Generating, evaluating and visualizing construction schedules with CAD tools

Ž .Automation in Construction 7 1998 433–447

Generating, evaluating and visualizing construction scheduleswith CAD tools

Kathleen McKinney a,), Martin Fischer b,1

a Center for Integrated Facility Engineering, Department of CiÕil and EnÕironmental Engineering, Construction Engineering andManagement Program, Building 550, Room 553-N, Stanford, CA 94305-4020, USA

b Department of CiÕil and EnÕironmental Engineering, Construction Engineering and Management Program, Stanford, CA 94305-4020,USA

Abstract

Collaborative AEC technologies centering around component-based CAD models support architectural and structuralperspectives. The construction perspective is often neglected because an important dimension for construction–time–ismissing. Construction planners are forced to abstract CAD model building components into schedule models representing

Ž .time. 4D-CAD 3D-CADq time removes this abstraction by linking a 3D building model and schedule model throughassociative relationships. Adding time to 3D-CAD models extends the use of CAD tools from the design phase to theconstruction phase. Although commercial 4D tools exist that allow planners to build 4D models and create graphicsimulations of the construction process, these tools lack features to support analysis of these models, easy generation andmanipulation of such models, and realistic visualizations of the construction process. This paper discusses these shortcom-ings, highlights requirements for CAD tools to support construction planning tasks, and describes our efforts to develop 4Dtools that generate 4Dqx models that more realistically represent the construction process. q 1998 Elsevier Science B.V.

Keywords: 4D-CAD; Construction planning; Interaction; Visualization; Knowledge representation

1. Introduction

Construction managers develop construction plansto meet clients’ cost and time requirements, to com-municate a plan to project participants, and to pre-vent costly construction errors. Typically, construc-

Žtion planners interpret design documentation 2D or.3D drawings and specifications to produce a con-

struction schedule consisting of a set of activities and

) Corresponding author. Tel.: q1-650-723-1312; fax: q1-650-725-6014; e-mail: [email protected].

1 Tel.: q1-650-725-4649; fax: q1-650-725-6014; e-mail: [email protected].

Ž .sequential relationships Fig. 1A . While construc-tion schedules communicate time and the sequence

Žof construction activities, project participants gen-eral contractor, subcontractors, clients, designers,

.etc. must mentally associate this schedule informa-tion with the description of the physical building.This mental 4D model represents the associations

Ž . Žbetween time the schedule and space the building. Ž .model Fig. 1B . Without a visual representation of

this mental 4D model, participants must rely solelyon their ability to interpret the abstract schedule andthe 2D or 3D design documents. Furthermore, ifproject information changes, designers and plannersmust mentally visualize how design or schedulechanges affect the overall sequence of construction.

0926-5805r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0926-5805 98 00053-3

( )K. McKinney, M. FischerrAutomation in Construction 7 1998 433–447434

4D-CAD removes this abstraction by representingthe associations between schedule information and

Ž .CAD information through a 4D moÕie Fig. 1D thatvisually communicates the sequence of building con-struction. In this manner, CAD is used to generate avisual representation of the construction schedule

Žand enhances existing scheduling techniques net-. w xwork, line of balance, bar chart 3,7,37,39 . Retik et

w xal. 37 developed a research prototype that associ-ates CAD geometry with construction activities togenerate a 4D movie. In work performed at the

Ž .Center for Integrated Facility Engineering CIFE atw xStanford University in 1994, Collier and Fischer 8

applied similar techniques to a construction projectusing a commercial 4D tool. This 4D tool used abatch process to link layers in a 3D-CAD model to

Ž .construction activities Fig. 1C . We refer to thisŽprocess of associating time sequenced construction

. Ž .activities and space 3D-CAD entities as 4D mod-eling. The resulting graphic 4D model contains arepresentation of the building components, the con-struction activities and their associations and pro-vides the information necessary to generate a 4Dmovie. On the project in 1994, the 4D movie alertedconstruction managers to a major space–time con-

flict restricting access to portions of the site during aw x6-month construction period 9,11,17 . In another

project, a construction company used a chore-ographed 4D modeling process, where planners man-

Ž .ually produced each 4D state Fig. 1E with a 3D-CAD tool. This 4D movie effectively communicatedto subcontractors a complex sequence of construc-

w xtion 11,43 . These and similar research and industryw xefforts 34,36,44 show the benefits of using 3D-CAD

to generate a visual representation of an existingconstruction schedule.

