Computer-Aided Civil and Infrastructure Engineering 29 (2014) 60–74
INDUSTRIAL APPLICATION
Development and Implementation of an Industry Foundation Classes-Based Graphic Information Model for Virtual Construction Jianping Zhang, Fangqiang Yu & Ding Li Department of Civil Engineering, Tsinghua University, Beijing, China
& Zhenzhong Hu∗ Department of Civil Engineering, Tsinghua University, Beijing, China; Graduate School at Shenzhen, Tsinghua University, Shenzhen, Guangdong Province, China
Abstract: Virtual Construction (VC) applications encounter difficulty in sharing and exchanging information with one another due to the long periods of interoperability limitation. To address these issues, an Industrial Foundation Classes-based graphic information model (IFC-GIM) is developed according to the exchange requirement of VC, and using the representations of three models in the IFC schema and its extension by defining the dynamic property set and properties for animation. The three models include the physical object model, the construction information model, and the realistic model. An OpenGL-based VC platform is developed and applied to a 440-m-high building to implement the IFC-GIM. The results demonstrate that the proposed IFC-GIM lays the foundation for data sharing and exchange among VC systems and other IFC-compliant applications, which, in turn, significantly reduces the modeling effort for VC and increases the value of VC results. Furthermore, animation is applied to simulate construction activities by the VC platform in addition to color and transparency, enhancing realistic feelings in 4D applications. 1 INTRODUCTION Virtual Construction (VC) is a computer-aided construction and management approach that allows partic∗ To
whom correspondence
[email protected].
should
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addressed.
C 2012 Computer-Aided Civil and Infrastructure Engineering. DOI: 10.1111/j.1467-8667.2012.00800.x
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ipants to perform construction processes virtually using simulation technology (Lai and Kang, 2009). With VC, practitioners can identify problems that may occur in actual construction processes, enabling them to avoid these problems during actual construction (Ning and Tsai, 2010). VC helps construction managers improve efficiency, reduce costs and risks, and shorten the construction period (Waly and Thabet, 2003). However, due to its interoperability (Lu et al., 2009), VC applications encounter difficulty in sharing information with one another or with other applications in the architecture, engineering, construction, and facility management (AEC/FM) industry. As such, massive and professional modeling efforts are a challenge of applying VC, and VC results cannot be used directly in other applications. Data exchange uses a point-to-point conversion interface that entails a heavy workload and is difficult to extend to other applications. Thus, this article proposes a neutral interface for information sharing among VC platforms and other AEC/FM applications based on the Industry Foundation Classes (IFC) standard. IFC is a neutral data format typically used within the AEC/FM industry to describe, exchange, and share information (BuildingSMART, 2011). It is widely adopted as a specification for product models in the AEC/FM industry, especially for building information modeling (BIM) (Fu et al., 2006). IFC presents building information by a set of predefined classes, including entities and types. For example, physical
Development and implementation of an IFC-GIM for VC
components of a building (e.g., walls, doors, and windows) are represented by IfcBuildingElement class and its subclasses. The design and construction information associated with relevant physical building components are captured by attributes of the IfcBuildingElement and corresponding predefined entities in the IFC schema. Although the IFC contains definitions for graphic information, such as geometric representation and realistic information, few IFC-compliant applications can export realistic information of building elements to an IFC file to facilitate realistic information sharing and exchange. In this article, realistic information is the material definition for realistic visualization in graphic software, which contains color, material definitions, texture maps, and texture coordinates of physical objects’ surfaces. Developing an IFC-based graphic information model (IFC-GIM), which defines the exchange requirement for VC, is a key and basic method to improve the interoperability of VC systems, because every implementation of an IFC interface should follow an exchange requirement that specifies the required information when exchanging or sharing data ( BuildingSMART, 2012). Information sharing among VC systems and other AEC/FM applications features five potential benefits: (1) VC systems can adopt a three-dimensional (3D) design model constructed by architects and engineers in design applications to accurately and conveniently obtain geometric and spatial information of a building, thereby reducing efforts dedicated to modeling for VC. (2) It can share information about construction plans and cost assessment generated in schedule management and cost estimate systems by importing these data via IFC neutral files. (3) The proposed IFC-GIM can be adopted by building design applications to improve their IFC interfaces, so that the realistic information built in these applications can be exported to an IFC file along with other information. VC systems and other IFC-compliant applications can then share these data to render a virtual environment. Texture coordinates are particularly valuable because generating them through VC platforms alone, using 3D surface models, is difficult. (4) If IFC-GIM is widely adopted, the interoperability of different VC systems can be improved tremendously. For instance, building and construction information generated in four-dimensional (4D) CAD systems can be imported into operationlevel VC systems to simplify the modeling process. In addition, the results of construction operation simulations, such as optimized construction schedules and resource consumption, can be transferred back into 4D CAD systems, thereby achieving optimum 4D construction management and visualization of construction processes. (5) The model can be integrated into a total BIM
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as part of the project delivery information when construction is completed, thereby enhancing the postconstruction benefits of BIM, for example, providing realistic visualizations of the virtual environment for facility management. This article presents an IFC-GIM for VC, which is an open and unified information model, including all the information required by VC systems and VC results. Based on this model, a VC platform is developed on top of a 4D construction analysis and management system named 4D-GCPSU (Hu and Zhang, 2011). The platform is applied to a 103-story 440-mhigh building located in Guangzhou to verify the feasibility of the proposed model. The experimental results demonstrate that adopting IFC-GIM to represent VC information is a practical method to exchange and share data among VC systems and other applications in the AEC/FM industry. This proposed method can reduce the modeling efforts of VC and increase the value of VC results.
