeling
of buildings
In building design, priority is given to integra-
tion across different systems and assemblies; in
mechanical design, priority is often placed on
optimization of individual components for mass
production. This influences the functional and per-
formance requirements of parametric CAD systems
for building design, when compared with the design
of mechanical parts. Firstly, buildings are composed
of very large numbers of distinct parts, arranged in
various functional and production-specific assem-
blies, all of which must be modeled if the integrity
of a design is to be maintained. Many of the parts
are identical to other parts and used repetitively,
although minor variations between specific instan-
ces are common. Secondly, certain relations be-
tween building components hold true in the
general case for all buildings (e.g., doors must be
set within walls) and can be embedded in a
parametric building modeler. These are typically
ast concrete design and production [8].
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 297
different to the relations that are emphasized in
tools for mechanical design. This section details
specific technical considerations for successful imple-
mentation of parametric modeling software for build-
ings in general and for precast concrete construction in
particular.
4.1. Top-down vs. bottom-up modeling
Parametric solid modeling can be structured to
support top-down or bottom-up modeling. In this
context, we use the following definitions:
– Bottom-up modeling starts with explicit representa-
tion of distinct parts, with parametric relations
defining the details and components of those parts,
followed by successive association of the parts to
form aggregated assemblies.
– Top-down modeling is the explicit definition of a
total product, and then refinement of the product
design by iteratively replacing objects that repre-
sent whole assemblies with successively finer
grained parts until the level of detail required for
production is achieved.
In most engineering disciplines, such as the aero-
space and automotive industries, the performance
limitations of parametric systems and the complexity
of individual pieces traditionally restrict their use to
bottom-up modeling, rather than for modeling com-
plete assemblies. The computer systems and modeling
software used for the overall product and for detailing
the parts are typically quite different. Although the
overall shape and configuration of the product is
designed conceptually (i.e., top-down) and the perfor-
mance of the airplane or automobile is tuned at this
level, the individual pieces that make up the compo-
nents of the assembly are modeled separately, albeit
parametrically. Representation of all the individual
parts in one large aircraft assembly model, which is
not commonly done, required an array of mainframe
computers [2].
Building design benefits from both top-down and
bottom-up approaches in different contexts and at
different stages. Most buildings are one of a kind,
custom designed. During conceptual design, the
overall form and size of the building are set, design
progresses as functional systems are defined, and
lastly, individual components can be detailed. The
local geometry of each component is set to satisfy
the form and functional constraints imposed by its
position and role in the assembly of which it is a
part. In this case, the whole (or major aspects) of the
overall parametric model need to be related, allowing
revision and potential regeneration. This requires that
software support a top-down modeling approach.
Top-down parametric modeling of buildings imposes
corollary requirements—in terms of performance and
embedded behavior—that, given current hardware
capabilities, cannot be practically met by current
parametric mechanical computer-aided design sys-
tems (this is discussed in detail in Section 4.8
below).
In all areas of design, considerations of economy
make the use of standard ‘‘off-the-shelf’’ compo-
nents important. This is especially true of buildings.
For standard parts, the assembly must be adapted to
accommodate them, requiring bottom-up functional-
ity. Careful consideration of the rules and con-
straints must be given wherever such functionality
is required within an essentially top-down modeling
system. For example, if the corbel in Fig. 1 were
only available in standard widths (values of wc), the
rules would have to be redefined not only to enable
discrete selection of an appropriate corbel for each
location, but to rationalize the selection of multiple
corbels. While bottom-up design is more common
at the detailed design level, some industrialized
building systems are based entirely on bottom-up
design at the scale of the complete structure [35]. It
is worth noting that most artifacts, including build-
ings, are eventually produced and/or assembled
bottom up.
An intensive investigation of design and produc-
tion processes carried out by the PCSC [27–29] has
shown that the generic design process for precast
concrete projects can be broken down into three main
stages: assembly layout, assembly detail and piece
detailing (Fig. 3). The first two stages require top-
down modeling functionality; first, the overall layout
is defined, and the assemblies are fitted to the con-
ceptual layout; next, the assemblies are broken out
into individual pieces, and their shapes are defined to
fit all neighboring pieces. In the final stage, bottom-up
modeling is necessary, as standard components such
as lifting hooks, prestressing strands, connection
Fig. 3. The process of precast design.
2 Building walls in architectural design systems are a well-
understood example of such functional objects (e.g., Ref. [36]). The
walls grow or shrink in response to the movement of connecting
walls, floors and ceilings. Windows and doors inserted into the walls
are also updated automatically and attempt to maintain the intent of
their original location—a fixed distance from a sidewall, axial
symmetry, etc. Such behavior, in slightly varying forms, is embedded
in most current generation architectural parametric solid modelers.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312298
plates and reinforcing bars are detailed to fit in the
concrete pieces.