CAD has also been used to generate constructionw xplans. Cherneff et al. 4 developed a system that

interprets a CAD model to develop a description of aCAD drawing, i.e., geometry representing a wall issymbolically represented as a ‘wall’ component. Theplanning module of this system then uses this infor-mation to generate a list of construction tasks re-quired to build each component, e.g., ‘construct wall’.

w xWinstanley et al. 45 developed a system that uses adescription of a CAD drawing that includes relation-ships between components, e.g., ‘supported-by’, togenerate and sequence construction activities usingthese inter-component relationships. Commercial

Ž wtools Primavera Project Planner , KETIV’s

Fig. 1. Traditional planning process vs. 4D modeling process.

( )K. McKinney, M. FischerrAutomation in Construction 7 1998 433–447 435

ARCHT w , Precision Estimating—Extended Edi-w w .tion , AutoCAD R14 exist that allow planners to

extract quantity information and then link this infor-mation to a construction schedule. These systems,however, use traditional schedule representations,such as critical path networks, and do not use theCAD information from which the schedule was gen-erated to represent the schedule information Õisuallyin 3D.

These efforts demonstrate how 3D-CAD modelscan be used for construction planning and can pro-vide the opportunity to investigate how differenttypes of spatial situations constrain or control thesequence and duration of construction. Our 4D-CADwork continues this investigation by exploring howwe can use CAD information to generate more real-istic schedules and visualize planning information in

Žwhat we refer to as 4Dqx models time, space, andadditional types of planning information, e.g., cost,

.productivity, interference . This paper describes thisvision of 4D-CAD and the functionality of the nextgeneration 3D and 4D tools needed to generate4Dqx models. We use a construction planningexample that highlights planning situations that arenot adequately addressed with today’s planning tools

Ž .and methods traditional and 4D and that illustratethe functionality necessary to build, visualize, andrepresent 4Dqx models.

2. Motivation: a test case example

The construction test case is the roof constructionof three campus buildings, three to four stories high,

Ž .with steeply pitched roofs Fig. 2A . During roof

Fig. 2. Case study figures: building model, roof assembly detail and schedule scenarios.


installation, contractors discovered that the guttercould not be installed since the assembly pieces forthe gutter were not within the scope of any subcon-

Ž .tractor. Upon review of the gutter detail Fig. 1F ,the subcontractors observed that the current designwas inadequate. Thus, the architect and subcontrac-tors had to redesign the roof–gutter assembly andresequence the roof construction activities.

The type of gutter assembly, however, dependedon the sequence of construction. The gutter assemblyneeded to have a piece or pieces that supported themain gutter to the roof edge and supported the

Ž .bottom edge of the roof tiles Fig. 2D . If a singlec-channel had been chosen, then the sheet metalcrew would have had to install the entire gutterassembly prior to the roof tiles. If a double c-channelhad been chosen, the sheet metal contractor wouldhave had to install the connection piece during roofconstruction and the gutter after the roofing contrac-tor had completed its work.

The contractors, after considering such issues,selected the sequence shown in Fig. 2F. However,the subcontractors discovered that the roof soffitstucco was cracking from the deflection of the steelstructure that was caused by the weight of the rooftiles. Once again, the planners had to stop construc-tion work and resequence the construction so that thetiles could be installed prior to the stucco. As thesescenarios show, design as represented in 3D modelsand construction schedules as represented in time-based models are often inextricably linked, and inte-grated tools are necessary to explore the impact ofdesign and construction decisions.

In the following sections, we use a set of scenar-ios based on this example to demonstrate how weenvision construction planners using 4Dqx modelsfor planning and replanning the roof construction.We show how current tools do not adequately ad-dress the spatial and temporal issues presented inthese scenarios. The scenarios are grouped into threetask sets pertaining to the major tasks required forthe planners to generate, visualize, and represent4Dqx models of the roof construction.

2.1. Interaction tasks

These tasks include building and editing the 4Dmodels to evaluate alternative construction se-

quences of the roof and identify potential problems.We show that more interactive 4D modeling meth-ods will improve planners’ ability to generate 4Dmodels quickly and that multi-representation of 3D-CAD information will support the collaborative gen-eration of 4D models.

2.2. Knowledge tasks

These tasks include using the knowledge in the4D models to perform computer analysis of 4Dqxmodels to adequately understand planning criteria.We demonstrate the need for standard representationof 4D information and for mechanisms to capturesemantic relationships between components within a3D-CAD environment.

