Clash detection vs constructability review, and what each one finds

Clash detection vs constructability review is not a comparison of two methods doing the same job. One tests whether two objects occupy the same space. The other tests whether every object in the drawings appears in somebody contract. A model can pass the first perfectly and fail the second.

What clash detection actually does

Clash detection compares geometry between models. A duct passes through a beam. A sprinkler main occupies the same space as a cable tray. A door swing hits a column.

It is automated, it is fast, and it is very good at what it does. A coordination process running clash detection weekly will find spatial conflicts a human reviewer would miss, and it will find them in the thousands.

It produces a clash report, the report is worked through, and by the time construction starts the count is near zero. That is a genuine achievement and it prevents a great deal of rework.

What it establishes is that the building can be built as modeled. That is a necessary condition and it is frequently mistaken for a sufficient one.

The clash report says nothing about who is buying any of the objects in the model, and that is a different question with different consequences.

Clash detection vs constructability review, side by side

What is compared. Clash detection compares model against model. A constructability and interface review compares scope descriptions against drawings, and scope descriptions against each other.

What a failure looks like. A clash is two objects in one place. A scope gap is one object in nobody contract. The first is visible to software. The second is invisible to it, because the object exists exactly once in the model and is perfectly coordinated.

Who runs it. Clash detection is run by the coordination team, usually the contractor virtual design group. A commercial interface reading is run by somebody reading contracts, which is a different skill and usually a different person.

When it happens. Clash detection runs continuously through coordination, largely after the trade packages are awarded. An interface reading is worth most before award, when a gap is a clarification rather than a change order.

What it costs to fix a finding. A clash found in coordination costs a redraw. A gap found after award costs a change order plus whatever the sequence disruption is worth.

Why a clean clash report can sit above an expensive gap

Take the embed plates that support a curtain wall. They appear once in the structural model, in the correct location, at the correct spacing. No clash.

The structural trade package includes cast in place concrete and the embeds shown on the structural drawings. The curtain wall package includes the unit, the anchor and the installation. Neither scope description mentions who supplies embeds at the tolerance the curtain wall requires, because at the time the packages were written the curtain wall shop drawings did not exist.

The model is coordinated. The work is unbought. Both statements are true simultaneously and only one of them is tested by software.

The same pattern repeats at waterproofing transitions, at fireproofing patched after a penetration, at owner furnished equipment connections and at the ceiling access panel condition. These are the places where the drawings are correct and the contracts are silent. The full list is set out in scope gaps between trade packages.

What each process is worth

This is not an argument that modeling is overrated. Coordination prevents rework that used to be routine, and a project without it pays for that in the field.

The argument is about what a clean report entitles you to conclude. A coordination report with a low clash count tells you the geometry works. It does not tell you the scope is bought, and an owner who reads the second meaning into the first has stopped asking the question that costs money.

The two also answer to different parties. Coordination is run for the builder benefit, to make the work buildable. An interface reading is run for the owner benefit, to establish whether the price covers everything drawn. Those interests overlap substantially and not completely.

The practical position is that both are worth doing, they find different things, and the commercial one is worth doing earlier because its findings are cheap to act on for a shorter window.

There is also a question of who is looking. The people running coordination are looking for things that cannot be built. They are extremely good at that and they are not asked to check whether a scope description covers what they are coordinating, because it is not their document and not their contract.

The people who own the scope descriptions are in preconstruction and purchasing, and they wrote each package separately, often weeks apart, sometimes from a template used on a different building. Nobody in that chain has the two adjacent descriptions and the drawing in front of them at the same moment.

That is the whole finding, and it is organizational rather than technical. The gap survives because it sits in the space between two competent functions rather than inside either one of them.

A worked example

A five story building. Coordination runs for four months and closes 2,340 clashes, finishing at eleven open items, all minor. The report is circulated and is genuinely good work.

The mechanical package includes ductwork, equipment and hangers. The structural package includes the frame. Neither includes the supplementary steel required to support four rooftop units at the locations the coordinated model shows, because the structural drawings showed a generic support detail and the mechanical scope description excluded structural steel by name.

The supplementary steel is drawn. It is coordinated. It clashes with nothing. It is in nobody scope of work.

It surfaces in month fourteen when the mechanical subcontractor declines to install units with no support in its contract. The change order is $186,000 including the sequence disruption, against a fabrication cost of roughly $54,000 had it been in the structural package from the start.

The difference is not the steel. It is the crane time, the resequencing of the roofing work behind it, and the fact that a fabricator quoting a small package for a job already under way is quoting a price nobody is competing with.

Read the scope descriptions against the coordinated model before either package is awarded and the same item is two sentences added to the structural scope, at bid pricing, with no crane and no argument.

What to ask for

1. The scope matrix, showing every trade package against every building system.
2. The full scope of work for each package at the six major interfaces, not a summary.
3. Written confirmation of who owns each interface, by package name.
4. The exclusions list for each package, read against the drawings rather than against a template.
5. The clash report, understood as evidence about geometry and nothing else.
6. The list of owner furnished equipment with the connection scope named for each item.
7. The closeout scope: training, spare parts, attic stock, as built drawings and manuals.

Where a scope matrix does not exist, that absence is the finding. It means nobody has looked at the interfaces as a set, and the software that everybody trusts was never testing for them. The scoping options are in the review packages.

What we do

We read the trade package scope descriptions against the drawings and write down what nobody has claimed. Each finding names the two packages, the drawing reference and the likely exposure in dollars. We do not run the model, we do not redesign anything and we do not duplicate the coordination process. It is a commercial reading of documents, and it is the constructability and interface review.

Questions people ask