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Virtual experiment of innovative construction operations
Automation in Construction 12 (2003) 561 – 575 www.elsevier.com/locate/autcon
Virtual experiment of innovative construction operations Heng Li a,*, Zhiliang Mab, Qiping Shen a, Stephen Kong a a
Department of Building and Real Estate, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China b Department of Civil Engineering, Tsinghua University, Beijing, China
Abstract The planning of construction operations is a complicated activity involving abstraction of construction activities from the drawings, choosing of suitable plants and falseworks, allocation of resources on site, planning of safe working place for labourers, and the scheduling of activities sequence. The increasing competition among contractors demands them to adopt innovative construction methods, which have not been used or tested previously. It is not until the beginning of actual construction that the construction planner can realize the validity of his construction operations planning. The lack of tools for the construction planner to evaluate and validate his planning can result in incorrect construction plans, which cause a lot of rework in the construction phase. Virtual Reality (VR) technology, on the other hand, is very likely to provide a solution to the above problem. VR system generates virtual environment containing objects with real world properties and allows user/planner to interact with the objects. This paper proposes an integrated VR system that generates near to reality construction environment for the construction planner to perform construction activities in a real world manner in order to plan, evaluate and validate the construction operations. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Virtual reality; Construction planning; Experiment
1. Introduction High land cost and stiff competition in the Hong Kong construction industry constantly demand innovative construction methods. Because it is both costly and risky to conduct real-life experiment on using innovative construction methods, the industry has been searching for an alternative method for experimenting innovative construction methods. Recently, virtual reality (VR) has been applied to simulate construction processes and environments to support construction
* Corresponding author. Tel.: +852-2766-5111; fax: +8522764-2572. E-mail address: [email protected] (H. Li).
planning and control [1,2]. VR allows its planner to intuitively interact with the virtual environment and objects as if they were real by immersing them in a 3D computer-generated simulation. It also promotes understanding of complex construction systems even in people with limited past experiences or knowledge about that system, and facilitates the evaluation of different scenarios with limited expense and effort. VR tools produce virtual environment that can allow the planner to interact with every element in a construction site. The virtual construction site model enables a realistic assessment of automation and alternatives, construction processes and strategies [3]. A number of researchers adopted VR tools in the construction planning area. Boussabaine et al. [4] proposed using VR as a tool for site layout and
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planning. VR’s ability to dynamically visualize the construction site environment enables the construction planner to identify any possible health and safety problems of the site layout before construction begins [4]. Adjei-kumi and Retik [5] presented an integrated construction planning system that uses VR tools to arrange location of site facilities and sequence of construction of building elements and visualize construction process. Retik and Shapira [6] also presented integrated construction planning systems that use VR tools for visualization of construction process according to a planned schedule. The benefits of using VR in the planning process have been proved in a recent experiment by Ye et al. [7], which investigated the potential benefits of VR environments in supporting assembly planning. The experiment shows that planners spent much longer time in the traditional engineering environment than that in the VR environment. Apart from time, the results also revealed advantages of the VR environment over the traditional environment in improving the planners’ overall assembly planning performance and in minimizing the handling difficulty, excessive reorientation and dissimilarity of assembly operations. Maruyama et al. [8] have proposed a concept of virtual and real-field construction management systems, which is integrated with virtual construction simulation, planning, scheduling and performance management systems to evaluate productivity and safety in virtual-simulated and real-field constructions. All of these systems mainly use VR as a tool for visualization and dynamic navigation of construction site but the interactivity function of VR is not utilized. In this paper, we present a knowledge-based VR system called Virtual Construction Laboratory (VCL), which enables the planner to conduct virtual experiments of innovative construction technologies and processes. We firstly describe the conceptual framework of the knowledge-based VR system, then we give an example of using the system for experimenting the 4-day cycle which is used in public housing projects in Hong Kong.
2. Overview of system architecture The knowledge-based VR system we developed has four major subsystems: system interface, database,
3D models and knowledge base, as shown in Fig. 1. The system interface provides a platform for the planner to set up the virtual construction site by selecting 3D building objects, plant and non-plant models from the 3D models subsystem. Information, such as productivity rates, resources usage and heuristics for site layout and control, are drawn from the Database and Knowledge Base subsystems for configuring time and resources used in construction activities. Once the virtual construction site is established, the system allows the planner to virtually experiment site operations according to a predefined construction schedule. The four subsystems are further described as follows: 2.1. System interface The system interface comprises a user’s menu bar including four functions: System, Library Preparation, Site Setting Up and Site Operation (see Fig. 2). These functions are further discussed in later sections. 2.2. 3D models The three types of 3D models include plant, nonplant and building models. They can be produced using the commercial modelling tools like MicroStationk, AutoCADR and 3D Studio MAXR. WorldToolKitR supports many 3D model file formats including WRL, DXF, NFF and 3DS, and thus, allowing the loading of 3D models made by various modelling tools into the virtual environment. To facilitate the ‘plant control’ operation in the site operation stage, the components of the movable part of a plant model has to be separated from other components. For instance the rib, trolley and hook components of a tower crane model must be individual 3D models (Fig. 3). The building model may contain the prefabricated elements of the non-plant models. For instance, the building model of a semi-precast building may include precast facades, precast staircases, semi-precast slabs and in situ RC walls and slabs. Apart from showing progress of construction works in the Status Window, the building model is used for getting location information of prefabricated elements and identifying the part of the building for fixing rein-
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Fig. 1. System architecture.
forcement and services conduits and for concreting. To facilitate this function of the building model, the continuous in situ-made parts of the building model
have to be divided into small components. Fig. 4 shows the division of a continuous in situ RC building structure.
Fig. 2. System interface.
Fig. 3. The three separated components of a tower crane.
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Fig. 4. Division of a RC structure.