2 EXISTING VC APPLICATIONS Construction planning and control can take place at the project and/or operation levels (Halpin, 1992), which correspond to two major VC categories: project-level VC (e.g., 4D CAD) and operation-level VC (e.g., discrete event simulation [Lu, 2003]). As a typical project-level VC, 4D CAD focuses on the 3D visualization of constructed products over a construction schedule by animating the evolution of buildings over time. The animation is accomplished by linking 3D models that represent complete product designs and construction schedules (McKinney and Fischer, 1998). In 1996, the Center for Integrated Facility Engineering (CIFE) at Stanford University formally used the 4D CAD concept for the first time (McKinney et al., 1996). Given its considerable potential benefits in the construction field, numerous researchers have improved 4D CAD tools since the mid-1990s. First developments include the 4D planner TM (Williams, 1996) and the CIFE 4D CAD (McKinney et al., 1996), which are based on the original 4D model. From then on, researchers started exploring new application fields for 4D CAD. New-generation tools such as the 4D Annotator developed by CIFE (Liston et al., 1998), nD modeling tools (Aouad et al., 2005), and the 4D-GCPSU series (Hu and Zhang, 2011) provide enhanced 4D models. Aside from 3D models, schedules, and their associations, the 4D models also contain information about resource consumption, cost, site layout, mechanical properties, and safety-related information. In existing 4D systems, color and transparency are mainly used
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to present different activities and construction states of building components, which are often too blurry to demonstrate the dynamic construction process. Operation-level VC is a computer technology where computational methods are applied to model, simulate, and visualize construction operation scenarios. Since the inception of CYCLONE technology (Halpin, 1977), many successful applications have been presented worldwide, such as RESQUE by Chang (1987), HSM by Sawhney and AbouRizk (1995), STROBOSCOPE by Martinez and Ioannou (1999), SDESA by Lu (2003), and hybrid SD-DES simulation by Alvanchi et al. (2011). Construction simulation systems create models that represent how construction processes and operations are conducted. These models contain the following data: building elements, different resources required to carry out construction operations (Cheng and Yan, 2009), rules on how operations are performed, plan schedules, managerial decisions made during operations, and the stochastic nature of events (Kamat and Martinez, 2001). To illustrate the process and results of VC, some of these models also embody the 3D geometric representation for the 3D visualization of buildings and sites, as well as 3D animation (Kang and Miranda, 2009) to depict the motion of construction equipment and building components during construction. However, few applications of both categories can export entire VC models with realistic and animated information to a neutral file for information sharing. Although some data exchange methods such as developing a point-to-point conversion interface (Lu et al., 2009) were reported, most of them entail a heavy workload and are not easily extendable. Thus, based on IFC standards, a neutral GIM is proposed to represent such information during information exchange for VC.
3 THE IFC-BASED GIM Given the foregoing state of affairs, the exchange requirement for VC considers four parts of information: (1) physical objects in construction, such as building elements and construction equipment; (2) construction information such as quantity, tasks, schedule, resource, and cost; (3) 3D realistic models that include geometric and realistic information; and (4) animation depicting the operation process. According to the exchange requirement, the total schematic framework of the IFCGIM is shown in Figure 1. The arrows in Figure 1 represent the dependencies among the partial models: (1) a realistic model extends the physical object model by geometric representations and realistic information and (2) an animation model combines the construction information model and realistic model, and fulfills itself
Fig. 1. Total schematic framework of the IFC-GIM.