2.3. Visual tasks

These tasks include the viewing of planning infor-mation represented by a 4Dqx model to understandand gain access to planning information. We illus-trate how current 4D movies do not realisticallyvisualize the construction process and describe theneed for visualizing the results of the 4D analysis inthe form of visual annotation and temporary con-struction components such as scaffolding, and zonesor stages of construction.

3. Interaction tasks

Today, the purpose of building 4D models isprimarily for visualization and communication. Cur-rent commercial 4D tools require planners to planand schedule before they use a 4D tool since theyhave to generate and coordinate a priori a 3D-CADmodel and construction schedule. As a result, thesetools simply provide features to ‘associate’ or ‘link’CAD and schedule information for the purpose ofgenerating a 4D movie. This kind of 4D modeling isnon-interactive and does not truly provide the oppor-tunity for planners to use 4D tools for planning andto explore the relationships between the design andthe construction schedule. Current 4D tools make itdifficult for planners to feasibly use 4D models forconstruction planning in the sense that they cannoteasily generate and compare alternative 4D models.


In this section, we present planning scenarios thatillustrate these limitations and show how more inter-active features that support generation, manipulation,and elaboration of 4D content can improve the use of4D tools for construction planning.

3.1. Interacting with 4D content

Consider the following scenario: ‘‘How can theplanners rapidly build 4D models of the roof?’’

Construction on the roof has stopped. The generalcontractor and subcontractors decide to build three4D models to Õisualize and compare their options. A

detailed 3D-CAD model of the roof and the originalroof schedule exist as shown in Fig. 2C and E. Usingcommercially aÕailable software, the planners try tobuild three alternatiÕe 4D models.

One option for the planners is to create a series ofimages depicting the state of construction on a par-

Ž .ticular day. When the images or 4D states Fig. 1Eare shown in sequence, they visually communicatethe sequence of roof construction. This method re-quires up front planning or story boarding to designeach 4D state according to the construction schedule.This can be a time consuming process and providesthe planners with little opportunity to explore alter-nate construction sequences. Nevertheless, planners

Ž .Fig. 3. 4D modeling approaches batch, link and interactive .


can create accurate and realistic 4D movies usingw xthis method as shown by Dillingham in a video 11 .

Another option is to use a 4D tool that enables aconstruction planner to ‘associate’ or ‘link’ 3D-CADentities with construction activities. There are severalways to perform this ‘association’ or ‘linking’ pro-cess with commercial 4D tools. One method is a‘batch process’ where the 4D tool associates animported construction activity with an imported CAD

Ž .layer or CAD entity Fig. 3A . Tools using thisw xmethod 10 require the planner to organize the CAD

model to match the construction schedule. For exam-ple, the planner must assign the CAD entities repre-senting the gutter building component to a CADlayer or a CAD group or block. When the CADinformation is imported into the 4D tool, the plannerassociates the construction activity ‘install gutter’with this CAD layer or group via a dialog box.Another option for the planners is to use rules thatautomatically perform the association. For example,a rule could associate the CAD layer ‘install gutter’to the construction activity ‘install gutter.’ Thismethod requires the planners to carefully coordinatethe layer names and construction activity names. If achange is made to the design of the gutter or to theschedule the planner must update the CAD and

schedule information independently and perform thislinking or 4D modeling process again.

Another method is to use a tool that allows theplanner to interactively ‘link’ a construction activity

w x Ž .with a CAD layer or entity 10,44 Fig. 3B . 4Dtools that use this method provide varying degrees ofinteraction with the CAD and schedule information.With some tools, the planner must import both theCAD and schedule information into the 4D environ-ment. Within the 4D environment, the planner candirectly select CAD entities and associate those enti-ties with a construction activity. For example, theplanner can select the entities visually representingthe metal deck to associate them with the construc-tion activity ‘install metal deck’. This linking pro-cess can be somewhat tedious and slower than a‘batch process’ method since the planner must man-ually assign an association between each construc-tion activity and a CAD entity. However, the plannercan edit these associations and some 4D tools allow

w xthe planner to edit the schedule information 44 .These tools provide a way to automate to varying

degrees the 4D modeling process. In doing so, how-ever, the tools provide little opportunity for theplanner to interact directly with the 4D content.None of the tools allow the planner to interact with

Fig. 4. CIFE 4D-CAD: screen shot of 4D environment within AutoCAD and example of semantic model in Dqqw.