2.3. Databases The three databases needed to be prepared are plant, non-plant and labourer database. The plant database stores the path of motion, speed of movement and loading capacity of plants, while the nonplant database stores the dimension and weight of the non-plant resources. Lastly, the labourer database stores the productivity of labourer in performing different construction operations.
according to a predefined sequence, the system produces construction schedule according to activities performed by the planner in the virtual environment. This approach fully utilizes the interactive ability of VR technology. A more feasible and realistic construction schedule can be produced since the effect of every construction activity to the overall construction process can be evaluated and validated immediately, and modification to the construction method or sequence of the activity can be made accordingly inside the virtual environment. The system also provides an interface for the planner to perform different construction activities and to control the operation of the involved plants in real time. All activities performed inside the virtual environment are recorded by the system and the records become the bases for the automatic production of construction schedule. A knowledge base is provided for checking the validity of the site layout and construction sequence. Before the introduction of the system architecture, an example of constructing a semi-precast building is presented to demonstrate the idea of the VCL. The operation of the system involves two stages: site setting up stage and site operation stage. 3.1. Site setting up stage
2.4. Knowledge base The knowledge base contains heuristics and relevant regulations that provide guidance and assistance on site planning and layout, site operations and arrangement. For example, the knowledge base gives a warning message to the ‘fasten large panel formwork’ activity, if the ‘fixate reinforcement’ activity has not been completed. The following section further describes the four subsystems through an example of applying the VCL to virtually experiment a new 4-day construction cycle.
3. System implementation and application The system is developed using the functions of WorldToolKitR from Sense8 and it is run on the SGI Onyx2 InfiniteReality System. Different from the other construction visualization systems that use CAD or VR as a tool to visualize construction process
In this stage, the system allows the construction planner to build up the site layout using the ‘virtual resources’ in the Resources Library (see Fig. 1). Then, the planner can choose the required resources from the library and put them into the virtual environment in appropriate locations. Table 1 shows the organization of resources in the library. The resources are predefined and shapeable 3D CAD models. They can be divided into two types: plant and non-plant. The motion path, speed of movement and capacities of every plant resource are predefined and stored into a database. After loading the plant into the virtual environment, the planner can activate the plant to perform construction operations according to the defined capacities, speed, etc. For the non-plant resources, the dimensions of every resource are stored in a database and the weights of the elements of falsework, materials and prefabricated components are also stored in the same database. The non-plant type can be further divided
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Table 1 The resources library Temporary facilities
Plant
Falsework
Materials
Prefabricated components
Site hut, hoarding, site entrance, car park area, access road, etc.
Tower crane, hoist, mobile crane, concrete pump, truck, etc.
Large panel formwork, scaffolding, working platform, shoring, etc.
Reinforcement, sand, aggregate, cement, etc.
Precast facßade, semi-precast slabs, fabric reinforcement, etc.
into made-on-demand type and ready-made type. The made-on-demand type includes small elements (such as sand, cement and reinforcement), access road and site entrance. 3D cubes with appropriate volumes are used to represent the small elements. For instance, the planner can choose 50 m3 ‘ready-mixed concrete’ from the Resources Library. The ready-mixed concrete is modelled by the system a cube with the volume of 50 m3 in the virtual environment. Upon choosing the required resource from the resource library, the planner can position it into the appropriate location in the virtual environment using mouse or other pointing device (Fig. 5). Accurate positioning of an object by pointer in 3D environment can be difficult. To facilitate the positioning operation, some restrictions have to be applied to the movement of objects like movement only in horizontal plane in a predefined height and rotation only by a predefined angle. Throughout the site setting up process, the type and amount of resources loaded and their locations
Fig. 5. Loading of resources to the site.
in the virtual environment are recorded and stored into a database. A knowledge-based system is used to evaluate the site layout plan and to identify potential problems. For instance, the knowledgebased system may find that the path for delivery of materials is too long and may give suggestions on how to improve it. 3.2. Site operation stage The site operation stage allows the planner to construct the building in the virtual environment using the resources available on site. Before the start of construction, the planner has to set the working hours in the system. The VCL has an accelerated time scale to simulate the daily operation: starts to run from 8:00 AM after the starting of the site operation stage and switches to 8:00 AM of the next day after 6:00 PM. The planner can pause the time and restart again as he likes and the activities in the virtual environment stops as the time stopped. Whenever the planner clicks on the menu at the top of the virtual environment window, the time stops automatically and starts again after the planner has finished his instruction. Table 2 shows how the operations are categorized. Every operation performed is recorded in a database. The data recorded includes the resources used, the starting and the duration of the operations. Fig. 6 shows the site operation environment. The three windows available to the planner are the Main Operation Window, Plant Operation Window and the Status Window. The Main Operation Window allows the planner to view and interact with the virtual environment in any position and direction. The Plant Operation Window provides the view of the virtual environment from the plant operator. Finally, the Status Window shows the progress of the construction works and allows the planner to pick major element of the building like wall, slab and precast element. Each
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Table 2 Types of operation Transporting
Fixating
Dismantling
Concreting
Plant operation
Prefabricated elements, falsework, shoring, working platform, materials, etc.
Prefabricated elements, falsework, shoring, working platform, etc.
Falsework, shoring, working platform, etc.
Wall, slab, column, beam, etc.
Tower crane, mobile crane, lorry mounted concrete pump, etc.
operation shown in Table 2 is elaborated in the following section. 3.2.1. Transporting The transporting operation involves the identification of element to be transported, the final position of the element, the starting time and method for horizontal and vertical transport. For instance, the planner may want to transport 12 scaffoldings from the ground floor storage area to a place in first floor using two labourers for horizontal transportation and one material hoist for vertical transportation at 10:00 AM. The planner first uses a pointer device to identify the scaffoldings in the Main Operation Window (Fig. 7). Then, a copy of the 12 scaffoldings is produced next to the original one and the planner can then position the scaffoldings in the correct position (Fig. 8). After the starting time and means of transportation are set, the horizontal and vertical distances are calculated by
the system based on information of the initial and final position of the scaffoldings, while the times required for the horizontal and vertical transport are calculated based on the capacity of the labourers and the material hoist in handling the job. Warning messages appear in the Main Operation Window if the materials hoist is not available at the specified period and the planner may examine the resources usage information window (Fig. 9) to evaluate when there are sufficient resources for the task or to decide whether there is a need to add more resources. In the case of labour, warning message will not appear because the number of labourers cannot be planned in the site setting up stage. Instead, the number of labourers used for each operation is recorded by the system and the information is available to the planner through the labour usage information window (Fig. 10). Fig. 11 shows the visualization of the above operation. The scaffoldings in the initial and final
Fig. 6. The site operation environment.
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Fig. 9. Materials hoist usage information window. Fig. 7. Selection of scaffolding.
positions are highlighted during the operation and the scaffoldings in the initial position are deleted from the virtual environment after completion. 3.2.2. Fixating Fixing involves the identification of elements to be fixed and the positioning of the elements to their final locations. For instance, the planner may want to fix 10 scaffoldings previously stored in the first floor by two labourers to support the soffit formworks of the third floor slab. The planner first selects the scaffoldings and then a copy of the scaffoldings placed next to the original one is produced by the system (Fig. 12).
