The Research of BIM in the Design Stage of Precast Concrete Structures

This paper presents a study on the application of BIM (Building Information Modeling) technology based on CDM (Component Data Model) theory in the design stage of precast concrete structures from three aspects of standard formulation, designing by filling numbers and software development, which is used to solve the problem that IFC(Industry Foundation Classes) cannot be implemented in the traditional mode. BIM technology, with its advantages of visualization, parameterization and integration, is widely used in the full life-cycle of prefabricated buildings. Initially, this paper analyzes the disadvantages of traditional BIM technology based on IFC format, such as the defect in developing IFC plug-ins and low data interaction efficiency for the lost of a large amount of data in the model. Then the process of BIM technology based on CDM theory is introduced for the design stage of precast concrete structures and from the comparative analysis of CDM theory and IFC theory, we can find that CDM theory can break the data island in the process of project design and improve the efficiency of data interaction among each BIM software. Finally, the feasibility of the proposed technical route is verified by a practical prefabricated engineering case, and this method can be universally applied to the full life-cycle management of the project construction.


Introduction
At present, in each stage of project construction, information exchange between BIM (Building Information Modeling) software is mainly through IFC (Industry Foundation Classes) format. However, in this mode, there are many unavoidable and difficult problems in information interaction. Such as a large amount of data in the model is lost, resulting in low data interaction efficiency, and software developers generally do not have the ability to develop IFC plug-ins. Aiming at the disadvantages of IFC format, Lai Huahui et al. [1] proposed a technology route of BIM data sharing and exchange based on IFC standards, and imported the data into SJTUBIM platform for integrated management. Wang yong et al. [2] proposed the method of automatic model transformation in architectural structure design, and studied the automatic transformation among architectural model, structural model and computational model based on IFC standard. Saeed et al. [3] believed that the current construction industry lacks industrial component standards based on BIM Technology, specific frameworks and standards need to be developed and embedded in BIM software to integrate industrial buildings with BIM. Gao Qiuya et al. [4] proposed the production information model of prefabricated components based on IFC standard, and divided the information of the prefabricated components in the production stage into product information, process information, resource information, cost information and site information. The above study has initially solved the problem of data sharing in the full life-cycle of buildings from the perspective of optimizing IFC format, but it has not fundamentally solved the difficulty in implementing IFC standards [5].
CDM (Component Data Model), was proposed by Huang Qiang, Chairman of China BIM Union, to realize open and interoperable information exchange by establishing component database needed for engineering construction and engineering management [6]. This paper applies BIM technology based on CDM theory to the design stage of precast concrete structure, solves the problem of internal differences of IFC format files in different BIM software, breaks the data island, and truly realizes data interaction and sharing. At the same time, based on the research building of prefabricated parts production base in Chizhou, Anhui Province, this paper demonstrates the advantages of CDM technology in the design stage of precast concrete structures through the formulation of CDM standards and the development of CDM software.

IFC Standard
IFC (Industry Foundation Classes) standard is an object-oriented data model standard developed by BuildingSMART. As an open specification for building information modeling (BIM), IFC aims to support data transfer and sharing among project participants in building construction and facility management [7]. It consists of data represented as an EXPRESS schema and reference data represented as an XML (Extensible Markup Language) schema [8].

Data Analysis and Transfer Under IFC Standard
Currently, there are many mainstream BIM software, both domestic and foreign, that can support IFC standard format, and can generate IFC data interchange and read imported IFC data [9]. However, in the actual project application, a large amount of information is lost when IFC files between different software are exchanged. Even if they are exported in the same software and then imported, the information is lost and the transformation is incomplete [10]. For example, IFC 21 files passed from Software 1 to Software 2 may lose entity types, IFC 31 files passed from Software 1 to Software 3 may lose attribute information, and Software 1 cannot pass IFC files to Software 4, etc. (Figure 1). There are many reasons, such as different classification standards at home and abroad, differences in the definition of engineering behavior description, and the fact that BIM software vendors do not make IFC interfaces according to IDM (Information Delivery Manual) standards for their own interests [11]. Therefore, in the development of IFC standards for more than 20 years, there are still many problems in the application of engineering. Under such circumstances, it is difficult for China to realize the application of BIM based on IFC model, and a large number of differentiation problems at home and abroad need to be solved [12].

