Expert interview on the topic of BIM: Michael Gehrlein talks about the difficulties and opportunities of Building Information Modelling.

Mr Gehrlein, how would you define BIM in a nutshell?

BIM is the computer-based collection of data on all the resources used in a building by all companies involved in the construction to form a data model. Performance and load calculations, drawings and specifications (dimensions) need to operate bidirectionally with the data model to make changes to the design with less effort, cost-effectively and without transmission errors.

BIM is on everyone's lips – but somehow it still doesn’t seem that popular. It seems to overwhelm many people. Why?

Workflows in design have hardly changed for decades. 2D drawing creation and mass compilations are common, but so is the habit of manually annotating drawings with calculation data and then writing the dimensions in a specification. The 3D CAD systems that emerged at the turn of the millennium are still only being systematically used by half of all building services consulting engineers – due to more exacting user requirements and greater processing complexity. BIM requires completely new approaches, disciplined processes and, above all, 3D. Associated with this are new data formats and data interfaces – for example, IFC or gbXML – which in most cases require moving to a new generation of software, which in turn needs first to be learned. Technical resources associated with the exchange of pure drawing data, such as two-dimensional DWG and PDF documents, which have been used for many years, are not BIM-compliant due to the lower quality information content and will fall by the wayside. So now in order to answer your question: apart from extensive software changes and further training requirements, over the coming years there also needs to be a recognition of the potential for a fundamentally new experience. The costs for this are difficult to assess. There is also doubt about managing this in ongoing project business.

What do you recommend to architects and consulting engineers who are new to the idea of adopting the issue?

The roots of BIM lie in the creation of a digital, three-dimensional building model. This applies in equal measure to the architect, structural engineer and building services engineer of a joint project. As already mentioned, this requires a complex transition for all employees of any project team who are involved. They need to learn to use a fundamentally new software. Based on my previous experience, there first needs to be a shift in the imagination of consulting engineers about what BIM actually is. You should be aware that the introduction process needs a completely new mindset. People are much more advanced with this in the UK. There are virtually no tenders or specifications these days on the island without BIM.

Why is a technological country like Germany lagging behind?

"Germany is a land of poets and thinkers. A new rule, like the Ten Commandments, will be set in stone and will not be changed". I heard this sentence from a Norwegian, who had been working with BIM in building design for over ten years. I do not want to do a disservice to the work of our architects’ and consulting engineering associations at this juncture. In my opinion, the reason for this is the fact that our fee structure (HOAI) only provides for the provision of 3D/4D building models in a single sentence under "Special services” and that is not even legally binding. And until we change this ruling in Germany, that is to say change our fee structure to become BIM-compliant, then the majority of consulting engineers and developers will avoid using BIM.

And by when do you think BIM will have become accepted throughout Germany?

The Federal Ministry of Transport and Infrastructure wishes to make the use of BIM binding above a total construction cost of €5 million from 2020. Some corporations already specify the use of BIM for their own new builds and renovations. This means that not even the most experienced consulting engineers can take part in a tender if they are not yet capable of working with BIM. As a result, more and more consulting engineers will have to work with BIM to remain competitive. As experience of BIM grows, in future projects may possibly be worked with lower construction costs, as many companies will recognise the benefits after they have overcome the “teething problems”. Moreover, I expect the HOAI fee structure will also be adapted accordingly in the foreseeable future. I could therefore imagine that, with the given circumstances, by 2030 BIM will have prevailed for all construction projects bigger than the size of a 4-apartment block.
What does the BIM file for a single product look like? What does it contain?

Michael Gehrlein

The person

Michael Gehrlein is a CAD and building services expert alike and, as such, is exactly the right person for us in terms of BIM. He is, among other things, a technical draughtsman and state-certified heating and air conditioning technology engineer. For six years Gehrlein worked in the building services design department of a leading industrial company handling major projects. He then had a further six years as team leader in an engineering company with nationwide projects. Since 2004, Gehrlein has been working freelance as a service provider for the efficient introduction and use of computer-aided design in building services. More information at

Let’s take a Kampmann trench heater. Apart from its geometry, a file contains all the requisite drawings, performance and connection data. Compiled into a BIM model, we thereby learn the dimensions that can be directly transferred into a program to generate a specification. We can tell from the requisite volumetric flow whether the connected pipes are correctly sized and meet the heat load. If the size of a room or the number of convectors changes, this can be identified in real time on the objects dependent on it and you can react to it. By comparison, up to now a geometric body or a 2D block has been used that contains hardly any information. Any text about the product was written by hand into the CAD drawing and needed to be explicitly taken into account in the event of changes.

And you can then build an entire digital house from the individual BIM data from thousands of structural elements?

Yes, of course - if you can do so! All the objects contained in the model are subdivided into model categories, have dependencies on each other, can be filtered and evaluated. Today's software technologies provide for extremely passable modelling of all sorts of objects. However, when designing, you have to handle the data systematically and responsibly as any design errors and incomplete designs can be simply checked and identified in a data model, unlike the CAD plans used to date.

Sophisticated BIM systems permit collaboration: literally. That is to say that different parties, different disciplines can all access a central model. If a party has changed something – how to the other participants see it?

In my view, real collaboration is only possible when all companies involved in a project work on a single data model or with a link to different data models. Currently this is only possible if they are all working with the same software. You can identify changes made by one or other party in that, for example, dependencies of your own objects to the objects changed by the other party can no longer be maintained and the software therefore issues an error message. As the IFC format only serves as a transfer medium for importing and exporting the different CAD/CAE systems, changes to a newly imported BIM model may need to be determined by a collision check. Nevertheless, as before, there should be personal communication between all the companies involved in the design and all changes discussed.

When the building is finished, can I then file the digital building model as “done”?

No – that is the beauty of the system. Up to now design statuses and revision documents disappear into the archive after the build. By contrast, BIM data should continue to be used! There is a significant amount of administrative work for facilities management teams, particularly with industrial buildings. The locking system for often several hundred doors often needs to be administered. Building cleaning services and building operators want to know how many square metres of floor need to be washed and vacuumed. How many windows need to be cleaned? Which lamp is meant for which room? Are ladders high enough to change the light bulbs? Where are the smoke detectors and fire dampers to be serviced located? And so on ... This can all be seen in the BIM data. Renovations occur after several years: walls are moved, which might also mean that pipes need to be re-routed and re-calculated. How much waste will be produced and what materials needs to be disposed of properly? There are costs behind all these figures, which can be precisely determined using BIM data.

To sum up – what are ultimately the advantages that BIM offers for construction projects?

More efficient planning processes and the avoidance of design errors by “the software co-thinking”. Faster implementation of changes made by clients during design phases, with the software independently taking into account dependencies to adjacent areas. Exact calculation of the dimensions of all equipment and elements. All these things contribute a significant amount to design, construction and operating costs.

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