Then, the planner can place the scaffoldings to the required locations one by one (Fig. 13). The planner is asked by the system to determine the number of labourers required for the fixing and the starting time of the operation. The system then calculates the time required for the fixing. Fig. 14 shows the visualization of the fixing operation. For the fixing, dismantling and concreting operations, the number of labourers used is visualised in the Main Operation Window due to their long time interference with the surrounding activities. The visualization method is shown in Fig. 15. For the fixing of reinforcement, services conduits and precast element, the planner can identify the location of fixing from the Status Window by picking
Fig. 8. Production and positioning of copy of selected elements.
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Fig. 10. Labour usage information window.
the appropriate elements of the building model. Then, the reinforcement 3D models, conduits 3D models and the precast element 3D models are loaded into the correct location at the starting time and are highlighted during operation. Fig. 16 shows the reinforcement fixing process.
Fig. 12. Selection and copy of scaffolding.
3.2.3. Dismantling Dismantling operation is same as fixating, which involves the identification of elements to be dismantled, the positioning of elements to their final position and the input of number of labourers required and starting time.
precast facade, to position the copy of the facade in the correct location for fixing, to select a tower crane as the means for horizontal and vertical transport, to set the number of labourers for fixing and to determine the starting time of the operation. The system then activates the tower crane to pick the precast facade at the starting time (Fig. 17) and transport it to the required location (Fig. 18). The facade is then highlighted until the fixing operation was finished (Fig. 19).
3.2.4. Transporting and fixing Sometimes, construction elements may be transported to the exact location for immediate fixing. For instance, the planner may want to transport a precast facade by a tower crane to the required location for fixing. In this case, the planner has to identify the
3.2.5. Dismantling and transporting Sometimes, the construction elements may be dismantled and be transported immediately to other location. For instance, the planner may want to dismantle a large panel formwork and then immediately transport it to other place by a tower crane for future
Fig. 11. Visualisation of transportation operation.
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Fig. 15. Labourers visualization method.
Fig. 13. Positioning of scaffolding.
fixing. In this case, the inputs by the planner to the system are similar to the fixing and transporting operation. The system instructs the tower crane to transport the formwork after the completion of the dismantling work. 3.2.6. Concreting Concreting involves placing of fresh concrete into a mould and vibrating the concrete to form a building element. Transportation of concrete from the place of production to the place of pouring is needed immediately before concreting. The means of transportation can be concrete pumps, cranes, trucks and barrows. The system combines transportation of concrete and concreting into one single function. For instance, the planner may want to use the concrete stored in a ready-mixed concrete truck to concrete two wall
elements in second floor using three labourers and a tower crane for transportation. The planner first selects the elements of the building to be concreted from the Status Window, identifies location of the concrete source in the virtual environment and inputs means of concreting and transportation and starting time of operation. The system calculates the time required for the transportation according to the distance of travel and capacity of the tower crane and the time required for concreting according to the volume of the building elements and the capacity of the tower crane and labourers used. Fig. 20 shows the visualization of the concreting operation. During the concreting operation, the building elements under concreting are highlighted and the labourers used are shown. 3.2.7. Plant operation The system provides interface for the planner or the plant operator to control the plant in the virtual
Fig. 14. Visualization of fixating operation.
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Fig. 16. The reinforcement fixing process.
environment in real time. For instance, the planner may want to use a tower crane to transport a precast facade from the ground floor to the 15th floor for fixing. Firstly, the element to be transported has to be identified. Then, the system checks whether the tower crane can handle the operation according to the weight of the element, the distance between the element and the tower crane, and the capacity of the tower crane. If the checking is passed, the tower crane can start to be operated. From the Plant Operation Window, the controller of the tower crane can appreciate what he can see inside the tower crane. The controller may find that he cannot see the facade inside the tower crane. Then, the planner may have to choose a safe place in the virtual
Fig. 17. Picking of facßade by tower crane.
environment for the labourers to use gesture to give signal to the controller on the movement of the tower crane for the operation. Head-mounted Display and Joystick is provided with the system for viewing inside the plant and for control of the plant, respectively. The usage of the plant can be checked from its usage information window. 3.3. Other planning tools 3.3.1. Displaying of planning schedule As mentioned in the previous section, every activity performed during the site operation stage is recorded by the system. The system provides schedule of activities in the form of Gantt chart for the planner to review his planning.
Fig. 18. Transporting of facßade to the defined location.
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Fig. 19. Fixing facßade.
3.3.2. Time control Time in the system can run in real time or in accelerated time according to the need of the planner. In addition, the planner may switch the time of the system backward or forward to examine the site condition in the virtual environment and to make correction to the planned activities. 3.3.3. Correction of planning The system provides functions for the planner to add or delete resources and activities at any time within the working hours. The starting time of the construction activity to be added or to be deleted must be later than or equal to the present time of the system to ensure the planner can appreciate the site condition before carrying out of the activity.
Authority is required by the government to achieve large production, high quality, low construction cost and fast construction through standardisation of housing block design, mechanisation of construction methods, use of prefabricated building components and fast track construction initiatives. Prefabrication system has been adopted in public housing block of Hong Kong for over 10 years. Common prefabricated elements include precast facade, precast staircase, semi-precast slab and fabric reinforcement. In this particular construction project, a new prefabricated element, precast bathroom, is introduced. The contractor of this project is aware of the complexity involved in adding installation of the precast bathroom in the already tight 6-day floor cycle, and thus, they decided to experiment the construction sequences in virtual environment. 4.1. The 6-day floor cycle Each of the housing blocks has four wings and each wing has five flats (four large flats and one small flat, as shown in Fig. 21). A total of 20 bathrooms have to be installed in each floor. The schedule of 6day floor cycle is shown in Fig. 22. The floor cycle starts with placing precast facade and fixing fabric reinforcement in day 1. In day 2, large panel metal wall form will be placed and concreting of wall will be on day 3. In day 4, the wall form will be stripped and the precast bathroom installed followed by placing the semi-precast slab. Fixing slab formwork,
3.3.4. Stereoscopic view The system provides stereoscopic view for all windows showing the virtual environment of the construction site to let the planner appreciate the distance and size of the objects and to give the planner a better understanding on the space use.
