Automation in Construction 7 Ž1998. 433–447
Generating, evaluating and visualizing construction schedules with CAD tools Kathleen McKinney
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, Martin Fischer
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Center for Integrated Facility Engineering, Department of CiÕil and EnÕironmental Engineering, Construction Engineering and Management Program, Building 550, Room 553-N, Stanford, CA 94305-4020, USA 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 structural perspectives. The construction perspective is often neglected because an important dimension for construction–time–is missing. Construction planners are forced to abstract CAD model building components into schedule models representing time. 4D-CAD Ž3D-CADq time. removes this abstraction by linking a 3D building model and schedule model through associative relationships. Adding time to 3D-CAD models extends the use of CAD tools from the design phase to the construction phase. Although commercial 4D tools exist that allow planners to build 4D models and create graphic simulations of the construction process, these tools lack features to support analysis of these models, easy generation and manipulation of such models, and realistic visualizations of the construction process. This paper discusses these shortcomings, highlights requirements for CAD tools to support construction planning tasks, and describes our efforts to develop 4D tools that generate 4D q x 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 plans to meet clients’ cost and time requirements, to communicate a plan to project participants, and to prevent costly construction errors. Typically, construction planners interpret design documentation Ž2D or 3D drawings and specifications. to produce a construction schedule consisting of a set of activities and
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Corresponding author. Tel.: q1-650-723-1312; fax: q1-650725-6014; e-mail:
[email protected]. 1 Tel.: q1-650-725-4649; fax: q1-650-725-6014; e-mail:
[email protected].
sequential relationships ŽFig. 1A.. While construction schedules communicate time and the sequence of construction activities, project participants Žgeneral contractor, subcontractors, clients, designers, etc.. must mentally associate this schedule information with the description of the physical building. This mental 4D model represents the associations between time Žthe schedule. and space Žthe building model. ŽFig. 1B.. Without a visual representation of this mental 4D model, participants must rely solely on their ability to interpret the abstract schedule and the 2D or 3D design documents. Furthermore, if project information changes, designers and planners must mentally visualize how design or schedule changes affect the overall sequence of construction.
0926-5805r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 5 8 0 5 Ž 9 8 . 0 0 0 5 3 - 3
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K. McKinney, M. Fischerr Automation in Construction 7 (1998) 433–447
4D-CAD removes this abstraction by representing the associations between schedule information and CAD information through a 4D moÕie ŽFig. 1D. that visually communicates the sequence of building construction. In this manner, CAD is used to generate a visual representation of the construction schedule and enhances existing scheduling techniques Žnetwork, line of balance, bar chart. w3,7,37,39x. Retik et al. w37x developed a research prototype that associates CAD geometry with construction activities to generate a 4D movie. In work performed at the Center for Integrated Facility Engineering ŽCIFE. at Stanford University in 1994, Collier and Fischer w8x applied similar techniques to a construction project using a commercial 4D tool. This 4D tool used a batch process to link layers in a 3D-CAD model to construction activities ŽFig. 1C.. We refer to this process of associating time Žsequenced construction activities. and space Ž3D-CAD entities. as 4D modeling. The resulting graphic 4D model contains a representation of the building components, the construction activities and their associations and provides the information necessary to generate a 4D movie. On the project in 1994, the 4D movie alerted construction managers to a major space–time con-
flict restricting access to portions of the site during a 6-month construction period w9,11,17x. In another project, a construction company used a choreographed 4D modeling process, where planners manually produced each 4D state ŽFig. 1E. with a 3DCAD tool. This 4D movie effectively communicated to subcontractors a complex sequence of construction w11,43x. These and similar research and industry efforts w34,36,44x show the benefits of using 3D-CAD to generate a visual representation of an existing construction schedule. CAD has also been used to generate construction plans. Cherneff et al. w4x developed a system that interprets a CAD model to develop a description of a CAD drawing, i.e., geometry representing a wall is symbolically represented as a ‘wall’ component. The planning module of this system then uses this information to generate a list of construction tasks required to build each component, e.g., ‘construct wall’. Winstanley et al. w45x developed a system that uses a description of a CAD drawing that includes relationships between components, e.g., ‘supported-by’, to generate and sequence construction activities using these inter-component relationships. Commercial tools ŽPrimavera Project Planner w , KETIV’s
Fig. 1. Traditional planning process vs. 4D modeling process.
