STEP and IGES Import
Importing STEP and IGES Data
Introduction
STEP (Standard for the Exchange of Product Data, ISO 10303) and IGES (Initial Graphics Exchange Specification) are long-established neutral file formats for exchanging 3D CAD and geometric data between different engineering systems.
Unlike IFC, which encodes semantic and relational information for BIM and facility modeling, STEP and IGES primarily describe pure geometry — solids, surfaces, curves, and assemblies — without detailed object classification or material context.
Both formats are widely used in mechanical, manufacturing, and plant design domains, and FRI3D provides import support for them to facilitate integration of geometry-only models (e.g., mechanical enclosures, ducts, or equipment housings) into the simulation environment.
⚠️ Important Note:
STEP import functionality is currently limited in FRI3D. While basic geometry import is supported, advanced STEP features such as complex assemblies, advanced BREP operations, parametric data, and some STEP application protocols (APs) may not be fully supported. Import reliability may vary depending on the source CAD software and export settings. For production workflows requiring robust geometry import, consider using IFC format for building models or verify STEP import compatibility with your specific files.
Format Overview
| Format | Standard | Description | Typical Use |
|---|---|---|---|
| STEP (.stp, .step) | ISO 10303 | Modern, extensible schema for 3D product data. Represents solids, assemblies, and topology with boundary representation (BREP). | Mechanical design, industrial parts, and equipment layouts |
| IGES (.igs, .iges) | ANSI Y14.26M / NIST | Legacy exchange format storing surfaces, wireframes, and annotations. | Older CAD data, wireframe and surface models |
STEP files are structured as entities (e.g., ADVANCED_FACE, EDGE_LOOP, BREP_SOLID) referencing topology and geometry.
IGES files store lists of directory entries and parameter data sections defining curves, surfaces, and annotations.
Supported Entities and Classes
FRI3D’s importer focuses on extracting geometry rather than full product semantics.
The following entity types are commonly recognized:
| Category | STEP Entities | IGES Entities | Description |
|---|---|---|---|
| Solids | ADVANCED_BREP_SHAPE_REPRESENTATION, MANIFOLD_SOLID_BREP, CLOSED_SHELL | Type 186 (Manifold Solid BREP) | Closed solids used for volumetric representation |
| Surfaces | ADVANCED_FACE, SURFACE_OF_LINEAR_EXTRUSION | Type 128 (NURBS Surface) | Faces or patches representing part boundaries |
| Curves / Edges | EDGE_CURVE, VERTEX_POINT, CIRCLE, LINE | Type 126 (NURBS Curve), 100 (Circular Arc), 110 (Line) | Boundary edges and wireframes |
| Assemblies | SHAPE_DEFINITION_REPRESENTATION, NEXT_ASSEMBLY_USAGE_OCCURRENCE | — | Hierarchical organization of parts and sub-assemblies |
| Attributes (Limited) | PRODUCT_DEFINITION, MATERIAL_DESIGNATION | — | Optional naming or material metadata (often incomplete) |
⚠️ Note:
Many STEP and IGES exporters differ significantly in how these entities are organized.
The same part may be represented as anADVANCED_FACEgroup in one file and as a nestedBREP_SOLIDin another.
As a result, import reliability may vary between software sources.
Import Workflow in FRI3D
-
File Detection
FRI3D automatically detects.stp,.step,.igs, or.igesextensions. -
Parsing
Geometry is parsed using the Open Cascade (OCCT) kernel, which handles BREP and NURBS surfaces. -
Topology Reconstruction
Solids, shells, and surfaces are reconstructed into watertight meshes suitable for simulation or visualization. -
Tagging and Classification
Because STEP/IGES lack inherent semantic tagging, FRI3D infers categories based on naming conventions (e.g., part names, layer identifiers, or color attributes). -
Simplification and Conversion
Geometry is optionally simplified to triangular meshes or bounding volumes for use in fire, smoke, or CFD modules.
Import options
File → Import → BIM/CAD opens the import dialog. Below the file list is a CAD import options group. It shows only the options that apply to the file type you select:
| Option | Applies to | Default | What it does |
|---|---|---|---|
| Zone | all CAD files | the current zone | The zone (compartment) that receives the imported geometry. |
| File unit | OBJ, STL | Metres | OBJ and STL files do not record their unit. Choose the unit they were exported in: metres, centimetres, millimetres, inches or feet. |
| Up axis | OBJ, STL | Format default (OBJ Y-up, STL Z-up) | OBJ and STL files do not record which axis is up. Most OBJ files come from 3D tools (Y up) and most STL files from CAD tools (Z up). Change it if the model comes in lying on its side. |
| Mesh tolerance | STEP, IGES | 5 mm | How closely the triangles follow curved surfaces. Smaller is smoother but heavier to display. Simulations use the items' bounding boxes, so they are unaffected. |
STEP, IGES and glTF/GLB files record their own unit and orientation, so FRI3D converts them to metres itself. If an imported model looks implausibly large or small, the import log says so; check the file unit.
The file filters list only the formats FRI3D reads: BIM/IFC, 3D CAD (STEP/IGES) and 3D mesh (glTF/GLB/OBJ/STL). FBX, DAE, USD, PLY and DXF files are refused with a message saying which format to convert them to.
Command line: Import_CAD
The same options are available from the command line:
Import_CAD.exe <cad file> <project> [zone] [T] [-unit m|cm|mm|in|ft] [-upAxis y|z] [-meshTolerance <metres>]
zoneis the zone for the geometry;Tcreates a new project at<project>first.-unitand-upAxisapply to OBJ and STL only.-meshToleranceis in metres (for example-meshTolerance 0.005for 5 mm) and applies to STEP and IGES.
Import_CAD.exe C:\Models\pump_skid.step C:\Projects\plant.fri3d Room101
Import_CAD.exe C:\Models\scan.stl C:\Projects\plant.fri3d Room101 -unit mm -upAxis z
Reliability and Limitations
STEP and IGES import in FRI3D are considered best-effort features.
How STEP and IGES assemblies are imported:
- Every placed part becomes one item, at its position in the assembly, including parts placed several times and mirrored parts. Nested sub-assemblies are imported once.
- Geometry is converted to metres from the unit recorded in the file.
- Part names are kept; unnamed parts and loose faces are imported too.
⚠️ Limitations:
- Parametric data and feature-based modeling information are not preserved.
- Some STEP application protocols (APs) beyond AP203, AP214, and AP242 may have limited support.
- Results vary with the source CAD software and its export settings.
For building models, IFC format is recommended.
General Limitations
- There is no guaranteed semantic consistency across files — identical parts exported from different CAD tools may have entirely different structures.
- Many files include non-manifold geometry or incomplete topology that can lead to gaps or misclassified solids.
- IGES often lacks surface orientation and volume closure, requiring post-processing or manual cleanup.
- Attribute and material data are usually not preserved reliably.
Therefore, while FRI3D can import these formats for visualization and approximate geometric context, IFC import is recommended for robust and semantically consistent workflows.
Supported Versions
| Format | Supported Versions | Notes |
|---|---|---|
| STEP | ISO 10303-21 (AP203, AP214, AP242) | Best results with AP214 or AP242 files containing solid BREP definitions |
| IGES | Version 5.3 and later | Limited to surface and solid entity types only |