4.2. Modeling function
Parametric solid modeling allows representation of
functional relations between parts as well as geomet-
ric relations. The functional properties of building
parts include structural, thermal, acoustic, safety and
other well-defined criteria (potentially including aes-
thetic criteria). Parametric behavior, implemented
with attributes and topological relations, defines the
way in which parts will interact with one another in
support of their function. For example, the function
of a structural spanning element is to accept loads
and transfer them to columns, walls or to higher order
spanning parts; its parametric behavior must insure
that it always be supported by at least two supports
(or one fixed support for a cantilever.) Fig. 1 reflects
this behavior. A window-to-wall joint is another
example: its function is to insulate the joint against
transfer of heat, moisture and sound and to provide
structural rigidity to the window. As such, its para-
metric behavior must insure that it is always
connected to both a wall and a window and extend
through the full perimeter of their contact. In each of
these examples, different specific solutions, with
different geometric details, may be developed to
satisfy the same functional requirements. Beams
may be of different materials, cross-sections, etc.;
window-to-wall joints will involve different techno-
logical solutions depending on the window and wall
materials and their geometry. Despite the variation in
possible solutions, the same functional behavior is
required.
How to satisfy these relations between objects in
response to one or more functions is an important kind
of design expertise. Functional knowledge of this
nature can be embedded in parametric solid modeling
software.2 The resulting software can select, apply and
adjust object relations in response to various condi-
tions, either as an initial layout, or later when the
design is revised. It also allows the capture of best
practice and systematic refinement of those practices.
Thus, functional behavior in objects enables users to
assemble 3D building models quickly and accurately,
greatly expediting design development and revisions.
It also enables automated matching of analysis meth-
ods to building assemblies, and the functional require-
ments against which analysis results must be com-
pared can be stored. When alternative technical
solutions to any part or assembly are sought, the
parametric objects determine the scope of alternatives
that can be applied.
Additional examples of functional behaviors that
could be programmed into objects include
� the ‘external envelope’ of a building with thermal
transfer, self-shading and enclosure; parametric
behavior insures that the envelope encloses a space
completely, and the object carries properties
necessary for energy analysis;� piping systems that have given flow requirements;
they resize themselves in response to routing
changes and detail themselves in response to given
connections;� stairways, which connect building floors with runs
and landings, where a run is a sequence of risers
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 299
and treads; risers, treads and landings must adjust
to code and good practice requirements.
Since the behavior can be defined parametrically,
that is, as variable rather than fixed values, many
details can be user controlled, such as spacing, sizes,
etc. For example, the size of a door and its offset from
the end of the wall in which it is located can be easily
varied. However, if the range of solutions that can be
generated excludes a specific case that a designer
wants, or the object itself is to be a new case, these
capabilities become a limitation, rather than a help.
Users trying to customize such parametric details find
they are fighting the system, rather than working with
it. It must be possible to override such object behav-
ior. More generally, it should be possible to customize
such object behavior and extend it.
4.3. Abstract functional objects
In order to make such functional behavior open-
ended, the behavior can be abstracted and represented
separately from the objects whose behavior they
define. This allows the behavior to be applied to
new object geometry and, when desired, to revise
the behavior. We call the objects that carry the
functional behavior alone abstract functional objects
(AFOs). AFOs represent, store and implement the
functional properties of building assemblies and parts
and carry their fundamental parametric behavior and
design intent. Similar constructs have been proposed
for use in contexts other than parametric modeling,
such as building product modeling [12], automated
development of early design alternatives [11] and
automated structural assembly layout [31]. The ‘con-
tainer spaces and functional features’ defined by
Csabai et al. [5] for top-down 3D layout design in
mechanical engineering reflect similar intent.
AFOs can be provided to users in parametric
modeling systems as abstract classes from which parts
with geometric form inherit function and behavior. In
most cases, predefined generic or specific parts may
be laid out in a design, with the parts carrying the
functional and physical definitions together. However,
if a custom part or means to realize some function is
desired, then the AFO provides an open-ended way to
realize that function within a new object class. AFOs
are expressed in the model as objects with attributes
and methods that define or can check their function;
they must also encapsulate the behavior of any new
object class they define, so that objects of the new
class will ‘know’ how to lay themselves out and
update themselves whenever their parameter values
are changed.
In a sense, we propose that 3D parametric building
modeling systems be viewed as collections of pre-
defined abstract functional objects, with embedded
analytical capabilities, that include an initial library of
the common physical objects that embody them.
Sophisticated users should be able to define new
physical objects that incorporate the functionality of
existing functional objects by using the AFOs to
define new objects and behavior (as new subclasses
with new or refined methods).
We distinguish between AFOs and abstract anal-
ysis objects (AAOs), which are often used to represent
real objects for engineering analysis, such as ‘stick-
and-node’ representations [4,15,19]. AFOs express
the function and design intent of physical objects that
have structure; AAOs are simply convenient ideal-
izations of real-world systems that are required by
specific analysis techniques. In general, AAOs are not
manipulated directly by users, but are generated—and
deleted—within analysis software modules. Examples
of AAOs include idealized diaphragms and shear
walls for structural analysis, space boundaries and
mass concentrations for thermal analysis and finite
and boundary elements for various analyses.
