JP7016086B2 - Model conversion device for steel structures and model conversion program for steel structures - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act (1) At the "74th Annual Academic Lecture Meeting of the Japan Society of Civil Engineers in the First Year of Reiwa" on September 3, 1st year of Reiwa, "Seismic analysis using shell elements" "Development of programs for sophistication" is released

The present invention relates to a model transformation device for a steel structure and a model transformation program for the steel structure, which converts a beam model represented by a simple line segment into a shell model represented by a surface.

Generally, in seismic design of steel structures, a beam model consisting of approximate beam elements based on the basic assumptions of the beam is used. Due to the basic assumptions of the beam, the beam element cannot take into account damage to the member with local buckling. Therefore, it is not possible to accurately evaluate the behavior of a steel structure in which a member is damaged with local buckling by a beam model. In ordinary seismic design, the seismic motion for design is input to the beam model in one horizontal direction, and the design is performed within the range that does not reach the above-mentioned damaged state.

Recently, in addition to the above seismic design, new studies are being conducted to predict and control the collapse behavior of steel structures for seismic motions larger than the design seismic motions with the aim of reducing unexpected damage. .. For this purpose, in order to consider the damage of the member accompanied by the above-mentioned local buckling, the finite element method (FEM (Finite Element Method)) analysis using the shell model consisting of the shell elements based on the plate theory is performed with high accuracy. Simulation is required.

However, when modeling a steel structure, it takes time to manually construct a new shell model. Therefore, it is convenient to have a tool (program) that can convert an existing beam model to a shell model.
As a technique capable of converting such a model and performing a highly accurate simulation, for example, the method described in Patent Document 1 is known.

In the method of designing a seismic reinforcing member for a bridge in Patent Document 1, a front view of an I girder is drawn by two-dimensional CAD, and two-dimensional CAD data having a drawing and coordinate values for an arbitrary axis is created for each cross-sectional shape. It is described that the 3D coordinate data is converted based on the 2D CAD data of the above, and the FEM analysis model is constructed based on the 3D data.
According to the method described in Patent Document 1, it is described that mesh division, plate thickness, and material property information can be given and used for FEM analysis.

Japanese Unexamined Patent Publication No. 2006-195713

However, in the method described in Patent Document 1, since a partial FEM analysis model is constructed in which only the seismic retrofitting member that is a part of the steel structure or the member in the peripheral portion thereof is independently taken out, the other structure is constructed. It is necessary to separately evaluate the force and displacement transmitted from the member to the member and give it to the above model as a boundary condition. In such a method, the influence of the damage behavior of some members on the entire steel structure cannot be considered. That is, in reality, there is always a force and displacement interaction between other structures and the members that are part of them, so some members need to be incorporated into the model of the entire structure for evaluation. be.

Therefore, the present invention provides a model transformation device for steel structures and a model transformation program for steel structures, which enables detailed simulation by the shell model by converting to a shell model considering the entire structure of the steel structure. The purpose is.

The model conversion device for a steel structure of the present invention includes a member constituting the steel structure, a beam element obtained by dividing the member into a plurality of parts, a node of the beam element, and a cross section at the position of the node. Based on the beam model of the entire structure and the replacement designation information in which the member to be replaced with the shell element from the beam model is specified, the cross section is linearly interpolated in the axial direction of the member to perform the member. It is characterized in that it is provided with a model building means for generating a shell model of the entire structure in which the member designated by the replacement designation information is replaced with a shell element by constructing the plate surface of the above.

In the model conversion program of the steel structure of the present invention, the computer is divided into a member constituting the steel structure, a beam element obtained by dividing the member into a plurality of parts, a node of the beam element, and a cross section at the position of the node. The cross section is linearly interpolated in the axial direction of the member based on the beam model of the entire structure included and defined and the replacement designation information in which the member to be replaced with the shell element is specified from the beam model. By constructing the plate surface of the member, the member is made to function as a model building means for generating a shell model of the entire structure in which the member designated by the replacement designation information is replaced with a shell element.

