# Automated Conversion of Building Information Modeling (BIM) Geometry Data for Window Thermal Performance Simulation

Authors: Changmin Kim; Changyoung Park; Changho Choi; Hyangin Jang
Journal: Advances in Civil Engineering
 (2019)

---

## Abstract

A window set is defined as a window where the frame and the glass are combined and is used at the part that comes into contact with the air. As the performance evaluation of window sets has gained significance, the need for software that can simulate window set performance has also increased accordingly. However, the simulation of window sets is not carried out efficiently due to the difficulty in the window set modeling. Meanwhile, the design of building information modeling has recently proliferated so that the window set BIM library is distributed online. If such a window set BIM library is utilized in the window set simulation, it is expected that the productivity issue that occurs in the simulation process could be improved. Therefore, this study proposes a method to automatically convert the information required in the simulation of the window set heat transfer coefficient from the BIM. In order to achieve the purpose of this study, the following procedure is carried out. First, the framework for converting the information required in the simulation of the window set heat transfer coefficient from the BIM is suggested. Second, the method to extract and convert BIM data based on the suggested framework is proposed. Lastly, the BIM data conversion program is developed, and its performance is validated by applying the window set BIM case. The case study result showed that the information converted and entered from the window set data BIM conversion program coincided with the information entered in the window set BIM. It is expected that the result of this study will increase the productivity of window set simulations, which will lead to the increased use of certification through these simulations.


---

## Body

## 1. Introduction

Since the performance evaluation of window sets has gained significance, the need for software that can simulate window thermal performance has also increased accordingly. In the case of applying an evaluation method through a test, costly testing equipment is necessary for measuring the heat transfer coefficient of window sets. However, a simulation can evaluate the thermal performance of a window set without the need for separate testing equipment. Also, data indicating small changes, which cannot be determined due to uncertainties that occur in the evaluation method through a test, can be obtained from the simulation [1]. In addition, the effect of input variables related to the heat transfer coefficient of the window set can be determined [2]. In particular, the determined information can be utilized in the design process for achieving the target performance of the window set by a manufacturer [1]. As the burden of expenses and time required for the certification of window sets can be reduced, the window set simulation has been introduced and implemented in various countries, including the United States and the Republic of Korea.However, the window set simulation is not efficiently applied despite its various advantages. In many studies, inconvenient user interfaces of window set simulation programs and difficulty in window set modeling are cited as examples of some obstacles [3, 4]. In fact, the user should draw each edge of all window set members for window set modeling. The user should also specify each material composing the respective window set member, select an edge inside and outside the window set, and specify the boundary condition. Although the modeling time varies by program, window set modeling takes approximately one hour to complete [5]. Therefore, it is essential to develop a method that can automatically provide modeling information required by the window set simulation program in order to maximize the window set simulation.Meanwhile, the use of the building information modeling (BIM) design is currently proliferating in the construction industry [6]. Along with the BIM design, the establishment of a library for the BIM design is being proliferated, and window set manufacturers are already making the window set BIM library accessible online. If such a window set BIM library is utilized in a window set simulation, it is expected that the productivity issue that occurs in the simulation process could be improved. Therefore, this study proposes the method to automatically convert the information required in the window set simulation from the BIM.In order to achieve the purpose of this study, the following procedure is carried out. First, the framework for converting the information required in the simulation of the window set from the BIM is suggested. Second, the method to extract and convert BIM data based on the suggested framework is proposed. Examples of BIM data for the window set’s heat transfer coefficient simulation include the information on the geometry and boundary of the window set. Lastly, the BIM data conversion program is developed in consideration of user convenience for the window set’s heat transfer coefficient simulation, and the performance of the developed program is verified by applying the window set BIM case.


## 2. Literature Review

The Industrial Foundation Classes (IFC) data model is the standardized method to save information included in BIM. IFC includes the geometry and semantic information of the members composing a building as well as the exchange of information that is available during the building’s life cycle [7, 8]. Also, the information included in BIM can be utilized without specific BIM software [9]. Due to such advantages, IFC is widely utilized in various fields, including building energy simulation [6], construction quality evaluation [10], and progress measurement [11]. Some studies for mapping the information included in the IFC to another file format as required by an application are being carried out based on the field of 3D geographic information system.In the field of 3D geographic information system, many studies for mapping an IFC file into the CityGML format have been carried out. Typical studies that were recently conducted were summarized in this study. Cheng et al. [12] suggested a method for mapping an entity included in the IFC and an entity in the format of CityGML to each other. For such purpose, the relationship between entities was discovered by applying the text-mining method to the name and definition of the entity in the IFC and CityGML. Donkers et al. [13] suggested a method for automatically converting the property information and geometric information included in the IFC into CityGML. For such purpose, relevant entities for each format were defined in advance according to the data structure, and the geometric transformation of the building envelope was carried out according to the information required by CityGML. Deng et al. [14] suggested a framework for the bidirectional mapping between the IFC and CityGML. The mapping rule between the IFC and CityGML was created by applying the instance-based method in order to enable the bidirectional mapping between the IFC and CityGML. Stouffs et al. [15] suggested a method for defining the entity relationship between the IFC and CityGML to be applicable to various cases. For such purpose, the entity relationship was defined by applying the triple graph grammars.Apart from the studies regarding the conversion of an IFC file into CityGML, other related studies have been carried in other fields. Kim et al. [16] suggested a method for converting an IFC file into an IDF file which was the format supported by EnergyPlus, a building energy simulation program. Also, Kim et al. [17] suggested a method for converting an IFC file into an INP file which was utilized in the DOE-2-based building energy analysis.In order to map the information included in the IFC to a file in a format required by each application, it is necessary to carry out the process to analyze the data structure of two different formats and to convert and map the relevant entities in order to match the value and representations [12]. Such studies presented the possibility that an IFC file in building units could be converted into and utilized in an appropriate format for each application. However, the conversion in building units was mainly carried out in previous studies. Since the conversion in building units is the methodology based on a simple geometry, the previously suggested building unit method is not appropriate for the window simulation, which requires detailed geometry information. Moreover, studies for converting the information included in the BIM into an appropriate format for the window set simulation have not yet been carried out within the scope of the author’s understanding. Therefore, a method for converting the detailed geometry information included in the BIM into an appropriate format for the window set simulation is proposed in this study.