by extending animated movements. Although a prototype of the IFC-based realistic model was proposed in a previous publication (Zhang et al., 2010), the remainder of this section describes the four submodels in detail. 3.1 The IFC-based physical object model A physical object model defines physical objects in the construction field, including building elements (e.g., beams, columns, and slabs) and construction resources (e.g., cranes, dump trucks, concrete, and soil). Physical objects are substantial and basic components of the IFC-GIM because the IFC model is a component-based information model. Other information sets are associated via relation instances. Therefore, a physical object model in IFC-GIM should be shared among all BIMbased applications, so that it need not be recreated in different applications. In IFC, physical objects are represented by the IfcObject class and its subclasses, and other information associated with relevant physical objects is captured by the attributes of IfcObject and its subclasses. Corresponding entities are linked through relation entities represented by IfcRelationship class and its subclasses. The details of object representation in IFC are described in “IFC 2x Edition 2 Model Implementation Guide” (“IFC Guide”) (Liebich, 2009). In addition, this article proposes a method of assigning realistic models to resources because the IFC standard does not include a direct method for that yet. As described in the IFC standard, building elements are characterized by IfcBuildingElement, which is a subtype of IfcObject. The attribute Representation of IfcBuildingElement is used to specify the geometric
Development and implementation of an IFC-GIM for VC
Fig. 2. The structure of the IFC-based physical object model in EXPRESS-G language.
model and realistic information of a building element. However, as another subclass of IfcObject, IfcResource, which represents construction resources, does not have an attribute similar to the attribute Representation. This article suggests that relationship entities can be used to attach products with 3D realistic model definitions to construction resources. The structure of the IFC-based physical object model is shown in Figure 2.
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cal building components, schedule plan, resource consumption, cost, site layout, safety management, and quality evaluation, are involved and created during the construction phase (Adeli and Karim, 2001; Hu and Zhang, 2011). However, this article only focuses on quantity, schedule, resource consumption, and cost, following the exchange requirement among VC applications. Construction information is also very valuable for facility management, such as determining maintenance periods of building objects (Zhang and Gao, 2012). Many researchers, such as Froese and Yu (1999), Tanyer and Aouad (2005), Owolabi et al. (2006), and Ma et al. (2011) have analyzed the IFC model framework for construction information. Generally, in IFC, the quantity of physical objects is represented by the IfcElementQuantity class, construction tasks are represented by the IfcTask class, schedules are represented by the IfcWorkSchedule class, construction resources are represented by the IfcResource class, and costs are represented by the IfcCostItem class, respectively. Figure 3 shows the structure of the IFC-based construction information model.
3.3 The IFC-based 3D realistic model 3.2 The IFC-based construction information model Construction is a dynamic and complex process over a long period. It requires multiple participants and numerous tasks. Many types of information on building and construction, such as quantity information of physi-
The IFC-based 3D realistic model, which includes the geometric representations and realistic information of physical components, is used to render 3D virtual scenes of buildings. The IFC-based 3D realistic model is valuable during the construction phase as well as during the design and maintain phases (Qin and Faber, 2012),
Fig. 3. The IFC-based construction information model in EXPRESS-G language.
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because 3D visualization is a basic function of most BIM applications. In IFC, the geometric representation is defined by the IfcProductRepresentation class with other relevant classes. More than one geometric representation item can be assigned to a physical object because IfcProductRepresentation constrains a list of IfcShapeRepresentation that represents a particular geometric representation of a product. Different types of geometric representation can be attached to a single object. Thus, both solid and surface models should be included in an IFCbased 3D model if both models exist. In practice, a 3D solid model built in BIM-based design applications, such as Revit Architecture 2012 and ArchiCAD 15, can already be shared through IFC neutral files. In this research, the solid model is generated by BIM design applications and exported to an IFC file. Then, the surface model is generated according to the solid model using a model converter developed by our previous research, and added to the IFC file. Therefore, various applications in construction phases are able to obtain both solid model and surface model from the IFC file. A detailed description of the geometric representation in IFC is presented in the “IFC Guide.” Thus, this section only focuses on the representation of realistic information. In computer graphics, the realistic information of physical objects includes three aspects, that is, color, material definition, and texture maps. In practice, a realistic model with materials and texture maps is often built by BIM-based design applications to demonstrate the design scheme. The IFC schema defines all the entities required to represent realistic information. However, such information cannot be shared through an IFC file along with physical objects because the exchange requirement of realistic information is unclear and no widely adopted IFC-based realistic model exists. Considering that, an IFC-based 3D realistic model is constructed in this section. This model lays the foundation for sharing a 3D realistic model during the design, construction, and operation phases. In general, realistic information is represented by a select type, IfcSurfaceStyleElementSelect, in IFC, and its different subtypes are used to represent different aspects of realistic information: IfcSurfaceStyleShading represents color, IfcSurfaceStyleRendering represents material, and IfcSurfaceStyleWithTextures represents texture. Therefore, color, material, and texture can all be linked to the product model through the IfcSurfaceStyleElementSelect class (BuildingSMART, 2007). Additional details about the representation are described as follows. 3.3.1 Color model. The RGB color model is the most widely used color model to represent and display