K. McKinney, M. Fischerr Automation in Construction 7 (1998) 433–447
ARCHT w , Precision Estimating—Extended Edition w , AutoCAD R14 w . exist that allow planners to extract quantity information and then link this information to a construction schedule. These systems, however, use traditional schedule representations, such as critical path networks, and do not use the CAD information from which the schedule was generated to represent the schedule information Õisually in 3D. These efforts demonstrate how 3D-CAD models can be used for construction planning and can provide the opportunity to investigate how different types of spatial situations constrain or control the sequence and duration of construction. Our 4D-CAD work continues this investigation by exploring how we can use CAD information to generate more realistic schedules and visualize planning information in
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what we refer to as 4D q x models Žtime, space, and additional types of planning information, e.g., cost, productivity, interference.. This paper describes this vision of 4D-CAD and the functionality of the next generation 3D and 4D tools needed to generate 4D q x models. We use a construction planning example that highlights planning situations that are not adequately addressed with today’s planning tools and methods Žtraditional and 4D. and that illustrate the functionality necessary to build, visualize, and represent 4D q x models. 2. Motivation: a test case example The construction test case is the roof construction of three campus buildings, three to four stories high, with steeply pitched roofs ŽFig. 2A.. During roof
Fig. 2. Case study figures: building model, roof assembly detail and schedule scenarios.
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installation, contractors discovered that the gutter could not be installed since the assembly pieces for the gutter were not within the scope of any subcontractor. Upon review of the gutter detail ŽFig. 1F., the subcontractors observed that the current design was inadequate. Thus, the architect and subcontractors had to redesign the roof–gutter assembly and resequence the roof construction activities. The type of gutter assembly, however, depended on the sequence of construction. The gutter assembly needed to have a piece or pieces that supported the main gutter to the roof edge and supported the bottom edge of the roof tiles ŽFig. 2D.. If a single c-channel had been chosen, then the sheet metal crew would have had to install the entire gutter assembly prior to the roof tiles. If a double c-channel had been chosen, the sheet metal contractor would have had to install the connection piece during roof construction and the gutter after the roofing contractor had completed its work. The contractors, after considering such issues, selected the sequence shown in Fig. 2F. However, the subcontractors discovered that the roof soffit stucco was cracking from the deflection of the steel structure that was caused by the weight of the roof tiles. Once again, the planners had to stop construction work and resequence the construction so that the tiles could be installed prior to the stucco. As these scenarios show, design as represented in 3D models and construction schedules as represented in timebased models are often inextricably linked, and integrated tools are necessary to explore the impact of design and construction decisions. In the following sections, we use a set of scenarios based on this example to demonstrate how we envision construction planners using 4D q x models for planning and replanning the roof construction. We show how current tools do not adequately address the spatial and temporal issues presented in these scenarios. The scenarios are grouped into three task sets pertaining to the major tasks required for the planners to generate, visualize, and represent 4D q x models of the roof construction. 2.1. Interaction tasks These tasks include building and editing the 4D models to evaluate alternative construction se-
quences of the roof and identify potential problems. We show that more interactive 4D modeling methods will improve planners’ ability to generate 4D models quickly and that multi-representation of 3DCAD information will support the collaborative generation of 4D models. 2.2. Knowledge tasks These tasks include using the knowledge in the 4D models to perform computer analysis of 4D q x models to adequately understand planning criteria. We demonstrate the need for standard representation of 4D information and for mechanisms to capture semantic relationships between components within a 3D-CAD environment. 2.3. Visual tasks These tasks include the viewing of planning information represented by a 4D q x model to understand and gain access to planning information. We illustrate how current 4D movies do not realistically visualize the construction process and describe the need for visualizing the results of the 4D analysis in the form of visual annotation and temporary construction components such as scaffolding, and zones or stages of construction.
