Clash detection that gets fixed: a coordination workflow

A clash report with 900 lines is not a coordination result. It is a list of things nobody has decided yet. Teams that fix clashes do three things differently: they agree the rules before modelling, they turn raw clashes into a short list of issues with one owner each, and they re-run the tests until the list is closed.

This article sets out a workflow that works on typical building projects, from the first model exchange to shop drawings.

Step 0: agree the rules before anyone models

Coordination fails early when disciplines work to different assumptions. Put these into the BIM execution plan (BEP) and get them signed by every discipline lead:

• Origin, units and levels. One shared coordinate system, one level naming scheme.
• Level of development per element. A duct modelled as a line cannot clash meaningfully. State the LOD for each element and each stage.
• Reserved space. Allocate corridors, shafts and ceiling zones to disciplines. Most clashes are two disciplines using the same space.
• Clash tolerances and clearances. Agree which tests include clearance for maintenance, insulation and access. Take the values from the specification, not from habit.
• Exchange dates. Models are published on fixed days, so the coordinator tests the same set every cycle.
• Who decides. Name the person who can tell a discipline to move.

The ISO 19650 series is the international framework for managing information on built-asset projects. Its guidance expects the appointing party to set information requirements and the delivery team to plan how information is produced and exchanged. Whatever the employer's BIM requirements are, write your BEP against them.

Step 1: check each model before you federate

Do not test unhealthy models. Before each cycle, every discipline runs its own checks:

• Elements on the correct level and workset
• No duplicate or overlapping elements
• No elements far from the origin
• Warnings reduced to an agreed number
• Linked files at the agreed revision

Federating a model with duplicates multiplies clashes. Ten minutes of checking saves hours of review.

Step 2: set up clash tests as pairs, with a purpose

Create tests as pairs of systems, not as one large "everything against everything" run. A typical set is:

Run hard clashes (geometry overlaps) and clearance clashes (an element enters a required buffer) as separate tests. They are different problems with different owners.

Step 3: group clashes by cause

Raw output of 900 clashes often comes from 40 root causes. A duct that crosses a row of beams creates a clash per beam. Group by level and zone, then by cause. Name each group with an issue title that describes the decision needed, such as "Level 3 corridor: supply duct vs transfer beam line C".

Aim for one issue per decision. This is the single biggest step in making clashes manageable.

Step 4: assign an owner and a due date

Each issue needs:

• A single responsible discipline
• A due date tied to the next model exchange
• A short instruction stating what is expected to change
• A status the whole team can read

BIM Collaboration Format (BCF) is the open buildingSMART standard for exchanging issues between BIM tools. A BCF topic can carry a viewpoint, a comment thread and a status, so each issue travels with its picture and is closed in the same place it was opened. The standard is more than the .bcf file: it also defines an API, so the same issue can be seen in different software.

Use common priority rules as a starting point and write them down: the structure is the reference, gravity drainage has priority over pressurised systems, large ducts move last. Agree your own list in the BEP, because the right rule depends on the building.

Step 5: run the coordination meeting on issues, not on clashes

Hold a short meeting each cycle with the discipline leads and the person who can decide. Review only open and new issues. For each, confirm the owner, the decision and the date. Leave the meeting with an updated register. If the meeting needs to look at geometry, use the viewpoint attached to the issue.

Two habits keep meetings short. First, issues that need no discussion are closed offline. Second, a discipline that does not attend accepts the decision taken.

Step 6: re-run, verify and close

After each exchange, run the same tests again. An issue closes when the clash no longer appears, the fix does not create a new problem in another discipline, and the change is recorded in the model. Close with evidence: a before-and-after viewpoint attached to the issue.

Track the open count by discipline and by age. Do not use the raw number of clashes as a measure of success. A large drop might just mean a test was switched off.

From coordination to shop drawings

Shop drawings should come from a model that has passed coordination, not from a model still in negotiation. Agree a coordination freeze for each zone. After that point:

• Sleeve and opening locations are issued from the coordinated model
• Hangers and supports can be designed against known routes
• Shop drawings are produced from the same model, so drawings and model match
• Late changes follow a change process with a stated cost and programme impact

Reinforcement follows the same logic. When the structure carries reinforcement in the model, bar bending schedules can be extracted rather than re-drawn, and the schedule updates when the model does.

Common mistakes

• Testing before models reach the agreed LOD
• Treating the clash count as a progress indicator
• No one with authority to make a decision at the meeting
• Closing issues without checking that the fix works in all disciplines
• Letting the coordination model and the issue register live in separate places

If you want a coordination plan for a specific building, send us the discipline list, the LOD requirement and the exchange dates, and we will propose a BEP and a cycle.

Sources

1. buildingSMART International, BIM Collaboration Format (BCF)
2. UK BIM Framework, Information management according to BS EN ISO 19650, Guidance Part 2