images in IT systems. In VC systems, the RGB color model is sufficient and widely used to distinguish different construction activities. In IFC, color is specified by the IfcColourRgb class, which has three attributes, namely, Red, Green, and Blue corresponding to the RGB lights, respectively. In an IFC-based 3D realistic model, an instance of IfcColourRgb is assigned to IfcSurfaceStyleShading such that the color of a physical object is represented. 3.3.2 Material definition. In computer graphics, material defines surfaces’ reflection and transmission factor of various lights. Computer graphics adopt different kinds of lights (ambient, diffuse, and specular) to approximate lights coming from different light sources in the real world (Shreiner et al., 2009). The RGB model also characterizes the color of light sources. Similar to lights, materials have ambient, diffuse, and specular factors to determine their reflectance of different kinds of lights. Each factor has red, green, and blue components. Furthermore, transparency is applied to characterize the transmission factor of surfaces. The properties mentioned above are the material definition information required of a VC platform to render a 3D virtual environment using a simple illumination model. In fact, a simple illumination model is applied by most VC platforms because it is effective enough to represent construction scenes. Global illumination models such as ray tracing algorithm can render better scenes but are too complex to render dynamic construction scenes rapidly. In IFC, IfcSurfaceStyleRendering, which represents material, has five attributes, namely, SurfaceColour, DiffuseColour, SpecularColour, SpecularHighlight, and Transparency, that determine ambient, diffuse, specular reflectance, shininess, and transparency of materials, respectively. Type of SurfaceColour, DiffuseColour, and SpecularColour is IfcColourRgb, such that their red, green, and blue components are specified by relevant attributes of IfcColourRgb. 3.3.3 Texture and texture mapping. Texture technology, including texture generation and mapping, is used to represent the details of a surface for constructing realistic models (Zalama et al., 2011). Textures are simply rectangular arrays of data, such as color and luminance, which are used to represent the fine structures of surfaces. Texture mapping maps the data to surfaces of 3D models. It determines which pixel’s color or luminance data can be used to render each pixel of a surface (Shreiner et al., 2009). Texture mapping is determined by the object and texture coordinates of each surface vertex. For every vertex, the object coordinate is its 3D Cartesian coordinate, whereas texture coordinate is the coordinate of the texture applied to the map.
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Fig. 4. IFC-based 3D realistic model in EXPRESS-G language.
Another method of assigning texture coordinates of each pixel is through the automatic calculation using a specified function. These properties are enough to achieve a standard texture mapping that is effective and efficient for VC, without considering advanced effects
of texture mapping, such as multitexture and combined textures. IFC uses IfcSurfaceTexture and its subtypes to represent texture maps (BuildingSMART, 2007). Subtype IfcPixelTexture has an attribute, Pixel, used to
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Table 1 Attributes of the three single actions Action Attributes
Move
Rotate
Scale
StartTime: DateTime EndTime: DateTime MoveVector: (float, float, float)
StartTime: DateTime EndTime: DateTime CenterPoint: (float, float, float) RotateAxis: (float, float, float) RotateAngle: float
StartTime: DateTime EndTime: DateTime CenterPoint: (float, float, float) ScaleValue: (float, float, float)
Table 2 Definition of custom property sets Property set name
Application entities
Pset Move
IfcProduct
Pset Rotate
IfcProduct
Pset Scale
IfcProduct
Definition It contains properties capturing a move action of physical objects It contains properties capturing a rotate action of physical objects It contains properties capturing a scale action of physical objects
determine an array of pixel values. Another subtype, IfcImageTexture, refers to an existing image determined by the URL of the image. Texture mapping is represented by the IfcTextureCoordinate and its subtypes. The subtype IfcTextureCoordinateGenerator presents the automatic calculation of texture mapping while another subtype, IfcTextureMap, presents the mapping of texture coordinates to an object. Although
IfcTextureMap contains a set of IfcVertexBasedTextureMap instances, each of them is applied to present the mapping of texture coordinates to a surface of the object. Overall, the IFC standard can represent a 3D realistic model. Figure 4 illustrates the structure of the IFC-based 3D realistic model, which can be applied to define geometric representations, color, material definition, texture, and texture mapping of physical components. Figure 4 shows two ways of correlation between a realistic model and a product model. In the first way, realistic information presented by IfcSurfaceStyle and IfcSurfaceStyleElementSelect is assigned to the geometric representation presented by IfcRepresentationItem of physical objects through IfcStyledItem. IfcAnnoationSurface is also applied to associate IfcTextureCoordinate with IfcGeometricRepresentationItem, which is a subtype of IfcRepresentationItem, so that texture mapping data can be associated with the corresponding geometric model. In the second way, an IfcSurfaceStyle instance can be associated to various products through IfcMaterial instances because IfcStyledItem instances can be styled representation items of IfcMaterial.