4. A case study in Hong Kong This case study involves construction of three 41storey public housing blocks. The client of this project is the Housing Authority of Hong Kong. The Housing
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Fig. 20. Visualization of concreting operation.
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Fig. 21. Floor plan of the housing block.
Fig. 22. The schedule of the 6-day floor cycle.
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Fig. 23. The bathroom fixing visualisation in virtual environment.
fixing slab reinforcement and concreting slab are performed in days 5 and 6. 4.2. The experiment In order to visualise and experiment the 6-day floor cycle, 3D CAD models of plants, falseworks, prefabricated building components and in situ-made building components like walls and slabs have to be made first. The 3D CAD models of in situ-made walls and slabs are divided into small components according to the predefined bay of concreting. The 3D CAD models are then imported into the VCL prototype system. The VCL prototype system has the basic functions to carry out different construction operation described in the previous sections but do not have knowledge base to show resource usage and cannot check and suggest errors or problems in the construction sequence. The design manager and a team of site personnel of the contractor arrange and discuss the 6-day floor cycle
using the VCL prototype system. Fig. 23 shows the bathroom faxing operation in the virtual environment. Before experimenting the 6-day floor cycle, the duration of bathroom installation is planned to be 30 min. However, after experimenting the floor cycle, the design manager and the site personnel realise that the installation time can actually reduced by half to 15 min. Their thought is validated in the actual construction stage because the actual installation takes around 5 –10 min. After experimenting, the floor cycle, a video showing the construction operation is produced. Fig. 24 shows some scenes in the video. The video is then presented to the client, Housing Authority, and site personnel to let them better understand the 6-day floor cycle. 4.3. Advantages of experiment Apart from realising a shorter installation time for the precast bathroom, the most obvious advan-
Fig. 24. Scenes from video presentation of 6-day floor cycle.
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tages of this virtual experiment is better understanding of the 6-day floor cycle by all participants in this construction project. The client, project manager, site foremen and workers already have a good understanding on the whole working procedures before commencement of the actual construction works. Communications between site personnel are improved and the workflow is smooth. Although no breakthrough on the construction schedule can be achieved in this experiment, the rehearsal of every construction operation in the virtual environment further enhances confidence of the project manager and design manager on the constructability of the 6-day floor cycle.
5. Conclusions and further studies The system has two main features that distinguish it from traditional planning software packages. Firstly, it has real-time performance while maintaining acceptable realism and presence for construction planners. Secondly, it consists of objects that have two distinct and inter-related aspects like geometry/ structure and function/behaviour. The system provides user-friendly functions for the construction planner to construct building in the virtual environment and allows him/her to experiment different construction methods. The authors believe that VR technology can be a very useful tool for the construction planner to plan, evaluate and validate construction operations. The abilities of VR technology to provide interactive interface to 3D models and to provide information on location of 3D models and time of an event happened in the virtual environment make it a perfect tool for doing virtual construction operations. The VR based virtual experiment system provides a safe and inexpensive alternative for testing innovative construction operations, which is difficult if not impossible to conduct in real-life situations. Our pilot study indicates that the system can provide users with a useful appreciation of construction operations. These include the constructability, appropriateness of the planning sequences, as well as the resource deployment requirements. However, the system, in its current form, can only simulate major building objects and processes. Much work is still needed in
order to provide a realistic environment to better simulate construction operations. This project has also made a major step forward towards collaborative working in construction which is becoming a reality as many activities are performed globally with players based in various geographical locations [9]. Through web-based facilities, such as shared applications, this VRL tool provides great flexibility for construction professionals in different countries/locations to work together in planning and managing their projects. As far as further studies are concerned, we are planning to conduct a comparative study to further investigate the potential benefits of VR environment in supporting construction planning. The study will be between two controlled groups of planners: one group will use VR environment and the other without. The effects of using the VR environment on the planners’ performance will be measured and analysed, in terms of time taken the planning task, the number of difficult assembly tasks, the number of reorientations, and the number of dissimilar construction steps.
References [1] A. Retik, Visualization for Decision Making in Construction Planning, Visualization and Intelligent Design in Engineering and Architecture, Southampton; Boston: Computational Mechanics Publications; London: Elsevier Science Publishers, 1993, pp. 587 – 599. [2] A.A. Morad, Y.J. Beliveau, Knowledge-based planning system, Journal of Construction Engineering and Management, ASCE 117 (1) (1991, March) 25566. [3] C. Hendrickson, D.R. Rehak, The potential of a virtual construction site for automation planing and analysis, Automation and Robotics in Construction, vol. X, Elsevier, Oxford, 1993. [4] A.H. Boussabaine, S. Cowland, P. Slater, A virtual reality system for site layout planning, Innovation in Civil and Construction Engineering, CIVIL-COMP, Netherlands, 1997, pp. 233 – 239. [5] T. Adjei-Kumi, A. Retik, A library-based 4D visualisation of construction process, Proceeding of Virtual Reality Annual International Symposium, vol. 1997, IEEE, UK, 1997, pp. 315 – 321. [6] A. Retik, A. Shapira, VR-based planning of construction site activities, Automation in Construction, vol. 8, Elsevier, Oxford, 1999, pp. 671 – 680. [7] N. Ye, P. Banerjee, A. Banerjee, F. Dech, A comparative study of assembly planning in traditional and virtual environments, IEEE Transactions On Systems Man And Cyber-
H. Li et al. / Automation in Construction 12 (2003) 561–575 netics, Part C—Applications and Reviews 29 (4) (1999) 546 – 555. [8] Y. Maruyama, Y. Iwase, K. Koga, J. Yagi, H. Takada, N. Sunaga, S. Nishigaki, T. Ito, K. Tamaki, Development of virtual and real-field construction management systems in innovative, intelligent field factory, Automation In Construction 9 (5 – 6) (2000) 503 – 514. [9] I. Faraj, M. Alshawi, G. Aouad, T. Child, J. Underwood, An industry foundation classes Web-based collaborative construction computer environment, Wisper, Automation in Construction 10 (1) (2000) 79 – 99.
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Further reading [1] M. Alshawi, Virtual reality: future implication on construction, TG 24 State of the Art Report: Virtual Reality in Construction, CIB, UK, 1999. [2] K. McKinney, M. Fischer, Generating, evaluating and visualizing construction schedules with CAD tools, Automation in Construction, vol. 7, Elsevier, Oxford, 1998, pp. 433 – 447. [3] J. Vince, Virtual Reality Systems, Addison-Wesley Publishing, USA, 1995.