According to the present invention, the model building means is a beam of the entire structure including a member constituting the steel structure, a beam element obtained by dividing the member into a plurality of members, a node of the beam element, and a cross section at the position of the node. The model and the member to be replaced with the shell element from the beam model are specified by constructing the plate surface of the member by linearly interpolating the cross section in the axial direction of the member based on the specified replacement specification information. Generate a shell model of the entire structure in which any member is replaced with a shell element. Therefore, it is possible to automatically convert the entire structure beam model in the steel structure to the shell model of the entire structure in which any member specified by the replacement designation information is replaced by the shell element. Therefore, a new shell model can be obtained without manually constructing it. In addition, since the shell model is a shell model of the entire structure in which any specified member is replaced with a shell element, the specified member can be incorporated into the model of the entire structure system for evaluation. The effect of the damage behavior of the member on the whole can be considered.

The cross section of the end portion of the member may be constrained by a rigid body to provide a boundary node processing means for the boundary node of the member.
The boundary node processing means restrains the cross section of the end portion of the member with a rigid body to obtain the boundary node of the member. Therefore, the motion at any one point on the rigid plane can be subordinate to the motion at another position on the rigid plane, so that the degree of freedom at both ends of the member can be significantly reduced. In addition, by consolidating the degrees of freedom at the end of the member at the boundary node, it is possible to match the design method of the beam model.

Based on the additional definition information that is added to the beam model and defines the residual stress applied as the initial stress, it is possible to provide an additional information processing means for applying the residual stress to the coupling position of the plate element. ..
Since the additional information processing means applies residual stress to the coupling position of the plate element when the shell model is generated, the simulation can be performed in consideration of the residual stress when the simulation is performed using the shell model.

When the beam element is a plate element, the distance between the plate element and the other plate element is calculated based on the relationship between the node of the plate element and the plate thickness of the other plate element, and the plate is used. It is possible to provide an element coupling processing means for determining contact between elements.
By determining whether the plate elements are in contact or separated state in this way, the joined state or separated state between the plate elements can be reflected in the shell model.

When the shell model is generated, the model building means can generate an output file for each member as a calculation node for parallel calculation.
By dividing the output file for each member, it is possible to suppress an increase in the size of the output file of the analysis result, and shorten the visualization process on the post processor and the extraction process time of the result data of interest. Furthermore, the affinity with the design method and the verification method by the beam model can be improved.

Since the present invention can be converted into a shell model considering the entire structure of the steel structure by generating a shell model of the entire structure in which the specified arbitrary member is replaced with the shell element in which the mesh is constructed. , Can enable detailed simulation with shell model.

It is a figure which shows the model conversion apparatus which concerns on embodiment of this invention, the storage apparatus which stored the beam model, the storage apparatus which stored additional information, and the storage apparatus which stored the converted shell model. .. It is a block diagram for demonstrating the model transformation apparatus shown in FIG. It is a figure which shows an example of a beam model by two columns and a beam connecting columns. (A) is a diagram for explaining a beam model and additional information as an input file, and (B) is a diagram for explaining a shell model as an output file. (A) and (B) are general box-shaped cross sections, (C) and (D) are stiffened box-shaped cross sections, (E) is an I-shaped cross section, and (F) is an H-shaped cross section. It is a figure for demonstrating the model of the residual stress distribution, (A) is a box-shaped cross section, (B) is a stiffening box-shaped cross section, (C) is an H-shaped cross section (TYPE-A), (D) is H. It is a cross section (TYPE-B). It is a flowchart for demonstrating the model transformation process of the model transformation apparatus shown in FIG. It is a figure for demonstrating the connection between each plate, (A) is a figure which shows an example of the cross section of a beam element, (B) is an enlarged view for demonstrating the connection process. It is a figure for demonstrating the construction of the mesh in a multi-linear member. It is a figure for demonstrating the process of a boundary node, (A) is a figure for demonstrating the conventional process when each member is connected by a rigid body, (B) is a figure for demonstrating the end portion in a member. It is a figure for demonstrating that the cross section is restrained by a rigid body. It is a figure for demonstrating that the cross section of the end part of a member by a shell model is restrained by a rigid body. It is a figure for demonstrating that the area is divided for each member for parallel calculation. It is a figure of an example which converted the pillar of the beam model shown in FIG. 3 into a shell model. It is a figure for demonstrating that a part of the beam model of the whole structure in a bridge was designated and replaced with the shell element to make the shell model of the whole structure.

A model transformation device for a steel structure according to an embodiment of the present invention will be described with reference to the drawings.
In this specification, the component unit constituting the steel structure is referred to as a member, and the member divided into a plurality of members for FEM analysis is referred to as an element (beam element, shell element). Further, the member is a single bar having a constant cross section, and includes a case where the member is formed by multiple straight lines in addition to being linear.

