Digital design gives us extraordinary levels of precision. The building site is a little less accommodating.

On screen, everything can line up perfectly. A precast wall meets a floor slab exactly where it should. A façade panel sits neatly against the structure. Openings align. Connections fit. Services pass through the spaces allocated to them.

Then construction starts. Every element of a real building is manufactured, positioned and installed within an allowable tolerance. Individually, those variations may be small. The challenge comes when different elements, structures and trades meet. That is where a few millimetres in several different places can suddenly become a much bigger issue.

Tolerance is part of the design
Precast manufacture offers a high level of dimensional control, but that does not remove tolerance from a project. Nor should it. The important question is how those tolerances have been considered across the complete building.

A precast panel does not connect to a digital model. It connects to an actual slab edge, steel frame, in-situ core, adjoining panel, window system, balcony connection or another part of the structure. Each comes with its own fabrication, construction or installation tolerances. The interface between them needs to accommodate that reality.

For example, a façade may be manufactured accurately within its specified tolerances, while the structure behind it sits slightly differently from its nominal position. The individual components can both be acceptable, yet the connection between them still needs sufficient adjustment to bring everything together correctly. That is why tolerances should never be treated simply as numbers buried within specifications.

They need to be designed into the building.

The interface is often where the risk sits
The individual precast element is rarely the complicated part. It is what happens around it. Bearing areas need sufficient allowance. Connections need appropriate adjustment. Joints have to work aesthetically and technically. Openings need to coordinate with windows, doors and services. Façade panels need to relate correctly to the structure behind them.

And all of this has to work while allowing for construction sequence, access and installation. This makes the interfaces between systems one of the most important areas for the project team to resolve.

Rather than asking only: “Does it fit?” The better questions are:

What happens if this element is at one end of its tolerance and the adjoining element is at the other?
Where is adjustment built into the detail?
Which element controls the setting-out?
Who owns the interface between the two systems?

Those questions are considerably easier to answer during design than when a crane is waiting on site.

Precision still needs flexibility
There can sometimes be a temptation to assume that greater factory precision means less allowance is needed at connections and interfaces. In practice, good detailing often does the opposite. The objective is not simply to make everything dimensionally exact. It is to understand where variation will occur and design enough flexibility into the connections and interfaces to accommodate it.

That might mean an adjustable fixing, an appropriate joint width, sufficient bearing, a defined tolerance zone or simply a clearer definition of responsibility between different design packages. None of these things is particularly dramatic. Which is rather the point.

When tolerances and interfaces have been properly considered, nobody notices them. When they haven’t, everybody does.

Designing for the building, not only the model
Digital modelling has transformed how construction projects are coordinated. But the model should not create a false sense that every element will occupy one mathematically perfect position when construction begins. The real value of digital coordination is in helping the project team understand how the building will actually come together, including the movement, tolerances, connections and interfaces that exist between its component parts.

For precast construction, that thinking is particularly important because many decisions are transferred from the site into the design and manufacturing process. Once production begins, changing those decisions becomes considerably more difficult.

So the objective is not simply to create a model in which everything fits.
It is to design a building that still fits when the model meets reality.

Bringing precast expertise into the design
At O’Reilly Precast, our design work goes beyond the precast element itself. It includes how elements connect, bear, align and ultimately come together on site.
Our in-house design and technical teams work with architects, structural engineers, contractors and project teams across the design, detailing, manufacture and installation process, including the development of individual elements and their connections.

That gives us a practical view of how decisions made on screen eventually translate into components that have to be manufactured, transported and assembled on site.
For projects considering structural precast or architectural precast façades, involving the precast team while key interfaces are still being developed provides the opportunity to review tolerances, connections, sequencing and buildability before those details become fixed.

Because ultimately, a successful precast design isn’t the one that looks perfect in the model.
It’s the one that works when it reaches site.

Talk to O’Reilly Precast early about your next project.