BIM fundamentals

The "B" places the method in construction, the "I" puts information at the centre. A BIM model is therefore not just a 3D model but a model of building elements with geometry and linked information. The information itself is mostly the same as in conventional design. What is new is that it can be derived, such as quantities from the elements, and linked, such as elements with the schedule. The "M" has four meanings:²

Software is the tool, not the goal. What matters is that it is agreed beforehand who delivers which information when and in what quality, and that everyone works on the same, current data.

A building model consists of discipline models, for instance for architecture, structure and building services, created by different designers with different software. For clash detection and overall views they are temporarily brought together in a coordination model.³ At the end comes the as-built model with all changes from construction; for operation it becomes the operator model.⁴

Depending on the use case the model is extended by further dimensions:⁵

In conventional delivery part of the digital information is lost at every handover: from design to tender, to construction, into operation. What is missing is entered again, with effort and errors. BIM is meant to close these gaps so that the information can be used throughout.⁶

"Build digitally first, then for real".¹ Following this principle, effort shifts to the early phases, where the influence on cost is greatest and a change is cheapest. Design takes more effort, but uncertainties show up in the model rather than on site. Reacting to design changes turns into acting ahead.⁷

Guideline VDI 2552 Part 1 describes the introduction of BIM in performance levels, after the British maturity model: from cooperation with individually exchanged files through collaboration with standardised processes to the integration of all data in the organisation.³

A BIM model is not built in stock. The client first has to know which information they need. The DIN EN ISO 19650 series derives this in stages: from the organisation’s goals the requirements for an asset and for a project, and from these the contractual requirements for the appointed parties.^{8, 9}

The diagram follows the presentation in Markus Boden’s bachelor’s thesis.¹⁰ Two documents matter most in the project. The exchange information requirements (EIR) are the BIM specification in the contract: they set which information is delivered in what quality, level of detail and currency, not how. The BIM execution plan (BEP) is the appointed parties’ answer on how they meet the requirements; it grows with the project.¹¹

On a drawing the scale tells you how precisely something is meant. A BIM model is always 1:1, so the level of detail has to be set explicitly. In practice it is called the level of development (LOD), in stages from LOD 100 to LOD 500, based on the phases from conceptual design to the record of the finished building; the phases are meant as guidance, not as a rule.⁸ Between two data drops the level of detail (LoD) grows step by step, from geometry (level of geometry, LoG) and information (level of information, LoI).³ What is actually needed at a given time is now called the level of information need (LOIN).¹²

For operation LOD 600 is added: the as-built model, enriched with maintenance, repairs and asset histories; the geometry may be coarser than at LOD 500. Which element reaches which stage at which milestone is recorded in a model development matrix, structured for instance by the cost groups of DIN 276.⁴

The common data environment (CDE) is the platform on which graphical and non-graphical information is collected, organised and exchanged centrally. It sets responsibilities and delivery processes, avoids duplicate data and makes every change traceable.¹³ Every piece of information carries a status there. That way it is always clear whether you are looking at a draft, a coordinated or a binding version.⁹

BIM is introduced step by step. Two questions place how far: does one designer use BIM only for their own task (little BIM), or does everyone work on the model across all phases (big BIM)? And does everything stay in one vendor’s software (closed BIM), or do open formats run between different programs (open BIM)?⁶

The goal is big open BIM, because a lack of continuity costs money. A study by the US National Institute of Standards and Technology put the extra cost of inadequate interoperability in the US capital facilities industry at USD 15.8 billion in 2002; about two thirds were borne by owners and operators, almost 60 per cent arose in operation.¹⁴

The open formats are developed by buildingSMART, founded in the early 1990s as the International Alliance for Interoperability: IFC for the models, plus standards that set which information a given exchange needs, how terms are named consistently and how to coordinate issues in the model.^{6, 8} More in openBIM formats: IFC, BCF and IDS.