Same bridge, three IFC files: which one will your client send back?

Two renders of the same 87 m concrete bridge. Same geometry to the millimetre, same colours, same camera. One of them will come back from the client with corrections. Nothing on the screen tells you which one.

I exported this bridge (BR-01, a real Tekla model) three ways and read the files side by side. This article is the written version of that comparison: the numbers, the real lines from the files, five searches you can run on your own model in a text editor, and two mistakes I found in my own “best” file.

Read the last column twice. The file that knows nothing is three times larger than the file that knows everything. We will see why.

1 · Five searches in a text editor

An IFC file in STEP format is plain text. You do not need any software to start checking it. Open it in Notepad++, VS Code or any editor and count these five words:

2 · Four questions to the same bridge

A client asks four questions without thinking. Each answer lives at a different address in the file.

What is this object?

The same pier P1, in the three files:

01  #92=IfcBuildingElementProxy('1Iw7Gbn7TFke4P2HpYRTgJ',#5,'Solid',$,$,#90,#91,$,$)
02  #201=IfcColumn('0orICgdHPSlR49FPc2z3O2',...,'Pier wall P1',...,'Pier wall',...,'PW-P1')
03  #252=IfcColumn('0orICgdHPSlR49FPc2z3O2',...,'Pier wall P1',...,'PW-P1',.PIERSTEM.)

File 01 says “Solid”. File 02 says column, and “Pier wall” is free text that a machine can only guess at. File 03 says it in the schema: PIERSTEM. Notice the GlobalId too: identical in 02 and 03, random in 01. Random GlobalIds break every issue list and every comparison between two versions of a model.

The bearings show the limit of IFC 2×3. The best it can do is a generic IfcDiscreteAccessory with a label. IFC 4.3 has IfcBearing with a predefined type (here ELASTOMERIC).

Where does it belong?

Open the tree. In the IFC 2×3 file the bridge is an IfcBuilding and pier P1 is an IfcBuildingStorey. It works, and it reads wrong. In IFC 4.3 it is IfcBridge → IfcBridgePart SUBSTRUCTURE → IfcBridgePart PIER 'P1', and four physical elements (the pier, its footing, blinding and stone bed) are contained in that part.

More on this in Your road is living in a building storey.

Where along the bridge?

IFC 2×3 has no alignment. So the station of P1 is a number somebody typed into a property set:

02  #11315=IfcPropertySingleValue('Station',$,IfcLengthMeasure(21.826),$)

In IFC 4.3 the station is a position on a real IfcAlignment: a horizontal and a vertical layout with real segments, an IfcReferent carrying Pset_Stationing, and IfcRelPositions tying pier P1 to station 0+021.826.

Where on Earth?

This was the result that surprised everybody. File 01 has no georeferencing. Instead, the national grid coordinates are baked into every vertex, in millimetres:

01  #37=IfcCartesianPoint((145717840.5,6566197332.7,63000.))

Many viewers and engines draw in 32-bit floats, roughly seven significant digits. At 6,566,197,333 mm the smallest step they can represent is about half a metre. The geometry jitters, clash tests lie, federation breaks, and the screenshot still looks perfect.

In IFC 4.3 the offset lives once, at file level, and the geometry stays small:

03  #46=IfcMapConversion(#33,#45,145700.,6566100.,0.,1.,0.,1.)
    #45=IfcProjectedCRS('EPSG:5110',...,'NN2000',...)

And the file size? File 03 replaces thousands of triangles with extrusions and defines one railing post once for 98 posts. Geometry entities drop by 95 %. More information, a smaller file.

3 · Even a “best” file hides mistakes

4 · Five steps before you deliver

1. Understand. Write down what the client must find in the file, and where. That list is your future IDS.
2. Structure. Decide the facility, the parts, the classes and the predefined types in the authoring tool. The exporter cannot invent them.
3. Export. Same geometry, different mapping: schema, classes, properties, units, coordinates, GlobalIds.
4. Inspect. Open the file. The five searches above take two minutes.
5. Verify. Three gates: the schema, the buildingSMART Validation Service, and your IDS. A valid file is not yet a complete file.

IFC 4.3 will not clean a messy model. It removes the excuse: infrastructure finally has the vocabulary. What you say with it is still your decision.