Table 3 Definition of properties of Pset Move Name StartTime EndTime Value
Property type
Data type
Definition
IfcPropertyReferenceValue IfcPropertyReferenceValue IfcPropertyListValue
IfcDateAndTime IfcDateAndTime List Value: IfcReal
The time when an action starts The time when an action stops Indicates MoveVector for a movement
Table 4 Definition of properties of Pset Rotate Name StartTime EndTime Axis CenterPoint Angle
Property type
Data type
Definition
IfcPropertyReferenceValue IfcPropertyReferenceValue IfcPropertyListValue IfcPropertyListValue IfcPropertySingleValue
IfcDateAndTime IfcDateAndTime List Value: IfcReal List Value: IfcReal IfcReal
The time when an action starts The time when an action stops Indicates RotateAxis for a rotation Captures CenterPoint for a rotation The angle of a rotation
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Table 5 Definition of properties of Pset Scale Name StartTime EndTime ScaleValue CenterPoint
Property type
Data type
Definition
IfcPropertyReferenceValue IfcPropertyReferenceValue IfcPropertyListValue IfcPropertyListValue
IfcDateAndTime IfcDateAndTime List Value: IfcReal List Value: IfcReal
The time when an action starts The time when an action stops Indicates ScaleValue for a scale action Indicates CenterPoint for a scale action
Fig. 5. Architecture of the OpenGL-based VC platform.
3.4 The IFC-based animation model Computer animation is a technique in which movements are displayed on a screen (Buss, 2003). A complex animation is usually composed of numerous simple actions that are implemented through geometric transformations of entities in a virtual world with the passage of time (Vigueras et al., 2011). Three basic geometric transformations are applied to illustrate three kinds of single action, that is, move, rotate, and scale (Shreiner et al., 2009). A complex animation is implemented through combinations of these geometric transformations. This article focuses on the IFC-based representation of single actions in key-frame animation, which is the most accepted method in VC systems. In
VC, move and rotate actions are mainly used to simulate the construction process, such as the installation of building elements. Scale actions are sometimes used to simulate bulge of building components, dynamic processes of pouring concrete from bottom to top, and changing the material stack in the construction site. Animation in VC is also used to represent assembly and adjustment processes in mechanical, electrical, and plumbing (MEP) systems, such as a series of switching actions of valves in adjustment processes of complex chilledwater systems. Such information is valuable for operating and maintaining MEP systems. Thus, the IFC-based animation model can enhance the value of the VC animation by facilitating its delivery to the facility management phase. The attributes of each action for key-frame animation are listed in Table 1. Each action has StartTime and EndTime attributes with DateTime type to capture the time points when the action starts and stops, respectively. Furthermore, in Move, the MoveVector attribute is used to capture the distance and the direction of a movement. In key-frame animation, a curved path movement is simulated by a series of straight path movements. In Rotate, the CenterPoint attribute specifies the rotate center point, which is a 3D Cartesian point. RotateAxis refers to the axis around which an object rotates, which is a 3D vector. Meanwhile, RotateAngle specifies the angle of a rotation. In Scale, the CenterPoint attribute determines the scale center point, which is also a 3D Cartesian point. ScaleValue determines the scale rates and is a 3D vector with variables x, y, and z that represent scale rates in the X, Y, and Z directions, respectively. The widely accepted version of the IFC standard, IFC 2 × 3, does not provide properly predefined classes to represent animation. Therefore, this article adopts the property definition mechanism provided by the IFC standard to extend the IFC schema by defining the dynamic property sets and the properties for animation. In IFC, the IfcPropertySet class declares the dynamically extended property set, while the IfcProperty class and its subclasses are used to represent dynamically extended properties. Further details on the property definition mechanism are
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Fig. 6. GIM for VC of the West Tower.