3. Interaction tasks Today, the purpose of building 4D models is primarily for visualization and communication. Current commercial 4D tools require planners to plan and schedule before they use a 4D tool since they have to generate and coordinate a priori a 3D-CAD model and construction schedule. As a result, these tools simply provide features to ‘associate’ or ‘link’ CAD and schedule information for the purpose of generating a 4D movie. This kind of 4D modeling is non-interactive and does not truly provide the opportunity for planners to use 4D tools for planning and to explore the relationships between the design and the construction schedule. Current 4D tools make it difficult for planners to feasibly use 4D models for construction planning in the sense that they cannot easily generate and compare alternative 4D models.
K. McKinney, M. Fischerr Automation in Construction 7 (1998) 433–447
In this section, we present planning scenarios that illustrate these limitations and show how more interactive features that support generation, manipulation, and elaboration of 4D content can improve the use of 4D tools for construction planning. 3.1. Interacting with 4D content Consider the following scenario: ‘‘How can the planners rapidly build 4D models of the roof?’’ Construction on the roof has stopped. The general contractor and subcontractors decide to build three 4D models to Õisualize and compare their options. A
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detailed 3D-CAD model of the roof and the original roof schedule exist as shown in Fig. 2C and E. Using commercially aÕailable software, the planners try to build three alternatiÕe 4D models. One option for the planners is to create a series of images depicting the state of construction on a particular day. When the images or 4D states ŽFig. 1E. are shown in sequence, they visually communicate the sequence of roof construction. This method requires up front planning or story boarding to design each 4D state according to the construction schedule. This can be a time consuming process and provides the planners with little opportunity to explore alternate construction sequences. Nevertheless, planners
Fig. 3. 4D modeling approaches Žbatch, link and interactive..
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can create accurate and realistic 4D movies using this method as shown by Dillingham in a video w11x. Another option is to use a 4D tool that enables a construction planner to ‘associate’ or ‘link’ 3D-CAD entities with construction activities. There are several ways to perform this ‘association’ or ‘linking’ process with commercial 4D tools. One method is a ‘batch process’ where the 4D tool associates an imported construction activity with an imported CAD layer or CAD entity ŽFig. 3A.. Tools using this method w10x require the planner to organize the CAD model to match the construction schedule. For example, the planner must assign the CAD entities representing the gutter building component to a CAD layer or a CAD group or block. When the CAD information is imported into the 4D tool, the planner associates the construction activity ‘install gutter’ with this CAD layer or group via a dialog box. Another option for the planners is to use rules that automatically perform the association. For example, a rule could associate the CAD layer ‘install gutter’ to the construction activity ‘install gutter.’ This method requires the planners to carefully coordinate the layer names and construction activity names. If a change is made to the design of the gutter or to the schedule the planner must update the CAD and
schedule information independently and perform this linking or 4D modeling process again. Another method is to use a tool that allows the planner to interactively ‘link’ a construction activity with a CAD layer or entity w10,44x ŽFig. 3B.. 4D tools that use this method provide varying degrees of interaction with the CAD and schedule information. With some tools, the planner must import both the CAD and schedule information into the 4D environment. Within the 4D environment, the planner can directly select CAD entities and associate those entities with a construction activity. For example, the planner can select the entities visually representing the metal deck to associate them with the construction activity ‘install metal deck’. This linking process can be somewhat tedious and slower than a ‘batch process’ method since the planner must manually assign an association between each construction activity and a CAD entity. However, the planner can edit these associations and some 4D tools allow the planner to edit the schedule information w44x. These tools provide a way to automate to varying degrees the 4D modeling process. In doing so, however, the tools provide little opportunity for the planner 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 D q qw .
