Overview: In this lesson, you will learn how to generate, edit and export an analytical model from a fully classified BIM model.
After completing this lesson, you will be able to:
- Generate an analytical model from a fully classified BIM-model with different deviation settings
- Remove connection nodes
- Add or remove restrictions
- Edit the nodes or axes of an analytical model
- Export the analytical model to an IFC file or a CIS/2 file
An Analytical Model is in essence a simplified wireframe model that only contains the load-carrying building elements. The linear building elements like columns, beams and members are represented by lines. The planar building elements like walls and slabs are not yet supported in the analytical model.
In BricsCAD, the BIMAnalyticalModel command allows you to generate an analytical model from a full 3D model of a structure. The command starts with an automatically generated proposal but allows you to further adapt the model to your desire. The automatic proposal will try to minimize rigid links (eccentricities) as much as possible by shifting, extending and even rotating some axis lines. The number of such changes can be controlled by the deviation settings.
You can modify the generated proposal afterward by using the options when you select an element (nodes or axes). To propagate the effect of a local change further you can use recalculate. This will launch a global solution that again tries to minimize rigid links but will respect the changes you made.
After editing the model you can export it to an IFC file and a CIS/2 file. This analytical model can then be used for further structural analysis after import in a dedicated analysis software package.
It's important to emphasize that the input model has to be fully classified since the BIMAnalyticalModel command assumes that all the load-carrying elements are indicated in the building element properties.
In the procedures, the analytical model is explained with an example model. You can find this model in the appendix.
Procedure: Creating an analytical model with strict deviation settings
First, you have to classify your BIM model. This is necessary for using the BIMAnalytical model command.
Use the Bimify command on your model or classify elements manually.
You are prompted: run Bimify on entire model or [Advance] <model>
Press Enter if you want to classify everything in your model or type A if you want a selection of elements.
Launch the BIMAnalyticalModel command in the commandline.
A pop-up window opens.
This window allows you to set the level of freedom used when generating an analytical model. When hovering over one of the options, a tooltip shows you what type of restrictions are imposed for that option. In this case, choose the Restricted parallel deviation with a maximal deviation in profile directions of 0 mm. No axis is allowed to deviate from its central position in the profile.
An interactive preview of the analytical model is constructed based on the chosen global deviation setting.
Also, the visual settings window appears.
In this window, you can check and uncheck different features of axes and nodes in order to highlight (in yellow) the axes and nodes which correspond to those features. This allows you to explore the current properties of the axis and nodes of the analytical model. Based on what you find you can determine if any adjustments need to be made.
You can also change the size of the disks representing the nodes. This is useful in combination with the node highlighting, which can be used for example to find nearly coinciding nodes.
- Select the checkbox Nearly coinciding with another node in the visual settings window. You will see that the center axes at these nodes do not align perfectly. As a result, small rigid links (eccentricities) had to be introduced. This is because of the previous setting in step 4. These small rigid links are displayed in purple.
To avoid this, restart the command with a slightly more flexible setting. In the Allowed deviations from center window, choose again for restricted parallel deviation but this time with a maximal deviation in profile directions of 30 mm.
- For the detection of nodes in the preview of the analytical model, the proximity of structural elements is used. Because of this, it is possible that there arise nodes in the model where tie rods cross. You can easily remove these nodes by clicking on the node and select remove.
Note: you can select several nodes at once using box selection.
- If you take a look a the remaining coinciding nodes, you see that the tie rods are not aligned perfectly with the center lines of the columns and small rigid links are formed. You can remove these rigid links by relaxing the position of the axes of the tie rods to a non-parallel position. You can do this by clicking on the tie rod, click relax restriction and click non-parallel.
The extra freedom will be used to remove the rigid links by moving the axes of the tie rods a bit.
- The only coinciding nodes that are left are at the front truss of the hangar structure. If you take a look at the side of the front truss, you see that a tilted member is connected to the horizontal beam rather than the column.
You can disconnect the tilted member by clicking on it and choose disconnect from node.
You are prompted: Select node disconnect from axes [Ok]
Select the node you want to disconnect from.
Type O to choose Ok.
Now select the two axes that you want to connect and click Connect axes.