1) At different stages of the project, information between
BIM software is basically transmitted through the IFC format. However, due to the disadvantages of redundancy and information loss, the model cannot be reused in all stages of the project, and often needs to be re-modeled to continue the work, which greatly increases the repeated workload of engineering personnel. 2) It is difficult for engineers to understand the complicated IFC format (Figure 2), and different software vendors have different interpretations of the IFC format, making it difficult for engineers to find and modify relevant error information in the IFC format [13]. 3) In the design phase, due to the great differences between kinds of BIM software, makes the establishment of the model often require multiple software, such as Revit, TEKLA, CATIA, and plug-ins based these software [14], which requires designers to master a large number of design software for integration design. This greatly increase the cost of software acquisition, personnel training and the difficulty of software modeling. It can be seen that these disadvantages make it difficult to implement the BIM technology based on IFC format in engineering application.

CDM Theory
CDM (Component Data Model) is Chinese BIM solution to realize BIM landing. The aim of the CDM standard is to provide the construction industry with an intermediate data standard that doses not depend on any specific software and connects the design to the operation and maintenance of the construction process, suitable for the information creation during the construction process and the information exchange during the full life-cycle of the project.
CDM parameter coding rules are composed of the form HcdmClass and HcdmNumber (Figure 3), where the HcdmClass format is AXYYY (A stands for construction project; X is the professional distinction code: 1 Foundation, 2 Structure, 3 Electrical, 4 Interior Trim, 5 Exterior Trim, etc. YYY stands for each component type). For example, A2001 stands for CDM standard of prefabricated rectangular column and A2002 stands for CDM standard of prefabricated circular column; The format of HcdmNumber is XX-FYY-ZZZ (XX represents the component category; FYY represents the floor; ZZZ represents component serial number), such as A2001-01-F12-002 stands for CDM standard for layer 12, Number2 prefabricated rectangular column with size 400×400.

Data Analysis and Transfer Under CDM Theory
Based on CDM theory, the data creation and interaction of the full life-cycle of the construction project only need to pass through: 1) Establish CDM standards for each component.
2) Design by filling in Numbers.
3) Develop CDM data software. 4) The CDM standard automatically forms the data model after the CDM data is read by the CDM data software. 5) Transfer CDM standards from the current phase to the next phase of the project.

Establish Component CDM Standards
According to the property characteristics of components and relevant provisions in the flat atlas, establishing component CDM standards. Taking CDM standard of prefabricated rectangular column in design stage as an example, the longitudinal information of standard table includes External Parameters, Coordinates and Surface Geometry Parameters, Reinforcement Relation Parameters and Reinforcement Design Parameters. And the horizontal information includes Instructions, Field coding, Field name, Unit and Field values (Figure 4-Figure 7).

Design by Filling in Numbers
According to the actual component information of the project, fill in corresponding field values in the CDM  Research and development of data software based on CDM format, the software's functions include reading the CDM set after filling in numbers, delivering the CDM set to each BIM software for model display and deep application losslessly, exporting the models in each BIM software to CDM set, and realizing one-key calculation of quantities and one-key calculation of price based on CDM set. This software not only gets through the non-destructive transmission of data between BIM software, but also realizes the concept that BIM technology does not need to rely on any 3D software and only needs to fill the number in the EXCEL data sheet (Figure 8).

