Research ArticleDeveloping an Extended IFC Data Schema and Mesh GenerationFramework for Finite Element Modeling

Zhao Xu ,1 Zezhi Rao,1 Vincent J. L. Gan,2 Youliang Ding,1 Chunfeng Wan ,1

and Xinran Liu 1

1Department of Civil Engineering, Southeast University, Nanjing 210096, China2Department of Civil and Environmental Engineering, #e Hong Kong University of Science and Technology, Hong Kong, China

Correspondence should be addressed to Zhao Xu; bernardos@163.com

Received 5 May 2019; Revised 21 July 2019; Accepted 6 September 2019; Published 3 October 2019

Academic Editor: Giovanni Garcea

Copyright © 2019 ZhaoXu et al.*is is an open access article distributed under the Creative CommonsAttribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Mesh generation plays an important role in determining the result quality of finite element modeling and structural analysis.Building informationmodeling provides the geometry and semantic information of a building, which can be utilized to support anefficient mesh generation. In this paper, a method based on BRep entity transformation is proposed to realize the finite elementanalysis using the geometric model in the IFC standard. *e h-p version of the finite element analysis method can effectively dealwith the refined expression of the model of bending complex components. By meshing the connection model, it is suggested toadopt the method of scanning to generate hexahedron, which improves the geometric adaptability of the mesh model and thequality and efficiency of mesh generation. Based on the extension and expression of IFC information, the effective finite elementstructure information is extracted and extended into the IFC standard mode. *e information is analyzed, and finally thevisualization of finite element analysis in the building model can be realized.

1. Introduction

*ere is a global trend in evaluating the overall performanceof buildings by means of the building information model(BIM) among structural design participants, such as de-signers, fabricators, and construction managers. However,the building models store the complete geometry of aconstruction, and they are not directly adequate for nu-merical simulation, as they only implicitly describe the to-pology and the mutual connections of different structuralcomponents [1]. In order to make the semantic data as wellas the connection explicit and elaborate interoperabilityamong building design participants, initiatives are putforward by extending the Industry Foundation Classes (IFC)model, which represent a promising approach toward en-suring software interoperability in the building industry. IFCstandard extensions to the data model provide three ex-tension mechanisms: extensions based on adding entitydefinitions, extensions based on IfcProxy entities, and ex-tensions based on attribute sets. Users can refer to the model

architecture of the IFC standard system and extend the IFCdata model according to their actual needs. Treeck and Rank[2] developed an algorithm based on the graph theory foranalyzing and interpreting building model-based geometryin order to support CAD (computer-aided design) tools withnumerical simulation techniques, which are applicable toany building energy simulation tool. Furthermore, this re-lation obtained by the developed spatial and topologicalanalysis may serve as the basis for queries by means of aspatial query language for BIMs. Nevertheless, anotherproblem arose that despite various simulation applicationsare available for analysis, data input from CAD drawingscannot be efficiently and precisely read by those applications.To solve this problem, Fazio et al. [3] designed an IFC-basedframework for implementing the performance evaluationprotocol. To refine the IFC instance files, Ying and Lee [4]proposed a rule-based automatic checking approach dedi-cated to the validation of IFC 2nd level—proper spaceboundaries for energy analysis. Similarly, a technology-transfer model named shared computer-aided structural

HindawiAdvances in Civil EngineeringVolume 2019, Article ID 1434093, 19 pageshttps://doi.org/10.1155/2019/1434093

design (SCASD) model is developed, affording a newmethodology of interaction, namely, electronic interaction(e-interaction) [5]. All of these studies have expanded theuse of IFC standards in different areas.

What is equally worth discussing is that, to fully explorethe three-dimensional structure model described by the IFC,the finite element method (FEM) is introduced to thestructural analysis. A triangle mesh is a common objectrepresentation adopted in scientific computing [6] as well asanimation [7] adopted in scientific computing, but the ac-curacy of triangular mesh calculation in finite elementcalculation is not as high as that of the quadrangle mesh.While mesh generation is extremely crucial for the accurateFEM analysis and computation, Lee and Lee [8] detailedabout how to design and develop a full-quadrilateral finiteelement mesh generator (Graph FEM V1.0). However,Nagakura et al. [9] mentioned that hexahedral meshes haveobvious advantages in terms of calculation error, the numberof cells, and resistance to distortion, so hexahedral meshesare becoming a mainstream mesh in FEM analysis. Lai et al.[10] improved this proposal to identify different many-to-one sweepable volumes to generate hexahedral elements. Inaddition, to be more specific, Miranda and Martha [11]described a hexahedral mesh generation algorithm ideallysuited for transition subdomain meshes in the context of anydomain decomposition meshing strategy. *e algorithmdecomposes hierarchically a volume until all volumes shouldbe mappable to generate elements by the 3D mapping ap-proach. Based on the generation of hexahedral mesh,Elhaddad et al. [12] presented a numerical discretizationtechnique for solving 3-dimensional material interfaceproblems involving complex geometry with the multilevelhp-FEM. By using a separate hexahedral mesh for eachmaterial subdomain and applying weak constraints on theinterface conditions between different meshes, the weakdiscontinuity of the material interface is solved.

Considering the three-dimensional modeling techniquesand the meshing methods in mind, it should focus on how toovercome the gap between BIM and FEM in buildingstructural analysis. In this paper, attempts have beenmade todevelop an effective extended IFC schema and mesh gen-eration framework to improve the architectural modelmeshing and finite element modeling. Section 2 brieflydiscusses the trends of research and the current range ofapplications of BIM and FEM. Section 3 introduces thetheoretical methods involved in this paper and shows thecomplete process of the whole project implementation. InSection 4, the case study is discussed and the researchmethods and future research direction are analyzed.

2. Literature Review

2.1. IFC Standard. *e application of BIM in the con-struction industry to enhance productivity has becomewidespread. Open BIM data schemas include the IndustryFoundation Classes (IFC) [13] and the green-building XMLschema. Among these two most popular BIM schemes,gbXML and IFC, IFC appears to be a suitable choice as it isricher in content, enables interoperability among different

software environments, and can be updated according to thebuilding’s modifications [14]. With the rising adoption ofBIM, a range of research studies have focused on IFC, theofficial data schema of BIM, to fully utilize and analyze theBIM data. Each IFC object contains its individual boundaryrepresentation model (BRep model), from which the at-tributed objects of the building can be derived via the IFC-Eurostep Toolbox in combination with the geometricmodeler ACIS from Spatial Corp [1]. However, Krijnen andBeetz [15] stated that the objectified relationship model andlow-level nature of IFC render many interesting relation-ships implicit, such as spatial connectivity and type in-formation. *erefore, additional inference steps are oftenapplied to increase its ease of use, querying efficiency, in-teroperability, and the discoverability of IFC information.

A wide variety of promising attempts have been pro-posed to develop the application and to improve the in-teroperability of IFC. To extend the interoperability of aconstruction quality database in the evaluation process byemploying IFC, Xu et al. [16] proposed integrated solutionsto improve current quality management processes with theassistance of an IFC-based working environment. Byexpanding the property set of IFC data, nontraditionalbuilding information is embedded into the BIM to build amapping relationship between BIM information and ex-ternal database information. Another aspect is noticed thatat present, getting the data out of BIM is still a hard task, andno BIM authoring tool able to properly export 2nd levelspace boundary geometry information contained in IFC canbe found. To conquer this problem, Lilis et al. [17] proposedan approach to extract such information correctly, which isnecessary for the setting up of building energy simulations.