Fig. 5. Examples of 4D tool functionality for building, analyzing, and visualizing planning information.


both the CAD and schedule information within one4D environment. Consequently, for the planners tobuild three alternative 4D models, the planners haveto edit the original CAD and schedule informationand then perform the 4D modeling process again.For example, the planners have to change the type ofgutter and add a new connection piece. For eachoption, then, the planners must reassociate the CADentities with the schedule activities. This process isrepetitive and time-consuming if there are manyactivities.

To overcome these problems, we developed aprototype 4D tool, CIFE 4D-CAD, where plannerscan ‘interactively’ generate CAD, schedule, and 4D

w x Ž .content within one environment 29 Fig. 3C . Thisprototype is built on AutoCADw and linked to a

w Žknowledge-based engineering system, Dqq Fig.. w x4A 21 . The planner can open the 3D-CAD model

of the roof–gutter assembly and edit that model,generate or edit the schedule information, and associ-ate CAD entities with construction activities withinthe CIFE 4D-CAD environment. CIFE 4D-CAD

Žstores this information in a semantic 4D model Fig..4B that represents CAD entities as 4D product

Ž .components Fig. 4D and schedule information asŽ .4D process components Fig. 4C within the knowl-

edge-based environment. Consequently, the plannerhas access to all of the 4D content—the 3D-CADgeometry, the schedule information and their associa-tions—within one 4D environment. With CIFE 4D-CAD, the planner can redesign, re-sequence, or re-associate CAD geometry with construction activitiesto quickly develop alternative construction se-quences. For example, building the three 4D modelsof the roof construction took a total of 30 min.

Lessons learned. 4D tools based on graphic 4Dmodels, such as the commercial 4D tools describedabove, make it difficult to interact directly with the4D content. 4D tools that store information graphi-cally and semantically make it easier for planners tomanipulate all of the 4D content.

3.2. Interacting with 4D models

During the project construction, the general con-tractor’s overall goal was to finish roof constructionand fireproof the steel structure as quickly as possi-ble. The subcontractors’ goals were to finish their

own work in a steady and continuous fashion. Thus,when the planners had to resequence the roof, theyhad to coordinate these conflicting goals. 4D toolstoday, however, allow planners to build 4D modelsthat represent only one perspective of the project.Consequently, planners must coordinate the level ofdetail of the design and schedule before the 4Dmodel is built. We envision the use of 4D tools tohelp contractors manage various levels of planningdetail to effectively coordinate subcontractors’ workwith overall project schedule objectives.

Consider the following scenario: ‘‘How could 4Dtools help the planners to coordinate the productionof 4D models of the roof?’’

A construction planner for the general contractor(starts with a model of the campus project Fig.

( ))5C 1 and wants to use a 4D tool to plan theproject. First, the planner breaks the building into20 work packages: excaÕation, foundation, steel,sheet metal, roof, etc. The planner then proÕides thesubcontractors responsible for each work packagewith releÕant design documentation and access tothe 4D project model. Subcontractors produce a 4Dmodel of their respectiÕe portions of the constructionproject. For example, the roofer builds a 4D model

(from a detailed 3D-CAD model of the roof Fig.( ))5C 3 and a detailed schedule. When subcontractors

finish, they ‘merge’ their 4D models with the oÕerall4D project model.

In this scenario, the project 4D model contains3D-CAD components or entities that represent thehigh-level building sections, such as the roof repre-

Ž .sented by a single surface entity Fig. 4 . The roof isrepresented in greater detail in CAD models pro-vided by the roof, sheet metal, and stucco subcon-

Ž Ž ..tractors Fig. 5C 3 . The general contractor andsubcontractors generate unique but related graphicand semantic views of the roof assembly. We refer tothis representation of multiple forms of a buildingcomponent, i.e., representing the roof assembly in

Ž .multiple levels of detail Fig. 2A , multiple domain-Ž .specific views Fig. 5C , and multiple function views,

w xas multi-representation 24 .Various research efforts have described and

demonstrated the value for multi-representationŽ w xsometimes referred to as multiple-views 31 , multi-

w x w x.ple perspectives 20 , or multiple abstractions 40 to


represent multifunctional and dynamic nature of de-sign and construction information. A few commer-cial CAD tools provide functionality for ‘graphic’multi-representation of CAD entities or components.These tools allow designers to generate one or moregraphic representations of the roof that are associatedwith a viewing scale. For example, the roof is repre-

