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IFC Workflow in Revit

Are you still cursing or are you getting it done?

With the current liNear Solutions 20.03, significant improvements for the MEP workflow based on IFC architectures have been added in response to numerous customer requests. With the example of an ARCHICAD file, we demonstrate the most important steps for standard-compliant dimensioning.


WHAT CAN AND CAN’T IFC DO?
When working with workflows based on Industry Foundation Classes (IFC), there is one thing you need to understand in order to start with realistic expectations: The IFCs don’t currently specify a universal exchange format for building models, meaning that the user practically has a free choice of authoring platform. So you can’t expect to model a building in ARCHICAD, modify it in Allplan and then load it into Autodesk Revit for further processing. A workflow like this rarely works with market standard text documents, so why should we expect it to work smoothly with complex parametrically constructed buildings? Apart from that, within the framework of an integrated design project, you should of course also ask what the point is. When you receive an IFC file, you are usually also receiving someone else's work. This other person is responsible for changes and should make them in the original platform.

Even some IFC developers draw comparisons to a PDF, which is an appropriate analogy. The PDF has to look visually correct and provide the recipient with the necessary information. However, its basic structure doesn’t have to be editable. At best, you can attach pages, mark passages, or comment on the available information. If you want to make structural changes, you must either make them in the original model or be prepared for a lot of manual work.

So, IFC is not an exchange format for further processing. Then what is IFC for? Where are the pitfalls in the MEP Workflow and how do we deal with them?

As already indicated, IFC is well suited for freezing and documenting the design status of a (partial) model. When outputting to an IFC file, the parametric building model is transferred from the authoring software into a uniform geometric representation. Additionally, previously defined datasets are attached to the various building elements. Using these elements, the data can be clearly identified and can be filtered, merged and further enriched in the further process. As long as the labeling (IfcGuid) of a component is not changed, each participant in the design process can find it again, which is a prerequisite for technologies such as the BIM Collaboration Format (BCF). Especially when working with a coordination tool, it is essentially irrelevant from which authoring platform an IFC comes, as long as it adheres to the IFC specifications in general and the agreed project standards in particular.

An IFC-based workflow can also be advantageous for designing technical building equipment if the architect involved does not use Revit. Unfortunately, the first problem arises once the architectural model is handed over to the MEP designer, which primarily concerns the analysis of the building geometry and the addressability of the installed components.

OPEN OR LINK IFC FILES?
The first decision must be made at the very beginning: Do you open the IFC file in Revit or just link it? And the choice matters, because two very different things happen in the background.

Opening (or rather importing) an IFC file initially sounds like an interesting option, because it offers the option of working on individual components later on should it become necessary. That's the theory at least. However, problems are often encountered here, that are similar to those that occur when a Word document is opened in Open Office: The imported model often doesn't have much to do with the actual architecture. Rooms aren’t placed correctly, components are not on the axis, and all kinds of other irritating things happen. For several years, rumors have been circulating that the IFC-Import isn't actually a problem if you know how to use it. Unfortunately, it was not possible for us to verify these statements. In recent years we have spoken to countless users in Germany and abroad and have never been able to derive a workflow that works for non-idealized building models. The export settings on the source platforms and the import settings on the target platform are too diverse to be of any real use. True, for each individual use case there is always a combination of import/export settings, combined with restrictive modeling guidelines, which make post-processing efforts less painful. Ultimately, however, many users were only left with the decision whether to re-model their building for the thermal load calculations or adapt the imported model to their own requirements manually. Both approaches leaves the MEP designer with problems that are difficult to solve as soon as the architect publishes changes later on (Fig. 1).

The second way to use an IFC file as a basis for the rest of the workflow is to simply "link" it in a Revit project. This certified method for processing an IFC file in Revit is very similar to the actual intention of IFC-Workflows and has been developed by Autodesk as an Open-Source-Project. Here, the IFC model is understood as a substructure, i.e. it only requires a correct graphical representation of the architecture and the associated data for the individual model elements. This is sufficient to establish dimensional references for the construction of the MEP and enables a precise modeling of the planning task, the results of which are later merged in the coordination environment.