K. McKinney, M. Fischerr Automation in Construction 7 (1998) 433–447
Fig. 5. Examples of 4D tool functionality for building, analyzing, and visualizing planning information.
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both the CAD and schedule information within one 4D environment. Consequently, for the planners to build three alternative 4D models, the planners have to edit the original CAD and schedule information and then perform the 4D modeling process again. For example, the planners have to change the type of gutter and add a new connection piece. For each option, then, the planners must reassociate the CAD entities with the schedule activities. This process is repetitive and time-consuming if there are many activities. To overcome these problems, we developed a prototype 4D tool, CIFE 4D-CAD, where planners can ‘interactively’ generate CAD, schedule, and 4D content within one environment w29x ŽFig. 3C.. This prototype is built on AutoCAD w and linked to a knowledge-based engineering system, D q qw ŽFig. 4A. w21x. The planner can open the 3D-CAD model of the roof–gutter assembly and edit that model, generate or edit the schedule information, and associate CAD entities with construction activities within the CIFE 4D-CAD environment. CIFE 4D-CAD stores this information in a semantic 4D model ŽFig. 4B. that represents CAD entities as 4D product components ŽFig. 4D. and schedule information as 4D process components ŽFig. 4C. within the knowledge-based environment. Consequently, the planner has access to all of the 4D content—the 3D-CAD geometry, the schedule information and their associations—within one 4D environment. With CIFE 4DCAD, the planner can redesign, re-sequence, or reassociate CAD geometry with construction activities to quickly develop alternative construction sequences. For example, building the three 4D models of the roof construction took a total of 30 min. Lessons learned. 4D tools based on graphic 4D models, such as the commercial 4D tools described above, make it difficult to interact directly with the 4D content. 4D tools that store information graphically and semantically make it easier for planners to manipulate all of the 4D content. 3.2. Interacting with 4D models During the project construction, the general contractor’s overall goal was to finish roof construction and fireproof the steel structure as quickly as possible. The subcontractors’ goals were to finish their
own work in a steady and continuous fashion. Thus, when the planners had to resequence the roof, they had to coordinate these conflicting goals. 4D tools today, however, allow planners to build 4D models that represent only one perspective of the project. Consequently, planners must coordinate the level of detail of the design and schedule before the 4D model is built. We envision the use of 4D tools to help contractors manage various levels of planning detail to effectively coordinate subcontractors’ work with overall project schedule objectives. Consider the following scenario: ‘‘How could 4D tools help the planners to coordinate the production of 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 the project. First, the planner breaks the building into 20 work packages: excaÕation, foundation, steel, sheet metal, roof, etc. The planner then proÕides the subcontractors responsible for each work package with releÕant design documentation and access to the 4D project model. Subcontractors produce a 4D model of their respectiÕe portions of the construction project. 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 oÕerall 4D project model. In this scenario, the project 4D model contains 3D-CAD components or entities that represent the high-level building sections, such as the roof represented by a single surface entity ŽFig. 4.. The roof is represented in greater detail in CAD models provided by the roof, sheet metal, and stucco subcontractors ŽFig. 5CŽ3... The general contractor and subcontractors generate unique but related graphic and semantic views of the roof assembly. We refer to this representation of multiple forms of a building component, i.e., representing the roof assembly in multiple levels of detail ŽFig. 2A., multiple domainspecific views ŽFig. 5C., and multiple function views, as multi-representation w24x. Various research efforts have described and demonstrated the value for multi-representation Žsometimes referred to as multiple-views w31x, multiple perspectives w20x, or multiple abstractions w40x. to
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represent multifunctional and dynamic nature of design and construction information. A few commercial CAD tools provide functionality for ‘graphic’ multi-representation of CAD entities or components. These tools allow designers to generate one or more graphic representations of the roof that are associated with a viewing scale. For example, the roof is represented as a single surface entity in 1:100 scale ŽFig. 2A. and as multiple entities at 1:20 scale ŽFig. 2B.. For construction planning, we need ‘semantic’ multi-representation of building components to generate views of subcontractor-specific work, such as a sheet-metal view of a project model, or views of site-logistics, such as the representation of storage or trailer areas. Generating and coordinating multi-representations of CAD-based planning information requires ‘mating’ mechanisms w32x to semantically relate one feature of a component to another feature of a component. For example, the roofer’s 3D roof assembly might contain a ‘connected-to-gutter’ feature edge ŽFig. 5E. that ‘mates’ with a ‘connectedto-roof’ feature edge of the sheet metal’s gutter assembly. These mating mechanisms, then, manage the coordination and ‘merging’ of the individual 4D models. Furthermore, the design of the gutter and roof assemblies can easily be changed and redesigned as long as they maintain these ‘mating’ features. Lessons learned. Planners will need 4D tools that enable the collaborative generation of 4D models that represent various levels of detail and provide planners with more opportunities to identify potential problems at any scale. To do so, CAD tools will need to support multi-representation of CAD entities and features and have ‘mating’ functionality.
