Isn't BIM just 3D CAD?

In a follow-up to his article BIM: what is it, and how does it affect you? published last month Dr Stephen Hamil, Director of Design and Innovation and Head of Building Information Modelling at NBS, sheds more light on the advantages of Building Information Modelling (BIM) by tackling one of the most common misconceptions.

Editor's note

Dr Stephen Hamil will be sharing what BIM means to manufacturers at the forthcoming FREE* RIBA Insight Consultancy Days in Manchester on 18 October and London on 25 October 2011. Meet him and other industry experts by reserving your place now.

Studio Klaschka. More about BIM at http://blog.studioklaschka.com and @studioklaschka

Quite often when I'm out and about I'll hear professionals talking about “the BIM” when what they really mean is “the 3D CAD model”. This was also highlighted recently in the LinkedIn discussion thread “Where is the ’I’ of BIM if a model is used only for clash detection and generating 2D drawings?”

Here’s a simple example that demonstrates the benefits of BIM (Building Information Modelling), as well as how it differs from 2- and 3D CAD:

Consider an external wall (as illustrated in Figures 1 and 2). Within the latest CAD systems, walls are now three-dimensional objects. The wall object is then broken down into the key products that make up its structure: for example, render, external brick leaf, cavity insulation, internal block leaf and plasterboard dry lining. Each of these is an object in itself; this allows the creation of automatic schedules and quantity take-off. For example, with just a click of a button, the number of bricks or the sheets of plasterboard within the building can be calculated.

Figure 1 – An 'out of the box' wall in 3D CAD

Figure 2 – Products that make up this wall's structure in 3D CAD

In addition to automatically generated quantities, 3D CAD models from different disciplines can be combined for clash detection. Users can add quite complex constraints so that the objects interact as expected. And, of course, spectacular visualisations can be created. However, can the full benefits of BIM be realised in these present 3D CAD models?

Consider the same external wall in a master specification system, for example: NBS Domestic Specification, our product for domestic new build, refurbishment and alteration work. Figure 3 displays the template description of the wall as a system in NBS Domestic Specification.

Figure 3 – External wall description in NBS Domestic Specification | Click to expand

Additional key products

Immediately it is apparent that, in addition to the key materials such as bricks, blocks and insulation, there are a number of other products that are not described as objects in the 3D CAD model. The external wall in NBS describes wall ties, cavity trays, weep holes, and lintels, and it's also evident that key products vary above and below the damp proof course.

Product definitions

Below this system description of the external wall, each of the products that make up the wall may be defined in greater detail, as for blocks in Figure 4.

Figure 4 – Specification clause for one of the many products that make up the wall

In the vast majority of cases this detailed information would not be in your 3D CAD mode: for example, the standard a particular product must comply with, or what its compressive strength and thermal conductivity is.

The question arises: "Will this information one day be included in CAD”? If so, the follow-up question is: "Who will maintain its currency"?

Workmanship

A true BIM must contain all of the information required to build and maintain the building. The expected standards for workmanship are a crucial part of this. Figure 5 displays a sample of the level of detail required to specify workmanship for our external wall.

Figure 5 – Specification clause to ensure high quality workmanship

Performance requirements and their verification

The final component of a true BIM – and one that is crucial in many designs – is the performance requirement. In modern procurement designers quite often produce outline schemes and describe the included systems in terms of their performance requirements. For instance, our external wall will not be specified in terms of the materials it is made from, but in terms of its structural, acoustic, thermal or aesthetic performance. However, listing performance requirements is only half the picture. How these requirements are to be verified once the wall is complete is also essential. Master specification systems worldwide are increasingly providing and maintaining this content. This information must now be linked to the corresponding objects in 3D CAD systems.

Summary

It’s clear that the use of 3D object-based CAD packages provides huge benefits over traditional 2D CAD. However, to really appreciate the true benefits of BIM, the information in 3D CAD models must be coordinated with information in master specification systems.

Figure 6 is an illustration from the very first edition of NBS, dating back to 1973. It shows the information on the drawing coordinated with the specification, quantities, standards, regulations and manufacturer information.

Figure 6 – Coordinated project information

Technology now allows us to accurately and more efficiently coordinate this information. This process has adopted the buzzword 'BIM'. The 3D CAD example described here is a simple wall, but multiply this across all of the systems and products that make up a building and its surrounding landscape, and it’s clear that integrating CAD and specification information is a vital step to truly adopting BIM.

 

Author Dr Stephen Hamil is director of design and innovation and head of building information modelling at NBS. He will also be a guest speaker at the forthcoming FREE* RIBA Insight Consultancy Days in Manchester on 18 October and London on 25 October 2011. Reserve your place now.

This article was originally published in Dr. Stephen Hamil’s blog, Construction Code.

 

* This free event is open to all customers of RIBA Insight products with annual subscriptions (excluding the Standard listing on ribaproductselector.com).

 

Back to top