Žsented as a single surface entity in 1:100 scale Fig.. Ž .2A and as multiple entities at 1:20 scale Fig. 2B .For construction planning, we need ‘semantic’

multi-representation of building components to gen-erate views of subcontractor-specific work, such as asheet-metal view of a project model, or views ofsite-logistics, such as the representation of storage ortrailer areas. Generating and coordinating multi-rep-resentations of CAD-based planning information re-

w xquires ‘mating’ mechanisms 32 to semantically re-late one feature of a component to another feature ofa component. For example, the roofer’s 3D roofassembly might contain a ‘connected-to-gutter’ fea-

Ž .ture edge Fig. 5E that ‘mates’ with a ‘connected-to-roof’ feature edge of the sheet metal’s gutterassembly. These mating mechanisms, then, managethe coordination and ‘merging’ of the individual 4Dmodels. Furthermore, the design of the gutter androof assemblies can easily be changed and re-designed as long as they maintain these ‘mating’features.

Lessons learned. Planners will need 4D tools thatenable the collaborative generation of 4D modelsthat represent various levels of detail and provideplanners with more opportunities to identify potentialproblems at any scale. To do so, CAD tools willneed to support multi-representation of CAD entitiesand features and have ‘mating’ functionality.

4. Knowledge tasks

While these interaction features help plannersbuild 4D models, they focus on the ‘4D’ aspects of4Dqx models. That is, the tools focus only ongenerating the temporal and spatial components ofthe models. In some cases, these models are suffi-cient for discovering potential problems. However,even careful review of the 4D movies of the roofconstruction does not necessarily reveal a missing

connection piece nor does it alert the planner topossible cracking of the stucco. In this section weshow how planners can use 4D models to study theseand similar planning criteria with 4D analysis.Specifically, we illustrate how standard representa-tion of 4D components, functionality to define andacquire relationships between components providethe knowledge necessary for ‘temporary support’analysis.

4.1. Assignment of standard representation of 4Dcomponents

Temporary support refers to whether or not abuilding component has adequate support at the timeof installation. A static analysis of the 3D-CADmodel of the roof may show that all parts havesupport. However, if a part is scheduled to be in-stalled prior to its supporting piece or a supportingpiece is missing, then the building component tem-porarily does not have support.

To perform temporary support analysis, the 4Dtool needs a semantic 4D model to reason aboutinformation from 4D components and their relation-

w xships 27 . The research prototype CIFE 4D-CADgenerates a semantic 4D model, but the 4D productcomponents contain references to their graphic repre-sentations only and not a true description of thebuilding components. As a result, CIFE 4D-CADcannot infer a building component’s type or its geo-metric attributes, such as length. This componentrepresentation was sufficient to generate 4D moviesrapidly from an existing non-component based CADmodel but is not sufficient for 4D analysis tools thatneed specific types of information about the form,function, or behavior of a particular building compo-

w xnent 16 .Various research and industry efforts are working

towards standard data models of building and con-struction information. These efforts include models

w xdesigned from an architectural perspective 12 , si-w xmultaneous engineering perspective 30 , and con-

w xstruction management perspective 2,13 , as well asw xgeneric building product model 22,23 . Our goal is

to add to these efforts by generating 4D informationmodeling requirements based on case studies andexamples of 4D analysis.


Consider the following scenario: ‘‘How can plan-ners discover that a connection piece for the gutter ismissing from the 4D model?’’

Let us assume that three 4D models are generatedusing a next-generation CAD tool that uses standard

[ ]building components 22 . The planners need addi-tional information to select one of the proposed roofschedules. The planners decide to use a 4D tool thatperforms ‘temporary’ support analysis. The plannerobserÕes each 4D moÕie, and during the original

( )roof sequence Fig. 2E a message notifies the plan-ner that the gutter and tiles need edge support.

The first task for the planner is to ensure that eachcomponent in the model is specified or assigned to aspecific component type so that the 4D analysis canreason about specific planning information for eachbuilding component. Several research projectsdemonstrate methods to assign component type. Onemethod is for planners to assign component typeduring modeling by selecting a component, such as‘double copper gutter’ from a component libraryŽ Ž ..Fig. 5A 1 . Another method is to assign component

w xtype after modeling with ‘interpretation’ 15 . Forexample, the planner selects geometry representingthe gutter and assigns to the geometry the component‘double copper gutter.’