The difficulties inherent in separating the function-
al and physical aspects of objects representing build-
ing parts are apparent in the example of structural
connections between precast concrete pieces. They
could be modeled simply as relationships between
structural elements [15,24], although a more useful
representation is achieved by objectifying the rela-
tionship such that connections are modeled as distinct
objects. If a connection object inherits the functional
behavior of a ‘structural connection AFO,’ then when
first instantiated, it can apply topological relationships
between the pieces it connects and model structural
behavior for analysis. Once reactions are calculated, it
can be detailed in terms of distinct physical compo-
nents (the steel plates and other components with
which they are fabricated) and in terms of the geo-
metric relationships between its components and the
pieces they connect. It is worthwhile noting that
Fig. 5. Parametric dimensions of a spandrel beam.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312300
connections per se are not built as distinct physical
objects, but rather have their components embedded
in the pieces that they connect, as shown in Fig. 4.
4.4. Embedding contextual design intent
The ability to constrain the shape, size, composi-
tion, location and orientation of pieces and connec-
tions enables designers to embed their intent in
parametric building models. The first level of design
intent is the explicit function of the objects them-
selves, as described above. A second, higher order
level can be defined for situations in which a semantic
relationship between building parts can be inferred
from their spatial context. Minimum headroom be-
tween a floor and any object overhead, minimum
corridor widths for egress and maximum slope for a
floor required to serve as a ramp are examples of
these. Software should enable designers to apply this
type of design intent in terms of dimensional con-
straints driven by equations or in terms of offset
mating relationships between elements. Consider, for
example, the spandrel beam in Fig. 5: if the designer
intends that the spandrel function as a railing above
the slab, then the full depth of the spandrel could be
constrained using an equation (note that the transverse
beam is assumed to be functionally constrained to rest
on the spandrel ledge):
d1 ¼ rmin þ d2 þ d3 ð1Þ
where d1 = spandrel depth, d2 = ledge depth, d3 = beam
depth and rmin =minimum railing height.
Fig. 4. Precast corbel connection: (a) a
Any change in the depth of either the ledge or the
beam would cause the spandrel to be updated such
that the railing height is kept constant at its minimum
allowed value. Similarly, constraints can be used to
ensure axial loading conditions between pieces and
within connections, e.g., the axis of a beam along its
base might be constrained to be collinear with the
centers of the bearing pads of the corbels on which it
rests.
However, design intent may vary from one project
to another, even for similar design situations. Thus,
careful consideration must be given to selecting con-
straints for automated insertion by design software.
An additional problem is that in certain cases, design
intents may conflict with one another—attempts to
model conflicting constraints will result in an over-
defined parametric model. In such situations, design-
ers must select which constraints to apply and which
to release. The spandrel beam in Fig. 5 provides an
example. The construction plane defining the second
s designed and (b) as fabricated.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 301
floor has been set at a height of H ( = L1� L0) above
the ground, and the depths of the beam (d3) and the
ledge (d2) are set according to strength or stiffness
requirements; the clear height between the ground and
the base of the spandrel beam (hc) is therefore a
derived value. In this situation, a minimum clear
height requirement cannot be implemented as a para-
metric constraint.
An alternative is to use specialized error-check-
ing functions to detect such problems as clear
height for vehicles, minimum headroom in stair-
cases, sufficient sloping for drainage, etc. Post hoc
functional evaluation is appropriate for handling
effects that span multiple assemblies, including
nonadjacent and potentially nonstandard parts and
arrangements, but it requires that any evaluation
software be able to recognize the parts relevant to
it. On the other hand, it has shortcomings: the
functions must be preprogrammed, and they are
prone to human error because the operator must
call them. Resolving conflicts of this nature will
often require adaptive change and not simply di-
mensional change; if the floor-to-floor height in Fig.
5 cannot be increased, an alternative (shallower)
mechanism must be found for supporting the beam
on the spandrel.
Care must be taken in setting up constraints that
implement either functional behavior or design in-
tent. In most cases, there is more than one way in
which a parametric constraint can be responded to.
However, a software designer embedding intent
through rule-based or other automated routines can-
not predict a priori all of the possible design config-
urations that could arise in practice. Selecting the
correct constraints to implement demands careful
analysis of the relationships between parts and the
ways in which the relationships change in different
design contexts. A trade-off exists between the
necessity to automate insertion of constraints be-
tween parts as far as possible on the one hand, and
on the other, the need to leave final control in the
hands of the user for situations in which nonstandard
design solutions are appropriate. For example, a
common situation in precast design arises when
pieces intersect spatially as they are placed in an
assembly. In practice, one must be cut away (in fact,
it must be defined with a block out to effect the cut)
to accommodate the other. Applying a cutting profile
to the cut piece is one possibility, but in fact, the
cutting profile is dependent on the location and shape
of the full piece and must be positioned at a specified
clearance distance from it. A straightforward solution
is to generate an ‘envelope’ around each piece and
then apply cuts from that envelope to any intersect-
ing piece according to a set of priority rules (e.g., a
column would always ‘cut’ a hole in a double tee).
While the default solution would be applied in
accordance with a rule set, the user must be able to
override that solution and apply a different one in
certain situations.
4.5. Design automation
A parametric building modeling system, as out-
lined up to this point, would provide all the func-
tionality needed to quickly define a building design.