## 3. BIM Data Conversion Method for Window Set Simulation

THERM is the program developed by the Lawrence Berkeley National Laboratory (LBNL), and it simulates the 2D heat transfer of the window set. In previous studies, experiment results showed that the THERM’s simulation result was sufficiently accurate when compared with the evaluation method through test [18, 19]. Moreover, THERM is approved by the National Fenestration Rating Council (NFRC) of the United States. This program has been certified and utilized for the performance evaluation and grade assessment of the window set in various countries, including the U.S. and the Republic of Korea. As THERM is particularly easy to use in comparison with other 2D heat transfer simulation programs and requires less time for users to learn, it is widely used [20]. Therefore, THERM, which is the representative program for simulating the thermal performance of the window set, is highlighted as this study is conducted. The file formats supported by THERM include THM and THMX, and the information included in these two file formats is identical. THM is THERM’s own unique format, which is not an open format. On the other hand, THMX is the XML (Extensible Markup Language) format, which is simple to use for file creation and modification. Therefore, this study suggests the framework to extract the window set BIM information saved to IFC and to convert and output such information into a THMX file supported by THERM. The framework suggested in this study is shown in Figure 1.Figure 1
BIM-based framework for window thermal performance simulation.First, the geometry data are extracted from the window set BIM. The step that takes the longest time in the modeling process for the window set simulation is window set modeling. Modeling includes the generation of a cross-section and a boundary as well as the setting of the boundary condition. The geometry data of the window set are necessary for the generation of the cross-section and boundary of the window set as well as the setting of the boundary condition. Therefore, the method to extract data regarding the geometry of the members that compose the window set from the BIM is proposed.Second, the data extracted from the window set BIM are converted into data that are required for the window set heat transfer coefficient simulation. The geometry of the window set is expressed according to the IFC’s geometry representations. However, the geometry of a member that composes the window set is defined as a set of outside vertices in the window set simulation program. Therefore, the complicated geometry of the frames that compose the window set changes to a set of outside vertices. Next, the 3D geometry extracted from the BIM is converted into 2D cross-sections, such as the head, sill, and the jamb, all of which are required by the window set simulation program. Additionally, the outline of the window set is extracted and converted into information regarding the boundary condition based on the converted cross-sections.Third, the converted information is outputted in a format that can be inputted to the window set simulation program. A unique format that can be inputted to the window set simulation program has been specified. The information regarding the geometry and boundary of the window set is outputted automatically in a format that can be inputted in the THERM, according to the method proposed in this study.


### 3.1. Geometry Data Extraction from BIM

The BIM expresses the geometry of an object as a plane or a solid, which is a set of a number of planes [21]. It is determined in various expression methods, including “Body SweptSolid,” “Body Brep (Boundary Representation),” and “Body CSG (Constructive Solid Geometry)” in IFC according to the geometry of the object [22]. In the case of the window set, a simple geometry such as a wood frame and glass as well as a complicated shape exists. In the IFC file, a simple geometry of the window set is expressed in the “Body SweptSolid Geometry” method. However, a complicated geometry of the window set is expressed in the “Body Brep Geometry” method [13]. As the method to extract data on a simple geometry has been discussed in previous studies [23], this study will primarily focus on and discuss the method to extract complicated geometry data from the IFC file.Figure2 shows the process to extract geometry data in “Brep” format. For the geometry data in the IFC, the coordinate in the local coordinate system can be extracted after converting it to the global coordinate system [14]. While the method to extract a coordinate in the local coordinate system from an IFC file varies for each geometry expression method, the information of the “Brep” geometry in the IFC is expressed as shown in Figure 2. IfcWindowStyle is an entity that defines the style of the window set and includes the geometry data. IfcRepresentationMap and IfcShapeRepresentation can be referred from IfcWindowStyle sequentially, and the IfcShapeRepresentation entity includes information showing whether the geometry type of the relevant object is “SweptSolid” or “Brep.” If the geometry type in IfcShapeRepresentation is “Brep,” IfcFacetedBrep can be referred from IfcShapeRepresentation, and IfcFacetedBrep includes the list of separate members that compose the window set. The list of faces composing each member can be checked from IfcClosedShell, and IfcClosedShell includes some IfcFace information. IfcFace has a polygonal geometry, and the coordinate of vertices composing each face can be extracted from IfcCartesianPoint. It is possible to extract the geometry data of the window set saved in the “Brep” method by extracting the coordinate of vertices of all faces composing the window set.Figure 2
Data structure of “Body Brep Geometry” in IFC.


### 3.2. Data Conversion

#### 3.2.1. Simplification of Window Set Geometry

A more complicated geometry is expressed as a larger number of faces in the design tool [24]. In order to utilize a model that has such a complicated geometry in the simulation, the simplification of the model should first be conducted [25]. The top drawing in Figure 3 indicates the frame that composes the window expressed with a number of faces. For the first triangle, the edges that compose the outline of the member are the edges shown in green as shown in the figure. As the red edge is not an edge that composes the outline of the member, it should be removed. Unlike the edges in green, the first and second triangles share the red edge. From the understanding of this information, this study suggests the simplification method to decimate an edge that is shared with an adjacent triangle among the edges composing the triangle.Figure 3
Geometry simplification algorithm.To achieve this purpose, the following is carried out. First, the three vertices composing the triangle are converted into three edges, including two vertices for all the triangles that compose the shape of the member. Second, a triangle for simplification is selected arbitrarily. Third, all the triangles that compose the figure are checked to determine whether a triangle that includes the same edge among the three edges that compose the selected triangle exists. If the triangle that includes the same edge exists, the vertices that compose all the other edges, except for the relevant edge, are saved. When this process is completed, an edge that overlaps with the other triangles will be removed. Next, it is possible to remove an edge inside the figure and extract the coordinate of vertices composing the outline of the member sequentially by repeating the above process for a triangle that shares the vertex composing the triangle as the next triangle.