Once all of the components are associated with astandard 4D product component type, the 4D analy-sis can start. As each 4D component is virtuallyconstructed, the system checks each 4D productcomponent for the temporary support necessary forinstallation. Each component type stores the support

Ž .conditions necessary for installation Fig. 5F , in theslot support_____conditions. For the 4D component

Ž .gutter1 Fig. 5F the slot support_____conditions inher-its the values ‘edge support’ and ‘continuous sup-port’ from the library component ‘double copper

Ž .gutter’ Fig. 5G .For each component, the 4D temporary support

analysis tool checks the component’s support_____con-ditions slot and searches for components which maysatisfy these conditions. For the ‘edge support’ con-dition, the tool fires the method edge_____support? thatlooks at the value in the edge_____support slot to checkif any components are related to the gutter1 compo-nent with the ‘edge_support_for’ relation. In thisexample, the value is ‘null’ since no component in

the model satisfies the ‘edge-support-for’ relation. Ifthe slot edge_____support contained a component, suchas a c-channel, then the analysis would check to seeif the component had been virtually constructed atthe time of gutter installation. If any of the tempo-rary support conditions are not met then a ‘NO’value is returned and assigned to the slot tempo-rary_____support for the gutter1 4D product compo-nent. For example, in scenario 1 the gutter is in-stalled prior to the c-channel and thus the gutter doesnot have adequate temporary support.

Lessons learned. This example of 4D analysisshows how planners can use knowledge in the 4Dmodel to generate schedule evaluations. In additionto the temporary support example, we have alsogenerated information models for cost, damage, and

w xproductivity analysis 1 . By using such case studieswe plan to develop iteratively a 4D informationmodel that utilizes industry standard models, yetextends them for construction planning.

4.2. Functionality for acquiring relationships be-tween components

Defining standard representations of 4D compo-nents is only one part of the challenge in realizing4D analysis. Another challenge is generating andacquiring the relationships between the components.Consider the following scenario: ‘‘How do we knowthe gutter is ‘supported-by’ the c-channel?’’

Let us assume that designers and planners used anext generation CAD tool that complies with Indus-

[ ]try Foundation Class standards 22 and functional-ity to acquire and represent relationships betweencomponents. The architect builds the 3D model withpre-defined components. The gutter component con-tains a ‘support edge’ feature. As the architect addsthe gutter to the roof model, this ‘support edge’feature searches for a ‘proÕide support’ edge featurein the 3D model. When the planner moÕes the gutternear the edge of the roof, the gutter snaps to thec-channel since the c-channel edge contains a ‘pro-Õide support’ edge feature. Concurrently, the seman-tic 4D model stores this ‘supported_by’ relationshipin the gutter component.

This scenario illustrates capturing relationshipsas the 3D model is produced. Some CAD tools


capture or infer geometric relationships betweenw xgraphic entities or components 25 . For example, the

drafting tools Ashlar Vellumw or Imagineer w inferthat a line is drawn perpendicular to another line.The Builder System captures the ‘part-of’ relation-ship between a door and a wall as the modeler adds

w xthe door to the model 4 . Other research efforts haveexplored methods to capture relationships in archi-

w xtectural drawing tools 19,26 , but commercial sys-tems are slow in adopting such functionality. Part ofthe problem is that the inference engines require a lotof memory and drastically reduce the speed of themodeling tool. Furthermore, this option depends uponthe use of pre-defined components and relationalfeatures.

Another method is to deriÕe relationships throughgeometric and knowledge-based reasoning. Thismethod uses information about CAD components,e.g., geometric location, to derive geometric-basedrelationships, e.g., beam1 is ‘connected-to’ column2w x w x18 or a pump is ‘close-to’ a control space 5 .Inference of these and other semantic relationships isnot always determined by geometry or a set of rules.A variety of support conditions exist, such as ‘ad-herence-to’ or ‘hanging-from’, that are difficult toinfer using rules and require highly domain specificrepresentations of building components within CADmodels.

Finally, another option is to manually interpret3D-model components and assign relationships. In-terpretation is a useful method for assigning seman-

w xtic or functional meaning to graphic content 6,15 .For example, the planner could specifically assign‘supported-by’ relationships between the main gutter

w xpiece and the c-channel. Cherneff et al. 4 use thismethod to assign ‘connected-to’ relationships be-tween walls. This method provides the flexibilitynecessary to account for the unique nature of build-ing construction but also requires construction plan-ners to understand the purpose and process of assign-ing such relationships. For a small detail like the testcase example, this method is feasible. For an entireconstruction project, however, manual interpretationadds an extraordinary amount of work required tobuild a 4D model.