For example, in the case of a precast concrete
structure, a designer could establish a grid, select
and insert building elements relative to the grid, size
all the pieces, select and place connections, set
reinforcing parameters for each part, layout external
systems that intersect with precast pieces and so
forth. The model can then be adjusted by changing
parameter values at any level—any geometric adap-
tation required to maintain integrity is handled auto-
matically by the system. This facilitates exploration
of design alternatives, potentially enhancing the qual-
ity of the resulting design. Drawing production can
also be almost entirely automated. Together with
automatic maintenance of integrity, this can contrib-
ute to significant reductions in engineering and draft-
ing hours and in human error related to production
drafting [28].
However, much more automation is possible. Con-
sider three levels of building design, in order of
increasing degrees of sophistication.
4.5.1. Piece and connection design
Standard engineering analysis and design calcula-
tion methods exist for a wide variety of building
components. These are typically encoded in building
regulations and design handbooks, and software mod-
ules are available for many of them; for example,
there are multiple existing applications to calculate the
prestress strand required for various cross-sections
under user-defined loading conditions. Such applica-
R. Sacks et al. / Automation in Con302
tions can easily be linked to read and write parameter
values to a precast piece object, allowing the piece to
‘design itself’ automatically. In a more integrated
solution, using AFOs as a basis, these existing soft-
ware modules can be adapted to function directly as
methods of the objects. For example, a corbel con-
nection object, as typified in Fig. 6, has well-defined
structural analysis procedures that could be invoked to
set its dimensions and its rebar and steel plate com-
ponents automatically, based on the loads it must
carry. Libraries of such objects can be maintained at
the operational level of individual projects, across a
company, or at the level of an entire sector of the
building industry. Because building parts and assem-
blies may differ from company to company or from
region to region, parametric modeling software must
allow for efficient definition of new products, con-
nections and features.
4.5.2. Assembly level engineering analysis
If loads and loading conditions are applied directly
to the building model, the input for structural (and
other) analysis can be prepared and transferred auto-
matically. The resulting reactions can then be sent
back to the parametric model, where they form the
basis for automated refinement of the design param-
eters of the individual pieces and connections. This
greatly reduces the amount of engineering work
required. This practice has already been implemented
in structural steel packages (e.g., Tekla Xsteel and
Design Data SDS/2).
Fig. 6. Library representation of a corb
4.5.3. Knowledge-based assembly layout
For repetitive and rule-driven design work, a floor
system, a building core, or even in some cases an
entire building could be laid out automatically, from
only a few simple parameters. For example, a typical
all-precast parking garage can be specified based on
building site perimeter, maximum number of levels
and the desired number of parking spaces (other
parameters, such as parking bay size, traffic clearan-
ces, minimum headroom requirements, etc., are all
driven by building regulations that can be encoded).
Automated high-level layout and analysis of this
nature has long been a goal of computer-integrated
construction research. Prototype implementations such
as HI-RISE [17], IBDE [10], SEED [11] and ABS [30]
all explored automated building design. The lack of a
parametric building modeling platform to build upon,
together with the limited performance of earlier hard-
ware generations, were major impediments to com-
mercializing these efforts. The recent development of
fully parametric 3D building modeling platforms and
the recent increases in hardware performance may
therefore open the way to production implementation
of high-level design automation. Some aspects will of
course require further research, as discussed in the next
section.
4.6. Limits on parametric dependencies
We are proposing parametric solid models of
buildings that have deeper chains of recomputation
struction 13 (2004) 291–312
el connection (Tekla Xengineer).
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 303
dependencies than have generally been applied to
date. These have the computational cost implications
already discussed, but they also have definitional
complexities. There are logical limits to the extent to
which automated design and embedded design intent
can be imposed using parametric dependencies.
Three distinct techniques can be applied to change
a design model [21]: parametric adaptation, substitu-
tion of a part and topological adaptation. The first
level of change, parametric adaptation, is straightfor-
ward: for example, a precast concrete slab section
could have its depth set to a mathematical function
dependent on other geometrical parameters, such as its
clear span (as shown in Figs. 1 and 3). Substitution of
a part requires more sophisticated rules, but can also
be implemented, e.g., a specific connection used to
connect the slab to a supporting wall could be
replaced with a different connection when the load
to be transferred exceeds a certain value. Topological
adaptation, on the other hand, cannot be implemented
using the first two methods and requires replacement
of a module (incorporating one topology) with a
different module (incorporating another topology),
which requires human intervention. The intended
design behavior for a double tee support illustrates
this limitation.
The two stems of a double tee floor sections are
commonly supported in ‘pocket connections’ recessed
into a spandrel beam. As the stem depth increases, the
pocket depth must increase. At some point, the
stresses in the beam preclude cutting away for the
pocket, and so a ‘corbel connection’ (a support which
protrudes from the supporting member, as in Fig. 6) is
often substituted to support the extreme stem of a field
of double tees. However, should the load that must be
transferred increase more, engineers often decide to
shift the double tee (or enlarge the column), in order
to support the double tee stem directly on the adjacent
column, instead of transferring the load through
the beam first. This is a topological change, with
implications beyond the local scope of the elements
involved.
