Most of what gets written about BIM is written about buildings. Structural frames, foundations, towers with forty floors of services to sort out. Interior fit-out is a different problem, and I think the case for modelling it gets explained badly as a result.
On a fit-out you inherit a building. The slab is where it is. The core is where it is. Your floor-to-ceiling height is whatever the previous tenant and the base build left you. Nothing about the shell is yours to move, so the coordination problem is not about the structure at all. It happens in the ceiling.
The two hundred millimetres nobody drew
Think about what has to live in a typical ceiling zone. Supply and return ducting. Sprinkler pipework with its own gradient rules. Light fittings, which need a specific depth above the ceiling line. Cable trays. Fire detection. Access panels, which have to be reachable and are usually remembered last. And then the architectural intent on top of all of it, a cove, a drop, a shadow gap that only works if the ceiling is flat at exactly that point.
Here is the part that catches people out. Every one of those drawings can be correct on its own. The electrical layout is right. The HVAC layout is right. The reflected ceiling plan is right. Each consultant has done their job properly. The problem only appears when you put all four on top of each other and discover that three of them want the same space.
That is what a coordinated model does. Not renders, not walkthroughs, not a spinning 3D view for a presentation. It puts every layer in the same space and shows you where they collide, while everything is still on a screen and nobody has ordered anything.
Finding it early is a different kind of cheap
There is a real difference between a clash found in the office and a clash found on site.
In the office it is an afternoon. Somebody notices the sprinkler main and the supply duct are fighting for the same run, the ceiling drops sixty millimetres in one corridor, a light is moved half a metre, and the drawing is reissued. Nobody outside the design team ever knows it happened.
On site it is a stop. The ceiling contractor is already on the floor with material. Now you need a revised drawing, a decision from the client about a visible ceiling drop they never agreed to, possibly a variation, and a trade rescheduled around work that has not finished. The cost is rarely the material. It is the day everyone stood still.
A model also does quieter useful things. When a layout changes, the quantities change with it, which makes a revised BOQ something you check rather than something you rebuild. And it gives the design team and the site team the same picture to argue over, which removes a surprising amount of email.
What it does not do
I would be careful about promising too much here.
A model does not improve a design. It will coordinate a badly planned floor to three decimal places quite happily. It does not replace a site survey either, and on a fit-out the survey matters more than usual, because you are modelling a building somebody else built. What was drawn in the base build and what actually got constructed are often two different things. If you model from the landlord's old drawings rather than from a measured survey, the model will be confidently wrong.
Tolerances still exist, and existing conditions are still discovered by somebody on a ladder with a torch.
It also costs time up front, at exactly the point when everyone wants drawings issued. That cost is real. It gets repaid on the projects where it prevents a stop, and not on the ones where it would not have.
Where we stand
We do not currently model our fit-outs in BIM. We coordinate services the way most practices of our size do, in two dimensions, in a room, with the people who will actually install it. On a small floor plate that works.
It gets weaker as scope grows, which is why we are working towards it rather than defending the way we do it now. When it becomes part of how we deliver, this page will say so, and it will say what changed.
What I would not do is claim it early because the word tests well. If a studio tells you they work in BIM, ask them to send you the model.
The honest summary is simpler than the software makes it sound. BIM moves decisions off the site and into the planning stage, where a mistake costs an afternoon instead of a week.
- BIM
- Coordination
- Fit-out