These options complement each other, e.g., thederivation method benefits from pre-existing ‘cap-tured’ or ‘manually applied’ relationships. Since

planners will not want to manually define all of thenecessary planning relationships 4D tools will needto provide the functionality outlined above.

Lessons learned. Current 4D tools generate tem-poral relationships between spatial components.Other types of relationships, however, are necessaryfor 4D analysis. Our goal is to define those relation-ships and investigate methods to capture them during3D and 4D modeling.

5. Visual tasks

We have now described methods for building4Dqx models. To make full use of the informationin these models, the visualization of a 4Dqx modelof the roof construction should alert planners topotential planning problems. This section discusseshow to visualize the ‘x’ aspects of the model. Wepresent two visual features: 4D annotation and repre-sentation of temporary construction components.

5.1. 4D annotation

Effectively communicating the 4D analysis resultsis critical for the planners to assess the planningcriteria and evaluate the alternatives. Currently, 4Dtools provide visual feedback based only on a criticalpath method evaluation. As the animation plays,planners can see when a component is under con-struction, complete, or on the critical path by thechanging color of the component. This feedback,although useful, displays only some temporal aspectsof the installation of spatial components. We suggestthe use of annotation for displaying and explainingthe results of the 4D analysis as illustrated in thefollowing scenario:

The construction planners haÕe built three alterna-tiÕe 4D models. They are considering three criticalissues: the temporary support of the building compo-nents, congestion during roof work, and the cost ofeach alternatiÕe. As the planners Õiew the 4D moÕie,at the time the gutter is installed the edge of thegutter flashes to warn the planners of a support

( )problem Fig. 5B . Concurrently, the 4D moÕie dis-plays the cost of each alternatiÕe and shows the


Õarying degrees of congestion between the crews( )Fig. 5D .

The scenario describes examples of 4D annotationor the visual display of planning information withinthe time–space context. These annotations can beused to display the results of 4D analyses. Ratherthan simply displaying textual-based messages, anno-tation directly relates planning information to the

Žvisual depiction of the construction process Fig..5B . Well-designed visual cues will eventually en-

able planners to quickly identify problem areas in thesame manner that colorful images show stress ratioson structures.

Current CAD tools, however, do not provide themechanisms to annotate 3D and 4D models. CADinformation must be exported into a third party toolthat provides more graphic functionality, such as

w xinterference checkers 44 . Annotation requires thefollowing mechanisms.

Ž .1 Dynamic bi-directional links between CADand analysis tools. Most examples of linking analy-sis and CAD tools involve importing CAD informa-

Ž .tion into a knowledge-based environment KBE orgenerating CAD information within the KBE. Theselinks are typically uni-directional and are difficult to

w xmaintain 35 . To produce 4D annotation we needfirst to extract information from the CAD model to

Ž .an analysis tool as described in Section 5.2 andthen export the information back to the CAD tool.

Ž .2 Visual mechanisms to support a Õariety ofannotation forms. Examples of this are flashing,highlighting, color changes, generating text, etc.Since CAD tools are designed for representation of alimited set of geometry in a static state, they do notsupport behavioral functionality of that geometrysuch as changing colors, transparency, etc. SomeCAD systems support the manual generation of an-notation in the form of red-lines, bubbles, highlight-ing, etc. Most systems, though, do not have adequate

Ž .application protocol interfaces API that allow thirdparty developers to use the CAD environment formore sophisticated visual information displays.

Ž .3 Visual representation of inter-component rela-tionships. Even CAD tools that support representa-tion of relationships provide little functionality tovisualize those relationships. For example, to anno-tate the support problem for the gutter would involve

visualizing the edge of the gutter that requires sup-port. Visualizing this edge would show the plannerwhere the problem is in the time–space context.

Lessons learned. Current CAD tools are designedto visualize building information and do not visual-ize annotative information well. Annotation function-ality is needed to visually associate analysis resultswith the 4D model.

5.2. Representation of temporary construction com-ponents

Consider the following scenario: ‘‘How can plan-ners use 4D models for visualizing logistics of siteconstruction?’’