Topological changes can be automated if the rules
for selecting and changing between different topolog-
ical structures can be specified. Engineers learn about
the boundary conditions defining the limits within
which a particular calculation is appropriate; similarly,
a parametric model has boundary conditions, which
have to be included within any modeling structure.
Thus, a building model could be broken into a large
set of layout modules, each of which includes AFOs
and their physical design subtypes, with each module
able to maintain its own integrity. Small adjustments
can be automatically recomputed, but larger changes
will require human judgment to deal with part substi-
tution within a module or with the effects of topolog-
ical change between modules. However, the expertise
regarding rules for parametric boundary conditions is
not widely available and will become a growing area
of research. Progress in this area will require resolu-
tion of a number of open issues:
– What is the optimum nature of layout modules?
They may simply be assemblies of parts with sets
of topological relations embedded by inheritance
from AFOs, or they may be abstract assembly
constructs with different AFOs assigned to them,
and to which physical objects that correspond to
the AFOs can be assigned.
– What are the appropriate breakpoints between
layout modules? Breakpoints define the ways in
which building parts should be grouped in
assembly layout modules, such that integrity of
the design can be maintained through wide ranges
of parameter values.
– What techniques can be used to deal with the
boundary conditions at the breakpoints and to
include them in the parametric behavior? At
breakpoints, simple parametric adaptation can no
longer maintain integrity—part substitution or
topological change is necessary. The techniques
may include context-specific algorithmic or rule-
based approaches or more sophisticated AI
techniques.
– What topological relations need to be established
between the AFO-based objects across breakpoints
between layout modules, and how can their
integrity be maintained?
4.7. Identity, collections and change propagation
The use of multiple identical or near-identical parts
in any given building, such as doors, windows,
structural elements, stairs and many others, is com-
mon. Similarity tends to reduce design, production
and maintenance unit costs. Thus, a design tool must
R. Sacks et al. / Automation in Construction 13 (2004) 291–312304
enable management and tracking of series of parts.
The way in which collections of identical parts are
modeled in CAD systems has bearing on the size of
the data store, as well as on the functionality the
system provides. In precast concrete, groups of iden-
tical pieces are termed ‘piece-marks.’3 At least four
strategies are possible for managing identical or nearly
identical parts:
1. All parts retain their own identity and can
therefore be edited independently. The disadvantages
of this strategy are that any edits common to a group
of identical parts must be made separately to each
instance, and the maximum amount of memory is
required. Identical objects must be identified and
collected after design is complete.
2. A single definition is stored in a non-displayed
section of the model for all identical parts in a
collection. Each instance is an exact copy of the
original with its own geometric transformation. Geo-
metric variation of an individual part can only be
achieved by disassociating the instance from its orig-
inal. The user explicitly groups the original unit
definitions.
3. As in strategy 2, a single definition is stored and
original units are pre-grouped, but three improve-
ments are incorporated: (a) parametric variations of
any of an original unit’s defining dimensions are
allowed, (b) the general part model is defined as a
set of features and (c) each part instance references
the set of features it includes and its particular pa-
rameters (sometimes called a configuration of the
original unit). In this context, features are ‘semanti-
cally significant and distinct entities’ [32] and can
express both geometry and nongeometric attributes
(such as material). Each configuration can set features
to be present or absent and can apply independent
values to any subset of the parameters of the original
unit.
4. Every displayed object is independent, but each
is a composite of one or more instanced parametric
3 ‘Piece-marks’ are groups of identical precast concrete pieces.
The term is derived from the annotation that is commonly ‘marked
on the piece.’ Two levels are possible—pieces with only minor
variations in geometry can be grouped for production runs, while
only identical pieces are interchangeable in terms of the locations
they can occupy in the building. The product type is often also
encoded in the piece-mark text.
features. Unlike in strategies 2 and 3, there is no pre-
grouping of original units. The features are instanced
in the non-displayed part of the data model and
incorporated in composite objects in the assembly.
Individual objects in a model are grouped by asso-
ciation with identical features, not with identical
parts. For example, a set of beams may all have
identical cross-section, but some may have holes in
them and others not. Any change to the cross-section
is automatically propagated to all members of the
collection; on the other hand, holes could be added
to each independently. Changes to a composite
object may involve changes to one or more of its
instanced features. If any change results in a new
instance of a feature that is identical to an existing
feature instance, the new instance is not created;
instead, the change is made by redirecting the
reference from the composite object to the existing
feature instance.
The first strategy is rarely used. The second
strategy is common in nonparametric CAD systems
(as defined above). The third and fourth are relevant
for parametric systems for modeling buildings. They
differ in (a) the ways in which similar parts of a
building can be edited within the context of an
assembly, including the functionality they enable
for editing parts and propagating edits to similar
parts and (b) in how they enable grouping of parts
for design and prefabrication or production on site.
In systems of the third type, the user is responsible
for initiating and maintaining collections of parts.