In previous studies, BIM data standard still has manyshortcomings in dealing with multiprofessional collabora-tion. Each major only involves information related to themajor and does not implement the process of extractingcomplete data. As a result, the uniform IFC format does notwork as well as it should, and it is difficult to achievesynergies between different application levels. Such prob-lems are not unrecognized. To handle this problem, thisresearch focused on defining a schema using popular re-lational database structure following an established starschema model and on transformation rules to bring BIMdata into a form that is easily queried. Likewise, Daum andBorrmann [18] described a system for topological and spatialquerying on IFC building models. Elements intersecting orcontained within boundaries of other elements can be re-trieved. Kang and Hong [19] developed an automaticmapping method to improve the efficiency of IFC dataconversion process to CityGML, aiming at the problem thatthe mapping model in BIM and GIS conversion processtakes more time to calculate.

2.2. Mesh Model. At present, the technology of grid seg-mentation is improving, and the precision and applicabilityof grid segmentation are increasing and expanding. *esuccessful applications of the use of the information modelas a standard practice for buildings led to an increase in

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demand for its use in other structures as well. In addition,there is neither IFC supports road structures nor specificroad BIM execution plan exists. Consequently, a continuousactive effort has been made to develop an IFC-based dataschema for effectively representing the design information ofcivil engineering structures other than buildings for pro-ductivity improvement. Yabuki [20] proposed the IFC-shield Tunnel and included items for representing compo-nents, facilities, and geologic information targeted at shieldmethod tunnels. *ereafter, Lee and Kim [21] proposed anIFC data schema expansion plan targeting integrated roadstructures, which include bridges and tunnels. Nevertheless,these studies focused more on the relationship from amacroperspective and from a holistic perspective, re-spectively, rather than considering the elements of thetunnel structure in detail. Lee et al. [22] presented a tunneldata schema to efficiently manage and exchange the designinformation, by identifying the applicable IFC elements anddefining new items for the additional necessary elements.Similarly, Park et al. [23] put forward a method to improvethe usability of the IFC data schema-based bridge in-formation model that can control not only the 3D geometricinformation but also nongeometric attributes for an openstandard-based bridge information model at present.

Besides above application and enhancement of both IFCand BIM authoring tools, another problem arose whendealing with the cooperation of BIM and FEM. At present,there is only a few of studies on cooperation of BIM and FEM[24]. Xiao et al. [25] analyzed the combined parametricmethod of BrIM models, built up structural properties forFEM, and highlighted that the most essential factors for finiteelement analysis are element types, material properties, meshmodes, and restraint system. Moreover, as for FEM, Romberget al. [1] discussed the advantage of the fully three-di-mensional structural analysis performed by p-version of thefinite element analysis. *e main method adopted in thispaper is to establish the BRep entity geometric model formesh generation, which is due to the unique advantages of theBRep geometric model in the relationship between points,lines, and planes representing the structure. Huang et al. [26]proposed the geometric topology model of CSG-BRep todescribe the three-dimensional topological relationship of thestructure, aiming at the limited topological informationexpressed by the BRep geometric model. *is paper in-troduces the data structure of the BRep model in detail andunifies the topological relationship between models by usingthe Boolean operator.

*e literature reviewed above concludes a wide variety ofpromising approaches proposed to enhance the in-teroperability and the discoverability of IFC information.Nevertheless, attempts have to be made to further overcomethe limitations imposed by lack of consistency in data and thedeclining interoperability in the field of building construction[22]. IFC is a data model that relies on human interpretationin the sense that the meanings of its modeling elements arenot constrained bymeans of formal semantics, leading to callsfor such a formalization of IFC in some formal language to befrequently used for computational reasons [27]. *us, de-veloping a method to transfer the architecture model to the

finite element model effectively is the key to fully utilizing andanalyzing BIM data. *e data transformation can provideeffective support for the finite element analysis process of IFC-format BIMs created on the web browser. By using the webtechnology, BIMs can be linked with external information likeFEM-data from other systems on the internet to improve thevisualization performance.

3. Methodology

*e traditional transformation from the architectural modelto the structural model is usually to establish a generalstructural finite element model and to convert the generalstructural analysis model to the architectural model, whichhas a very good effect in the past when the interactionbetween architectural and structural model is not strong.With the development of BIM technology, the interactiondegree of the building structure model is continuouslyimproved. *e direct finite element analysis of the buildingmodel can better realize the use of a model in multipleaspects, which greatly facilitates the management of buildinginformation. Moreover, there are a series of incompatibilityproblems in the transformation between the traditionalgeneral structural analysis model and the architecturalmodel, and the mismatch of the model after the trans-formation, which are also the shortcomings exposed by thetraditional transformation method.

In this study, a novel mesh segmentation method isproposed to support the data conversion from the BIM tothe finite element model. *e proposed method firsttransforms a given IFC-based BIM into a connection model,which is then meshed to obtain a refined finite elementanalysis model for the subsequent calculations and simu-lation. *en, the meshed model is used to set boundaryconditions and load, and the finite element analysis is carriedout. In the process of grid division, the first thing is to ensurethe extraction of components based on the whole model soas to facilitate the view of the analysis results of components.*en, the relative coordinate and local coordinate trans-formation of the components are defined, and the com-ponents are meshed using the refined grid algorithm, at lastthe coordinate data of the unit section are encoded andcompiled. *e information transformation about non-geometric attributes, especially the loads and constraints,generally exists in the model. If these nongeometric attri-butes cannot be compiled in the model, the nongeometricinformation under IFC standards should be embedded in theoriginal geometry file. At the same time, this paper alsoexpanded and expressed the IFC attribute set and processedCAE results with WebGL technology combined withcompressed data to facilitate the calculation means andresults of finite element analysis to be applied and displayedin the BIM. By processing the results, the control of lineframe display and selection of display result list can beachieved. In addition, the selection and display positioncontrol of color bar can also be achieved. After calculatingthe data of the finite element model, it is able to display theresults of finite element analysis in the BIM. Figure 1 showsthe proposed BIM-based framework for mesh generation.

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3.1. Transformation Method

3.1.1. hp-Vision Finite Element Method. �e transition be-tween architectural and structural models is in uenced bymany factors. For traditional architectural models, the col-lection method based on scanning volume and constructionentity is generally adopted [26]. In the surface model, BRep(boundary representation) model can be used to expresscommonly [28]. In theory, the classical ­nite element method(h-vision) is very suitable for the simulation of 1, 2, and 3-dimensional structural problems [29]. However, it has certainrequirements on the aspect ratio of the structure shape, andlow-precision locking may occur when the case of plate andshell structure is encountered [1]. For simple materials, thegrid generator can follow the material page exactly, but it canbe cumbersome at the composite interface. In order to avoidtoo complex mesh, a p-version ­nite element method whichextends the original complex geometry to simple shapethrough the virtual domain is introduced into the ­nite el-ement analysis method so that the cartesian mesh can bedivided easily. While using the p-vision, it can well simulatethe complex boundary. However, materials with complexgeometric shapes need to obtain local high gradients orsingular points, and the problem must re­ne the local grid,which greatly increases the space occupied by the hard disk. Amultilevel hp-­nite cell method is proposed to enhance thestructural analysis model to bemore accurate in ­nite elementexpression. �e hp-vision ­nite element method combinesh-vision and p-vision, which can deal with complex 3d ge-ometry. �is scheme is especially suitable for dealing withdiscontinuity of interface of di�erent materials.