The subcontractors and general contractors haÕefinished building seÕeral 4D models and are tryingto choose one alternatiÕe. Before making a decision,the sheet metal subcontractor wants to know who isresponsible for erecting the scaffolding and when thescaffolding will be aÕailable. The roofing subcon-tractor obserÕes that the 4D model does not showwhere he can stage the tiles and other roof supplies.

Temporary construction components, such as tem-porary structures, equipment, staging or supply ar-eas, are just as critical to the planning and construc-tion process as the permanent building componentsw x42 . However, since they are not part of the perma-nent building structure they are not designed by thearchitects and engineers and often depend on themethod of construction chosen by the contractor.Thus, they are typically not represented in a 3D or4D model of the building. The 3D or 4D tools shouldeither supply templates for these temporary construc-tion components for planners to add them to 3Dmodels or provide mechanisms to generate thesecomponents. The Interactive Visualizer research pro-ject at the Georgia Institute of Technology is explor-ing ways to visualize construction equipment within

w xa CAD environment 33 . Incorporating such featuresinto a CAD-based environment will provide a moreaccurate visualization of a construction schedule.

Additionally, the visualization of work spacesŽ .Fig. 5 , such as zones, and staging areas, is criticalfor planners to coordinate subcontractors on sitew x38,41 . The location of these areas often changes


throughout a construction project. For example, anaccurate representation of the roof construction needsto include the area where the roof components arebeing installed and the area where the roofers arestoring the roofing materials. The location of thisarea is related to the location and duration of roofwork and can therefore be represented with 4D-CAD.Planners should be able to assign functional uses ofoutlined areas and view the impact of work spacesand storage areas on the flow of work for projectconstruction. The roofers may require a clearancearea for safety. This area, then, should be con-strained for the time of roof installation to excludeany other construction work. However, an area forstaging materials may be less restrictive, i.e., it mightbe possible for other work crews to share the space.

Lessons learned. Today’s CAD tools are designedto produce a static state of a building design. Typi-cally, this is the completed or final-state of thebuilding. Planners want to visualize the intermediatestages of the building and temporary constructioncomponents. CAD tools need to provide plannerswith components that are dynamic and have multiplestates that can be dependent upon time andror se-quence of construction.

6. Discussion: ongoing research work

Representing the construction perspective in aCAD-based environment is an ongoing effort for our4D-CAD research group at CIFE. We are extendingthe use of 4D-CAD from a communication tool usedby a single contractor on a limited number of pro-jects to a planning tool used by the project team.Overcoming the limitations described in this paper isa step in this direction. In addition to ongoing casestudies with industry, our current research includeswork in all of the areas discussed above, specifiedsubsequently.

6.1. Interaction

We are exploring different interaction techniquesfor building and manipulating 4D models. One suchproject is the use of an interactive workbench that

w xprojects the 3D model of the building 14 . Theplanners can gather around the workbench and inter-

actively select building components and sequencethem, quickly developing and evaluating sequencealternatives.

6.2. Knowledge

One research effort is developing a productivitymodifier and cost calculator that ‘utilizes time, space,and crew information to generate cost estimates that

w xincorporate time–space conflicts’ 1 . This researchinvolves the representation of workspaces and con-gestion. Another research project is investigatingvarious methods for capturing and viewing 4Dqxinformation within a componentrassembly browser

w xand editor 28 .

6.3. Visual

We are currently building a prototype in VRMLto demonstrate the use of features for annotation of

w x4D models 28 . This work includes issues such ashow best to display additional types of informationin a visually rich 3D environment and how to visu-ally assign construction planning features to CADcomponents.

By collectively pursuing research in these threeareas we plan to contribute to a standard representa-tion of 4Dqx models. Planners will be able to usethese models to investigate how their planning deci-sions impact the use of time and space during con-struction. This should lead to the discovery of poten-tial problems before actual construction and makethe construction process more efficient.

Acknowledgements

The authors gratefully acknowledge the support ofŽ .the Center for Integrated Facility Engineering CIFE

at Stanford University and its member companies, inparticular Nielsen Dillingham Builders. We thankTodd Zabelle of Pacific Contracting and HenselPhelps employees for giving us full access to projectinformation. We would also like to acknowledgeFlorian Aalami, Burcu Akinci, Eric Collier, AtulKhanzode, Jennifer Kim, Bart Luiten, Sheryl Staub,and John Kunz who have been involved in the4D-CAD research.


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Documents

Generating, evaluating and visualizing construction schedules with CAD tools