What's the catch? Well, if you look at a successfully linked IFC you will see that, while the linked components are classified accordingly (e.g. as wall, window, etc.), they no longer contain complete Revit parametrics. The resulting components have the categories Wall or Window and also appear as such but no longer behave that way. The consequence is that they cannot be correctly processed by most tools, such as by Revit's own gbXML-model export and many slot and void design solutions (Fig. 2).

For large projects, there is another aspect which makes IFC-linking even more complicated. The IFC-Spaces are represented in Revit, not as proper "rooms" or "spaces", but as "Generic Models" which are not suitable for further processing in the MEP workflow.

With on-board resources, it is almost impossible to conduct a cooling load calculation on the basis of an IFC linked in Revit to dimension the technical systems. Currently, the only alternatives are tabular inputs or graphical tools in which an IFC architecture can be manually defined for a calculation. This process is costly, prone to error and difficult to integrate into efficient design processes, because once the calculation is complete, the results must be laboriously transferred back into the MEP partial model. This is already tiresome in one iteration and becomes an unbearable task if there are any later alterations. It is high time for a rethink.

liNear’s development department has therefore thought long and hard about how proven workflows can also be enabled for linked IFC models in the future. Using ARCHICAD-Architecture as an example, we want to demonstrate how you as a MEP designer can bridge the gap between different platforms with the help of liNear software on the Revit platform.

WALKTHROUGH: 
FROM AN IFC-ARCHITECTURE TO A HEAT LOAD

In the following we describe the calculation of a heat load based on a single-family home model which is available in IFC4 format1. The general procedure is closely related to the workflow based on a Revit architecture, in detail only a few steps are different.

ASSIGN IFC-CLASSES TO REVIT-CATEGORIES
In the following we assume that the IFC is linked in the standard configuration. If you have special requirements for the translation of IFC classes and types in revision categories and subcategories, you can store these in the IFC class assignment beforehand (Fig. 3).


Info

In addition to the default configuration it may, depending on the export settings of the architecture authoring system, also be advisable to make your own adjustments, e.g. to the assignments for coverings (IfcCovering) and bare ceilings (IfcSlab), so that there are fewer problems during import. Assign these IFC-types to a suitable Revit-category (e.g. IfcSlab.FLOOR could be redirected to the category "storey ceiling"). An overview of predefined types and their meanings can be found in the IFC-specification2. Details on the configuration of the IFC interface in Revit can be found in the Revit IFC Handbook from Autodesk3.


LINK IFC FILE IN REVIT
Before we link the IFC file, we use an IFC viewer to ensure that the components in our building have been correctly classified by the exporting authoring solution and are as collision free as possible. We also need rooms in the form of IfcSpace elements to be able to transfer the model to Revit smoothly. If one of these preconditions does not apply, please first discuss the requirements with your architect. Depending on the circumstances, it may be necessary to adapt export classifications, output filters and properties for geometry generation to your needs (Fig. 4).  

We start with a new Revit project based on the liNear template (or another suitable MEP template). Then we open the "Manage Links" dialog in Revit and click on the second tab labeled "IFC". There we add a link to the desired IFC project. Revit now automatically generates a file with the ending "ifc.rvt" in the background. This file contains a Revit model which offers a representation of the geometries and attributes in the IFC file. It is worth mentioning that although the generated model can be opened and partially modified in Revit, comprehensive editing is not possible. Elements such as walls, windows, doors, etc. are correctly categorized in this model, but can no longer be edited with the usual Revit tools such as the Family or Sketch Editor (Fig. 5).

Once the import is complete and after closing the dialog, you see the model of the referenced IFC file. For an improved presentation, we recommend hiding the Revit category "Generic Modeles" using the visibility dialog. This usually causes the graphical representations of spaces (IfcSpace) and openings (IfcOpeningElement) to no longer be explicitly represented. It also means that you can easily detect possible errors in the IFC classification table: In the event of errors, the linking routine would generate "Generic Modeles" instead of the correct categories as an emergency solution, which can lead to technical problems in the subsequent workflows (heating/cooling load calculation, slot and void design, etc.) (Fig. 6).