presented in the “IFC Guide.” This method of extending the IFC schema is convenient and effective, and the extended animation model can be used by developers of AEC/FM applications as long as they adopt the definition of the custom property sets and the properties. Nevertheless, it will be very helpful for the wide application of a custom property set if the IFC standard adopts it as a predefined property set. According to the attributes of each action, three dynamic property sets, Pset Move, Pset Rotate, and Pset Scale, are defined in Table 2, which specify the move, rotate, and scale actions of physical objects. Dynamic properties for Pset Move, Pset Rotate, and Pset Scale are defined in Tables 3, 4, and 5, respectively. Complex animation can be indicated by attaching all property sets to physical objects, which represent simple actions that compose the complex animation.
4 DEVELOPMENT OF AN OPENGL-BASED VC PLATFORM OpenGL is a standard specification that defines a cross-language for programming applications, which produces 3D computer graphics. It is widely used in CAD, virtual reality, and scientific visualization (Shreiner et al., 2009). OpenGL adopts the surface
model that is easier to work with for the representation of 3D physical objects, as well as the simple illumination model to illustrate the virtual environment. Therefore, this research adopts OpenGL to develop a lightweight 3D graphic platform to visualize construction processes. To implement and verify IFC-GIM, an OpenGLbased VC platform using C# language is developed in addition to the 4D-GCPSU system. Figure 5 shows the architecture of the VC platform. As a basis for this platform, IFC-GIM stores and manages all building and construction data that are required by VC and VC results. IFC-GIM in the VC platform can be built through the importation of an existing information model such as a building design model, with the addition of necessary information that includes realistic information and animation. The 4D-GCPSU is a 4D CAD system developed in previous studies and improved by this research. It provides an integrated project-level construction management of planning, resource, cost, site layout, and safety in a 3D virtual environment, in addition to 4D construction simulation (Hu and Zhang, 2011). In 4D-GCPSU, schedule management and resource management modules provide the functionality of importing construction schedule and cost estimation data generated by other applications as well as the ability to attach construction information to the physical object
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Fig. 8. A view of the VC scene rendered by the VC platform with realistic information.
Fig. 7. Test processes of IFC-GIM.
model. These functionalities are significantly useful for building IFC-GIM in the VC platform. The IFC interface of the platform can import IFC models generated by other IFC-compliant applications, such as the 3D realistic model generated by BIM design applications, and optimize schedules and construction operation animations generated by a operation-level
VC system. As no IFC-compliant operation-level VC applications are available as of this writing, the authors attempt to develop an IFC model interface by working on the data mapping algorithm between SDESA and IFC-GIM to practice data exchange between projectlevel and operation-level VC. SDESA is an operationlevel VC platform using simplified discrete event simulation approach developed by Lu (2003), which consists of modeling module, site layout module, discrete event simulator, analysis module, and twodimensional demonstration module. Through the IFC interface, the IFC-GIM generated in this VC platform can also be exported to an IFC file, thus allowing other IFC-compliant applications to share the model.
Table 6 Statistical results about numbers of instances of various IFC types in the BIM design model IFC type IfcBuildingElement IfcBuildingElementProxy IfcFaceBasedSurfaceModel IfcSolidModel Total objects
Number
Description
25,267
Represent building elements
4,563
Represent realistic models of construction resource Represent surface models
29,830 29,830 2,438,642
Represent solid models Fig. 9. An animation illustrating the process of crane-lifting of construction equipment.
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tions and (2) whether the VC platform can handle realistic and animation data in IFC-GIM to carry out realistic and animated VC. The Guangzhou West Tower is a 432-m-high building with 102 floors. It is located in the CBD area of Guangzhou, capital of Guangdong province in China. Aggregated construction area of the building is approximately 450,000 m2 . The main structure comprises an inner core tube, an outer steel structure, floor slabs, core tube steel structure, and so on. Figure 6 shows the 3D models of the main structure and of the construction equipment. A typical personal computer equipped with a dual core 2.0 GHz central processor and 2 GB memory, which is a midrange 2010 computer in terms of comprehensive performance, was used to implement the application. The test processes are demonstrated in Figure 7 and are described in detail as follows.
Fig. 10. The 4D construction simulation using animation presenting construction activities.