With the improvement of users’ aesthetic requirements,attention has been paid to the appearance and style of ar-chitecture, and more complex curved surfaces and irregular

shapes have been applied to architecture. �e ­nite elementmethod based on hp-vision combination is adopted to es-tablish smaller grid surface at the microlevel of the structureand achieve more accurate e�ect through continuous sub-division so as to express the architectural information.

3.1.2. Model Transformation Algorithm Based on BRepBodies. In a simple basic structure described by the IFC ­les,it mainly contains the following contents:

(1) �e basic content of the project, which is mainlyprovided by IfcProject:IfcOwnerHistory. Creator, application, and creationtimeIfcGeometricRepresentationContext. Type, dimension,coordinate system, precision, etc.IfcUnitAssignment. Unit of measure

(2) �e basic spatial structure of the project, which ismainly provided by IfcBuilding:IfcLocalPlacement. Describe the relative spatial po-sition of an artifactIfcProductDe�nitionShape. De­ne geometry in thelocal coordinate system

In the geometric representation of the model, the IFCstandard provides several methods: construct solid model,construct solid geometry model (CSG model), surfacemodel, subdivision surface model, and boundary repre-sentation model. �e CSG model has the advantages ofsimple method, fast generation speed, simple data structure,small data volume, and easy modi­cation. CSG is the resultof a Boolean operation in which an object is represented as a

JSON

IFC file Geometricmodel

3D ACIS

BIMserver

BRep model

Scanningmethod

Connectionmodel FEM

Componentextraction

Coordinatetransformation

Nodecoding

Compiledatabase

Calculation

WebGL

Modeldisplaying

Developmentof IFC

information

Differenceobjects

Coupling objects

Step

Figure 1: Proposed BIM-based framework for mesh generation.

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series of simple elementary objects (such as cubes, cylinders,and cones). *e disadvantage of CSG is that the informationis simple and does not record the boundary, vertex in-formation, and so on. Limited by the type of basic voxel andthe type of voxel operation, it indicates that the coverage areaof the body has great limitations and the local operation ofthe body (e.g., chamfering) is not easy to realize. *e spacerequired to display the resulting shape represented by CSG isalso longer. *e scanning representation, also known as thestretch body representation, represents the two-dimensionalcontour drawn on the working plane and then stretches thecontour in a certain direction to form a three-dimensionalobject. *e advantage of scan representation is that it issimple and suitable for graphic input means. *e disad-vantage of the scanning representation is that it is difficult toperform geometric transformation, the boundary in-formation of the shape cannot be directly obtained, and thecoverage of the shape representation is very limited. *esurface model is a series of two-dimensional faces enclosedinto a three-dimensional form, which is expressed in a moredirect way without too much node information. *e sub-division surface model is composed of a triangular mesh,which is more detailed in expression than the surface model.

An object of the BRepmodel is represented as a region ofclosed space connected by many surfaces such as faces,triangles, and splines. *e advantage of boundary repre-sentation is that there is more information about faces,edges, points, and their relationships. It is good for gener-ating and drawing wireframe and projection diagram andeasy to be associated with 2-dimensional drawing softwareand surface modeling software. *e disadvantage ofboundary representation is that because its core informationis surface, it cannot provide information about entity gen-eration process. *e BRep model needs more information todescribe the object, and the expression form of the boundaryexpression method is not unique. *e BRep model has greatadvantages in finite element mesh segmentation because thesolid model contains many relationships between points andlines.

3.2. From BIM to Finite Element Model

3.2.1. Model Transformation Algorithm Based on BRepObjects. In recent years, the development of engineeringcollaborative design has become more and more urgent.Collaborative design requires a high degree of integration ofdata and information to avoid inefficient and repetitivework. In other words, when the building scheme is de-termined, the data information of the building model cre-ated can be extracted by the downstream participatingmajors (structure, energy saving, equipment, etc.) to formcorresponding data model, so as to reflect the concept ofcollaborative design. At present, IFC is generally used in thedata exchange of building models in the construction andfacility management industry. *e goal of this productmodel standard is to define an integral, object-oriented, andsemantic model of all components, attributes, properties,and relationships of and within a building model. *e IFC

product model is specified using the modeling languageEXPRESS, which has been used to define STEP-basedproduct models within ISO 10303 before. Since 2014, theIFC4 standard has been released.

*e methods of describing solid in IFC standard areintroduced, which mainly include scanning solid model,surface model, boundary representation model, solid ge-ometry model, and subdivision surface model. Althoughthe IFC standard building model can well express thegeometric shape of a complete structure, it is not suitablefor numerical simulation because it only implicitly ex-presses the topological structure and mutual relationsbetween different components. It is difficult to describe theconnection of structures using the general stretchingmethod in IFC.

In order to make the topology and connection of thestructure explicit, it should further express the geometry ofthe building. In this case, it needs to establish the BRepmodel of the object with attribute objects. BRep is a methodof using boundary surface slices to express entities. *eentity object expressed by BRep is surrounded by manysmall faces. Each facet is composed of many boundary lines.Each boundary line is formed by connecting two vertices.*e orientation of each face can be judged by its normaldirection. *e advantage of using BRep technology is thatthere is much information about points, edges, planes, andtheir mutual relations, and it is beneficial to generate anddraw wireframe, projection diagram, and so on. However, itsmain disadvantage is that the boundary form is arbitrary andnot unique, which is not conducive to the development ofthe data conversion interface.

In order to be able to correctly express the connectionand topology of the model, a BRep-model with attributedobjects is derived from the building model firstly. As thesecond step, the geometric model is decomposed into“connection model” which includes coupling objects anddifference objects. *e coupling objects are set in all locationin which structural components are arranged in planecontact, while difference objects only being in contact withother difference objects at nodes or along common edges. Inthe third step, the model is divided into finite elementmeshes by scanning solid quadrilateral meshes, as shown inFigure 2. *e advantage of this model is that it not onlycompletes the mesh division of the model but also de-composes the beam-slab-column wall effectively.

3.2.2. Connection Model Definition. Consider a consistentgeometric building model as a set of one or more BRepbodies.

*e following types of intersection of objects can bedistinguished in Figure 3:

(i) Type PLF. *e intersection of objects with adjacentfaces consists of nodes, edges, and faces

(ii) Type PL. *e intersection of objects with adjacentedges consists of nodes and edges

(iii) Type P. *e intersection of objects with adjacentnodes consists of nodes only

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Objects can thus be characterized according to theirintersections with other objects. Based on their di�erentsemantics, they are partitioned into a set of coupling objectsMK and into the set of di�erence objects MD (Figure 4).