IMPORT STOREYS
An important step for the further processing is the transfer of storeys from the reference into the current MEP project. Our storey table is ideal for this purpose. By using the function "Import/Synchronize ...", you can import the storeys from the IFC (Fig. 7). Select all the storeys and replace any existing storeys in your template with the imported data. Insert a work level above the attic storey at ridge height. This will help you later in the placement of MEP spaces. With the help of our view manager, floor plans can then be created for the two storeys.

CREATE MEP SPACES
Switch to one of the floor plans. Here you can see different characteristics of the IFC Import. For one thing, the IfcSpaces, i.e. the rooms in your IFC, are created as general models and are therefore unusable fur further application in the MEP workflow. To be able to continue with a cooling load calculation, you must first create corresponding spaces, similar to the Closed-BIM-Workflow. You can do this using the Revit basic functionality or with our tool "Create spaces" (Fig. 8).

In addition to presetting correct heights and adopting room names and numbers from the IFC, the liNear tool has the decisive advantage that virtual components are taken into account (see e.g. living room/kitchen (-> Wohnen/Küche) in Figure 9) and attention is paid to correct handling of door situations. Names, numbers and other parameters are also transferred in this process (optional).

To be on the safe side, you should also check the spaces again in a work stage or a three-dimensional view. Click the corresponding spaces in our zoning tool and make sure that the room volumes are correctly represented (Fig. 10).

 

If the MEP space representation in your floor plan is incorrect, make absolutely sure that the "height for calculation" parameter of the respective storey level is set to a value at which the rooms on this storey assume their maximum expansion. In rooms with a constant cross-section, the default value can be maintained, provided the storeys have been constructed below the relevant storey level. If this is not the case, e.g. if the screed was modeled as an independent element above the storey level, please correct the input by specifying a relative height above the screed thickness (Fig. 11).

CONFIGURE PARAMTER SETTINGS
Similar to the interaction with Revit architecture spaces, the liNear solution offers the possibility to accept data from the IfcSpace objects into the corresponding MEP spaces. This is particularly interesting for room naming, since we can automatically take room names and numbers from the IFC spaces, for example from the parameters "LongNameOverride" and "IfcName" (Fig. 12).

Using the button next to the "Transfer additional parameters" setting, you can also configure which additional (jointly used) parameters you want to transfer to your MEP spaces. Here, you can see the IFC imported parameter sets (Fig. 13).

TRANSFER TO liNear BUILDING AND CALCULATION OF HEAT LOAD
When transferring to liNear Building, no building material properties are currently transferred from wall layers where IFC architectures are used. However, if U-values are already stored on components (e.g. windows/doors), the corresponding parameter (e.g. ThermalTransmittance) can be set in the configuration (Fig. 14). Here, however, we point out that it must be ensured that meaningful values are already stored in the information supply chain. If this can not be guaranteed, then you should not trust the specified values and should simply define the layered structures or U-values in the liNear Building project master tables instead.

The rest of the workflow is not different from the work with Revit architectures. In particular, even with IFC-based architectures, you can use our bidirectional connection as usual to compare data between models and analyze the results. The search command ("Pipette" tool) allows you to navigate through the selection of components in Revit to the corresponding model section in Building. Conversely, the "Zoom and Show" command also allow the component belonging to a component surface to be identified in linked models. Here, a 3D working view with semi-transparent shading, trimmed accordingly to the storey, can be used in which the category "Generic Models" has been hidden (Fig. 15).

SUMMARY
The new features in liNear Solutions 20.03 offer significant improvements for working with linked IFC architectures in Revit. This is made possible by Desktop functionalities specially extended to this workflow as well as corresponding further developments of our own Building Analyse core, i.e. the algorithm that divides a building model into individual components for the load calculations in accordance with standards. The latter is constantly being developed further, completely detached from the road map of the underlying Revit platform and allows us to react quickly and flexibly to your feedback.

Christian Waluga


1 Source: Institute for Applied Information/Karlsruhe Institute of Technology;
   www.ifcwiki.org/index.php?title=KIT_IFC_Examples

2  technical.buildingsmart.org/standards/ifc/ifc-schema-specifications/

3  www.autodesk.de/campaigns/inter­operability/ifc-handbuch


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