4. Knowledge tasks While these interaction features help planners build 4D models, they focus on the ‘4D’ aspects of 4D q x models. That is, the tools focus only on generating the temporal and spatial components of the models. In some cases, these models are sufficient for discovering potential problems. However, even careful review of the 4D movies of the roof construction does not necessarily reveal a missing
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connection piece nor does it alert the planner to possible cracking of the stucco. In this section we show how planners can use 4D models to study these and similar planning criteria with 4D analysis. Specifically, we illustrate how standard representation of 4D components, functionality to define and acquire relationships between components provide the knowledge necessary for ‘temporary support’ analysis. 4.1. Assignment of standard representation of 4D components Temporary support refers to whether or not a building component has adequate support at the time of installation. A static analysis of the 3D-CAD model of the roof may show that all parts have support. However, if a part is scheduled to be installed prior to its supporting piece or a supporting piece is missing, then the building component temporarily does not have support. To perform temporary support analysis, the 4D tool needs a semantic 4D model to reason about information from 4D components and their relationships w27x. The research prototype CIFE 4D-CAD generates a semantic 4D model, but the 4D product components contain references to their graphic representations only and not a true description of the building components. As a result, CIFE 4D-CAD cannot infer a building component’s type or its geometric attributes, such as length. This component representation was sufficient to generate 4D movies rapidly from an existing non-component based CAD model but is not sufficient for 4D analysis tools that need specific types of information about the form, function, or behavior of a particular building component w16x. Various research and industry efforts are working towards standard data models of building and construction information. These efforts include models designed from an architectural perspective w12x, simultaneous engineering perspective w30x, and construction management perspective w2,13x, as well as generic building product model w22,23x. Our goal is to add to these efforts by generating 4D information modeling requirements based on case studies and examples of 4D analysis.
