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- In BIMpedia
Prefabrication moves work from the site into a factory. It ranges from components (a pipe spool, a precast stair) through assemblies (a riser section, a plant skid, a bathroom pod) to whole volumetric modules, rooms built and finished off site. The benefits are real: faster, safer and more consistent work. The catch for the BIM person is that decisions move earlier, and the model must describe the parts as they will be made and delivered.
- Components: single parts made off site (precast beams, pipe spools).
- Assemblies: several parts joined off site and installed as one (riser sections, plant skids, bathroom pods).
- Volumetric modules: complete, finished room-sized units, stacked and connected on site.
The services riser is designed as one model per discipline. The contractor decides late to prefabricate it in three-storey sections. The factory cuts the model into sections itself; nobody checks their weight against the tower crane's capacity at that radius. On the day, the top section is too heavy for the lift. It goes back to the factory to be split.
The decision to prefabricate is taken at the design stage. The model is split into the sections that will be delivered, each with its weight and lifting points. The 4D sequence shows each lift, and the logistics plan checks the crane's capacity at the radius of each one before the factory starts.
What changes in the model
| Topic | Site-built | Prefabricated |
|---|---|---|
| When design is fixed | Before the trade starts on site | Before the factory starts, often months earlier |
| How the model is split | By discipline and zone | Also by module, section or pod, as made |
| What each part carries | Type, size, system | Also weight, lifting points, part number, delivery slot |
| Main checks | Clashes, quantities | Also transport, lifting, route to position, tolerances at interfaces |
A clinic riser runs six storeys (3.6 m each). The crane can lift 4.0 t at the riser's radius. Each storey of riser (frame and services) weighs about 1.5 t. The contractor proposes two sections of three storeys.
How many sections?
Six storeys of riser at about 1.5 t per storey; the crane lifts 4.0 t at that radius. Sections must be equal and as few as possible. How many sections?
Key takeaways
- Prefabrication moves decisions earlier: the design must be fixed before the factory starts, not before the site does.
- Split the model the way the parts will be made and delivered: by module, by riser section, by pod.
- Check the route as well as the part: transport, crane, the path through the building.
- Interfaces between modules and site-built work need the most attention.
Your notes
Related tool
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