Each coupling object and di�erence object is composedof closed BRep objects being described by nodes, edges, andfaces. �e connection forms between di�erence objects aretypes of P or PL. �e connections between coupling objectsare types of P, PL, or PLF.�e connection forms between thedi�erence object and the coupling object are types of P, PL,or PLF.

To consider a geometric building model Ω⊂R3 con-sisting of a set of n BRep objects, the algorithm to decomposethe entity model into join models is shown in Algorithm 1,and the process is shown in Figure 5.

3.2.3. Automatic Mesh Generation with Hexahedral Elements.After the BRep model expressed the connection relationshipof the model correctly, it needs to start meshing thestructure. �e initial point of mesh generation is the modelthat is decomposed using the algorithm in the previoussection. Coupling and di�erence objects are meshed with

explicit given sequences and type-sensitive meshing algo-rithms. It can either use elementary 3D mesh macros orhexahedral meshes scanned using a 2D quad mesh. Firstly, ithas to de­ne a reasonable mesh size for each di�erent object.�e most critical element in the partitioning process is togenerate compatible elements. Only when the position ofnodes, edges, and faces of hexahedra on coupling objects isinherited to their adjacent di�erence objects, elementswithout hanging nodes can be guaranteed.

When the model is divided into coupling object anddi�erence object, the hexahedral meshing is generated. Inview of the characteristics of building structure shape rulesand easy to establish e¡cient model, this paper adopts thegeneration method of the hexahedral grid based on two-dimensional quadrilateral grid scanning. In this paper, amethod of generating hexahedral meshes based on two-dimensional quadrilateral meshes is proposed, aiming at thecharacteristics of building model shape rules and easy tobuild e¡cient models. In this method, an improved leadingedge advance algorithm is used for two-dimensionalquadrilateral meshing, and sweep method is used for three-dimensional meshing. �e meshing process is discussed in[2]. Multilevel hp-­nite cell method was used to generate

Sweepingdirection

Source surface

Targetsurface

Figure 2: Principle of the sweeping mesh division.

(a) (b) (c)

Figure 3: Intersection types: (a) P, (b) PL, and (c) PLF.

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meshes of di�erent sizes for the division of source surfacemeshes.

In the sweep method, the requirements for the model areas follows: ­rst, the source surface and the target surfacemust have the same geometric topology. Second, the sourceand target surfaces may not be two-dimensional, but theymust be parameterized. �ird, the connecting surface mustbe a quadrangle-like closed surface. Last, the meshing of theconnecting surfaces must be structured.

After sweeping a single component, it is transmittedthrough the coupling objects and di�erence objects betweenthe connectome. In the process of grid division, the size andshape of each component are di�erent, so it is generallynecessary to transfer the grid through the same boundary.�e transfer process is as follows:① Determine the elementdensity in the sweeping direction, which is determinedaccording to the ­neness of the ­nite element mesh; ②Determine the sweeping advance vector of the quadrilateralgrid by combining the hexahedral transition template. Be-cause there are di�erent sizes of meshes in the process ofmesh generation, di�erent advance vectors should be set upfor mesh segmentation; ③ Use the sweep method to gen-erate internal nodes layer by layer and generate hexahedralelements. For the quadrilateral grid element nodes of the

two-dimensional source surface, the corresponding sweepadvance vector is adopted to generate the internal nodeslayer by layer, and then the hexahedral elements are gen-erated layer by layer according to the geometric topologicalrelations of the hexahedral elements and their transitiontemplates;④ Restore of missing hexahedral mesh transitionelements. Hexahedral transition elements cannot be com-pletely divided by the sweep method, and a certain numberof mesh elements will be missing in the transition griddivided by the sweep method. After scanning, some nodesare repaired.

�e process of meshing between connectors is asfollows: ① Use the grid generator to cut the di�erenceobjects by the sweeping method, and then sweep in thethird direction; ② Assign values through common facecoupling objects, and then re­ne the coupling object; ③Pass through the common surface to the adjacent di�er-ence object. In this process, only when the position ofnodes, edges, and faces of hexahedra on coupling objects isinherited to their adjacent di�erence objects, elementswithout hanging nodes can be guaranteed. If the size of thehexahedra mesh is di�erent, the coupling part needs to bere­ned again, and part of the node information is nolonger transferred. For mesh re­nement, the formation

Difference objects

Coupling objects

Figure 4: Connection model with coupling and di�erence objects.

ni,j

ωj

fi,jωi

Figure 5: Connection model with coupling and di�erence objects.

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process of the connectome grid is shown in Figure 6. H-pfinite element mesh division can well maintain the uni-formity and continuity of the geometric gradient andimprove the efficiency of mesh discretization.

3.2.4. Finite Element Information Extraction Based on theIFC Standard. A building is composed of many compo-nents. When analyzing a single component, it should extractthe component information and then divide the structureinto grids. *e method of grid partitioning is given above.*ere are many basic structures for grid partitioning, andthey are often used together on a case-by-case basis. Hex-ahedral element has significant advantages in finite elementanalysis, so the hexahedral element should be used as far aspossible in the construction of the finite element model. *esize of the element is determined according to the accuracyof the calculation. It also needs to collect the element bodyinformation in order to complete the finite element analysisof the building model.

For a unit hexahedron, the coordinate information of thepoint is the main concern. After extracting the componentinformation, the frame point coordinates of the componentare found. *e point coordinates of the hexahedron can bedefined through geometric relation. In order to obtain thecoordinates of each point, the first step is to determine thecoordinates of the frame point of the component. *e ex-traction of coordinate information of frame points requiresnested references of multiple IfcLocalPlacement objects so asto obtain the geometric positioning of the local coordinatesystem of components in the absolute coordinate system.After obtaining the absolute coordinates of the frame points,the relative coordinates and absolute coordinates of eachpoint can be obtained through the geometric relationshipbetween the nodes in the process of mesh segmentation. *eshape expression of the IFC model is different from that ofcomponent structure. It is necessary to calculate the topologyinformation of each node of the element body according tothe shape expression of the structural component and thelocation of the local coordinate system. In the finite elementanalysis of the whole structure, for the frame beam andcolumn, the two ends of their topological expression are thetwo end points of the beam and column in the analysis model.For the wallboard component, the corner points expressed byits topology are the corner nodes in the analysis model.

(1) Component Extraction Based on the IFC. In the buildingmodel, many building components are continuous beams,cross-story columns, and cross-story walls, which need to besplit up and establish the relationship between structuralcomponents and structural connections when generatingstructural components. Before the finite element analysis, itshould extract the components in the IFC file first and thendivide the components into grids.*e internal data structurebased on IFC standards uses algorithms to extract in-dependent components. According to the types of buildingcomponents selected by users, such as walls, beams, andcolumns, the target components are extracted from IFCphysical files. In order to express the extracted IFC files of

components correctly, the algorithm is required to extractnot only the physical information of components but alsoother nonphysical information.