The model transformation device 10 of the steel structure according to the present embodiment shown in FIG. 1 generates a shell model based on the beam model stored in the storage device 20 and the additional information stored in the storage device 30. And store it in the storage device 40.

Although the storage devices 20 to 40 are shown as separate bodies from the model conversion device 10 in FIG. 1, the beam model, the additional information, and the shell model are divided into areas in the storage means of the model conversion device 10. It may be stored, or any or all of the storage devices 20 to 40 may be connected to the model transformation device 10 via a network.
In the present embodiment, it is assumed that the model transformation device 10 is connected to the storage devices 20 to 40 via a network.

The model transformation device 10 is a computer on which a model transformation program operates. The model transformation program functions as each means of the model transformation apparatus 10 by operating on a computer.

As shown in FIG. 2, the model transformation device 10 includes a communication means 11, a model construction means 12, an additional information processing means 13, an element combination processing means 14, a boundary node processing means 15, and a display means 16. The input means 17 and the storage means 18 are provided.
The communication means 11 has a function of receiving data from the network and outputting the data to each means of the model transformation device 10, inputting data from each means, and transmitting the data to the network.

The model building means 12 reads a beam model of the entire structure in which the replacement specification information in which the member to be replaced by the shell element is specified and the additional definition information indicating the additional definition added for each member and each element are incorporated. , Builds a mesh of shell elements based on the cross-sectional shape of the beam element of the member specified by the replacement specification information, and generates a shell model of the entire structure in a predetermined format. In the present embodiment, the shell model is described in a format that can be simulated by the seismic analysis software SeanFEM manufactured by Seismic Analysis Laboratory. It also generates an XML (Extensible Markup Language) format file to visualize the shell model.
The additional information processing means 13 adds additional information to the beam model and generates a shell model to which additional definition information is added based on the content of the additional information.
The element coupling processing means 14 determines whether the two beam elements are in a contact state or a separated state from the cross-sectional shape, and sets the two beam elements in a bonded state.
The boundary node processing means 15 performs processing in which both end faces of the member are constrained by a virtual rigid body to be a boundary node of the member.

The display means 16 may be an LCD panel, an organic EL panel, or the like.
The input means 17 can be a keyboard, a mouse, or a touch pad.
The storage means 18 stores programs such as an OS and application software, and working data of the model being converted. The storage means 18 can be, for example, an SSD (Solid State Drive) or a hard disk drive.

Next, the shell model and additional information will be described with reference to FIGS. 3 and 4.
For example, the beam model shown in FIG. 3 is composed of members P1a and P2a as columns and members P3a as beams straddling the tops of the members P1a and P2a.
The member P1a is composed of beam elements E1a to E4a, the member P2a is composed of beam elements E9a to E12a, and the member P3a is composed of beam elements E5a to E8a. Each beam element E1a to E12a has nodes N1a to N13a. Then, the node N1a and the node N13a are fixed.

As shown in FIG. 4A, such a beam model includes node definition information, element definition information, support condition definition information, and cross-sectional shape definition information, and is defined.
The node definition information is data in which coordinate information is associated with node numbers (1 to 13) for identifying nodes N1a to N13a of each beam element E1a to E12a.
The element definition information includes element numbers (1 to 12) for identifying each beam element E1a to E12a, a member number for specifying a member, a cross-section number for identifying a cross section, and a starting point node of each element. It is the data associated with the node number (1 to 13) indicating the end point node.
The support condition definition information is data that defines fixed nodes (nodes N1a, N13a). In this embodiment, the degree of freedom to constrain each node can be defined.
In the cross-sectional shape definition information, information indicating a material and coordinate information indicating a cross-sectional shape are associated with a cross-sectional number.

Further, the additional information shown in FIG. 4A includes replacement designation information for designating the beam element of the member to be replaced with the shell element by the element number, and additional definition information.
The additional definition information includes diaphragm definition information, mesh size definition information, initial irregular definition information, and contact determination definition information.
The diaphragm definition information sets the position of the diaphragm, and can be set for each member.
The mesh size definition information can be set as a mesh size in the plate width direction (dimensional) and a mesh size in the axial direction (dimensional).
Residual stress can be set for the initial irregularity definition information.
The contact determination definition information is a threshold value (permissible value for contact determination between plates) for determining whether or not the plates (plate elements) are in contact with each other.

