4D Straight From Your IFC File

Everyone who works with IFC has heard that the format carries more than geometry. Properties, classifications, relationships, materials. Fine.

Almost nobody mentions that it also carries time.

Not as a linked Primavera file. Not as an exported spreadsheet that has to be re-imported somewhere else. As proper entities, inside the same file as the geometry. Which means that if the data is there, you can watch the structure build itself – and you do not need a single commercial licence to do it.

This article walks the whole thing, start to finish, by clicking. Where the schedule sits in the file, how to play it back, how to build one from scratch if your model has none – and the three things that went wrong when I made this film.

Starting point – where the schedule actually sits in the file

Let us start with the thing that catches almost everybody the first time. You open an IFC file in a viewer, expand the tree, and there is no schedule anywhere. That is not a bug. It is not where you are looking.

The tree you see in a viewer is the spatial structure – project, site, facility, its parts, and the physical elements inside them. A schedule does not belong there, because a task is not a thing standing in space. It hangs off the project on a separate branch.

That is where the “there is nothing in the file” impression comes from. An ordinary viewer shows one branch and stops. To see the other one you need a tool that reads it – and that is what the rest of this article is about.

The links themselves are simple, but each one does a different job:

One direction is worth memorising, because it reads backwards from what you expect: in IfcRelAssignsToProduct the product is the physical element and the related objects are the tasks. You read it from the element to the task, not the other way round.

Part 1 – What 4D in IFC actually is

The most common misunderstanding goes like this: 4D is “the model plus a schedule sitting next to it”. It is not. There is no link to an external file, no ID matching, no shared spreadsheet. The schedule is in the file, as first-class objects, and the connection between a task and an element is an explicit relationship object – exactly like the one that puts a column inside a storey.

Five entity types carry the whole thing.

A task and its time

A task is deliberately thin. It holds a name, an identification code, a predefined type, and a pointer to a separate object that carries the dates. Two details are worth knowing, because they catch people out.

First, the predefined type – usually CONSTRUCTION – is not decoration. It says what kind of work this is: construction, demolition, logistics, installation. A 4D tool uses it to pick the colour. Leave it as NOTDEFINED and your demolition sequence animates in the same green as your new build.

Second, IfcTaskTime has twenty attributes and an animation needs three: duration, schedule start and schedule finish. The rest – actual start and finish, early and late dates, float, percentage complete – only matters once you compare against as-built.

Assigning tasks to elements

A schedule with no connection to geometry is just a schedule. The connection is IfcRelAssignsToProduct, and it is what turns dates into an animation. In the bridge file the assignments look like this:

Add it up and you get 51 elements. The bridge has 151. Hold that gap – we come back to it in Part 3.

Dependencies between tasks

Dependencies are their own objects. IfcRelSequence points from a preceding task to a following one and carries a type:

Each dependency can also carry a lag (IfcLagTime) – the offset between the two events. Concrete curing before striking formwork is a lag. None of the fourteen dependencies in this file have one, and in Part 3 you will see what that does to the animation.

Part 2 – Step by step in Bonsai

Bonsai – formerly BlenderBIM – is a free, open-source add-on that turns Blender into an IFC working environment. It reads the schedule natively and can animate the model directly from it. Everything below is clicking in the interface; the screenshots are from Blender 5.1 with Bonsai 0.8.5.

The numbers on each screenshot match the numbers in the list underneath it.

Step 1 – Install and load the model

Install Bonsai from Blender’s own extensions platform: Edit → Preferences → Get Extensions, search for Bonsai, install. Restart Blender. You get a new BIM tab in the top bar.

Then File → Open IFC Project and pick your file.

Step 2 – Open the scheduling panel

This is where most people get stuck, because it is not where you would look for it. The scheduling tools are not in the sidebar of the 3D viewport – they sit in the Properties editor, on the Scene tab.

1. 1
   Scene tab

   Everything Bonsai adds to the project lives here.

2. 2
   Dropdown → Costing and Scheduling

   Switches Bonsai to the 4D and 5D panel group.