#### 3.2.2. Cross-Section Generation

As mentioned above, THERM is the 2D-based simulation program. In THERM, a 2D cross-section is defined as head, jamb, or meeting rail according to its position [26]. In this study, the position of the glass is utilized in order to define the cut position according to the required cross-section. According to the cross-section of each member that should be extracted based on the glass, the members located at the top can be classified as the head, the members located at the bottom can be classified as the sill, and the members located on the left or right can be classified as the jamb.The first step for utilizing the coordinate information of the glass is to classify between the glass and the frame among the members composing the window set. In the case of the window set, the glass is thinner than the frame. Therefore, a member, which is the thinnest among the members of the window set, is classified as the glass, and other members are classified as the frames.The cross-section according to the window set type is created based on the glass that was previously classified. Figure4 shows an example of the cross-section generation of the head and the sill. As shown in the figure, a vertex, which is the coordinate of Z that is larger than the coordinate of Z based on the glass, is extracted for the head based on the profile. On the other hand, a vertex, which is the coordinate of Z that is smaller than the coordinate of Z based on the glass, is extracted for the sill based on the profile. In this way, a vertex, which is the coordinate of X that is larger than the coordinate of X based on the glass, is extracted for the jamb. For the meeting rail, all vertices existing between two windows are extracted as shown in Figure 5. The coordinate of the vertex that shows the geometry for the 2D cross-section of the head, sill, jamb, and the meeting rail can be extracted from the 3D geometry information extracted from the IFC file through this process.Figure 4
Cross-section generation algorithm (head and sill).Figure 5
Cross-section generation algorithm (meeting rail).


#### 3.2.3. Boundary Extraction

In the window set simulation, the boundary condition defines the temperature and the surface heat transfer coefficient of each member. For the window set simulation, the exterior and interior surfaces of the cross-section of the window set are specified as different boundary conditions to each other. The window set BIM includes the geometry information of each member. However, the information regarding the boundary of the window set is not included. Therefore, the method to extract only the geometry of the boundary of the window set among the geometry information of the window set and to specify the boundary condition of the relevant geometry is necessary for extracting the boundary condition of the window set.In this study, the following method is applied in order to extract only a geometry corresponding to the boundary among the geometry information of a member that composes the window set. First, the geometry information of all members that compose the window set is extracted. Second, all the coordinates that correspond to the vertex of each member are determined. Third, the top, or the start point of the window set, is connected to the next vertex that composes the figure. In the case of an intersection, the movement in the direction of the window is the priority. The second priority is to connect the vertex in the bottom direction (Figure6).Figure 6
Boundary extraction process.


#### 3.2.4. Boundary Condition Determination

The coordinate information of edges composing the boundary of the window set is extracted, and the boundary condition of each edge is matched. In the window set simulation, the boundary condition is classified as “Outside,” “Inside,” or “Adiabatic.” Interior indicates the inside boundary, Exterior indicates the outside boundary, and Adiabatic indicates the insulation boundary with no heat transfer. The direction of the window set is initially extracted from the IFC file to differentiate between the interior and exterior boundaries. In the IFC, the direction of the object is included in the IfcDirection entity [27]. Therefore, it is possible to confirm the direction of the window set by extracting the information included in IfcDirection, which is connected to IfcWindow. If the interior is south and the exterior is north in the IFC, IfcDirection does not exist. However, the values for the other directions are included. The interior and exterior directions, according to the direction information of the window set extracted from IfcDirection, are shown in Table 1. The interior side is set as the interior and the exterior side is set as the exterior for boundary condition according to the direction information among the edges composing the boundary that was extracted above. Next, the edges that are not in the interior and exterior directions are set as “Adiabatic,” which is the adiabatic boundary side for the boundary condition.Table 1
Boundary condition determination by IfcDirection.
IfcDirection
Boundary condition
N/A
IfcDirection (−1, 0, 0)
IfcDirection (0, 1, 0)
IfcDirection (0, −0, 0)


### 3.3. Export

The geometry and boundary condition extracted from the IFC file can be extracted as the relationship between specific entities. However, there is a difference between the format of the information extracted from the IFC file and the format required in THMX that is compatible with THERM. Therefore, it is necessary to convert each type of information extracted from IFC into the format that is required in THMX.In this study, the geometry information extracted from IFC was converted into the format that was required in THMX. The result through the cross-section generation algorithm that is converted into <CrossSectionType>, which is the cross-sectional type and a proper format for <Polygons>, which indicates the 2D geometry of the cross-section, is inputted to THMX. Here, the cross-sectional types including the sill, head, and the meeting rail are inputted as the cross-sectional type. For the boundary information in THMX, the information on the two vertices and property should be entered for each edge composing the boundary. For the geometry information in the boundary information, each vertex is connected and arranged in a way in which the starting point is connected to the endpoint. Then, this information is saved in the <Boundaries> class of THMX. In addition, the boundary information that corresponds to the Exterior, Interior, and Adiabatic classifications, as previously determined, is also saved together. The entity items extracted from IFC and the items of the result converted through the method suggested in this study that is outputted to THMX are shown in Table2.Table 2
Mapping from IFC to THMX.
IFC entity
Conversion data
THMX class
IfcWindowStyle
Cross-section generation results
<CrossSectionType>
Cross-sectional type
Boundary condition determination results
<BoundaryCondition>
Boundary condition name
Cross-section generation results
<Polygons>
Point index
Cross-section generation results
Nsides
Boundary condition extraction results
<Boundaries>
BC polygon ID
Boundary condition extraction results
Boundary condition
Boundary condition extraction results
Polygon ID
Boundary condition extraction results
Point index