Here, the residual stress applied as the initial stress will be described in detail.
The additional information processing means 13 is, for example, a general box-shaped cross section shown in FIGS. 5A and 5B, a stiffened box-shaped cross section shown in FIGS. 5C and 5D, and FIG. Residual stress as an initial irregularity can be applied to the I-shaped cross section shown in (E), the H-shaped cross section shown in FIG. 5 (F), or any other cross section.

The residual stress is set by the value of the residual stress ratio (σ _rt / σ _y , σ _rc / σ _y , σ _rt1 / σ _y , σ _rc1 / σ _y ) according to each cross section. For the residual stress, set a distribution model that satisfies the balance with respect to the stress component in the axial direction of the member.

When the member has a box-shaped cross section, based on the model of the residual stress distribution of the box-shaped cross section shown in FIG. 6 (A) which is a box-shaped cross section and FIG. 6 (B) which is a stiffened box-shaped cross section. It will be explained in detail.

For the panel portion forming the box shape, the tensile side residual stress σ _rt / σ _y (positive value) and the panel compression side residual stress σ _rc / σ _y (negative value) are given to the equation (1) as input values.
b _t / b = -σ _rc σ _rt / (σ _rt -σ _rc ) ² ... (1)
Note that b is the panel width (sub-panel width in the case of a stiffening plate).

For the stiffener, for example, the residual stress distribution with the following fixed values can be used.
σ _rst / σ _y = 0.25, σ _rsc / σ _y = -0.40 ... (2)
b _st / b _s = 0.19, b _sc / b _s = 0.61 ... (3)
Note that b _s is the height of the rib.

Next, when the member has an H-shaped cross section and an I-shaped cross section, FIG. 6 (C) is an H-shaped cross section (TYPE-A) and FIG. 6 (D) is an H-shaped cross section (TYPE-B). It will be described in detail based on the model of the residual stress distribution of the H-shaped cross section shown.
Regarding the residual stress on the tensile side of the flange, σ _rt1 / σ _y is given in the case of TYPE-A, and σ _rc1 / σ _y is given in the case of TYPE-B.
For the residual stress on the compression side of the flange, give σ _rc1 / σ _y .
For the residual stress on the tensile side of the web, give σ _rt / σ _y .
For the residual stress on the compression side of the web, give σ _rc / σ _y .

The operation and usage state of the model transformation apparatus according to the embodiment of the present invention configured as described above will be described with reference to the drawings.
The user operates the input means 17 while looking at the display means 16 shown in FIG. 2, operates the model conversion program on the computer, and causes the computer to function as the model conversion device 10.
In the model conversion device 10, the model building means 12 reads the beam model (node definition information, element definition information, support condition definition information, cross-sectional shape definition information, etc.) stored in the storage device 20 (see step S10 in FIG. 7). ). Further, the model building means 12 reads the additional information stored in the storage device 30 (see FIG. 1) (see step S20). When the beam model and additional information are stored in the storage means 18 in the model conversion device 10 in advance, the model building means 12 reads the beam model and additional information from the storage means 18.

The additional information processing means 13 reads the additional information and adds it to the beam model (see step S30). The beam model in this embodiment is described in a format that can be simulated by the above-mentioned SeanFEM. Therefore, the additional information processing means 13 includes, in the header of the input file serving as the beam model, a list of element numbers of the beam elements of the members to be replaced by the shell elements (replacement specification information), the position of the diaphragm (diaphragm definition information), and the shell. Describe the mesh size of the element (mesh size definition information), information on initial irregularity (initial irregularity definition information), and the like.

First, as a preprocessing of model transformation, the element coupling processing means 14 determines the node of the plate element and the plate thickness of another plate element when the beam element is a plate element, based on the definition of the cross section of the beam element. The distance between the plate element and the other plate elements is calculated based on the relationship of the above, the contact determination between the plate elements is performed, and the coupling between the plates is processed. The processing contents of the plate-to-plate coupling will be described below.
For example, in the case of the beam element of the cross section shown in FIG. 8A, the local coordinate values of the start point and the end point of the plate constituting the cross section and the plate thickness are given to the cross section shape definition information.

