Choosing an optimal level of details in a BIM-model is very important. Objects need to be modelled with enough details to be useful in clash control and understanding scope of work. At the same time, too many details will make model very large and software will start lagging and be difficult to control.

Post tensioning tendons and anchorages are important components in the Randselva bridge and are the reason why the slender design is possible. Due to the bridge curvature in plan view and the combination of two different structural systems, the post tensioning geometry is very complex, and the position of the components is not flexible. However, only the outer shape of the tendons geometry and anchorages is important to model correctly, as it will form the basis for clash control.

Randselva bridge contains over 200 tendons.

The steel strands and the inner geometry of the anchorages are taken care of by the company delivering the product and does not need to be modelled. An excerpt of the 200 tendons modelled at Randselva bridge is shown in the figure above.

In addition to the tendons, all structural reinforcement needed for the Randselva bridge project has been modelled. This gives a very good understanding of which clashes must specially be designed and fitted due to clashes and potential installation problems. All rebars in the BIM-model are however not clash free. A pragmatic approach is chosen where some clashes between rebars in the model are accepted as long as it is obvious that clashes can easily be adjusted at construction site. This has been thoroughly agreed with PNC Norge AS, the company responsible for the bridge works.

Randselva bridge contains over 200 000 rebars.

Another advantage of modelling all rebars is that reinforcement can be ordered directly from the BIM-model, eliminating the need for manually made bar bending schedules. In areas with heavy reinforcement and limited space like blisters for post tensioning anchorages, many projects traditionally produce 1:1 mock-ups at worksite to test constructability. For the Randselva bridge project, these mock-ups have been produced digitally and have proven to be a very efficient and cost-effective way of engineering.

In particular, due to the bridge curvature each blister position and corresponding reinforcement are almost unique. In the BIM-model, local adjustments on the general and blisters reinforcement were made for every blister. This would traditionally require a great amount of specific drawings to avoid extra work at site. A comparison between a mock-up assembled at worksite and a digital mock-up developed for Randselva bridge is presented in the figure below.

Digital mock-ups have replaced traditional 1:1 construction-site mock-ups.