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Consider the following scenario: ‘‘How can planners discover that a connection piece for the gutter is missing from the 4D model?’’ Let us assume that three 4D models are generated using a next-generation CAD tool that uses standard building components [22]. The planners need additional information to select one of the proposed roof schedules. The planners decide to use a 4D tool that performs ‘temporary’ support analysis. The planner obserÕes each 4D moÕie, and during the original roof sequence (Fig. 2E) a message notifies the planner that the gutter and tiles need edge support. The first task for the planner is to ensure that each component in the model is specified or assigned to a specific component type so that the 4D analysis can reason about specific planning information for each building component. Several research projects demonstrate methods to assign component type. One method is for planners to assign component type during modeling by selecting a component, such as ‘double copper gutter’ from a component library ŽFig. 5AŽ1... Another method is to assign component type after modeling with ‘interpretation’ w15x. For example, the planner selects geometry representing the gutter and assigns to the geometry the component ‘double copper gutter.’ Once all of the components are associated with a standard 4D product component type, the 4D analysis can start. As each 4D component is virtually constructed, the system checks each 4D product component for the temporary support necessary for installation. Each component type stores the support conditions necessary for installation ŽFig. 5F., in the _conditions. For the 4D component slot support_ _conditions inhergutter1 ŽFig. 5F. the slot support_ its the values ‘edge support’ and ‘continuous support’ from the library component ‘double copper gutter’ ŽFig. 5G.. For each component, the 4D temporary support _conanalysis tool checks the component’s support_ ditions slot and searches for components which may satisfy these conditions. For the ‘edge support’ con_support? that dition, the tool fires the method edge_ _support slot to check looks at the value in the edge_ if any components are related to the gutter1 component with the ‘edge_support_for’ relation. In this example, the value is ‘null’ since no component in
the model satisfies the ‘edge-support-for’ relation. If _support contained a component, such the slot edge_ as a c-channel, then the analysis would check to see if the component had been virtually constructed at the time of gutter installation. If any of the temporary support conditions are not met then a ‘NO’ value is returned and assigned to the slot tempo_support for the gutter1 4D product comporary_ nent. For example, in scenario 1 the gutter is installed prior to the c-channel and thus the gutter does not have adequate temporary support. Lessons learned. This example of 4D analysis shows how planners can use knowledge in the 4D model to generate schedule evaluations. In addition to the temporary support example, we have also generated information models for cost, damage, and productivity analysis w1x. By using such case studies we plan to develop iteratively a 4D information model that utilizes industry standard models, yet extends them for construction planning. 4.2. Functionality for acquiring relationships between components Defining standard representations of 4D components is only one part of the challenge in realizing 4D analysis. Another challenge is generating and acquiring the relationships between the components. Consider the following scenario: ‘‘How do we know the gutter is ‘supported-by’ the c-channel?’’ Let us assume that designers and planners used a next generation CAD tool that complies with Industry Foundation Class standards [22] and functionality to acquire and represent relationships between components. The architect builds the 3D model with pre-defined components. The gutter component contains a ‘support edge’ feature. As the architect adds the gutter to the roof model, this ‘support edge’ feature searches for a ‘ proÕide support’ edge feature in the 3D model. When the planner moÕes the gutter near the edge of the roof, the gutter snaps to the c-channel since the c-channel edge contains a ‘ proÕide support’ edge feature. Concurrently, the semantic 4D model stores this ‘supported_by’ relationship in the gutter component. This scenario illustrates capturing relationships as the 3D model is produced. Some CAD tools
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capture or infer geometric relationships between graphic entities or components w25x. For example, the drafting tools Ashlar Vellum w or Imagineer w infer that a line is drawn perpendicular to another line. The Builder System captures the ‘part-of’ relationship between a door and a wall as the modeler adds the door to the model w4x. Other research efforts have explored methods to capture relationships in architectural drawing tools w19,26x, but commercial systems are slow in adopting such functionality. Part of the problem is that the inference engines require a lot of memory and drastically reduce the speed of the modeling tool. Furthermore, this option depends upon the use of pre-defined components and relational features. Another method is to deriÕe relationships through geometric and knowledge-based reasoning. This method uses information about CAD components, e.g., geometric location, to derive geometric-based relationships, e.g., beam1 is ‘connected-to’ column2 w18x or a pump is ‘close-to’ a control space w5x. Inference of these and other semantic relationships is not always determined by geometry or a set of rules. A variety of support conditions exist, such as ‘adherence-to’ or ‘hanging-from’, that are difficult to infer using rules and require highly domain specific representations of building components within CAD models. Finally, another option is to manually interpret 3D-model components and assign relationships. Interpretation is a useful method for assigning semantic or functional meaning to graphic content w6,15x. For example, the planner could specifically assign ‘supported-by’ relationships between the main gutter piece and the c-channel. Cherneff et al. w4x use this method to assign ‘connected-to’ relationships between walls. This method provides the flexibility necessary to account for the unique nature of building construction but also requires construction planners to understand the purpose and process of assigning such relationships. For a small detail like the test case example, this method is feasible. For an entire construction project, however, manual interpretation adds an extraordinary amount of work required to build a 4D model. These options complement each other, e.g., the derivation method benefits from pre-existing ‘captured’ or ‘manually applied’ relationships. Since
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planners will not want to manually define all of the necessary planning relationships 4D tools will need to provide the functionality outlined above. Lessons learned. Current 4D tools generate temporal relationships between spatial components. Other types of relationships, however, are necessary for 4D analysis. Our goal is to define those relationships and investigate methods to capture them during 3D and 4D modeling.