*e IFC physical files are preprocessed first. *e algo-rithm is used to detect the IFC instances corresponding tothe target building artifacts, and then the circular iteration isused to extract the examples of the associated link with thetarget instance. During this period, redundant instances areeliminated until all relevant instances are extracted. Finally,the extracted IFC files are processed after processing, and theIFC files are checked for validity. *e whole procedure flowis shown as Figure 7.

*e BRep components extracted in the process are storedin IFC files as physical files. *e IFC nongeometric in-formation is written or accessed in the conversion process. Ifthe nongeometric information is not shown in the standardphysical files or is missing, the physical files need to besupplemented accordingly. *e BRep geometry model, asthe intermediate state of the running program, is containedin the IFC files.

(2) Local Coordinate SystemAnalysis. Structural components(including other entities in the structural analysis model)refer to the unified coordinate system of the structuralanalysis model, and the hierarchical coordinate referencemethod in the architectural model is not adopted. *estructural components are expressed by topological geom-etry so as to distinguish the connection relations efficiently.*erefore, the transformation between local coordinatesystems should be considered in the expression process.

*e unified coordinate system of the structural analysismodel does not use the hierarchical coordinate referencemethod in the building model, so the coordinates need tobe converted. *e IFC standard stores the position co-ordinate information of the object through the IfcLocal-Placement type. In IFC files, spatial structure is defined byIfcRelAggregates. *e location of the model component isdefined by local position. Each component has individualindependent coordinate system (IfcLocalPlacement). At-tribute PlacementRelTo corresponds to the coordinatesystem of an upper order, and attribute RelativePlacementcorresponds to the information of relative position. *en,through multiple references of the coordinate system, thecoordinate system of components is nested under multipleIfcLocalPlacement, and its position in global coordinatesystem of the whole project is finalized. *rough nestedreferences of multiple IfcLocalPlacement objects, the rel-ative position is obtained of the local coordinate system ofcomponents in the global coordinate system (the locali-zation of the origin and the axes of local coordinate systemin global coordinates). *e position of the object does notpoint the world coordinate system but points a relativeobject in the local coordinate system. For example, theposition of a wall is determined by the local coordinatesystem of an adjacent beam which is set in the overallcoordinate system of the entire building. All objects haveprecise position coordinates in the world coordinate sys-tem, so the coordinate systemmust be converted during themapping. Hence, the conversion of geometric information

8 Advances in Civil Engineering

from IFC contains the transformation of the coordinatesystem. Upon model display, the coordinate systememployed of model is the local cartesian coordinate system.BIMserver is a BIM-based server management platform.�is platform can be used as an example. Upload the IFC­le to the BIMserver server, parse the model throughIFCEngine, split the entire model according to the buildingcomponents, and obtain its component model (IFC ­le).After the above process, the original IFC ­le is convertedinto a smaller IFC ­le obtained by the component and a

JSON ­le containing the component properties. In theprocess of extracting components, it is necessary to traversethe various components in the IFC ­le, obtain the geo-metric positioning, shape expression, and related materialproperties of the local coordinate system of the componentin the entire model coordinate system, and store them in aJSON ­le. Generally, the origin of coordinates is on thesurface of ellipsoid and corresponding elevation zero. �ecalculation of transition matrix Tt between two coordinatesystems is as follows:

Use the mesh generator tomesh the difference

objects and sweep in thethird direction

Assign values throughcommon face couplingobjects and refine the

coupling object

Pass through thecommon surface to

the adjacentdifference object

Figure 6: �e meshing process.

Step 1:preprocess the IFC file

Step 2:input/identify target

components and instanceextraction

Step 4:eliminate redundant

association cases

Step 5:extract the instance

contained in the extractedinstance attribute value

Step 6:postprocess IFC files

Step 7:check the validity of the

extracted IFC physical files

Step 3:extract the instance

associated with the targetcase

Extract newcases?

Sear

chin

g lo

op

No

Yes

Figure 7: Component extraction based on IFC physical ­les.

Advances in Civil Engineering 9

Tt � RxRyRzT,

Rx �

1 0 0 0

0 cos θx sin θx 0

0 − sin θx cos θx 0

0 0 0 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

,

Ry �

cos θy 0 − sin θy 0

0 1 0 0

sin θy 0 cos θy 0

0 0 0 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

,

Rz �

1 0 0 0

0 cos θz sin θz 0

0 − sin θz cos θz 0

0 0 0 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

,

T �

1 0 0 0

0 1 0 0

0 0 1 0

x1 y1 z1 1

⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣

⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦

,

(1)

where it is assumed that the origin of the local coordinatesystem in the IFC model is (x1, y1, z1) in the global co-ordinate system; the Z-axis of local coordinate system isperpendicular to the ground; the Y-axis points to due north;the spin matrix corresponding to coordinate axes is Rx, Ry,and Rz; and θx, θy, θz are the intersection angles betweenrotation vector and X-axis, Y-axis, and Z-axis.

In the process of meshing the local components, thecoordinate information of the physical instance should bemapped to the world coordinate system, and only bymapping to the unified world coordinate system can themeasurement and comparison be made according to thesame standard. *e geometric representation of the Ifc-Product instance is based on the local coordinate system,and the local coordinate system in the world coordinatesystem will be affected by the reference coordinate system.To obtain the representation of its local coordinate system inthe world coordinate system, follow the reference coordinatesystem of IfcProduct and trace upward until its Place-mentRelTo is empty. *is paper takes the coordinatetransformation between a beam and a column as an exampleto illustrate the method of calculating the world coordinatesystem. *e column section is 500× 500mm, the height is3600mm, the beam section is 500× 300mm, and the lengthis 2000mm. *e instance fragment is shown in Figure 8.

(3) Calculation of Node Coordinates Based on IFC Standard.A node is the most basic finite element entity. A node de-scribes a physical location on the created structure and isused by the element to define the location and shape. It canalso be used as a temporary input to create geometric objects.*e three-dimensional finite element model element can beunderstood as a three-dimensional body composed ofseveral nodes, and each three-dimensional element is

connected to form a complete finite element model.*erefore, if the nodes in each finite element model arecreated and then connected together according to a prin-ciple, the finite element model can be created.

In the process of assembling components in the pointgraph, it needs to extract the component information firstlyand then obtain the position information of the frame pointsof the component in the absolute coordinates from the IFCstandard information so as to calculate the position in-formation of each point in the IFC context. According to thesolution method above, the absolute coordinate system ofeach component, the reference coordinate system of eachcomponent, and the relative position of the absolute co-ordinate system are obtained after nesting layer upon layer. Inthe process of finite element mesh segmentation, the meshelements involved include one-dimensional node element,two-dimensional quadrilateral element, and three-di-mensional hexahedron element. *e data structure of nodecell and tetrahedral cell is simple. No matter what kind of gridcan be represented by nodes arranged in a certain order. *equadrilateral element can be represented by its four nodes inorder. For the purpose of unifying the data of the subsequenthexahedral element, it is stipulated that the node order of thequadrilateral element should conform to the right-hand rule.*e data structure of nodes is similar to that of geometricpoints. *e data structure of the quadrangle includes twoparts, element number (Id) and element node set (Ele-met2D_Nodes). *e data structure of nodes and quadrilateralcells can be specifically expressed in Algorithm 2.