When inserting a new instance of a product in an
assembly, the user can choose to create an entirely
new part, a new configuration of an existing part, or
to simply add a new instance by reference. The
resulting structure directly determines the grouping
of identical parts for production. This can be oner-
ous: when a local change is made to a part, that part
must be separated from its group, and a new indi-
vidual part must be generated. Edits to any part are
automatically propagated to all instances of that part
in a building assembly. This has advantages and
disadvantages. Propagation of changes is efficient,
assuming it is desired; however, it can be argued that
editing at the part level, with automatic propagation
of changes to all identical parts, can result in
nonsensical designs because while making changes
to the ‘master’ part, the user is unaware of the impact
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 305
of the change in the context of each part in the
assembly.
In systems employing the fourth strategy, most
editing is performed at the assembly level. Collection
of parts into production groups can be delayed until
design is essentially complete. Automated functions
based on user-defined rules can identify all identical
parts and assign them the same production identifier
(in precast concrete construction, a ‘piece-mark’). In
general, production groups will include all parts that
reference a subset of identical feature instances (i.e.,
they are identical in all respects significant for pro-
duction, while they may vary in minor ways that can
be effected after mass production). For example, all
columns of the same cross-section and length could be
grouped for production, despite the fact that only
some of them have corbels added to support beams.
Because parts are defined as collections of parametric
features, the memory requirements for this method
can be very compact. This is similar to the approach
explored by Hakim and Garrett [13] using description
logic.
While the last two strategies for maintaining group-
ings of similar parts are both effective in dealing with
building components during design and production,
each has a quite different style of use.
4.8. Performance
The complexity of buildings, both in terms of the
sheer number of individual parts and the multitude
of constraints and dependencies between them,
makes performance a key concern for viable para-
metric 3D modeling of buildings with top-down
functionality (see Section 4.1). Each time a change
is made to any dimension or feature of a parametric
model, the software must reevaluate all of the
affected constraints and relations in order to update
the properties and the geometric form of the model
to a newly consistent state. The computational
intensity of reevaluation grows geometrically with
the growth in the numbers of constituent parts and
relational constraints in a parametric model. This has
been a limiting factor in the past, and some systems
delay reevaluation until the user explicitly initiates
reevaluation.
In the authors’ review of the proposals of candidate
CAD platforms to serve as a platform for a precast
concrete parametric modeling system, a test was
prepared to benchmark the performance of different
systems. In preparation for the benchmark test, a
precast concrete parking garage structure was ana-
lyzed in detail in order to determine the full number of
distinct parts it contains. The building has four floors
with a total of 18,161 m2 floor space and contains 497
precast pieces. It is relatively simple in that it has no
interior partitioning, no finishes and no window or
door details. The total number of distinct parts—
precast pieces, connection plates, reinforcing bars,
prestress strands, bearing pads, etc. in this building
is 33,182.
The test required modeling of a typical bay of a
hypothetical precast garage (similar in configuration
to the building described above), with four floors of
double tee pieces supported on inverted tee beams,
which rested on corbels on four corner columns (Fig.
7). The test specification required that the structural
connections be modeled with parametric relations,
such that changes in column locations would be
propagated to change the beams and double tees
appropriately. This bay was then replicated 1,000
times, resulting in a structure with 21,601 pieces
(the large number of precast pieces compensates for
the fact that no internal detail was required for any of
them). At each stage, local changes to pieces were
prescribed, such as cutting holes through them, mov-
ing one row of columns, moving an individual column
and cutting a block shape out of an arbitrary portion of
the structure, while maintaining the integrity of the
relationships between the pieces. Of the five commer-
cial CAD platforms tested (Bentley Structural Tri-
forma, Autodesk AutoCAD, Solidworks, Nemetschek
Allplan and Tekla Xengineer), three did not exhibit
parametric behavior as defined above. Although they
provide parametric behavior on part definition and
insertion, they did not maintain parametric relation-
ships between objects and did not reevaluate in
response to changes.
The two remaining systems, which fulfilled all of
the functional requirements, were evaluated for per-
formance. The time required to reevaluate the model
and return control to the user after each prescribed
change, at each stage of replication of the building’s
bays and floors, was measured. Only one—a system
developed especially for building modeling—was
able to maintain response times of less than 1 min
Fig. 7. Performance benchmark test building.
Fig. 8. Assembly layout with construction planes.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312306
throughout the predetermined test sequence; the test
continued beyond the prescribed 21,601 pieces, until
86,404 pieces were modeled in the assembly. The
performance of the other system—a generic paramet-
ric modeling system—degraded in relation to the
number of pieces in the assembly, reaching response
times in excess of 3 min at 21,601 pieces, despite
running on a high-end PC computing platform.
This test highlights the significance of performance
for parametric systems intended for modeling build-
ings. Building models differ from models of mechan-
ical parts in that they must incorporate large numbers
of parts in an assembly. We hypothesize that the basic
structure of the parametric data, as implemented in
each system, determines its ability to deliver the
performance required for a designer to work effec-
Fig. 9. Schematic structure topology.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 307
tively. Where the relationships between the objects are
restricted to those that model structural behavior only
(beams, columns, etc.) the sequence of reevaluation in
response to parametric change is predetermined. In a
generic system, the sequence of reevaluation must
first be established algorithmically, and this degrades
performance as the number of related objects within
the same assembly increases.