## 3.1. Geometry Data Extraction from BIM

The BIM expresses the geometry of an object as a plane or a solid, which is a set of a number of planes [21]. It is determined in various expression methods, including “Body SweptSolid,” “Body Brep (Boundary Representation),” and “Body CSG (Constructive Solid Geometry)” in IFC according to the geometry of the object [22]. In the case of the window set, a simple geometry such as a wood frame and glass as well as a complicated shape exists. In the IFC file, a simple geometry of the window set is expressed in the “Body SweptSolid Geometry” method. However, a complicated geometry of the window set is expressed in the “Body Brep Geometry” method [13]. As the method to extract data on a simple geometry has been discussed in previous studies [23], this study will primarily focus on and discuss the method to extract complicated geometry data from the IFC file.Figure2 shows the process to extract geometry data in “Brep” format. For the geometry data in the IFC, the coordinate in the local coordinate system can be extracted after converting it to the global coordinate system [14]. While the method to extract a coordinate in the local coordinate system from an IFC file varies for each geometry expression method, the information of the “Brep” geometry in the IFC is expressed as shown in Figure 2. IfcWindowStyle is an entity that defines the style of the window set and includes the geometry data. IfcRepresentationMap and IfcShapeRepresentation can be referred from IfcWindowStyle sequentially, and the IfcShapeRepresentation entity includes information showing whether the geometry type of the relevant object is “SweptSolid” or “Brep.” If the geometry type in IfcShapeRepresentation is “Brep,” IfcFacetedBrep can be referred from IfcShapeRepresentation, and IfcFacetedBrep includes the list of separate members that compose the window set. The list of faces composing each member can be checked from IfcClosedShell, and IfcClosedShell includes some IfcFace information. IfcFace has a polygonal geometry, and the coordinate of vertices composing each face can be extracted from IfcCartesianPoint. It is possible to extract the geometry data of the window set saved in the “Brep” method by extracting the coordinate of vertices of all faces composing the window set.Figure 2
Data structure of “Body Brep Geometry” in IFC.


## 3.2. Data Conversion

### 3.2.1. Simplification of Window Set Geometry

A more complicated geometry is expressed as a larger number of faces in the design tool [24]. In order to utilize a model that has such a complicated geometry in the simulation, the simplification of the model should first be conducted [25]. The top drawing in Figure 3 indicates the frame that composes the window expressed with a number of faces. For the first triangle, the edges that compose the outline of the member are the edges shown in green as shown in the figure. As the red edge is not an edge that composes the outline of the member, it should be removed. Unlike the edges in green, the first and second triangles share the red edge. From the understanding of this information, this study suggests the simplification method to decimate an edge that is shared with an adjacent triangle among the edges composing the triangle.Figure 3
Geometry simplification algorithm.To achieve this purpose, the following is carried out. First, the three vertices composing the triangle are converted into three edges, including two vertices for all the triangles that compose the shape of the member. Second, a triangle for simplification is selected arbitrarily. Third, all the triangles that compose the figure are checked to determine whether a triangle that includes the same edge among the three edges that compose the selected triangle exists. If the triangle that includes the same edge exists, the vertices that compose all the other edges, except for the relevant edge, are saved. When this process is completed, an edge that overlaps with the other triangles will be removed. Next, it is possible to remove an edge inside the figure and extract the coordinate of vertices composing the outline of the member sequentially by repeating the above process for a triangle that shares the vertex composing the triangle as the next triangle.


### 3.2.2. Cross-Section Generation

As mentioned above, THERM is the 2D-based simulation program. In THERM, a 2D cross-section is defined as head, jamb, or meeting rail according to its position [26]. In this study, the position of the glass is utilized in order to define the cut position according to the required cross-section. According to the cross-section of each member that should be extracted based on the glass, the members located at the top can be classified as the head, the members located at the bottom can be classified as the sill, and the members located on the left or right can be classified as the jamb.The first step for utilizing the coordinate information of the glass is to classify between the glass and the frame among the members composing the window set. In the case of the window set, the glass is thinner than the frame. Therefore, a member, which is the thinnest among the members of the window set, is classified as the glass, and other members are classified as the frames.The cross-section according to the window set type is created based on the glass that was previously classified. Figure4 shows an example of the cross-section generation of the head and the sill. As shown in the figure, a vertex, which is the coordinate of Z that is larger than the coordinate of Z based on the glass, is extracted for the head based on the profile. On the other hand, a vertex, which is the coordinate of Z that is smaller than the coordinate of Z based on the glass, is extracted for the sill based on the profile. In this way, a vertex, which is the coordinate of X that is larger than the coordinate of X based on the glass, is extracted for the jamb. For the meeting rail, all vertices existing between two windows are extracted as shown in Figure 5. The coordinate of the vertex that shows the geometry for the 2D cross-section of the head, sill, jamb, and the meeting rail can be extracted from the 3D geometry information extracted from the IFC file through this process.Figure 4
Cross-section generation algorithm (head and sill).Figure 5
Cross-section generation algorithm (meeting rail).


### 3.2.3. Boundary Extraction

In the window set simulation, the boundary condition defines the temperature and the surface heat transfer coefficient of each member. For the window set simulation, the exterior and interior surfaces of the cross-section of the window set are specified as different boundary conditions to each other. The window set BIM includes the geometry information of each member. However, the information regarding the boundary of the window set is not included. Therefore, the method to extract only the geometry of the boundary of the window set among the geometry information of the window set and to specify the boundary condition of the relevant geometry is necessary for extracting the boundary condition of the window set.In this study, the following method is applied in order to extract only a geometry corresponding to the boundary among the geometry information of a member that composes the window set. First, the geometry information of all members that compose the window set is extracted. Second, all the coordinates that correspond to the vertex of each member are determined. Third, the top, or the start point of the window set, is connected to the next vertex that composes the figure. In the case of an intersection, the movement in the direction of the window is the priority. The second priority is to connect the vertex in the bottom direction (Figure6).Figure 6
Boundary extraction process.