With the global deviation setting, you chose this can only be done by introducing a small rigid link again. You can remove this by merging the two nodes. Select both nodes and click Merge.
You see that the side member also is attached to the front beam instead of the column. Repeat step 9 for the side member.
- You can remove the last coinciding nodes in the front truss by merging the nodes. This will shift the axes of the tilted members and introduces rigid links on the other end.
Note: If you take a look at the axis not parallel to center axis by using the visual setting window, you will see that the tilted member in the front truss has been allowed to be non-parallel to the center axis. So you could also remove the small rigid links by relaxing the tilted member to a non-parallel position like as we did for the tie rods.
Procedure: Creating an analytical model with the least restrictive deviation settings
- Repeat the steps from the previous procedure until step 4.
- Choose Any deviation in the Allowed deviations from center axis. This allows all kinds of freedom to avoid any rigid links. In the preview of the analytical model, you will see that some axes have been tilted. For example, the small members in the trusses are not centered and the column axis is moved towards the profile bounds to find connections with smaller side members there.
- Remove the unwanted tie rod nodes as in step 7 of the procedure with a strict deviation setting.
- In the interactive preview of the analytical model, the column axes were moved towards the profile bounds. In this step, you will center the axis of the columns. You can do this by clicking on the axis of the column, click Add restriction and click To center. This introduces rigid links very locally.
- In the visual settings window, select the checkbox has rigid links.
Type R in to choose Recalculate.
Recalculate will propagate the effect of a local change. It will launch a global solution that again tries to minimize rigid links, but will respect the modifications already imposed. In this case, the rigid links will be removed and the hole side of the structure shifts inward to the center plane of the columns. The axes of the smaller side members are hereby moved outside their profile bounds to avoid rigid links.
You can repeat this for the other side.
- In the visual settings window, select the checkbox Not centered.
You will see that one of the columns is still not centered because the connection at its top includes not only the larger beams but also one of the smaller ones. You change this by imposing a center restriction on this column as well.
Procedure: Exporting an analytical model
Before starting the command, open the Settings dialog.
In the Settings dialog, click the option Structural Analysis Model and choose the export format of the analytical model. You can choose between IFC and CIS/2.
Launch the BIMAnalyticalModel command.
Change the analytical model to your desire.
When you have finished the model, type E to choose Export and exit.
The document window appears where you can name and save the file in the desired folder.
Note: There are some remarks about the exported result.
The node concept used by BimAnalyticalModel does not completely coincide with CIS/2 and IFC node concept:
In the BimAnalyticalModel, only connections between linear elements are modeled with a node. For example, columns will not have a floor node if there is no connection to another element there. During export, missing end nodes are added to be compatible with CIS/2 and IFC export.
In the BimAnalyticalModel, a node always lies exactly on every connected axis. Eccentricities are modeled with explicit visual rigid links (purple bars in preview). During export, each cluster of BricsCAD nodes that are interconnected with rigid links is translated into a single connection node with eccentricities in CIS/2 and IFC.
The axis concept used by BimAnalyticalModel does not always coincide with the CIS/2 and IFC axis concept:
In the BimAnalyticalModel, there will be one single axis for each physical element. An axis can be attached to more or less than two nodes. During export, an axis that has multiple nodes along its length gets split up into several analysis members, each with one end and one start node, for the CIS/2 and IFC export.
The concept of Base Position in BricsCAD is similar to cardinal points. Base positions in BricsCAD form a subset of the cardinal points specified in the CIS/2 standard. For now, only cardinal positions 1 to 9 are supported.
In the analysis model, as calculated by BricsCAD, an axis can shift away from the center of the physical entity, essentially to any point within certain bounds. This means that the final position of the analysis axis does not necessarily coincide with a cardinal point or base position of the profile. During export, the section profile is linked to the analysis axis using the closest base position of the profile to the final analysis axis, using the cardinal point concept in CIS/2 and IFC. This means that if you visualize the structure in a CIS/2 or IFC viewer that shows physical members as extrusions of the profile section along the analysis axes, you will see that the structure shown there can slightly deviate from the original structure as modeled or imported into BRICSCAD. The structure as described by the exported analysis model is therefore an approximation of the actual structure.