Using Building Information Modelling (BIM) on new buildings is easy to do – you are starting from scratch after all, and so you can make sure everything is laid out where it needs to be. But what if the building you are working on is an existing structure, such as a historical building, a renovation or an extension? Most of the things that are important to a structural engineer, in this case, are going to be hidden by existing features.  But the BIM model still needs to be created.

What does “existing BIM” mean?

Construction work on existing buildings can take many forms, such as:

  • Adapting a historic space for modern life
  • Refitting a space for a new tenant
  • Facilities management as soon as a new building is complete

This is why it is so important that at the start of any project, discussions take place to ensure all the trades involved understand the goals of the project and any limitations of the model.

How much of the existing building should be BIM modelled?

The original thinking behind this was that in the case of existing BIM, only the part of the building being worked on for that particular project should be modelled. But, in practice, it was found that this approach caused issues for the other disciplines if they had a wider scope – such as replacing the MEP systems throughout the entire building – as the structure they need to use for coordination just won’t be there. Or let’s say the scope of the project changes halfway through, which has been known, then you have to go back and start sifting through the original documents again.

So, it may be wise to model the entire building to ensure these sorts of problems do not arise, and to future-proof any further work on the same building, as your research effort will have been front-loaded and you don’t need to do it twice.

It is important to note that building owners tend not to ask for BIM, it is usually driven by the design team’s requirements. Coordination efforts are much easier if everyone is using 3D models, so you should do what you can to convince everyone that BIM should be used on all projects.

No project is really too small for BIM – projects of all scope can benefit from the use of BIM, both for their actual work and also in the cases of future work, or scope creep as noted earlier in this post.

Model element authorship

The architectural model is often the first building information model to be started, and the MEP and structural teams may request that they are therefore brought in on the model as early as possible so that they can ensure the elements are modelled to their own best practice. This is a good idea, as 3D models tend to turn out better if discipline experts are allowed to do their own modelling.

How to deal with uncertainty in existing building BIM

When it comes to existing buildings, there will always be some uncertainty, especially when it comes to MEP systems and structural elements as these are usually hidden inside architectural elements. So how do we handle this? We use LOD, LOA and LOC.

  • LOD (Level of Development) – not useful for existing elements as they are fully developed by definition – however, this does not mean that we know everything about them
  • LOA (Level of Accuracy) – this is a scale developed by the USIBD to set the accuracy of data gathering and for any elements that are modelled based on data that has been gathered
  • LOC (Level of Confidence) – some structural engineers are also working with this adaptable scale to describe how much information they have about a given element, in the simplest form LOC is a single digit:
    • 0 – assumed based on existing documentation or symmetry
    • 1 – identified based on non-destructive evaluations, such as radar
    • 2 – field-verified using a laser scan or probe

In general, precision levels should be set on a project-by-project basis as some buildings may be important enough to justify modelling small inch differences, whereas others can tolerate them. The age of the building being worked on is also relevant when it comes to deciding this, as buildings from the 1980s, for example, will be treated differently than those from the 1880s.

Existing documentation

In an ideal world, we would have all of the existing documents when we take on an existing building project, but more often than not, this is not the case – and even if we do get them they can be illegible or inaccurate. If the building is very old, technology has usually progressed faster than construction and so systems vary from floor to floor.

Laser scanning

There are many different ways to use point clouds as part of the design process:

  • Laser scan is overlaid on the model for verification
  • The laser scan is the starting point of the model and can be used as an initial deliverable to hand over to the design team to be used as the basis for new documents
  • Laser scan used as part of the final documentation to show connections to existing elements or supplemented with photographs

If you want to find out more about The CAD Room’s BIM for existing building services, or any of our other services, please get in touch.