Virtual experiment of innovative construction operations Heng Li a,*, Zhiliang Mab, Qiping Shen a, Stephen Kong a a
Department of Building and Real Estate, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China b Department of Civil Engineering, Tsinghua University, Beijing, China
Abstract The planning of construction operations is a complicated activity involving abstraction of construction activities from the drawings, choosing of suitable plants and falseworks, allocation of resources on site, planning of safe working place for labourers, and the scheduling of activities sequence. The increasing competition among contractors demands them to adopt innovative construction methods, which have not been used or tested previously. It is not until the beginning of actual construction that the construction planner can realize the validity of his construction operations planning. The lack of tools for the construction planner to evaluate and validate his planning can result in incorrect construction plans, which cause a lot of rework in the construction phase. Virtual Reality (VR) technology, on the other hand, is very likely to provide a solution to the above problem. VR system generates virtual environment containing objects with real world properties and allows user/planner to interact with the objects. This paper proposes an integrated VR system that generates near to reality construction environment for the construction planner to perform construction activities in a real world manner in order to plan, evaluate and validate the construction operations. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Virtual reality; Construction planning; Experiment
1. Introduction High land cost and stiff competition in the Hong Kong construction industry constantly demand innovative construction methods. Because it is both costly and risky to conduct real-life experiment on using innovative construction methods, the industry has been searching for an alternative method for experimenting innovative construction methods. Recently, virtual reality (VR) has been applied to simulate construction processes and environments to support construction
* Corresponding author. Tel.: +852-2766-5111; fax: +8522764-2572. E-mail address: [email protected] (H. Li).
planning and control [1,2]. VR allows its planner to intuitively interact with the virtual environment and objects as if they were real by immersing them in a 3D computer-generated simulation. It also promotes understanding of complex construction systems even in people with limited past experiences or knowledge about that system, and facilitates the evaluation of different scenarios with limited expense and effort. VR tools produce virtual environment that can allow the planner to interact with every element in a construction site. The virtual construction site model enables a realistic assessment of automation and alternatives, construction processes and strategies [3]. A number of researchers adopted VR tools in the construction planning area. Boussabaine et al. [4] proposed using VR as a tool for site layout and
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planning. VR’s ability to dynamically visualize the construction site environment enables the construction planner to identify any possible health and safety problems of the site layout before construction begins [4]. Adjei-kumi and Retik [5] presented an integrated construction planning system that uses VR tools to arrange location of site facilities and sequence of construction of building elements and visualize construction process. Retik and Shapira [6] also presented integrated construction planning systems that use VR tools for visualization of construction process according to a planned schedule. The benefits of using VR in the planning process have been proved in a recent experiment by Ye et al. [7], which investigated the potential benefits of VR environments in supporting assembly planning. The experiment shows that planners spent much longer time in the traditional engineering environment than that in the VR environment. Apart from time, the results also revealed advantages of the VR environment over the traditional environment in improving the planners’ overall assembly planning performance and in minimizing the handling difficulty, excessive reorientation and dissimilarity of assembly operations. Maruyama et al. [8] have proposed a concept of virtual and real-field construction management systems, which is integrated with virtual construction simulation, planning, scheduling and performance management systems to evaluate productivity and safety in virtual-simulated and real-field constructions. All of these systems mainly use VR as a tool for visualization and dynamic navigation of construction site but the interactivity function of VR is not utilized. In this paper, we present a knowledge-based VR system called Virtual Construction Laboratory (VCL), which enables the planner to conduct virtual experiments of innovative construction technologies and processes. We firstly describe the conceptual framework of the knowledge-based VR system, then we give an example of using the system for experimenting the 4-day cycle which is used in public housing projects in Hong Kong.
2. Overview of system architecture The knowledge-based VR system we developed has four major subsystems: system interface, database,
3D models and knowledge base, as shown in Fig. 1. The system interface provides a platform for the planner to set up the virtual construction site by selecting 3D building objects, plant and non-plant models from the 3D models subsystem. Information, such as productivity rates, resources usage and heuristics for site layout and control, are drawn from the Database and Knowledge Base subsystems for configuring time and resources used in construction activities. Once the virtual construction site is established, the system allows the planner to virtually experiment site operations according to a predefined construction schedule. The four subsystems are further described as follows: 2.1. System interface The system interface comprises a user’s menu bar including four functions: System, Library Preparation, Site Setting Up and Site Operation (see Fig. 2). These functions are further discussed in later sections. 2.2. 3D models The three types of 3D models include plant, nonplant and building models. They can be produced using the commercial modelling tools like MicroStationk, AutoCADR and 3D Studio MAXR. WorldToolKitR supports many 3D model file formats including WRL, DXF, NFF and 3DS, and thus, allowing the loading of 3D models made by various modelling tools into the virtual environment. To facilitate the ‘plant control’ operation in the site operation stage, the components of the movable part of a plant model has to be separated from other components. For instance the rib, trolley and hook components of a tower crane model must be individual 3D models (Fig. 3). The building model may contain the prefabricated elements of the non-plant models. For instance, the building model of a semi-precast building may include precast facades, precast staircases, semi-precast slabs and in situ RC walls and slabs. Apart from showing progress of construction works in the Status Window, the building model is used for getting location information of prefabricated elements and identifying the part of the building for fixing rein-
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Fig. 1. System architecture.
forcement and services conduits and for concreting. To facilitate this function of the building model, the continuous in situ-made parts of the building model
have to be divided into small components. Fig. 4 shows the division of a continuous in situ RC building structure.
Fig. 2. System interface.
Fig. 3. The three separated components of a tower crane.
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Fig. 4. Division of a RC structure.