### 3.2.4. Boundary Condition Determination

The coordinate information of edges composing the boundary of the window set is extracted, and the boundary condition of each edge is matched. In the window set simulation, the boundary condition is classified as “Outside,” “Inside,” or “Adiabatic.” Interior indicates the inside boundary, Exterior indicates the outside boundary, and Adiabatic indicates the insulation boundary with no heat transfer. The direction of the window set is initially extracted from the IFC file to differentiate between the interior and exterior boundaries. In the IFC, the direction of the object is included in the IfcDirection entity [27]. Therefore, it is possible to confirm the direction of the window set by extracting the information included in IfcDirection, which is connected to IfcWindow. If the interior is south and the exterior is north in the IFC, IfcDirection does not exist. However, the values for the other directions are included. The interior and exterior directions, according to the direction information of the window set extracted from IfcDirection, are shown in Table 1. The interior side is set as the interior and the exterior side is set as the exterior for boundary condition according to the direction information among the edges composing the boundary that was extracted above. Next, the edges that are not in the interior and exterior directions are set as “Adiabatic,” which is the adiabatic boundary side for the boundary condition.Table 1
Boundary condition determination by IfcDirection.
IfcDirection
Boundary condition
N/A
IfcDirection (−1, 0, 0)
IfcDirection (0, 1, 0)
IfcDirection (0, −0, 0)


## 3.2.1. Simplification of Window Set Geometry

A more complicated geometry is expressed as a larger number of faces in the design tool [24]. In order to utilize a model that has such a complicated geometry in the simulation, the simplification of the model should first be conducted [25]. The top drawing in Figure 3 indicates the frame that composes the window expressed with a number of faces. For the first triangle, the edges that compose the outline of the member are the edges shown in green as shown in the figure. As the red edge is not an edge that composes the outline of the member, it should be removed. Unlike the edges in green, the first and second triangles share the red edge. From the understanding of this information, this study suggests the simplification method to decimate an edge that is shared with an adjacent triangle among the edges composing the triangle.Figure 3
Geometry simplification algorithm.To achieve this purpose, the following is carried out. First, the three vertices composing the triangle are converted into three edges, including two vertices for all the triangles that compose the shape of the member. Second, a triangle for simplification is selected arbitrarily. Third, all the triangles that compose the figure are checked to determine whether a triangle that includes the same edge among the three edges that compose the selected triangle exists. If the triangle that includes the same edge exists, the vertices that compose all the other edges, except for the relevant edge, are saved. When this process is completed, an edge that overlaps with the other triangles will be removed. Next, it is possible to remove an edge inside the figure and extract the coordinate of vertices composing the outline of the member sequentially by repeating the above process for a triangle that shares the vertex composing the triangle as the next triangle.


## 3.2.2. Cross-Section Generation

As mentioned above, THERM is the 2D-based simulation program. In THERM, a 2D cross-section is defined as head, jamb, or meeting rail according to its position [26]. In this study, the position of the glass is utilized in order to define the cut position according to the required cross-section. According to the cross-section of each member that should be extracted based on the glass, the members located at the top can be classified as the head, the members located at the bottom can be classified as the sill, and the members located on the left or right can be classified as the jamb.The first step for utilizing the coordinate information of the glass is to classify between the glass and the frame among the members composing the window set. In the case of the window set, the glass is thinner than the frame. Therefore, a member, which is the thinnest among the members of the window set, is classified as the glass, and other members are classified as the frames.The cross-section according to the window set type is created based on the glass that was previously classified. Figure4 shows an example of the cross-section generation of the head and the sill. As shown in the figure, a vertex, which is the coordinate of Z that is larger than the coordinate of Z based on the glass, is extracted for the head based on the profile. On the other hand, a vertex, which is the coordinate of Z that is smaller than the coordinate of Z based on the glass, is extracted for the sill based on the profile. In this way, a vertex, which is the coordinate of X that is larger than the coordinate of X based on the glass, is extracted for the jamb. For the meeting rail, all vertices existing between two windows are extracted as shown in Figure 5. The coordinate of the vertex that shows the geometry for the 2D cross-section of the head, sill, jamb, and the meeting rail can be extracted from the 3D geometry information extracted from the IFC file through this process.Figure 4
Cross-section generation algorithm (head and sill).Figure 5
Cross-section generation algorithm (meeting rail).


## 3.2.3. Boundary Extraction

In the window set simulation, the boundary condition defines the temperature and the surface heat transfer coefficient of each member. For the window set simulation, the exterior and interior surfaces of the cross-section of the window set are specified as different boundary conditions to each other. The window set BIM includes the geometry information of each member. However, the information regarding the boundary of the window set is not included. Therefore, the method to extract only the geometry of the boundary of the window set among the geometry information of the window set and to specify the boundary condition of the relevant geometry is necessary for extracting the boundary condition of the window set.In this study, the following method is applied in order to extract only a geometry corresponding to the boundary among the geometry information of a member that composes the window set. First, the geometry information of all members that compose the window set is extracted. Second, all the coordinates that correspond to the vertex of each member are determined. Third, the top, or the start point of the window set, is connected to the next vertex that composes the figure. In the case of an intersection, the movement in the direction of the window is the priority. The second priority is to connect the vertex in the bottom direction (Figure6).Figure 6
Boundary extraction process.