5. Precast modeling example
The following simple test structure (Fig. 8) illus-
trates the complex parametric behavior required to
support the typical operations a precast designer must
perform in order to build and maintain a 3D model of
a building. The structure consists of one bay of a
building, defined by two sets of parallel grid lines
(denoted A, B, 1 and 2). Two columns, positioned
relative to the intersections of axes A-1 and A-2,
Fig. 10. Connection detailing—columns, inverted tee beam and corbel co
support a spandrel beam at the level of the first-floor
construction plane. The connections between the
spandrel beam and the columns are formed by creat-
ing pockets in the columns, with the constraint that the
exterior faces of the columns and the spandrel beam
remain collinear. Two more columns, located relative
to the intersections of axes B-1 and B-2, support an
inverted tee beam at the same floor level. The con-
nections in this case are corbels. A field of double-tee
sections rests on the ledge of the inverted tee beam; on
their opposite ends, the double tees have extended
stems which rest within pockets in the spandrel.
In the first stage—assembly layout—the designer
begins by establishing a layout for the building as a
whole (this may be copied or translated from an
architect’s design, or in the case of design-build,
originate with the precast designer). The layout con-
sists of vertical construction planes (designated Axis
A, Axis B, Axis 1 and Axis 2 in Fig. 8—these are
termed ‘building axes’ in plan drawings) and primary
nnections. Equivalent stick and node representation is shown inset.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312308
floor levels (horizontal planes—Plane 1 and Plane 2).
The locations and sizes of all the pieces in the
building will be derived directly or indirectly from
these planes.
Next, the designer selects precast products and
inserts instances of them into the assembly to repre-
sent precast pieces. The first pieces are positioned by
relating them to one another and/or to the construc-
tion planes (Fig. 8). The columns are located with x
and y offsets from vertical plane intersections. Their
top and bottom levels are fixed relative to horizontal
Fig. 11. User changes to piece geometry with cor
planes. At this stage, they are simply rectangular
prisms. The other pieces are placed relative to the
columns; in placing them in the assembly, the user
establishes their topological relationships to the pieces
already in the assembly. The beams are constrained to
lie such that their axes span between connection
points whose location is constrained relative to the
columns; the field of double tees is defined as lying
between the beams and between construction planes
‘Axis 1’ and ‘Axis 2.’ The resulting topological
dependencies are shown schematically in Fig. 9 (in
responding system updates of the assembly.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 309
the figure, dependencies are from right to left). Note
that although at this stage the connections have no
explicit geometry, they exist as logical entities with
location.
In the second stage—assembly detail—specific
connection types are selected from a library and
detailed for each existing connection object in the
model. In the example, recessed connections are
assigned to connections 1 and 2 (Fig. 9), corbels to
3 and 4, pockets to 5a and 5b and ledge bearings to 6a
and 6b. The connections now have internal geometry
(see Fig. 10). Aspects of their geometry can also be
constrained by the geometry of the pieces they con-
nect; for example, the width of a corbel may be
constrained to be equal to the width of the column
with which it is cast. Wherever necessary, the pieces
themselves are now cut out to accommodate the
pieces they are connected to and the connection
hardware (e.g., recesses in columns A1 and A2 to
accommodate the spandrel beam).
The practical result is that the designer may move
construction planes, move or rotate pieces, change the
proportions of the pieces (parametric adjustments to
their defining dimensions, such as their cross-sectional
dimensions), change the cross-sectional shapes of the
pieces, change connection types, etc (Fig. 11), and in
each case, the systemmaintains the overall geometry of
the assembly. A case of special interest is provided by
lowering one corbel supporting the inverted tee: as a
result, the field of double tees becomes warped (Fig.
11c). This is done routinely in precast buildings where
the slab must provide slopes for drainage. While each
double tee is warped in the assembly, they are cast
straight—thus, the piece production drawing for these
pieces must show them flat. Notice also that prestress-
ing induces both camber and elastic shortening in the
double tees, which further complicates the geometry of
their end connections. These are important capabilities
for any production 3D parametric modeling system for
precast engineering.
In the third stage—piece detailing–the designer
details each piece. This stage can be highly automat-
ed: load results from structural assembly analysis are
applied to each piece, the reinforcement and prestress
strands are calculated and detailed and embeds are
inserted. Before generating assembly and piece draw-
ings, the system identifies and collects identical pieces
into piece-mark groups. The components of each
precast piece are also related parametrically to the
concrete envelope geometry of that piece. Thus, once
piece detailing has been completed, changes at the
assembly level should be propagated to the reinforce-
ment and embeds of each piece. This is relatively
straightforward when the change to the piece geom-
etry is purely dimensional; it is far more complex
when the change is topological (e.g., change of cross-
section type). In such cases, detailing of the piece
from scratch may be necessary.
6. Conclusions
In the first CAD revolution in the AEC industry,
designers moved from manual drafting to computer-
aided drafting. This revolution is largely complete, as
most architectural and engineering practices have now
adopted the technology. New releases of commercial-
ly available software deliver gradually decreasing
levels of functional enhancements over their prede-
cessors. The second CAD revolution in the AEC
industry, introducing 3D parametric solid modeling,
has begun; research and development efforts are now
being focused on providing the appropriate function-
ality to be embedded in next-generation building
design software packages. Certain sectors of the
AEC industry, such as structural steel, already have
3D modeling systems, with extensive parametric ca-
pability. Others, such as the precast concrete industry,
are moving rapidly in that direction. The potential
benefits are both direct (reduced design and drafting
costs, enabling production automation, etc.) and indi-
rect (reduced error rates in construction, enhanced
ability to consider design alternatives, etc.).