For the end points (nodes, marked with ●) of the plate element shown in FIG. 8 (B), the distance from the plate thickness center line of another plate element is calculated, and the distance is the plate thickness of the plate element. The contact between each plate element is determined depending on whether or not it matches the value of 1/2. At this time, even if there is a disagreement, it is determined that the contact state is in contact if it is within the allowable value of the contact determination definition information. If it is determined that they are in contact with each other, a new coordinate value indicating an intermediate rating (x mark) is calculated, and processing is performed so that the intermediate rating and the end point of the plate are connected by a rigid body. Further, both ends (both ends of the member) of the region replaced with the shell element are processed so that the nodes constituting the original beam element are used as master nodes and are connected by a rigid body (see step S40).
In this way, the joined state or separated state between the plates (plate elements) can be reflected in the shell model.

Here, the determination of the bond between the plates (plate elements) will be described in detail. The determination is of the following two types, and residual stress is applied by the additional information processing means 13 according to the determination (see step S50).
First, the first point is the case where the end points of other plates coincide with the surface of the plate. In this case, it is determined that both plates are joined by welding, and when the residual stress is taken into consideration, tensile stress is applied to the joint portion of both panels.
The second point is the case where the end points of the plate and the end points of other plates coincide with each other.
In this case, it is determined that the plate is in a bent state, and the residual stress is not introduced into this joint even when the residual stress is taken into consideration. The joint between the plate ends is restrained by a rigid body. At this time, for example, in SeanFEM, since a rigid body requires a length (both ends of the rigid body cannot have the same coordinates), the ends of both plates are slightly shrunk (1/1000 of each plate thickness). ..

Next, the model building means 12 processes, for example, the building of the mesh (see step S60). The processing contents of mesh construction will be described below.
For example, when the member shown in FIG. 9 is multi-linear, the mesh is constructed inside the member based on the information about the nodes of the beam element (node definition information, element definition information) in the beam model. Nodes are provided and the normal vector of the cross section at these node positions is calculated. The normal vector of these cross sections is the average value of the normal vectors in the adjacent sections. Then, from the start point to the end point of the member, the shell element nodes on the cross section located at the diaphragm and the intermediate point are linearly interpolated in the member axial direction to construct a mesh that becomes the node of the shell element and the plate surface of the member.

At this time, the additional information processing means 13 can give a constraint (due to a rigid body element) imitating a plurality of diaphragms to an arbitrary cross-sectional position having the member axis as a normal for the member of FIG. 8 (B). By doing so, the basic assumptions of beam theory can be introduced. Further, this arbitrary cross-sectional position can be specified as a diaphragm interval by additional information.

Here, the processing of the boundary node by the boundary node processing means 15 shown in FIG. 2 (see step S70) will be described in detail.
As a treatment of the member ends of the beam model, for example, as shown in FIG. 10A, when the members A and B are connected by a rigid body R, the cross section (end face) of the ends of the members A and B is formed. Is constrained by the rigid body R (member C), the master node at the end faces of the members A and B is used as the boundary node, and the slave node at the end face of the rigid body R is used as the boundary node. , Slave node) The degree of freedom is erased, so it cannot be calculated by simulation.

Therefore, as shown in FIG. 10B, the boundary node processing means 15 automatically determines the connection with the rigid body at the node at the end of the members A and B, and cross-sections of the ends of the members A and B. Is constrained by rigid bodies R1 and R2 (virtual rigid bodies), and is automatically corrected so that the master node of the rigid body to which the node at the end of the members A and B belongs becomes the boundary node.

In this way, if the cross sections of the ends of the members A and B are constrained by the rigid body, the rigid body is not deformed. Therefore, the motion at any one point on the rigid body plane is dependent on the motion at another position on the rigid body plane. Can be done.
Therefore, as shown in FIG. 11, the degrees of freedom of the boundary nodes of the members A and B are set to u _x in the X-axis direction, u _y in the Y-axis direction, u _z in the Z-axis direction, θ _x in the X-axis direction, and θ in the Y-axis direction. It can be significantly reduced to 6 degrees of freedom in _y and the Z-axis direction θ _z , and 12 degrees of freedom in total at both ends.
In addition, since the beam theory is a theory in which the cross section is invariant, the shell model in which the end cross sections of the members A and B are constrained by a rigid body has a high affinity with the beam model and is designed by the beam model. You can use the law and the verification method.

