Thursday, March 18, 2010

BIM Serial 003: From a computer model to the real building

So far we have discussed the BIM concept in general and what does BIM mean to a consultant or an engineering office. Now it is time to make the move from the computer model to the real world.

Modelled to the last detail

In a typical construction process, the engineer is responsible for the conceptual design covering the materials to be used, their amounts, sizes and general arrangement. In other words: construction documents. However, before we can start actual fabrication or construction, somebody still has to define all the details like exact reinforcing bar arrangement, connection details for the structural parts, details for the HVAC etc. Not surprisingly this phase is called detailing. A detailer adds to the model all the bolts, stiffener plates, creates rebar pending schedules, and may also utilise building information models from different disciplines to detail, for example, connections between HVAC and structural elements.

Once the detailing has been completed, the model looks “as-built,” including even the smallest details. Since the model becomes very much like the building to be built, some issues to affect its construction will most probably become noticeable. This is one of the main benefits of BIM: these issues are found out in the model and not at the construction site, which often happens in a conventional drawing-based process.

A very important criteria for selecting a BIM application for fabrication and construction is to make sure that it is capable of holding all necessary building details. The application should not become slow or its data files impractically big no matter how complicated or large a building is.

From the model to the real world

So now the building is nicely modelled to the last detail. All the constructability issues within and between disciplines have been solved. It is time to start building for real.

The traditional process is to print out hundreds or thousands of drawings and deliver those to the contractor. The main contractor then uses these printed documents to build the building either at the constructions site or using various subcontractors like fabricators of pre-casters.

The BIM process aims to make the design and the construction more integrated:

1) In addition to the drawings, the model should be one – and actually the most important – deliverable from the consultant to the contractor. Let’s leave the legal and responsibility issues aside for the time being.

2) The 3D BIM environment should be used as a basis for all communication between the consultant and the contractor. It is much easier than looking by 2D drawings.

A modern fabrication facility includes computerised (CNC) fabrication machinery and IT-based material management. A BIM must integrate with those to avoid “print out and key it,” as far as possible. CNC data shall flow directly from the model to the controls of the fabrication machinery. The same applies to the information transfer between the model and the material management and procurement systems. Every “print out and key in” phase creates a risk to make mistakes. Another big benefit of BIM.
BIM integration does not end at fabrication. Information shall also flow directly between the model and construction site equipment used to measure and control the physical locations. Let’s return to the work on site in more detail in the next issue of Roof&Facade.

Ari Nassi@Tekla

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