## 3.2.4. Boundary Condition Determination

The coordinate information of edges composing the boundary of the window set is extracted, and the boundary condition of each edge is matched. In the window set simulation, the boundary condition is classified as “Outside,” “Inside,” or “Adiabatic.” Interior indicates the inside boundary, Exterior indicates the outside boundary, and Adiabatic indicates the insulation boundary with no heat transfer. The direction of the window set is initially extracted from the IFC file to differentiate between the interior and exterior boundaries. In the IFC, the direction of the object is included in the IfcDirection entity [27]. Therefore, it is possible to confirm the direction of the window set by extracting the information included in IfcDirection, which is connected to IfcWindow. If the interior is south and the exterior is north in the IFC, IfcDirection does not exist. However, the values for the other directions are included. The interior and exterior directions, according to the direction information of the window set extracted from IfcDirection, are shown in Table 1. The interior side is set as the interior and the exterior side is set as the exterior for boundary condition according to the direction information among the edges composing the boundary that was extracted above. Next, the edges that are not in the interior and exterior directions are set as “Adiabatic,” which is the adiabatic boundary side for the boundary condition.Table 1
Boundary condition determination by IfcDirection.
IfcDirection
Boundary condition
N/A
IfcDirection (−1, 0, 0)
IfcDirection (0, 1, 0)
IfcDirection (0, −0, 0)


## 3.3. Export

The geometry and boundary condition extracted from the IFC file can be extracted as the relationship between specific entities. However, there is a difference between the format of the information extracted from the IFC file and the format required in THMX that is compatible with THERM. Therefore, it is necessary to convert each type of information extracted from IFC into the format that is required in THMX.In this study, the geometry information extracted from IFC was converted into the format that was required in THMX. The result through the cross-section generation algorithm that is converted into <CrossSectionType>, which is the cross-sectional type and a proper format for <Polygons>, which indicates the 2D geometry of the cross-section, is inputted to THMX. Here, the cross-sectional types including the sill, head, and the meeting rail are inputted as the cross-sectional type. For the boundary information in THMX, the information on the two vertices and property should be entered for each edge composing the boundary. For the geometry information in the boundary information, each vertex is connected and arranged in a way in which the starting point is connected to the endpoint. Then, this information is saved in the <Boundaries> class of THMX. In addition, the boundary information that corresponds to the Exterior, Interior, and Adiabatic classifications, as previously determined, is also saved together. The entity items extracted from IFC and the items of the result converted through the method suggested in this study that is outputted to THMX are shown in Table2.Table 2
Mapping from IFC to THMX.
IFC entity
Conversion data
THMX class
IfcWindowStyle
Cross-section generation results
<CrossSectionType>
Cross-sectional type
Boundary condition determination results
<BoundaryCondition>
Boundary condition name
Cross-section generation results
<Polygons>
Point index
Cross-section generation results
Nsides
Boundary condition extraction results
<Boundaries>
BC polygon ID
Boundary condition extraction results
Boundary condition
Boundary condition extraction results
Polygon ID
Boundary condition extraction results
Point index


## 4. Case Study

The BIM data conversion program for the simulation of the window set heat transfer coefficient that was developed in this study can be used easily by the general public. Moreover, it is developed to facilitate the management of a material database. In case of a material database, the program is designed to allow the user to utilize the basic database and update the data for a newly developed material. The home item includes the function to import an IFC file and export the extracted information in the THMX format. The library item includes the function to manage the database content related to glass and frame material. The default screen of the developed program is shown in Figure7.Figure 7
Graphic user interface of the developed program.In order to verify the performance of the BIM data automatic extraction program developed in this study, the extraction result targeting the window set BIM was evaluated. The window set with a simple cross-section expressed as ‘Body SweptSolid Geometry’ and the window set with a complicated cross-section expressed as “Body Brep Geometry” were selected as the targets for the case evaluation. Also, the double-hung windows and casement windows are specific types of windows that are mainly used in residential buildings. Therefore, the double-hung windows expressed as “Body SweptSolid Geometry” and the casement windows expressed as “Body Brep Geometry” were selected as the targets for the case study in this study. The default libraries provided by Autodesk® Revit® and the window set BIM library accessible on the Internet were utilized as the window set BIM for the case evaluation. The window set BIM was modeled through the family creation, and the material and finishing materials of each member were specified as family parameters in order to connect the material of each member composing the window set.


### 4.1. Test Case 1

The first window set case for verifying the suggested method is shown in Figure8. This is the double-hung window in a wood frame that has the cross-section of a simple shape. The modeling of the window set BIM was carried out in Autodesk® Revit® and was saved to an IFC file format. The following is carried out for test case 1. First, the operational feasibility of the window set BIM conversion program developed in this study was verified. In order to verify the operation of the window set conversion BIM program, the IFC file is imported to the program and the type of the cross-section is specified. Second, whether the geometry information and the boundary condition that are “Body SweptSolid Geometry” are outputted correctly to THMX is verified. In order to verify the status of the output to THMX, the THMX file outputted in the window set BIM conversion program is analyzed to determine if it is outputted properly for the THMX format. Lastly, whether the information outputted to THMX can be imported to the THERM program is verified. To achieve this purpose, the THMX file outputted in the window set BIM conversion program is imported to THERM and the information on the geometry and boundary is checked from the inputted information.Figure 8
BIM model of test case 1.The window set BIM saved in the form of an IFC file can be imported to the program developed in this study. The program developed in this study imports an IFC file and simultaneously recognizes the type of window. Moreover, it provides an option to select a cross-section that is suitable for the window type. When the user selects a cross-section, the program provides the type of the proper cross-section corresponding to the middle window. In the case of a double-hung window, which is the first test case, the head, upper jamb, lower jamb, meeting rail, and the sill can be selected as the type of cross-section. Figure9 shows the result of importing an IFC file to the window set BIM conversion program developed in this study and the head selection as the cross-section. As shown in the figure, the geometry of the cross-section is outputted at the center of the program GUI. It was confirmed that the outputted geometry information was the same as the cross-section of the head in test case 1. The user can output the geometry and boundary information as a THMX file by selecting the type of cross-section and pressing the Save THMX button.Figure 9
Conversion program input result of test case 1.Figure10 shows the THMX file of the head cross-section outputted in the window set BIM conversion program. We can see that the <Polygons> class includes the geometry information of the window set, and the x coordinate and y coordinate of the members composing the cross-section of the member according to <Point index> are saved. This means that the geometry information previously saved in the IFC file has been properly converted into the form that is required in the THMX file. In addition, “Adiabatic,” “KS Interior,” and “KS Exterior” have been specified for the boundary condition in the <Boundaries> class, and the point index of the relevant boundary is saved.Figure 10
THMX output result of test case 1.A file saved by THMX can be imported directly from THERM. Figure11 shows the result of importing a THMX file that includes the cross-section information of the head outputted using the program developed in this study from THERM. We can see from the figure that the geometry information, whose cross-sectional type is a head, is imported to THERM and is the same as the geometry of the cross-section shown in Figure 9. In Figure 10, the green border that surrounds the geometry of the cross-section indicates the boundary of the cross-section. We can see that the boundary of the cross-section is outputted through the suggested method without configuring additional settings by the user. We can also see that when the boundary corresponding to the outside among the boundaries of the window set is selected, “KS Exterior” is entered. The type of the boundary is also automatically converted and outputted in BIM through the suggested method.Figure 11
THERM input result of test case 1.