5. Visual tasks We have now described methods for building 4D q x models. To make full use of the information in these models, the visualization of a 4D q x model of the roof construction should alert planners to potential planning problems. This section discusses how to visualize the ‘x’ aspects of the model. We present two visual features: 4D annotation and representation of temporary construction components. 5.1. 4D annotation Effectively communicating the 4D analysis results is critical for the planners to assess the planning criteria and evaluate the alternatives. Currently, 4D tools provide visual feedback based only on a critical path method evaluation. As the animation plays, planners can see when a component is under construction, complete, or on the critical path by the changing color of the component. This feedback, although useful, displays only some temporal aspects of the installation of spatial components. We suggest the use of annotation for displaying and explaining the results of the 4D analysis as illustrated in the following scenario: The construction planners haÕe built three alternatiÕe 4D models. They are considering three critical issues: the temporary support of the building components, congestion during roof work, and the cost of each alternatiÕe. As the planners Õiew the 4D moÕie, at the time the gutter is installed the edge of the gutter flashes to warn the planners of a support problem (Fig. 5B). Concurrently, the 4D moÕie displays the cost of each alternatiÕe and shows the
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Õarying degrees of congestion between the crews (Fig. 5D). The scenario describes examples of 4D annotation or the visual display of planning information within the time–space context. These annotations can be used to display the results of 4D analyses. Rather than simply displaying textual-based messages, annotation directly relates planning information to the visual depiction of the construction process ŽFig. 5B.. Well-designed visual cues will eventually enable planners to quickly identify problem areas in the same manner that colorful images show stress ratios on structures. Current CAD tools, however, do not provide the mechanisms to annotate 3D and 4D models. CAD information must be exported into a third party tool that provides more graphic functionality, such as interference checkers w44x. Annotation requires the following mechanisms. Ž1. Dynamic bi-directional links between CAD and analysis tools. Most examples of linking analysis and CAD tools involve importing CAD information into a knowledge-based environment ŽKBE. or generating CAD information within the KBE. These links are typically uni-directional and are difficult to maintain w35x. To produce 4D annotation we need first to extract information from the CAD model to an analysis tool Žas described in Section 5.2. and then export the information back to the CAD tool. Ž2. Visual mechanisms to support a Õariety of annotation forms. Examples of this are flashing, highlighting, color changes, generating text, etc. Since CAD tools are designed for representation of a limited set of geometry in a static state, they do not support behavioral functionality of that geometry such as changing colors, transparency, etc. Some CAD systems support the manual generation of annotation in the form of red-lines, bubbles, highlighting, etc. Most systems, though, do not have adequate application protocol interfaces ŽAPI. that allow third party developers to use the CAD environment for more sophisticated visual information displays. Ž3. Visual representation of inter-component relationships. Even CAD tools that support representation of relationships provide little functionality to visualize those relationships. For example, to annotate the support problem for the gutter would involve
visualizing the edge of the gutter that requires support. Visualizing this edge would show the planner where the problem is in the time–space context. Lessons learned. Current CAD tools are designed to visualize building information and do not visualize annotative information well. Annotation functionality is needed to visually associate analysis results with the 4D model. 5.2. Representation of temporary construction components Consider the following scenario: ‘‘How can planners use 4D models for visualizing logistics of site construction?’’ The subcontractors and general contractors haÕe finished building seÕeral 4D models and are trying to choose one alternatiÕe. Before making a decision, the sheet metal subcontractor wants to know who is responsible for erecting the scaffolding and when the scaffolding will be aÕailable. The roofing subcontractor obserÕes that the 4D model does not show where he can stage the tiles and other roof supplies. Temporary construction components, such as temporary structures, equipment, staging or supply areas, are just as critical to the planning and construction process as the permanent building components w42x. However, since they are not part of the permanent building structure they are not designed by the architects and engineers and often depend on the method of construction chosen by the contractor. Thus, they are typically not represented in a 3D or 4D model of the building. The 3D or 4D tools should either supply templates for these temporary construction components for planners to add them to 3D models or provide mechanisms to generate these components. The Interactive Visualizer research project at the Georgia Institute of Technology is exploring ways to visualize construction equipment within a CAD environment w33x. Incorporating such features into a CAD-based environment will provide a more accurate visualization of a construction schedule. Additionally, the visualization of work spaces ŽFig. 5., such as zones, and staging areas, is critical for planners to coordinate subcontractors on site w38,41x. The location of these areas often changes
K. McKinney, M. Fischerr Automation in Construction 7 (1998) 433–447
throughout a construction project. For example, an accurate representation of the roof construction needs to include the area where the roof components are being installed and the area where the roofers are storing the roofing materials. The location of this area is related to the location and duration of roof work and can therefore be represented with 4D-CAD. Planners should be able to assign functional uses of outlined areas and view the impact of work spaces and storage areas on the flow of work for project construction. The roofers may require a clearance area for safety. This area, then, should be constrained for the time of roof installation to exclude any other construction work. However, an area for staging materials may be less restrictive, i.e., it might be possible for other work crews to share the space. Lessons learned. Today’s CAD tools are designed to produce a static state of a building design. Typically, this is the completed or final-state of the building. Planners want to visualize the intermediate stages of the building and temporary construction components. CAD tools need to provide planners with components that are dynamic and have multiple states that can be dependent upon time andror sequence of construction.
6. Discussion: ongoing research work Representing the construction perspective in a CAD-based environment is an ongoing effort for our 4D-CAD research group at CIFE. We are extending the use of 4D-CAD from a communication tool used by a single contractor on a limited number of projects to a planning tool used by the project team. Overcoming the limitations described in this paper is a step in this direction. In addition to ongoing case studies with industry, our current research includes work in all of the areas discussed above, specified subsequently. 6.1. Interaction We are exploring different interaction techniques for building and manipulating 4D models. One such project is the use of an interactive workbench that projects the 3D model of the building w14x. The planners can gather around the workbench and inter-
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actively select building components and sequence them, quickly developing and evaluating sequence alternatives. 6.2. Knowledge One research effort is developing a productivity modifier and cost calculator that ‘utilizes time, space, and crew information to generate cost estimates that incorporate time–space conflicts’ w1x. This research involves the representation of workspaces and congestion. Another research project is investigating various methods for capturing and viewing 4D q x information within a componentrassembly browser and editor w28x. 6.3. Visual We are currently building a prototype in VRML to demonstrate the use of features for annotation of 4D models w28x. This work includes issues such as how best to display additional types of information in a visually rich 3D environment and how to visually assign construction planning features to CAD components. By collectively pursuing research in these three areas we plan to contribute to a standard representation of 4D q x models. Planners will be able to use these models to investigate how their planning decisions impact the use of time and space during construction. This should lead to the discovery of potential problems before actual construction and make the construction process more efficient.
Acknowledgements The authors gratefully acknowledge the support of the Center for Integrated Facility Engineering ŽCIFE. at Stanford University and its member companies, in particular Nielsen Dillingham Builders. We thank Todd Zabelle of Pacific Contracting and Hensel Phelps employees for giving us full access to project information. We would also like to acknowledge Florian Aalami, Burcu Akinci, Eric Collier, Atul Khanzode, Jennifer Kim, Bart Luiten, Sheryl Staub, and John Kunz who have been involved in the 4D-CAD research.
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