The basic criteria for 3D parametric modeling
software for design and engineering of buildings can
be summarized as follows:
(a) Effective support must be provided not only for
bottom-up assembly of components, but for top-down
modeling as well. To support initial design develop-
ment, to enable replacement of alternate functional
solutions at later design stages and to allow definition
of new objects that satisfy functional criteria, the
modeling constructs should include abstract function-
al objects.
(b) The software must support designers by
providing objects that model building function.
R. Sacks et al. / Automation in Construction 13 (2004) 291–312310
These should automatically insert the parametric
and topological relations and constraints between
parts, so that they behave correctly in all design
contexts. This is a challenging requirement, requir-
ing careful definition of each possible context and
the parameters that define it. Proper implementation
allows automatic maintenance of the geometric and
topological consistency of the design as changes
are made, within the limitations imposed by the
different natures of parametric versus topological
change.
(c) The semantics of functional objects imply that a
parametric building modeling system will have a
specific functional domain (i.e., it will incorporate
attributes and behavior to address functional consid-
erations such as structural, energy, fire code, acoustic
and others). If the object definitions are not open-
ended, a system may restrict the range of possible
design. Abstract functional objects offer a solution to
this requirement.
(d) Design and detailing tasks should be automat-
ed, through definition of sets of parametric objects
with embedded behavior for automated layout and
editing. The behavior can be based on functional
criteria expressed in structural design codes, industry
best practice and company policies. The development
of such automation, on top of the interactive capa-
bilities of parametric modeling, enhances the prod-
uctivity of the technology. The design of such
automation for building objects is likely to become
a major development undertaking over the next
decade.
(e) To date, one of the barriers to implementation
of building-level automated design modules has been
the lack of a parametric building modeling platform.
Development of a platform as described here may
facilitate the development of sophisticated tools of the
kind that have been envisaged by researchers. It is
likely that comprehensive tools will be composed of
assembly-level modules; research will be required to
establish the limits of such modules and appropriate
interactions between them.
(f) The data structure should provide users flexible
control over how changes are propagated among
identical or near-identical parts. A strategy in which
each individual part in a building model has inde-
pendent identity, but is modeled as a composite of
lower level features that are concentrated in sets of
unique objects, can best support the functionality
required.
(g) The software performance, in terms of re-
sponse time to user actions that require reevaluation
of the model, must be adequate to enable uninter-
rupted use. This requires that the strategy employed
for reevaluating a model, and its underlying data
structure, support assemblies with tens of thousands
of parts.
Additionally, with parametric 3D modeling, the
computer can be a true partner in managing a specific
set of functional criteria within a building design. A
designer using the system will not only oversee the
computer’s detailing, but must also be responsible for
adapting the design to functions or requirements not
embedded in the software. The flexibility to maintain
control of the design is an absolute necessity for the
acceptance of such software.
In theory, there are three alternative strategies for
development of specialized building modeling appli-
cations: (a) directly on a generic, general purpose
parametric CAD platform, which does not restrict
the range of possible parametric relationships be-
tween parts, (b) on the basis of a platform specif-
ically designed to support building modeling, or (c)
from scratch, without a generic platform, as stand-
alone software. The results of the performance
benchmark test indicate that option (a) is currently
unrealistic—only the system in which the functional
behavior of the parts was embedded at a low level
within the software data model succeeded in pass-
ing the test. To date, most commercial developers
of parametric modeling software for the AEC
industry have adopted the third alternative. They
have implicitly assumed that the parametric behav-
ior of specific building systems or types must be
embedded in special-purpose software. For example,
doors and windows are constrained to be located in
walls in architectural packages; steel detailing pack-
ages restrict connections to meeting points of steel
members, etc. They do not enable users to model
higher order arbitrary constraints between building
elements. Some of these behaviors are fairly
straightforward and have already been implemented
in software systems. Others will require much
specification, implementation, testing and refine-
ment before robust solutions become available.
These modelers will become increasingly special-
R. Sacks et al. / Automation in Construction 13 (2004) 291–312 311
ized, reflecting the expert knowledge that they
embed.
An alternative, broader approach would be to
provide a parametric building modeling platform with
functional building objects, but to add a basic set of
possible (and unrestricted) higher order parametric
relationships that can be applied between user-defined
entity types. Such a platform, if provided with an open
application programming interface (API), would en-
able third-party software developers to build specific
AEC automation applications with the possible desir-
able result that diverse building systems could be
modeled on a common platform. Clearly, providing
additional generic capabilities involves a nontrivial
increase in required computing power. The exact
nature of the trade-off between the generality of
applicability on the one hand, and performance on
the other, has yet to be established.
The next generation of CAD, using 3D parametric
building modeling, with embedded assembly, piece
and component function and behavior provides a new
level of support for building design automation.
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