Offsite’s tolerance challenge

29 February 2020 | By Mark Boyle and Giorgio Bianchi

Adopters of DfMA should understand that a different tolerance strategy is required from traditional construction. Mark Boyle, Giorgio Bianchi and Katerina Vatti explain.

Offsite construction and design for manufacture and assembly (DfMA) are  very much in vogue, as government and clients look for quicker and better quality ways to deliver projects. But this approach does present a major engineering challenge that is not always acknowledged: how to accommodate tolerances in all elements of the building project.

With traditional construction, tolerance issues are handled element by element, and there is the opportunity to control and correct them as the building goes up.

But with offsite manufacture, each element is much larger. The points where tolerances can be controlled are far fewer and, if not properly understood and managed, tolerance problems can rapidly accumulate, potentially leading to serious time delays and cost overruns.

For tolerances on offsite projects, there should be a strategy to predict, monitor and correct where necessary. This should provide a means to check and adjust as elements are manufactured and again as they are assembled.

Compared to other industries, where industrialisation of production was introduced many years ago, construction has less experience in tolerance management. Manufacturers and designers often overestimate the tolerances that can be achieved in current manufacturing environments or hardly consider tolerances during the design process.

A closer and more open relationship between designers and the manufacturing world is required. We need to have honest conversations about the tolerances that can be reasonably achieved and agree a strategy to monitor production and assembly and make corrections both off site and again during assembly on site.

Prototyping would also help, especially with the new and innovative solutions that are coming into the industry with DfMA systems. The manufacturer builds a prototype unit, then, with the engineers in the factory, sees how easily it fits together, and measures and monitors the tolerances so that it informs the final design.

This process helps to identify critical areas and revises the design to include more tolerance where required, ironing out problems before they go on site, when it is often too late to fix them. Fortunately, many clients now understand the importance of prototyping to reduce risks and to save cost and time.

Collaboration is the key word here. At an early stage, the construction and manufacturing teams should agree a project-specific tolerance strategy that everyone around the table is confident of achieving.

This is even more important when different materials are involved. Concrete naturally sets into the shape you want it to, but it does shrink with time. Steel can be machined to good tolerances, but the high-temperature manufacturing process means steel elements are often not straight and have a slight curve which should be designed for. Timber can also achieve good tolerances, but again has a natural slight curve and, because it absorbs water, will shrink like concrete.

Each of these materials have different needs and different approaches to managing tolerances.

As building taller becomes more common, the need for tolerance management will also become more important. Prefabricated high-rise buildings are increasingly being constructed or are at design stage. Here, tolerance strategy is even more vital, because risk of tolerance accumulation is greater than elsewhere. Other design features common to high-rise, such as transfer structures, may also lead to tolerance accumulation that requires a specific mitigation strategy.

If offsite and DfMA projects are properly planned and executed, with a tolerance strategy built in, all should go well. If tolerances are left to chance, then the prospect of delivering better quality buildings more quickly will disappear.

Mark Boyle and Giorgio Bianchi are directors and Katerina Vatti is an associate at Robert Bird Group

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