In this way, the beam model is converted to the shell model.
In the shell model modeled by the shell element, when there are many members and the degree of freedom of the analysis model is large, parallel calculation is performed when the simulation by numerical analysis is performed. In order to support this parallel calculation, in the model building means 12, for example, the members P1b to P8b of the analysis model M modeled by the shell element shown in FIG. 12 are assigned to one calculation node N1b to N8b, respectively, and FIG. As shown in (B), an output file is generated for each calculation node (member) (see step S80).

In the domain decomposition method for performing parallel calculation, the calculation efficiency is prioritized and the division is performed, and the members may be divided in the middle. However, in the present embodiment, the model building means divides each member to generate an output file, thereby suppressing an increase in the size of the output file of the analysis result, visualization processing on the post processor, and attention. The extraction processing time of the result data can be shortened, and the compatibility with the design method and the verification method by the beam model can be improved.

For example, when the beam model shown in FIG. 3 is the entire structure constituting the steel structure, the shell model of the entire structure in which the members P1a and P2a indicating the columns that are the members of the beam model are specified and replaced with shell elements. Is generated.
An example of conversion to this shell model is shown in FIG. In FIG. 13, the members P1a and P2a, which are pillars, are converted into a shell model having a plate surface.
Further, FIG. 14 shows an example of a shell model of the entire structure (entire bridge) in which a portion of the beam model of the entire structure in the bridge is designated and replaced with a shell element.
In the shell model, not only the entire structure is replaced with the shell element, but also the shell of the entire structure in which the beam element and the shell element are combined by designating an arbitrary member as shown in FIGS. 13 and 14. Can be a model.
Therefore, since the designated member can be incorporated into the model of the entire structure and evaluated, the influence of the damage behavior of the member on the entire structure can be considered. Therefore, it is possible to perform detailed and highly accurate simulation by the shell model.

INDUSTRIAL APPLICABILITY The present invention can convert a beam model representing a steel structure by a simple line segment into a shell model represented by a surface, and is therefore suitable for simulating the influence of seismic motion on the steel structure with high accuracy. It is particularly suitable for simulation of bridges, which are large-scale steel structures.

10 Model conversion device 11 Communication means 12 Model construction means 13 Additional information processing means 14 Element coupling processing means 15 Boundary node processing means 16 Display means 17 Input means 18 Storage means 20, 30, 40 Storage devices P1a to P3a, P1b to P8b, A to C members E1a to E12a beam element N1a to N13a node R, R1, R2 rigid body N1b to N8b calculation node M analysis model

Claims (6)

A beam model of the entire structure defined by including a member constituting a steel structure, a beam element obtained by dividing the member into a plurality of members, a node of the beam element, and a cross section at the position of the node, and the beam model. The replacement designation is made by constructing the plate surface of the member by linearly interpolating the cross section in the axial direction of the member based on the replacement designation information in which the member to be replaced with the shell element is designated. A model conversion device for a steel structure provided with a model construction means for generating a shell model of the entire structure in which the member specified by information is replaced with a shell element. The model transformation device for a steel structure according to claim 1, further comprising a boundary node processing means for constraining a cross section of an end portion of the member with a rigid body to serve as a boundary node of the member. The invention according to claim 1 or 2, further comprising an additional information processing means for applying a residual stress to the coupling position of the plate element based on the additional definition information added to the beam model and defining the residual stress applied as the initial stress. Model conversion device for steel structures. When the beam element is a plate element, the distance between the plate element and the other plate element is calculated based on the relationship between the node of the plate element and the plate thickness of the other plate element, and the plate is used. The model transformation device for a steel structure according to any one of claims 1 to 3, further comprising an element coupling processing means for determining contact between elements. The model of a steel structure according to any one of claims 1 to 4, wherein the model building means generates an output file for each member as a calculation node for parallel calculation when the shell model is generated. Converter. Computer,
A beam model of the entire structure defined by including a member constituting a steel structure, a beam element obtained by dividing the member into a plurality of members, a node of the beam element, and a cross section at the position of the node, and the beam model. The replacement designation is made by constructing the plate surface of the member by linearly interpolating the cross section in the axial direction of the member based on the replacement designation information in which the member to be replaced with the shell element is designated. A model conversion program for a steel structure that functions as a model building means for generating a shell model of the entire structure in which the member specified by information is replaced with a shell element.

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