2.3. Databases The three databases needed to be prepared are plant, non-plant and labourer database. The plant database stores the path of motion, speed of movement and loading capacity of plants, while the nonplant database stores the dimension and weight of the non-plant resources. Lastly, the labourer database stores the productivity of labourer in performing different construction operations.
according to a predefined sequence, the system produces construction schedule according to activities performed by the planner in the virtual environment. This approach fully utilizes the interactive ability of VR technology. A more feasible and realistic construction schedule can be produced since the effect of every construction activity to the overall construction process can be evaluated and validated immediately, and modification to the construction method or sequence of the activity can be made accordingly inside the virtual environment. The system also provides an interface for the planner to perform different construction activities and to control the operation of the involved plants in real time. All activities performed inside the virtual environment are recorded by the system and the records become the bases for the automatic production of construction schedule. A knowledge base is provided for checking the validity of the site layout and construction sequence. Before the introduction of the system architecture, an example of constructing a semi-precast building is presented to demonstrate the idea of the VCL. The operation of the system involves two stages: site setting up stage and site operation stage. 3.1. Site setting up stage
2.4. Knowledge base The knowledge base contains heuristics and relevant regulations that provide guidance and assistance on site planning and layout, site operations and arrangement. For example, the knowledge base gives a warning message to the ‘fasten large panel formwork’ activity, if the ‘fixate reinforcement’ activity has not been completed. The following section further describes the four subsystems through an example of applying the VCL to virtually experiment a new 4-day construction cycle.
3. System implementation and application The system is developed using the functions of WorldToolKitR from Sense8 and it is run on the SGI Onyx2 InfiniteReality System. Different from the other construction visualization systems that use CAD or VR as a tool to visualize construction process
In this stage, the system allows the construction planner to build up the site layout using the ‘virtual resources’ in the Resources Library (see Fig. 1). Then, the planner can choose the required resources from the library and put them into the virtual environment in appropriate locations. Table 1 shows the organization of resources in the library. The resources are predefined and shapeable 3D CAD models. They can be divided into two types: plant and non-plant. The motion path, speed of movement and capacities of every plant resource are predefined and stored into a database. After loading the plant into the virtual environment, the planner can activate the plant to perform construction operations according to the defined capacities, speed, etc. For the non-plant resources, the dimensions of every resource are stored in a database and the weights of the elements of falsework, materials and prefabricated components are also stored in the same database. The non-plant type can be further divided
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Table 1 The resources library Temporary facilities
Plant
Falsework
Materials
Prefabricated components
Site hut, hoarding, site entrance, car park area, access road, etc.
Tower crane, hoist, mobile crane, concrete pump, truck, etc.
Large panel formwork, scaffolding, working platform, shoring, etc.
Reinforcement, sand, aggregate, cement, etc.
Precast facßade, semi-precast slabs, fabric reinforcement, etc.
into made-on-demand type and ready-made type. The made-on-demand type includes small elements (such as sand, cement and reinforcement), access road and site entrance. 3D cubes with appropriate volumes are used to represent the small elements. For instance, the planner can choose 50 m3 ‘ready-mixed concrete’ from the Resources Library. The ready-mixed concrete is modelled by the system a cube with the volume of 50 m3 in the virtual environment. Upon choosing the required resource from the resource library, the planner can position it into the appropriate location in the virtual environment using mouse or other pointing device (Fig. 5). Accurate positioning of an object by pointer in 3D environment can be difficult. To facilitate the positioning operation, some restrictions have to be applied to the movement of objects like movement only in horizontal plane in a predefined height and rotation only by a predefined angle. Throughout the site setting up process, the type and amount of resources loaded and their locations
Fig. 5. Loading of resources to the site.
in the virtual environment are recorded and stored into a database. A knowledge-based system is used to evaluate the site layout plan and to identify potential problems. For instance, the knowledgebased system may find that the path for delivery of materials is too long and may give suggestions on how to improve it. 3.2. Site operation stage The site operation stage allows the planner to construct the building in the virtual environment using the resources available on site. Before the start of construction, the planner has to set the working hours in the system. The VCL has an accelerated time scale to simulate the daily operation: starts to run from 8:00 AM after the starting of the site operation stage and switches to 8:00 AM of the next day after 6:00 PM. The planner can pause the time and restart again as he likes and the activities in the virtual environment stops as the time stopped. Whenever the planner clicks on the menu at the top of the virtual environment window, the time stops automatically and starts again after the planner has finished his instruction. Table 2 shows how the operations are categorized. Every operation performed is recorded in a database. The data recorded includes the resources used, the starting and the duration of the operations. Fig. 6 shows the site operation environment. The three windows available to the planner are the Main Operation Window, Plant Operation Window and the Status Window. The Main Operation Window allows the planner to view and interact with the virtual environment in any position and direction. The Plant Operation Window provides the view of the virtual environment from the plant operator. Finally, the Status Window shows the progress of the construction works and allows the planner to pick major element of the building like wall, slab and precast element. Each
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Table 2 Types of operation Transporting
Fixating
Dismantling
Concreting
Plant operation
Prefabricated elements, falsework, shoring, working platform, materials, etc.
Prefabricated elements, falsework, shoring, working platform, etc.
Falsework, shoring, working platform, etc.
Wall, slab, column, beam, etc.
Tower crane, mobile crane, lorry mounted concrete pump, etc.
operation shown in Table 2 is elaborated in the following section. 3.2.1. Transporting The transporting operation involves the identification of element to be transported, the final position of the element, the starting time and method for horizontal and vertical transport. For instance, the planner may want to transport 12 scaffoldings from the ground floor storage area to a place in first floor using two labourers for horizontal transportation and one material hoist for vertical transportation at 10:00 AM. The planner first uses a pointer device to identify the scaffoldings in the Main Operation Window (Fig. 7). Then, a copy of the 12 scaffoldings is produced next to the original one and the planner can then position the scaffoldings in the correct position (Fig. 8). After the starting time and means of transportation are set, the horizontal and vertical distances are calculated by
the system based on information of the initial and final position of the scaffoldings, while the times required for the horizontal and vertical transport are calculated based on the capacity of the labourers and the material hoist in handling the job. Warning messages appear in the Main Operation Window if the materials hoist is not available at the specified period and the planner may examine the resources usage information window (Fig. 9) to evaluate when there are sufficient resources for the task or to decide whether there is a need to add more resources. In the case of labour, warning message will not appear because the number of labourers cannot be planned in the site setting up stage. Instead, the number of labourers used for each operation is recorded by the system and the information is available to the planner through the labour usage information window (Fig. 10). Fig. 11 shows the visualization of the above operation. The scaffoldings in the initial and final
Fig. 6. The site operation environment.
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Fig. 9. Materials hoist usage information window. Fig. 7. Selection of scaffolding.
positions are highlighted during the operation and the scaffoldings in the initial position are deleted from the virtual environment after completion. 3.2.2. Fixating Fixing involves the identification of elements to be fixed and the positioning of the elements to their final locations. For instance, the planner may want to fix 10 scaffoldings previously stored in the first floor by two labourers to support the soffit formworks of the third floor slab. The planner first selects the scaffoldings and then a copy of the scaffoldings placed next to the original one is produced by the system (Fig. 12).