*e hexahedral element is composed of six quadrilateralelements in geometric topological relation, so a hexahedralelement can be represented by 8 nodes of two relativequadrilateral elements. *ese eight nodes must be one-to-onein space. *e data structure of the hexahedral element in-cludes the hexahedral number (Id) and the eight nodes of thehexahedral element (Element3D_Nodes). Its data structure isexpressed in Algorithm 3.

X

Y

Z

X

Y

Z

X

Y

Z

A

B

C

Figure 8: Coordinate system transformation in IFC physical files.

10 Advances in Civil Engineering

After the nodes of the hexahedral element are defined,the coordinates of the base points of the components canbe found by looking up the coordinate information in theIFC file, and then the coordinate values of the other pointscan be calculated. Firstly, a rectangular coordinate systemfor the component is established, and the origin is the basepoint of the component. According to the method ofmeshing mentioned above, it can be found that the co-ordinates of all components are in the coordinate systemof the component. Tetrahedrons, hexahedrons, triangularprisms, and pyramid elements can be used in conjunctionwith complex, irregular entities. With the calculationabove, the graph of all node positions and coding in-formation can be formed. *en with consideration ofthe load information, the position change informationis obtained as the standard for structural analysis ofcomponents.

3.2.5. #e Extension of Nongeometric Properties. *e struc-tural analysis model mainly includes nodes, units, loads, andconstraints.*e IFC standard provides a comprehensive set ofpredefined entities for structural analysis, in which the rel-evant finite element elements are prefixed with IfcStructural.For the node connection, IfcStructuralConnection is mainlyused in the IFC standard to express the connection nodesbetween elements in the structural model. *e connectionhere mainly refers to the connection node or support conceptof the structural analysis model (finite element model); thestructural analysis model of the unit with IfcStructur-alMember entity to express, IfcStructuralMember have twochild class, don’t is the line unit (IfcStructuralCurveMember)and surface single yuan (IfcStructuralSurfaceMember); for thestructure’s load representation in IFC, there are multiple loadconditions and multiple load combinations according to thedesign specifications. *erefore, accurate expression of theload groups to the key. *e combination of IfcStructur-alAction and IfcStructuralLoadGroup is generally adopted toexpress the load condition and load combination; structuralanalysis model of constraint expression using IfcBoundar-yCondition entities.*is entity can express support boundaryconstraints (applies to IfcStructuralConnection) and canexpress the end of the bar to degrees of freedom (for use inIfcRelConnectsStructuralMember). About beam section in-cludes point about beam (IfcBoundaryNodeCondition), edge(IfcBoundaryEdgeCondition), and face about about bouquets(IfcBoundaryFaceCondition). *e writing or access of thisnongeometric information in IFC standard needs to be re-flected only in the process of transformation. If it is notreflected in the standard physical file or there is lack of in-formation, the physical file needs to be supplementedaccordingly.

3.2.6. Establishment of the IFC-Based Finite Element Model.In the process above, the BIM is subjected to the re-quirements of the finite element analysis model. To realizethe finite element analysis of BIM, on the one hand, itneeds to expand and express the finite element in-formation of the processed BIM; on the other hand, the

mapping relationship between data and model is in-tegrated, as shown in Figure 9. In this way, the informationof the finite element model is firstly extended in IFCstandard, and then the finite element information is in-tegrated into the BIM.

(1) #e Extensions Based on IfcProxy Entities. As an openstandard system, the IFC standard provides three extensionmechanisms for the extension of the model, namely, ex-tension based on IfcProxy entity, extension by adding entitydefinition, and extension based on attribute set. Each of thethree extension methods has its own advantages and dis-advantages, and different methods can be used for differentusers. For example, if the national standard is formulated,entity expansion is required, and the relevant attribute setsare supplemented. For project-level BIM applications, theIfcProxy extension or attribute set extension can be used.Compared with the extension method based on the attributeset, the operation is simple and convenient, the versioncompatibility is good, the type is safe, and the operationefficiency is high, which is the preferred expansion method.

In the IFC standard, an entity can have multiple attributesets. *ese attribute sets can be either a predefined attributeset or a custom attribute set, which lays the foundation for usto extend the attribute set. *e association of an entity withits attribute set is achieved by defining an attribute re-lationship entity. As a container of attribute information, theattribute set loads the structural force information that theengineers care about.

*is paper first establishes a basic information attri-bute set about a finite element body and defines the at-tribute content contained in the attribute set. Whendefining the content of an attribute, it needs to determineboth the attribute type and the attribute value type.Usually the attribute type is IfcpropertySingleValue. Afterthe definition of the basic information attribute set andrelated attributes of the structure is completed, theidentification information attribute set at the structuralcomponent level is established, including the relative andabsolute coordinate information of the individual pointsof the unit body. Since beams, columns, slabs, and walls ofstructural components all have these two attributes, in theactual extension, the common parent class of IfcBeam,IfcColumn, IfcSlab, and IfcWall, IfcBuildingElement canbe selected as the associated entity of the extended at-tribute set, which is also a reflection of the flexibility of IFCstandards in practice. *e rating and rating value prop-erties of beam, column, board, and wall are included in thePset_AppraisalResult property set, and the rating andrating values in the property set are defined.

After the definition of component attribute set andrelated attributes is completed, the specific method ofdefinition in the process of grid generation shouldbe defined to ensure the accuracy of information ex-pression as much as possible in the compilation process.*e grid attributes of components are defined as shownin Table 1.

*e set of encoding attributes of a cell body is shown inTable 2.

Advances in Civil Engineering 11

After the expansion of the attribute set, it is required toexpress the ­nite element analysis model based on IFCstandard further. IFC standard data need to be expressed inEXPRESS normalized language, which provides two ways ofdescription: EXPRESS language description in code formand express-g diagram in tree form (a graphical represen-tation of the product model corresponding to the EXPRESSlanguage, known in the standard as a “graphical subset ofthe EXPRESS language”). Because the steps of manually

modifying the code form of EXPRESS language are tedious,the workload is large, and it is easy to make mistakes.�erefore, after the IFC standard is extended a littleaccording to the user’s own needs, it is often described by theexpress-g diagram to complete the expression of the ex-tension information. �e EXPRESS language description inthe code form is mostly used after the IFC standard system isupdated or other IFC standards are extensively extended andmodi­ed. In this paper, IFC-based ­nite element analysis

The extension andexpression FEM

information based on IFC

The integration of geometricmaterial information in BIM

software

IFC file extension+

Express-g

Geometric materialproperties linked to BIM

software+

Adding quality parametersin BIM software

IFC data mapping in FEM basedon IFC

Standardizationcriteria

Informationintegration

Figure 9: Implementation of IFC data mapping in the FEM model.

(1) for i� 0; i< n; i++ do(2) for j� i+ 1; j< n; j++ do(3) if RC (i, j)� true then(4) Do decomposition:(5) Create imprint face fi,j(6) Create normal vector ni,j(7) Copy ωi to ωi′(8) Translate ωi′ along − ni,j by ∆d(9) Copy ωj to ωi′(10) Get connection objects:(11) a1:�wi′ ∩ bwj′ ⊆MK1(12) d1:�ωi⊆MD(13) d2:�wj/ba1 ⊆MD(14) end if(15) end for(16) end for

ALGORITHM 1: Decomposition into the connection model.