3. 3
   Construction Scheduling → Work Schedules

   Expand it. You should see how many schedules were found and the name of the one in your file.

4. 4
   First of the four small icons

   This is the one that opens the task tree. The others are rename, duplicate and delete – none of them will open the schedule.

Step 3 – Set the date range and the speed

Once you are editing the schedule, the task tree appears, and below it a section called Animation Tools. It is collapsed by default and does not exist at all until a schedule is active – if you cannot find it, step 2 did not take.

1. 1
   Animation Tools

   Expand the section.

2. 2
   Animation Settings

   Animation mode. The neighbouring Snapshot Settings gives you one date instead of a sequence.

3. 3
   Guess

   Reads the earliest start and latest finish from the tasks and fills in the date range. Do not type the dates by hand.

4. 4
   Speed

   Pick Frame-based and read it as “this many frames buys one week of site time”. 12 frames per week turns this programme into roughly ten seconds of video.

Step 4 – Create the animation and watch it

Same panel, further down:

1. 5
   Create Animation

   Writes the keyframes onto the model. This is the button.

2. 6
   Add Camera

   Drops in a camera aimed at the model. You will want it for the render.

Press Spacebar to play. Dragging the timeline jumps to any date.

Step 5 – Render the film

Bonsai builds the animation in the scene but does not render it – that part is plain Blender. The right engine is Workbench: it draws object colour directly, which is exactly what Bonsai animated, and it is by far the fastest.

1. 1
   Render Engine → Workbench

   Renders object colour, with no materials or lighting to set up.

2. 2
   Color → Object

   Without this the render ignores the animated colours and you get uniform grey.

Step 6 – And if your file has no schedule at all

Everything above assumed the schedule was already in the file. If you have a bare export – geometry only – you build it in the same panel, in this order. Each step names the entity it creates.

Part 3 – The three things that broke

Everything above is the documented path. What follows is what actually happened when I made this film. None of the three announces itself – the animation just comes out wrong.

The general lesson is worth more than the fix itself: after you build an animation, count the elements that actually have a task. Not the ones the schedule mentions – the ones that got animated. If the two numbers differ, something is on screen at the wrong time.

This one is not the tool’s fault. This one is the schedule, and it is the most useful of the three, because it is the one you will hit on real projects.

All six sub-tasks have identical dates, and the dependencies between them are START_START with no lag. That says, precisely and correctly: these six things start at the same moment. So the animation shows six foundations appearing simultaneously. It is not wrong – it is a faithful rendering of a programme that says all the crews pour on the same Monday.

There is a second symptom in the same table. The sub-tasks occupy 5 days of a 25-day stage. Nothing is scheduled in the remaining three weeks, so the film holds a static image while the calendar burns through a month.

Two ways out, and they are genuinely different decisions:

1. Stagger the dates. If the work really does move along the bridge – A1, then P1, then P2 – the sub-task dates should say so. This is a planning fix, and it is the honest one.
2. Add lag. Keep START_START and give each dependency a few days of lag, so the crews follow each other rather than starting together.

Which brings out the real point of building a 4D animation in the first place.

A programme in a Gantt chart with everything starting on one date looks perfectly normal – six bars, neatly aligned, nobody blinks. The same programme as a film is instantly, physically implausible: six separate foundations, seventy metres apart, materialising in the same frame.

4D is not a presentation tool. It is a review tool. The animation is how you notice that the schedule says something nobody meant.

Try it on your own file – five checks

What this actually demonstrates

A twelve-second film is a nice thing to have. It is not the point.

The point is that a single IFC file carried the geometry, the classification, the quantities and the schedule of one bridge – and a free add-on read the schedule out of it and animated the structure without anyone retyping a date.

That only works when the information is in the model. Click Export to IFC on a model with no types, no quantities and no schedule, and you get a shape. Everything in this article would have had nothing to read.

If you want to go further than one example, the IFC Infrastructure Masterclass works through the whole delivery workflow across six workshops – objects, relationships, properties, classification and information structure, on real infrastructure models.