Then, the planner can place the scaffoldings to the required locations one by one (Fig. 13). The planner is asked by the system to determine the number of labourers required for the fixing and the starting time of the operation. The system then calculates the time required for the fixing. Fig. 14 shows the visualization of the fixing operation. For the fixing, dismantling and concreting operations, the number of labourers used is visualised in the Main Operation Window due to their long time interference with the surrounding activities. The visualization method is shown in Fig. 15. For the fixing of reinforcement, services conduits and precast element, the planner can identify the location of fixing from the Status Window by picking
Fig. 8. Production and positioning of copy of selected elements.
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Fig. 10. Labour usage information window.
the appropriate elements of the building model. Then, the reinforcement 3D models, conduits 3D models and the precast element 3D models are loaded into the correct location at the starting time and are highlighted during operation. Fig. 16 shows the reinforcement fixing process.
Fig. 12. Selection and copy of scaffolding.
3.2.3. Dismantling Dismantling operation is same as fixating, which involves the identification of elements to be dismantled, the positioning of elements to their final position and the input of number of labourers required and starting time.
precast facade, to position the copy of the facade in the correct location for fixing, to select a tower crane as the means for horizontal and vertical transport, to set the number of labourers for fixing and to determine the starting time of the operation. The system then activates the tower crane to pick the precast facade at the starting time (Fig. 17) and transport it to the required location (Fig. 18). The facade is then highlighted until the fixing operation was finished (Fig. 19).
3.2.4. Transporting and fixing Sometimes, construction elements may be transported to the exact location for immediate fixing. For instance, the planner may want to transport a precast facade by a tower crane to the required location for fixing. In this case, the planner has to identify the
3.2.5. Dismantling and transporting Sometimes, the construction elements may be dismantled and be transported immediately to other location. For instance, the planner may want to dismantle a large panel formwork and then immediately transport it to other place by a tower crane for future
Fig. 11. Visualisation of transportation operation.
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Fig. 15. Labourers visualization method.
Fig. 13. Positioning of scaffolding.
fixing. In this case, the inputs by the planner to the system are similar to the fixing and transporting operation. The system instructs the tower crane to transport the formwork after the completion of the dismantling work. 3.2.6. Concreting Concreting involves placing of fresh concrete into a mould and vibrating the concrete to form a building element. Transportation of concrete from the place of production to the place of pouring is needed immediately before concreting. The means of transportation can be concrete pumps, cranes, trucks and barrows. The system combines transportation of concrete and concreting into one single function. For instance, the planner may want to use the concrete stored in a ready-mixed concrete truck to concrete two wall
elements in second floor using three labourers and a tower crane for transportation. The planner first selects the elements of the building to be concreted from the Status Window, identifies location of the concrete source in the virtual environment and inputs means of concreting and transportation and starting time of operation. The system calculates the time required for the transportation according to the distance of travel and capacity of the tower crane and the time required for concreting according to the volume of the building elements and the capacity of the tower crane and labourers used. Fig. 20 shows the visualization of the concreting operation. During the concreting operation, the building elements under concreting are highlighted and the labourers used are shown. 3.2.7. Plant operation The system provides interface for the planner or the plant operator to control the plant in the virtual
Fig. 14. Visualization of fixating operation.
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Fig. 16. The reinforcement fixing process.
environment in real time. For instance, the planner may want to use a tower crane to transport a precast facade from the ground floor to the 15th floor for fixing. Firstly, the element to be transported has to be identified. Then, the system checks whether the tower crane can handle the operation according to the weight of the element, the distance between the element and the tower crane, and the capacity of the tower crane. If the checking is passed, the tower crane can start to be operated. From the Plant Operation Window, the controller of the tower crane can appreciate what he can see inside the tower crane. The controller may find that he cannot see the facade inside the tower crane. Then, the planner may have to choose a safe place in the virtual
Fig. 17. Picking of facßade by tower crane.
environment for the labourers to use gesture to give signal to the controller on the movement of the tower crane for the operation. Head-mounted Display and Joystick is provided with the system for viewing inside the plant and for control of the plant, respectively. The usage of the plant can be checked from its usage information window. 3.3. Other planning tools 3.3.1. Displaying of planning schedule As mentioned in the previous section, every activity performed during the site operation stage is recorded by the system. The system provides schedule of activities in the form of Gantt chart for the planner to review his planning.
Fig. 18. Transporting of facßade to the defined location.
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Fig. 19. Fixing facßade.
3.3.2. Time control Time in the system can run in real time or in accelerated time according to the need of the planner. In addition, the planner may switch the time of the system backward or forward to examine the site condition in the virtual environment and to make correction to the planned activities. 3.3.3. Correction of planning The system provides functions for the planner to add or delete resources and activities at any time within the working hours. The starting time of the construction activity to be added or to be deleted must be later than or equal to the present time of the system to ensure the planner can appreciate the site condition before carrying out of the activity.
Authority is required by the government to achieve large production, high quality, low construction cost and fast construction through standardisation of housing block design, mechanisation of construction methods, use of prefabricated building components and fast track construction initiatives. Prefabrication system has been adopted in public housing block of Hong Kong for over 10 years. Common prefabricated elements include precast facade, precast staircase, semi-precast slab and fabric reinforcement. In this particular construction project, a new prefabricated element, precast bathroom, is introduced. The contractor of this project is aware of the complexity involved in adding installation of the precast bathroom in the already tight 6-day floor cycle, and thus, they decided to experiment the construction sequences in virtual environment. 4.1. The 6-day floor cycle Each of the housing blocks has four wings and each wing has five flats (four large flats and one small flat, as shown in Fig. 21). A total of 20 bathrooms have to be installed in each floor. The schedule of 6day floor cycle is shown in Fig. 22. The floor cycle starts with placing precast facade and fixing fabric reinforcement in day 1. In day 2, large panel metal wall form will be placed and concreting of wall will be on day 3. In day 4, the wall form will be stripped and the precast bathroom installed followed by placing the semi-precast slab. Fixing slab formwork,
3.3.4. Stereoscopic view The system provides stereoscopic view for all windows showing the virtual environment of the construction site to let the planner appreciate the distance and size of the objects and to give the planner a better understanding on the space use.
4. A case study in Hong Kong This case study involves construction of three 41storey public housing blocks. The client of this project is the Housing Authority of Hong Kong. The Housing
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Fig. 20. Visualization of concreting operation.