### 4.2. Test Case 2

The second window set case for verifying the suggested method is shown in Figure12. The window type in test case 2 is the casement window, and it has the cross-section of a complicated geometry. test case 2 was targeted to verify whether the geometry information, which is “Body Brep Geometry,” has been correctly outputted as the cross-section. To achieve this purpose, the head and the meeting rail were selected among the cross-sectional types of the window set and converted into a THMX file. Then, the THMX file outputted from the program was imported to THERM. Just as in the procedure described in test case 1, the modeling of the window set BIM was conducted in Autodesk® Revit® and was saved to an IFC file format.Figure 12
BIM model of test case 2.The cross-section created through the method suggested in this study is shown in Figures13 and 14. Figure 13 shows the head among various cross-sections, and Figure 14 shows the cross-section of the meeting rail. We can see from the result that the complicated geometry of the window set, which was converted into a 2D cross-section, was imported to THERM. We can see from the THERM input result figure below that the geometry information, expressed as “Body Brep Representation,” has been simplified through the conversion method suggested in this study. It was also observed that the information was outputted properly in a form required in the THMX file. This indicates that the window set BIM with a simple geometry, as well as a complicated geometry, can be converted into a proper geometry for the simulation program of the window set’s heat transfer coefficient in the future.Figure 13
THERM input result (head) of test case 2.Figure 14
THERM input result (meeting rail) of test case 2.


## 4.1. Test Case 1

The first window set case for verifying the suggested method is shown in Figure8. This is the double-hung window in a wood frame that has the cross-section of a simple shape. The modeling of the window set BIM was carried out in Autodesk® Revit® and was saved to an IFC file format. The following is carried out for test case 1. First, the operational feasibility of the window set BIM conversion program developed in this study was verified. In order to verify the operation of the window set conversion BIM program, the IFC file is imported to the program and the type of the cross-section is specified. Second, whether the geometry information and the boundary condition that are “Body SweptSolid Geometry” are outputted correctly to THMX is verified. In order to verify the status of the output to THMX, the THMX file outputted in the window set BIM conversion program is analyzed to determine if it is outputted properly for the THMX format. Lastly, whether the information outputted to THMX can be imported to the THERM program is verified. To achieve this purpose, the THMX file outputted in the window set BIM conversion program is imported to THERM and the information on the geometry and boundary is checked from the inputted information.Figure 8
BIM model of test case 1.The window set BIM saved in the form of an IFC file can be imported to the program developed in this study. The program developed in this study imports an IFC file and simultaneously recognizes the type of window. Moreover, it provides an option to select a cross-section that is suitable for the window type. When the user selects a cross-section, the program provides the type of the proper cross-section corresponding to the middle window. In the case of a double-hung window, which is the first test case, the head, upper jamb, lower jamb, meeting rail, and the sill can be selected as the type of cross-section. Figure9 shows the result of importing an IFC file to the window set BIM conversion program developed in this study and the head selection as the cross-section. As shown in the figure, the geometry of the cross-section is outputted at the center of the program GUI. It was confirmed that the outputted geometry information was the same as the cross-section of the head in test case 1. The user can output the geometry and boundary information as a THMX file by selecting the type of cross-section and pressing the Save THMX button.Figure 9
Conversion program input result of test case 1.Figure10 shows the THMX file of the head cross-section outputted in the window set BIM conversion program. We can see that the <Polygons> class includes the geometry information of the window set, and the x coordinate and y coordinate of the members composing the cross-section of the member according to <Point index> are saved. This means that the geometry information previously saved in the IFC file has been properly converted into the form that is required in the THMX file. In addition, “Adiabatic,” “KS Interior,” and “KS Exterior” have been specified for the boundary condition in the <Boundaries> class, and the point index of the relevant boundary is saved.Figure 10
THMX output result of test case 1.A file saved by THMX can be imported directly from THERM. Figure11 shows the result of importing a THMX file that includes the cross-section information of the head outputted using the program developed in this study from THERM. We can see from the figure that the geometry information, whose cross-sectional type is a head, is imported to THERM and is the same as the geometry of the cross-section shown in Figure 9. In Figure 10, the green border that surrounds the geometry of the cross-section indicates the boundary of the cross-section. We can see that the boundary of the cross-section is outputted through the suggested method without configuring additional settings by the user. We can also see that when the boundary corresponding to the outside among the boundaries of the window set is selected, “KS Exterior” is entered. The type of the boundary is also automatically converted and outputted in BIM through the suggested method.Figure 11
THERM input result of test case 1.