Type Mesh_NodeId as LongPosition3D (0 to 2) as Double

End TypeType Mesh_Element2DId as LongElement2D_Nodes (0 to 3) as Mesh_Node

End Type

ALGORITHM 2: Data structures for nodes and quadrilateral cells.

12 Advances in Civil Engineering

information extension did not define new entities, but ex-tended based on attribute set. *e express-g diagram isselected here to describe the quality information extendedabove as shown in Figure 10.

In Figure 11, among the five types of entities, IfcProduct,IfcElement, IfcBuildingElement, IfcPropertySetDefinition,and IfcPropertySet, the two adjacent types of entities areinheritance relations, which are expressed with bold solidlines and circles pointing to subclass entities. An associationrelationship is established between IfcPropertySet andIfcEntity through IfcPropertySetDefinition so that the con-struction quality status of IfcEntity can be expressed throughthe construction quality evaluation information contained inIfcProperty. IfcProperty includes two subclass entities, Ifc-ComplexProperty and IfcSimpleProperty, of which IfcSim-pleProperty includes six subclass entities.*e set of propertiesdefined in this article is the optional set of properties of theentity IfcBeam/Column/Slab/Wall (which inherits all theproperties of IfcBuildingElement), connected by a dotted line.Figure in the contents of the elliptic dotted box is defined as 4item properties: OnsiteConcreteBeam/Column/Slab/explicitattribute of the Wall, through the thin lines is connected to aset of properties.*e attribute values of the four attributes canbe divided into enumerated values and simple valuesaccording to their types, and the relationship between themand the attribute value entities is mandatory association,which is connected by thin solid lines. *e symbol “(ABS)”represents abstract class, which is an entity that cannot bedirectly displayed in the construction project, and exists sothat other entities can appear as its superclass.

(2) Database Preparation. In order to make the data storage,identification, association, call, and other management work in

the database more convenient and orderly, it is necessary toencode the relevant data, which should follow the principles ofstandardization, simplicity, uniqueness, and integrity. In ad-dition to the above coding principles, during the finite elementanalysis and calculation, the node and element number willhave an impact on the bandwidth and the number of wavefront segments of the total stiffness matrix of the structure andthen on the calculation time and database capacity. At thepresent stage, there are many optimizers that can improve thenode number twice, and the node number and unit number aremore conducive to calculation. In order to facilitate coding, thecoding order of nodes is carried out according to the right-hand rule, and the grid in the XY plane is corresponding to theadjacent one in the Z direction, and so on, to reach the codinginformation of each point. After finite element analysis of thestructure, a large amount of data will be generated, which is notconducive to data processing. In this study, it is proposed tolightweight the scalar data, vector data, and grid data in theresults, compress the data by using compression algorithm, andimprove the efficiency of displaying the results.

(3) Information Transfer between Database and BIMSoftware. For the data to be used in the finite elementanalysis of the BIM, in addition to the coordinate in-formation and element information of some sections andnodes contained in the IFC file mentioned above, thematerial information and load information of the structureare also included. In Figure 12, it describes how this in-formation is connected to the entire model through theexpress-g language. After encoding the node informationof components, the finite element information of eachcomponent into the database is compiled.*e processes areas follows:

(ABS)IfcBoundaryCondition

IfcStructureAnalysisModel

IfcStructuralResultGroup

IfcStructuralLoadGroup

IFCKERN

EL,IfcGroup,

( KERN

EL)

IfcStructuralLoad

IfcStructuralLoadTemperature

(ABS)IfcStructuralLoad

Static

Figure 10: *e express-g diagram description of IFC entities.

Advances in Civil Engineering 13

(i) Replace the component instance with the corre-sponding absolute coordinate information

(ii) De­ne the component attribute coding(iii) Enter the information about all artifacts(iv) Associate the absolute coordinates of a component

with its corresponding attribute information(v) Postprocess and check the database information,

validity check(vi) Associate database with IFC ­le

When conducting ­nite element analysis on the struc-ture, ­rst of all, it should locate the coordinate informationof the origin of the relative coordinate system of thestructural components in the absolute coordinate system.After ­nding the coordinate system of the basic points of thecomponents, the database should be traversed to ­nd all theinformation related to the ­nite element analysis of therelevant nodes. �e processes of extracting coordinate in-formation are as follows:

(i) Preprocess IFC ­le

(ii) Input, recognition, and absolute coordinate ex-traction of target components

(iii) Traverse the database to identify absolute co-ordinate information corresponding to the targetartifact

(iv) Extract information about mechanics, materials,lengths, and other correlations associated with ab-solute coordinates

(v) Extract database information postprocessing, checkvalidity

�rough the above process of database preparation, the­nite element analysis point information can be obtained.�e obtained information is exported to the simulator forcalculation, and the obtained results are fed back to themodel for display.

4. Discussion

4.1. Case Study. In Figure 13, a simply supported beambridge, which is box girder type, is taken as a sample. First,the BIM is based on IFC standard to generate the BRep entitymodel. �en, the model is meshed, and the results ofmeshing are exported to the dataset. �e database isestablished according to the way of component arrange-ment. �rough the ­nite element analysis, the cloud imageof the structure under stress can be obtained. Such resultscan be displayed in the BIM software, and the stress andstrain conditions of each point can be obtained so as toachieve the visualization of the ­nite element analysis of theBIM. �e deadweight, vertical live load, and surface load areconsidered in the structural analysis in Table 3. �e stress

IfcBeam/Slab/Column/Wall

(ABS)IfcProduct

(ABS)IfcElement

(ABS)IfcBuildingElement

(ABS)IfcPropertySetDefinition

OnsiteReinforcedConcreteBeam/Column/Slab/Wall

IfcPropertyIfcSimpleProperty

IfcPropertySet

IfcPropertySingleValue

IfcPropertyEnumeratedValue

IfcPropertyBoundedValue

IfcPropertyTableValue

IfcPropertyReferenceValue

IfcPropertyListValue

IfcComplexProperty

Cellnumber

Cellcoordinate

Celllength

Cellshape

......

Real

Real

Real

Real

(ABS)IfcObject

(ABS)IfcObjectDefinition

IfcRoot

Figure 11: Express-g diagram for ­nite element analysis.

14 Advances in Civil Engineering

and strain analysis of the beam under deadweight is cal-culated, and the node information is compared, and thee�ect diagram is generated.

WebGL technology can be used to perform CAE pre-sentation of the ­nite element calculation results of thebridge. In addition, users can choose to display a list of

Figure 13: Generation of mesh and display of ­nite element analysis results.

Type Mesh_Element3DId as LongElement3D_Nodes (0 to 7) as Mesh_NodeEnd Type

ALGORITHM 3: Data structure of a hexahedron.

Preprocess IFC file

Input, recognition, and absolutecoordinate extraction of target

components

Traverse the database to identifyabsolute coordinate information

corresponding to the target artifact

Extract information aboutmechanics, materials, lengths, andother correlations associated with

absolute coordinates

Extract database informationpostprocessing, check validity

Replace the component instancewith the corresponding absolute

coordinate information

Define the component attributecoding

Enter the information about allartifacts

Associate the absolute coordinatesof a component with its

corresponding attribute information

Postprocess and check the databaseinformation, validity check

Associate database with IFC file

Figure 12: Database compilation process.