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Fig. 21. Floor plan of the housing block.
Fig. 22. The schedule of the 6-day floor cycle.
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Fig. 23. The bathroom fixing visualisation in virtual environment.
fixing slab reinforcement and concreting slab are performed in days 5 and 6. 4.2. The experiment In order to visualise and experiment the 6-day floor cycle, 3D CAD models of plants, falseworks, prefabricated building components and in situ-made building components like walls and slabs have to be made first. The 3D CAD models of in situ-made walls and slabs are divided into small components according to the predefined bay of concreting. The 3D CAD models are then imported into the VCL prototype system. The VCL prototype system has the basic functions to carry out different construction operation described in the previous sections but do not have knowledge base to show resource usage and cannot check and suggest errors or problems in the construction sequence. The design manager and a team of site personnel of the contractor arrange and discuss the 6-day floor cycle
using the VCL prototype system. Fig. 23 shows the bathroom faxing operation in the virtual environment. Before experimenting the 6-day floor cycle, the duration of bathroom installation is planned to be 30 min. However, after experimenting the floor cycle, the design manager and the site personnel realise that the installation time can actually reduced by half to 15 min. Their thought is validated in the actual construction stage because the actual installation takes around 5 –10 min. After experimenting, the floor cycle, a video showing the construction operation is produced. Fig. 24 shows some scenes in the video. The video is then presented to the client, Housing Authority, and site personnel to let them better understand the 6-day floor cycle. 4.3. Advantages of experiment Apart from realising a shorter installation time for the precast bathroom, the most obvious advan-
Fig. 24. Scenes from video presentation of 6-day floor cycle.
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tages of this virtual experiment is better understanding of the 6-day floor cycle by all participants in this construction project. The client, project manager, site foremen and workers already have a good understanding on the whole working procedures before commencement of the actual construction works. Communications between site personnel are improved and the workflow is smooth. Although no breakthrough on the construction schedule can be achieved in this experiment, the rehearsal of every construction operation in the virtual environment further enhances confidence of the project manager and design manager on the constructability of the 6-day floor cycle.
5. Conclusions and further studies The system has two main features that distinguish it from traditional planning software packages. Firstly, it has real-time performance while maintaining acceptable realism and presence for construction planners. Secondly, it consists of objects that have two distinct and inter-related aspects like geometry/ structure and function/behaviour. The system provides user-friendly functions for the construction planner to construct building in the virtual environment and allows him/her to experiment different construction methods. The authors believe that VR technology can be a very useful tool for the construction planner to plan, evaluate and validate construction operations. The abilities of VR technology to provide interactive interface to 3D models and to provide information on location of 3D models and time of an event happened in the virtual environment make it a perfect tool for doing virtual construction operations. The VR based virtual experiment system provides a safe and inexpensive alternative for testing innovative construction operations, which is difficult if not impossible to conduct in real-life situations. Our pilot study indicates that the system can provide users with a useful appreciation of construction operations. These include the constructability, appropriateness of the planning sequences, as well as the resource deployment requirements. However, the system, in its current form, can only simulate major building objects and processes. Much work is still needed in
order to provide a realistic environment to better simulate construction operations. This project has also made a major step forward towards collaborative working in construction which is becoming a reality as many activities are performed globally with players based in various geographical locations [9]. Through web-based facilities, such as shared applications, this VRL tool provides great flexibility for construction professionals in different countries/locations to work together in planning and managing their projects. As far as further studies are concerned, we are planning to conduct a comparative study to further investigate the potential benefits of VR environment in supporting construction planning. The study will be between two controlled groups of planners: one group will use VR environment and the other without. The effects of using the VR environment on the planners’ performance will be measured and analysed, in terms of time taken the planning task, the number of difficult assembly tasks, the number of reorientations, and the number of dissimilar construction steps.
References [1] A. Retik, Visualization for Decision Making in Construction Planning, Visualization and Intelligent Design in Engineering and Architecture, Southampton; Boston: Computational Mechanics Publications; London: Elsevier Science Publishers, 1993, pp. 587 – 599. [2] A.A. Morad, Y.J. Beliveau, Knowledge-based planning system, Journal of Construction Engineering and Management, ASCE 117 (1) (1991, March) 25566. [3] C. Hendrickson, D.R. Rehak, The potential of a virtual construction site for automation planing and analysis, Automation and Robotics in Construction, vol. X, Elsevier, Oxford, 1993. [4] A.H. Boussabaine, S. Cowland, P. Slater, A virtual reality system for site layout planning, Innovation in Civil and Construction Engineering, CIVIL-COMP, Netherlands, 1997, pp. 233 – 239. [5] T. Adjei-Kumi, A. Retik, A library-based 4D visualisation of construction process, Proceeding of Virtual Reality Annual International Symposium, vol. 1997, IEEE, UK, 1997, pp. 315 – 321. [6] A. Retik, A. Shapira, VR-based planning of construction site activities, Automation in Construction, vol. 8, Elsevier, Oxford, 1999, pp. 671 – 680. [7] N. Ye, P. Banerjee, A. Banerjee, F. Dech, A comparative study of assembly planning in traditional and virtual environments, IEEE Transactions On Systems Man And Cyber-
H. Li et al. / Automation in Construction 12 (2003) 561–575 netics, Part C—Applications and Reviews 29 (4) (1999) 546 – 555. [8] Y. Maruyama, Y. Iwase, K. Koga, J. Yagi, H. Takada, N. Sunaga, S. Nishigaki, T. Ito, K. Tamaki, Development of virtual and real-field construction management systems in innovative, intelligent field factory, Automation In Construction 9 (5 – 6) (2000) 503 – 514. [9] I. Faraj, M. Alshawi, G. Aouad, T. Child, J. Underwood, An industry foundation classes Web-based collaborative construction computer environment, Wisper, Automation in Construction 10 (1) (2000) 79 – 99.
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Further reading [1] M. Alshawi, Virtual reality: future implication on construction, TG 24 State of the Art Report: Virtual Reality in Construction, CIB, UK, 1999. [2] K. McKinney, M. Fischer, Generating, evaluating and visualizing construction schedules with CAD tools, Automation in Construction, vol. 7, Elsevier, Oxford, 1998, pp. 433 – 447. [3] J. Vince, Virtual Reality Systems, Addison-Wesley Publishing, USA, 1995.