## 4 Realization of IFC-Based 4D Construction Information Model of Prefabricated Buildings
### IFC Standard and Its Extension Mechanism
#### 4.2.4 Realizing Construction Schedule Analysis Information's Extension Based on IFC Standard  
As stated previously, the IFC4 ADD2 TC1(ISO 16739-1:2018) does not provide predefined entities to represent construction schedule analysis's results. Hence, realizing schedule analysis information's extension based on the IFC standard is necessary. To achieve the objective of realizing the information model centered on IfcTask, this paper will introduce IfcPropertySet to describe the analysis information and associate it with IfcTask. Therefore, IfcPropertySet is determined by three categories, including Pset_ScheduleNetworkAnalysis, Pset_CriticalPath, and Pset_DelayAnalysis, as shown in Table 3.  
The Pset_ScheduleNetworkAnalysis is used to store results of the schedule network analysis. Schedule network analysis results are composed of three parameters: degree centrality, closeness centrality, and betweenness centrality, which are explained in detail in Section 6. Therefore, this property set contains three properties, namely_IfcDegreeCentrality,_IfcClosenessCentrality, and_IfcBetweennessCentrality, as shown in Table 4.  
The Pset_CriticalPath is used to store information of the critical path. This IfcPropertySet contains two properties to describe the critical path's information, namely_IfcCriticalPath and_IfcCriticalPathDate, as shown in Table 5. The_IfcCriticalPath's data type is IfcBoolean. The_IfcCriticalPath uses a Boolean representing whether the activity resides  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Attributes** & **Property Type** & **Entity Data Type** & **Description** \\ \hline \_IfcDegreeCentrality & IfcPropertySingleValue & IfcReal & A real value to express the degree centrality of construction tasks. \\ \_IfcClosenessCentrality & IfcPropertySingleValue & IfcReal & A real value to express the closeness centrality of construction tasks. \\ \_IfcBetweennessCentrality & IfcPropertySingleValue & IfcReal & A real value to express the betweenness centrality of construction tasks. \\ \hline \hline \end{tabular}
\end{table}
Table 4: Definition of the Pset_ScheduleNetworkAnalysis.  
Figure 4: Cost information’s extension model.  
on the critical path of the project. Furthermore, the _IfcCriticalPathDate's data type is IfcDateTime. This property is extended to record when the critical path is made since the critical path may be changed.  
The Pset_DelayAnalysis is used to store information of delay. This IfcPropertySet contains nine properties to describe the delay events information, as shown in Table 6.  
Finally, Figure 5 illustrates how to extend the schema of the schedule analysis information model. In this figure we can see that all the IfcProperty is related to the corresponding IfcPropertySet. Then, all the IfcPropertySet is associated with IfcTask by IfcRelDefinesByProperties.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Attributes** & **Property Type** & **Entity Data Type** & **Description** \\ \hline \begin{tabular}{c} _IfcDelayID \\ _IfcDelayStartTime_ \\ _IfcDelayFinishTime_ \\ _IfcDelayDuration_ \\ _IfcDelayStage_ \\ _IfcDelayFactor_ \\ _IfcDelayDescription_ \\ _IfcDelayParticipant_ \\ _IfcDaryAndCriticalPath_ \\ \end{tabular} & \
\begin{tabular}{c} _IfcPropertySingleValue_ \\ _IfcInterger_ \\ _IfcPropertySingleValue_ \\ _IfcPropertySingleValue_ \\ _IfcPropertySingleValue_ \\ _IfcPropertySingleValue_ \\ _IfcPropertySingleValue_ \\ _IfcDaryAndCriticalPath_ \\ \end{tabular} &
\begin{tabular}{c} _IfcInteger_ \\ _IfcDelayStartTime_ \\ _IfcDary