## 4.2. Test Case 2

The second window set case for verifying the suggested method is shown in Figure12. The window type in test case 2 is the casement window, and it has the cross-section of a complicated geometry. test case 2 was targeted to verify whether the geometry information, which is “Body Brep Geometry,” has been correctly outputted as the cross-section. To achieve this purpose, the head and the meeting rail were selected among the cross-sectional types of the window set and converted into a THMX file. Then, the THMX file outputted from the program was imported to THERM. Just as in the procedure described in test case 1, the modeling of the window set BIM was conducted in Autodesk® Revit® and was saved to an IFC file format.Figure 12
BIM model of test case 2.The cross-section created through the method suggested in this study is shown in Figures13 and 14. Figure 13 shows the head among various cross-sections, and Figure 14 shows the cross-section of the meeting rail. We can see from the result that the complicated geometry of the window set, which was converted into a 2D cross-section, was imported to THERM. We can see from the THERM input result figure below that the geometry information, expressed as “Body Brep Representation,” has been simplified through the conversion method suggested in this study. It was also observed that the information was outputted properly in a form required in the THMX file. This indicates that the window set BIM with a simple geometry, as well as a complicated geometry, can be converted into a proper geometry for the simulation program of the window set’s heat transfer coefficient in the future.Figure 13
THERM input result (head) of test case 2.Figure 14
THERM input result (meeting rail) of test case 2.


## 5. Discussion

The window set modeling process for simulating the heat transfer coefficient of the window set is labor-intensive due to an inconvenient user interface. Therefore, its utilization is limited regardless of the various advantages of the simulation. As the BIM design has recently been revitalized, various window set manufacturers have released the BIM libraries online. If such BIM libraries are converted and utilized for the window set simulation, the manpower required in the modeling process for the window set simulation can be reduced. In Section3, the method to convert BIM data for the window set simulation suggested in this study was described. In Section 4, the result of the case study was illustrated. The result of the case study indicates that the method suggested in this study has the following benefits.First, the method for extracting the Brep shape that indicates the detailed geometry of a member, not the geometry information of CSG and swept solid expressed mainly, which in case the previous building BIM, was suggested. The method for converting the extracted geometry information into an appropriate format for window simulation was also suggested. This has a significance as the basic study on the method to express the geometry information of the IFC. Moreover, the suggested model can be utilized in the extraction of the geometry information from other separate members that have a complicated geometry in the future in addition to that of the window set.Second, the geometry information and the boundary information were converted and entered into the simulation program automatically from the window set BIM through the suggested method as the result of the case study. In order to certify the window set, the modeling should be performed manually in the same way in a certified program based on the window set drawing. As this process accounts for a significant amount of time in the simulation, a window set certification agency can save the manpower required for the certification of a window set by automating such a process. In this way, a window set manufacturer does not require additional manpower to evaluate a change in the certification class according to a change in the geometry of a window set when developing a product to submit for certification.The BIM data conversion method suggested in this study can enter the geometry and boundary information of a window set to the simulation program automatically. However, the following limitations of this method should be investigated continuously in the future.First, THERM, which was developed by LBNL, was selected as the target in this study among window set simulation programs. In the future, a method applicable to other window set simulation programs in addition to THERM will be suggested.Second, a study related to the conversion of geometry and boundary information, which took the largest amount of time, was carried out in this study. However, the material information of the members composing a window set is also required for the simulation. Therefore, a method to identify the property information of materials through the establishment of a database will be suggested in the future.


## 6. Conclusions

There is a growing interest in the performance of the window set along with a growing interest in the improvement of energy efficiency in buildings. Therefore, a method for evaluating the performance of a window set is also gaining significance. The performance evaluation of a window set through simulation is highly practical as expensive equipment is not required. However, its utilization is limited due to the limitation of labor-intensive window set modeling. Therefore, a study for extracting the information required for window set modeling from BIM and converting the data into an appropriate format for THERM, a window set simulation program developed by LBNL, was carried out in this study.This study suggested the method to automatically extract, convert, and output the information on the geometry and boundary required in the window set’s heat transfer coefficient simulation from the window set BIM. To achieve this purpose, the method to extract the geometry information from the window set BIM saved as an IFC file, which was the standard format of BIM, was suggested. Since the window set is expressed as complicated geometry information, the method to extract “Body Brep Representation,” which was not previously suggested, is included. Second, the method to convert the geometry information extracted from the window set BIM into the geometry of the cross-section, geometry of the boundary, and the boundary type was suggested. The geometry information was simplified, and the cross-section was created in order to convert it into the geometry information of the cross-section required in the window set simulation. The boundary was also extracted based on the priority in order to convert the geometry information of the window set into the boundary information, and the boundary condition was determined based on the direction of the window set. Lastly, the window set BIM conversion program that took in consideration the user convenience based on the suggested method was developed, and its performance was verified by applying the window set BIM model to the developed program. The information on the geometry and property was extracted by applying the selected window set case to the program, and this information was converted to a THMX file. It was confirmed that the converted information inputted to THMX could be imported to THERM and that the converted geometry and boundary information was inputted.Since the information on the geometry and boundary required in the window set’s heat transfer coefficient simulation can be inputted automatically through the program developed in this study, the manpower and time required for the simulation can be reduced. It is expected that the program can be particularly useful in the window set design step to evaluate the performance repeatedly by changing the shape of some members. In conclusion, it is expected that the increased productivity of the window set simulation will lead to the increased use of certification through the window set simulation. The performance of the developed program was verified in this study based on the analysis of some cases.This study targeted THERM, which was developed by LBNL among window set simulation programs. This study has a limitation, in which only the geometry and boundary information was converted. A method that can be applicable to other window set simulation programs in addition to THERM and a method to automatically extract the property information of the material from the IFC will be suggested in the future.THERM, the window set simulation program that was developed by LBNL and discussed in this study, can be downloaded fromhttps://windows.lbl.gov/software/therm.



---

*Source: 7402089-2019-02-17.xml*