Sunday, February 16, 2014


This is the second post in a series.  I am trying to encourage students & teachers of Architecture & Building to use BIM as an active research & learning tool.  It could be a Drawing course, a Technology course, a Theory of Design course, a History of Architecture course, or even a BIM module :-)  The idea is to take an integrated approach.  Don't teach Revit with a series of abstract, disconnected exercises.  Use Revit as an investigative tool, as a way to learn other stuff.  Use it like you might use a pencil, exploring the way something works in a series of freehand sketches.

In this post I am presenting the first two passes of an exploration.  There is a Revit file to download, and a set of handouts or "summary sheets" to guide you through the process of creating this file from scratch.  The models you create will be abstractions: simplifications such as architects commonly use when analysing a design idea.  We are trying to capture fundamental relationships, underlying concepts and geometries.  In the next post we will be adding more subtle inflections, coming closer to reality.  But my focus will always be on the learning experience. 

The summary sheets show the main steps.  I'm trying to cater for fairly new users, but not for absolute beginners.  If you have no experience of Revit at all, you will need a guide, someone with more experience.  They are designed to be useful to someone leading a class.  I'm hoping it will reduce your preparation time.  If you have intermediate Revit experience, you should be able to work through the exercises once yourself, then launch straight into teaching, using the sheets as handouts.

I'm using a floor to floor height of 4.2m  This is almost certainly incorrect.  But it's not far off, and it's a convenient round figure.  I don't want you to be distracted by irrelevant details.  We're using a broad brush approach to gain some basic insights.  Similarly I have chose to offset the 3 shells (see previous post) by half a metre from each other.  This is surely wrong, but it makes the maths much easier and it works fairly well in practice for the models we are building.

I received a comment on my last post, pointing out that the Gherkin was conceived as a revolve, rather than a loft.  I'd like to thank Mwaraya for this contribution.  It's great to get feedback and I individuals or groups who use this material will contact me with more comments and corrections.  I will do my best to respond and improve my work.  It appears that the Gherkin profile was conceived as a series of arcs in section.  My lofting approach comes very close to the same shape, but is fundamentally quite different.

An analogy may help.  When setting out an ellipse, builders commonly use an approximation base on arcs.  This is easer to construct on site than a true ellipse.  Similarly you could approximate a parabola with a series of arcs.  Gaudi used a different approach, creating catenary curves by hanging chains and cloth, then freezing the result as a plaster cast.  It's a question of choosing the method to suit the situation. 

In my case, lofting from a series of circles is a very economical approach to generating 3 concentric Gherkin shells, my Russian dolls representing Skin, Frame & Floors.  I would also venture to suggest, that although the architects my have conceived the original form as a series of arcs constructed in section, the real building was not set out like this.  It was constructed from tubular steel A-frames, each 2 storeys high.  18 of these frames form a complete ring, leaning inwards or outwards slightly depending on the location.  In this sense the Gherkin is lofted from a series of circles, even though the diameters of these circles may have been decided by constructing a set of arcs in section using a CAD programme.

As for the skin it is neither a revolve nor a loft, but a large number of diamond-shaped facets, each perfectly flat.  (in as far as glass can be said to be perfectly flat)  It seems to me that this is a rather special type of unitised curtain wall system.  Each diamond is one story high and must be fixed back to the steel frame in some way.  There must also be a system of adjustments and tolerance allowances.  It would be fascinating to know more about how all that works.

In practice many of the diamonds are split into triangles by transomes that coincide with the floor slabs.  Some of these triangles hinge out to provide natural ventilation in hot weather.  Once again my Revit models diverge from reality slightly.  I am using a Rhomboid divided surface.  Triangles occur at the edges of the surface, but not at each floor level.  The illusion of triangles will be created by having a slab edge that penetrates the skin, appearing to divide the glass.

As long as we are aware that our model diverges from reality everything will be fine.  Problems arise when people start to imagine that CAD & BIM allow you to create "reality" inside the computer.  It's no more real than a pencil sketch.  It may be more complex.  It may capture more aspects of reality.  But it's still a model, an abstraction.  It's important to remember what we are abstracting and why.  Blindly modelling every nut and bolt, "just because we can" is a recipe for disaster.  Be aware that you are simplifying.  Think carefully about what you want to achieve.  Choose appropriate methods, including shortcuts and assumptions.  Structural Engineers make stick models.  They assume that joints are either rigid or perfect hinges.  This is not the case, but it allows them to do their job effectively.

You can download the various files from the Autodesk 360 links below.

Basic Geometry diagram 

Summary Sheets            

2nd pass Revit Model     

More developed Revit model



  1. Good information and ideas to share with readers.

  2. Awesome info Mr. Andy Milburn....
    Im a Fan of you... Keep the good work up and encourage the youngsters like us...


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