This platform also provides a material definition interface that defines and modifies realistic information of physical objects as well as an animation definition interface that manually assigns the animation of building components and construction equipment to simulate construction operations. In summary, to implement IFC-GIM, the VC platform is improved in three aspects compared with existing 4D systems: (1) the platform can share all of the information required by VC while VC results can be shared by other IFC-compliant AEC/FM applications; (2) realistic information is managed to achieve realistic 4D simulation; and (3) construction activities can be represented by operation animations in addition to color, transparency, and texture maps, which are applied to distinguish different activities in 4D systems.
5 AN APPLICATION EXAMPLE The OpenGL-based VC platform was applied in a realworld project named Guangzhou West Tower to evaluate the following: (1) whether the IFC-GIM is sufficient to represent the information exchange requirement of VC systems and is effective to improve interoperability of the VC platform and other IFC-compliant applica-
1. An existing design model built by architects and engineers in Revit Architectural was exported to an IFC file, the size of which is about 32 MB. Within the file, physical object definitions of the main structure and of the construction equipment, including self-lifting steel shuttering, steel scaffolds, wood platform, tower cranes, and so on, were included. Geometric representation and quantity data were also provided. Although realistic information was created in Revit Architectural, the data were not successfully exported to the IFC file along with other design information because of the limitation of its IFC interface to map realistic information in its native database to the IFC-GIM. Table 6 shows a portion of the statistical results on the number of instances of various IFC types in the IFC file. 2. The IFC file was imported into the VC platform, while an IFC-based physical object model with geometric representation and quantity data was rapidly generated in the VC platform. 3. The construction schedule created in the MS Project was imported into the VC platform through the schedule management module. At the same time, the construction schedule was added to IFC-GIM by the VC platform. An IFC-based 4D model was then built by linking each component of the 3D model to the correlative work breakdown structure node. 4. Quantity bills, including labor, materials and machines required, and comprehensive price, were imported through the resource management module since the quantity bill method (Ma et al., 2011) was adopted in this project to estimate construction cost. Each physical object was then associated to a corresponding quantity bill to attach resource
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Fig. 11. Query and analysis of resource and cost. Table 7 Statistical results about numbers of instances of various IFC types in the IFC-GIM IFC type IfcTask IfcCostItem IfcConstructionEquipmentResource IfcSurfaceStyle IfcVertexBasedTextureMap IfcPropertySet (Animation) Total objects
Number 2,448 42 13
Description Represent construction tasks Represent quantity bills Represent construction equipment
23
Represent realistic information 24,571 Represent texture mappings of surfaces 43 Represent construction animation 2,528,319
and cost data to the physical object model. The VC platform also converted the quantity bills and resource data for the IFC-based construction information model. An IFC-based enhanced 4D
model was eventually established in the VC platform. 5. Realistic information was added to selected objects in the physical object model through the material definition interface. The realistic model was completed by defining all necessary material styles and associating them to corresponding physical objects according to their actual material. For a material with a texture image, the texture coordinates of each physical object, to which the material was assigned, were automatically calculated by the VC platform. Figure 8 shows the realistic model rendered in the VC scene with material and texture effects. In comparison with the scene rendered with geometric model and color only, the virtual scene rendered with texture and material is more realistic. Therefore, IFC-GIM can sufficiently represent realistic information that is required by a VC platform to provide realistic senses for most users on the necessary facility and immersed feeling in VC.
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6. Based on the construction schedule and technology, animation of construction equipment and a crane was created through the animation definition interface to illustrate the activity of crane-lifting for IFC-based animation model testing. StartTime, EndTime, action type, and other attributes of each action were inputted through the interface. Properties and property sets of actions were simultaneously created and associated to all selected physical objects to build the IFC-based animation model. Figure 9 demonstrates the six key frames of crane-lifting animation. Animation of other construction operations, such as lifting the platform steel shuttering and pouring core tubes from bottom to top, was also added. 7. Based on the IFC-GIM built on the VC platform, project-level construction simulation was carried out. The VC platform produced a realistic and dynamic visualization of construction scenes and processes according to a construction schedule by animating the evolution of buildings over time. Statistics of resource consumption for each task per day/week/month during the construction period was provided for construction management. Figure 10 illustrates a construction process of a one-floor concrete core tube that comprised four primary activities. Figure 11a presents a summary of resource consumption while Figure 11b shows the analysis curves of cost using the earned value method. 8. Finally, IFC-GIM was exported to an IFC file for other IFC-compliant applications to obtain the data, the size of which is approximately 40 MB. Table 7 displays a portion of the statistical results. The IFC file was successfully imported into the DDS-CAD Viewer 6.0 for IFC-GIM checking and viewing. However, only physical objects, materials without surface styles, geometric representations, and animation properties were viewed. That is as predicted, since a widely accepted IFC-GIM is a necessary prerequisite for sharing realistic information, and animation data among IFC-compliant applications. In summary, the experimental results proved the following: (1) IFC-GIM can represent realistic information, animation data, construction schedule, and resource data for the VC platform to provide realistic scenes for various stakeholders. By doing so, the stakeholders can understand the actual construction sites and processes as well as achieve an integrated analysis and management of schedule, resource, and cost. (2) With the help of IFC-GIM, the VC platform can share and exchange the physical object model, geo-
metric information, and construction information with other IFC-compliant applications, which result in the reduced modeling effort for project-level VC implementation and the increased value of VC results. (3) The animation function of this platform can effectively illustrate the construction process in which animation is added to represent different construction activities in 4D simulation.