CDM Data Transmission Method at Each Stage of the Project
In the precast concrete structure system, CDM data are transmitted in order of design stage, production stage, transportation stage, installation stage and other stages. Taking CDM standard of prefabricated rectangular column as an example, in the design stage, the designer fills the values of CDM set from KZ001 to KZ170. After the completion of the filling, the designer passes it to the engineer in the production stage for the filling in the field code of KZ201-KZ204, and then fills in the field code of KZ251-KZ258 in the transport stage ( Figure 9). In this way, the disadvantages of IFC data transmission are solved.

1) Engineers do not need to master a large number of BIM
software, and only need to fill in the form with numbers, realizing that BIM technology does not depend on any 3D software. Moreover, CDM data tables are in a format that engineers can understand without having to deal with the complicated IFC data codes. 2) By developing the API of each BIM software in CDM data software, the same CDM set can be imported into different software to generate the same model. The same model can be exported to the same CDM set in different BIM software. This method has high efficiency of data interaction and realizes the lossless transfer of data among software.
3) The data information of each stage of the project only needs to be transmitted through the CDM data table.
With the extension of the project stage, the CDM standard will be filled in one by one, and there will be no data islands of a certain stage of the project. The downstream of the project can intuitively understand the data from the upstream, so as to truly realize the the full life-cycle management of the project construction.

General Situation of the Project
The research building of prefabricated parts production base in Chizhou, Anhui Province, is located in Qianjiang Industrial Park. The construction unit is China Power Chizhou Construction CO., LTD., and the design unit is SINOHYDRO BUREAU 8 CO., Ltd.,. This building is a public office building with a structural form of integrated frame structure. This building has a design life of 50 years and a seismic grade of 6. And the building is 17.10m high with 4 floors and the building area is 5114.78m 2 .
The precast components of the project include: precast sandwich exterior wall, precast exterior wall board, precast interior wall board, precast steel truss laminated floor, precast laminated beams, precast columns, precast balconies, precast air conditioning board and precast stairs, etc. Among them, there are 648 precast boards, 466 precast laminated beams, 132 precast columns and 16 precast stairs.

Establish CDM Standards for Each Component
In design phase, the project made the CDM Standard of precast rectangular column (A2001), the CDM Standard of precast circular column (A2002), the CDM Standard of precast laminated floor (A2003), the CDM Standard of precast stair (A2004), the CDM Standard of precast laminated beams (A2005) and the CDM Standard of precast polygon laminated floor (A2006) ( Table 1).

Design in CDM Standard by Filling in Numbers
According to the CDM standards formulated in this project and the actual design parameters of each component, the CDM field values of each component in design phase is filled (Figure 10), and the CDM set -∑CDM of project components is formed after the number is filled.

CDM Software Development
Due to the superiority of TEKLA software in component assembly and the fact that all core functions of TEKLA have program instructions to correspond to, that is, to provide users with personalized interface handlers for TEKLA platform development [15]. This project adopts C# language to develops CDM software HUANG-CDM, and uses TEKLA API to display the model of data in TEKLA platform.

Model Generation
Through HUANG-CDM software and read in the CDM set, click the code name of any CDM component, and this component can be auto-generated on the Tekla platform ( Figure 11). Then click the "Import all CDM data into TEKLA" button, the whole project model driven by CDM data can be formed on the TEKLA platform without any damage ( Figure 12). Moreover, the 3D model dynamically corresponds to the CDM data in the HUANG-CDM software, that is, when any data is be added, modified and deleted in the CDM set, the model can automatically update this information without tedious operation by the designer in the 3D design software.

Conclusion
This paper presents a study on the application of BIM technology based on CDM theory in the design stage of precast concrete structures from three aspects of standard formulation, designing by filling numbers and software development, which is used to solve the problem that IFC cannot be implemented in the traditional mode. And this method can be universally applied to the full life-cycle management of the project construction, so as to solve the problem that IFC-BIM cannot be implemented in the traditional mode. Finally, the feasibility of the proposed technical route is verified by the actual prefabricated engineering case, which solves the problems such as impassability of BIM data in the actual work and heavy repetitive workload of modelers, and greatly improves the application efficiency of BIM technology in the project.