Advances in Civil Engineering 15

results that can be viewed both on the surface and inside ofthe component. *e results of the cloud image displayed canbe switched by selecting the color bar and control the po-sition.*e strain cloud image in the specific direction can beselected based on the effects to facilitate the more detailedanalysis of the structure. Figure 14 shows the node dis-placement cloud image of Uy and sum directions based onthe structure on the website.

Figure 15 shows the deformation of beam-columnconnections under the uniformly distributed load. Underthe different mesh densities, the beam and column meshsegmentation are also transmitted through the couplingjoint. *e analysis results of the structure can be dis-played and manipulated on the web side as the casesabove.

4.2. Technology Comparison. At present, in the finite ele-ment analysis of BIM, there are two main technologies incontinuous implementation and application. One way toachieve the goal is to use finite element analysis software toenhance the application of building models. Now somecommercial software claims that they have developedrelated software applications and platform systems.However, this kind of platform is more based on the ex-pansion of the product chain of its own software and doesnot have strong universality. In contrast, the IFC standard-based BIM introduced in this paper is more versatile in thedevelopment of finite element analysis. In the finite ele-ment analysis based on the IFC standard, some studiesdevelop the IFC standard interface directly. *rough datatransformation, the building model can be better generatedand imported into the structural model for calculation,which is also a relatively simple and convenient way at thepresent stage. In the studies so far, direct analysis of IFC

files for structural analysis has not received more attention.*e IFC mesh segmentation method in this paper caneffectively calculate the information of each point in thefinite element mesh segmentation. Other information suchas material information, load information, and node in-formation is stored in a database according to the EX-PRESS language standard for computing purposes.Because the grid segmentation of components is based onIFC standards, when the finite element calculation of thesenodes is carried out, the results can be directly displayed inthe BIM.

4.3. FutureWork. Although the finite element analysis of thestructure by using the BRepmodel is effective, there are still alot of work to be improved in the future, including thefollowing:

(1) Improvement of Algorithms about Meshing Need.*e boundary constraint condition for constructingharmonic scalar fields in our algorithm, namely, thesource surface and the target surface mentioned inthis paper, is constructed by the user through in-teractive selection tool on the boundary surface ofthe swept body. *is not only increases the user’sinconvenience but also leads to the large de-formation difference between the source surface andthe target surface, which is not conducive to thegeneration of high-quality hexahedral mesh. In thefuture, it is proposed to consider using the surfacesegmentation method and shape analysis technologyon the surface grid of the model to automaticallydivide the surface of the model into surface regionswith the same characteristics, and then automaticallyset boundary constraint conditions of harmonic

Table 1: Grid attributes of components definition.

Attribute name Attribute type Attribute set type Attribute definitionsCellShape IfcpropertySingleValue Boolean Shape of elementCellLength IfcpropertySingleValue Boolean Length of element

Table 2: Cell body information definition.

Attribute name Attribute type Attribute set type Attribute definitionsCellCoordinate IfcpropertySingleValue Boolean Coordinate informationCellNumber IfcpropertySingleValue Boolean Coded informationUx IfcpropertySingleValue Boolean Displacement of X-axisUy IfcpropertySingleValue Boolean Displacement of Y-axisUz IfcpropertySingleValue Boolean Displacement of Z-axisUsum IfcpropertySingleValue Boolean Integrate displacement

Table 3: Parameter information of the bridge.

E (N/m2) Section shape Length (m) Surface load (kPa) Deadweight per cubicmetre (kg/m3)

3.25× e10 25 500 2600

16 Advances in Civil Engineering

scalar field according to the corresponding relationsof these subsurfaces.

(2) Expansion of Entity Type into the Finite ElementModel. In the process of transforming the buildingmodel to connection model, other solid con-struction methods can be extended to conduct fi-nite element mesh division. If the building modelcan be generated and optimized with different al-gorithms according to different entity expressiontypes, the use efficiency of the building model willbe greatly improved, especially for buildings andbridges with complex grids. Moreover, the appli-cation of h-p finite element method can also begreatly improved.

(3) Grid Generation of IFC Files in Specific Algorithms.In this paper, the information of IFC files is an-alyzed and the necessary information in thestructural analysis is obtained. In future studies,direct grid compilation of IFC files will become amore critical method. Grid generation can beachieved by directly writing the coordinate in-formation of points in IFC files or by means ofentities. *is paper mainly manually segments IFCcomponents into grids, which seems not in-efficient. At present, not enough research has beendevoted on analyzing IFC documents, and theintegrated study in the field of structure still needsto explore new methods to expand the applicationlevel of BIM technology.

5. Conclusion

*is paper mainly focuses on IFC standard finite elementstructure analysis. Considering the advantages of BRep entities,this study uses the BRepmodel to express all geometric entitiesthrough the transformation of the 3D ACIS model. *e entityis segmented into the finite element mesh applying the meshrefinement algorithm, and the finite element information of thestructure is obtained based on step format data. *rough theanalysis and expression of IFC files, the finite element analysiscan be finally displayed in the IFC standard.

In the transformation from the architectural model tothe geometric model, it was found that the entity structuremodel with BRep entities had unique advantages. *e modelis transformed into a connected model by analyzing thestructure of the model, and the mesh of the model is dividedinto hexahedrons by the calculation method based on thesweep method. In the process of transformation, althoughthe local construction precision is not enough, the error canbe held within the scope of modification. In the field of BIMinformation, relevant requirements are collected for com-ponent extraction firstly to ensure that a single componentcan be selected for analysis and calculation. *e informationof the nodes in the IFC file is defined and analyzed to providenecessary data for the finite element analysis. *e IFC in-formation was expanded and expressed on the BIM port,and the connection between the IFC and the database wasestablished to ensure that the finite element analysis resultswere displayed in the IFC standard-based software. In thiscase, a section of the simply supported beam bridge is

Figure 14: Cloud image of node displacement in Uy/sum direction.

Stress_mises7.522e + 01

60

45

30

15

1.193e – 01

Spatial_displacement magnitude4.319e – 09

3.6e – 9

2.7e – 9

1.8e – 9

9e – 10

0.000e + 00

Figure 15: Stress and strain diagrams of frame beams and column grids.

Advances in Civil Engineering 17

analyzed and processed, and the displayed results can bevisualized in the BIM software.*emain contribution of thispaper is to develop a method to transfer architecture modelto finite element model effectively. *e grid model is ob-tained by meshing IFC files and the results of finite elementanalysis and then is displayed based on the extension of IFCstandards. In this process, the extraction of building com-ponents can be completed to provide effective support forstructural analysis. In the outlook, it puts forward severaldirections for future research, which will also provide newthinking for future research in related fields.

Data Availability

*e data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

*e authors declare that they have no conflicts of interest.

Acknowledgments

*e authors’ special thanks go to all survey participants andappreciation goes to the National Science Council of P. R. C.for financially supporting this research (NSFC-71302138).

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