6 CONCLUSIONS Based on the findings, the following conclusions are derived. 1. The exchange requirement for VC consists of four parts: definition of physical objects in the construction (e.g., building elements and construction equipment), construction information model (including task, schedule, resource, and cost), 3D realistic model of physical objects, and animation depicting the operation process. 2. In general, appropriate predefined classes exist to represent the physical object model, the construction information model, and the 3D realistic model in IFC schema. However, no predefined class is suitable to represent animation information. Therefore, three dynamic property sets and correlative properties that represent the three basic actions are defined to expand the IFC schema to IFC-GIM that sufficiently represents the exchange requirement for VC systems. 3. As verified by an actual project, the adoption of IFC-GIM to represent and exchange VC information is a practical method to achieve interoperability in VC systems and in other applications in the AEC/FM industry. Such interoperability can reduce the modeling effort of VC and increase the value of VC results. 4. On the basis of IFC-GIM, the OpenGL-based VC platform sufficiently achieves realistic and animated construction simulation on the project level, whereas animation of construction operations is applied to represent construction activities in addition to color and texture maps that enhance the realistic sense in 4D applications. As the first step, this article proposes and verifies IFCGIM as the foundation of data sharing in VC systems and in other AEC/FM applications. Further research should be conducted to develop the potential benefits of IFC-GIM in terms of interoperability, such as the following: (1) develop a converter between the IFC model and the operation-level VC system to achieve data sharing between the operation-level VC system and the
Development and implementation of an IFC-GIM for VC
OpenGL-based VC platform, despite the absence of the IFC-compliant operation VC system and (2) import IFC-GIM into an IFC-compliant facility management system so that a realistic virtual building with complete and well-organized construction and graphic information can be provided for facility managers, while potential benefits of IFC-GIM during the postconstruction phase can be exploited. These issues will be the focus in the next phase of this research. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51008168), the Tsinghua University Initiative Scientific Research Program (No. 2011THZ03), and China Postdoctoral Science Foundation Funded Project (No. 201104112). The authors would also like to thank the anonymous reviewers for constructive comments that helped in improving the quality of this article. REFERENCES Adeli, H. & Karim, A. (2001), Construction Scheduling, Cost Optimization, and Management – A New Model Based on Neurocomputing and Object Technologies, Spon Press, London. Alvanchi, A., Lee, S. & AbouRizk, S. (2011), Modeling framework and architecture of hybrid system dynamics and discrete event simulation for construction, ComputerAided Civil and Infrastructure Engineering, 26(2), 77–91. Aouad, G., Lee, A. & Wu, S. (2005), From 3D to nD modeling, Journal of Information Technology in Construction, 10(special issue), 15–16. BuildingSMART. (2007), IFC2x Edition 3 HTML Documentation. Available at: http://www.buildingsmarttech.org/specifications/ifc-releases/ifc2×3-tc1-release, accessed June 10, 2012. BuildingSMART. (2011), IFC – Industry Foundation Classes. Available at: http://www.ifcwiki.org /index.php /Main Page, accessed June 10, 2012. BuildingSMART. (2012), The BuildingSMART Data Model. Available at: http://www.buildingsmart.com /standards/ifc, accessed June 10, 2012. Buss, S. R. (2003), 3D Computer Graphics: A Mathematical Introduction with OpenGL, Cambridge University Press, London. Chang, D. (1987), RESQUE. Ph.D. thesis, University of Michigan, Ann Arbor. Cheng, T. & Yan, R. (2009), Integrating messy genetic algorithms and simulation to optimize resource utilization, Computer-Aided Civil and Infrastructure Engineering, 24(6), 401–15. Froese, T. M. & Yu, K. Q. (1999), Industry foundation class modeling for estimating and scheduling, in Proceedings of Durability of Building Materials and Components 8, Vancouver, Canada, 2825–35.
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