GHERKIN REVISITED

I am revisiting the Gherkin following a request from David at the University of Ulster.  He wanted to share my Revit model to use with his first year Architecture students.  As usual I felt it was necessary to clean things up again before uploading, and learnt quite a bit along the way that seems worth sharing.

So the idea here is for a research project using BIM processes.  It's a learning experience for students of Architecture, or just anyone who is fascinated by what makes buildings tick.  It's supposed to be hands on.  You model the Gherkin in Revit and you get a much deeper insight into the design & the technology along the way.  Also maybe learn some new Revit skills.  There are several earlier posts you might want to flip through.  Just type "Gherkin" into my search box and you'll get a list of links something like this.

Design is a bit like calculus.  You start with a rough approximation, which then points the way to a more exact definition, leading to something closer still, and so on.  As the iterations increase, you get closer to your goal.  When I was at school we called it tending towards infinity. So I'm going to start with a broad brush interpretation of the Gherkin & when you understand the geometry of that we'll start to introduce various subtleties to draw you closer & closer to the actual thing.  We'll never actually get their of course.  It's just a learning exercise.

So let's get to it.  We will treat the building as a set of Russian dolls representing the glazed skin, the diagrid steel frame and the floor plates. The first two will be half-dolls in order to reveal what's inside (and to save on processing load)  The two halves will be rotated at 90 degrees to each other to even better see the relationships between the 3 major elements.

I'm not going to describe every step in the process here.  Instead there will be a pdf "handout" to download.  Getting into teacher mode here.  There will also be a download of the Revit file.  Be patient it may take a couple of posts before the downloads are ready.  Here are the parts, shown separately and then combined.  This is the first approximation.

Each of the shells is made in the same way.  A series of circles with different radii and vertical offsets are used to loft a form.  Revit has a habit of breaking circular volumes into two halves.  You can see the seam in the image below.  Using "tab select" you can pick one of these curved surfaces and divide it.

Divided surfaces start off as a 10x10 grid, but then you can change the numbers (U & V) and you can also select a different pattern.  We will be using "Rhomboid" in this first exercise.  Later we will use "Half-Step" for the structure.  For the frame we need an 18x18 grid, for the skin 72x72. It's a bit more complicated than that, but we'll get to that later.  4 panes of glass for each leg of the frame.

Next step is to make simple curtain panel families.  I keep them simple as possible, because when you load them the computer is going to go into thinking mode for a couple of minutes.  Don't want to overload systems which may not be state of the art.  The structural skeleton is based on "A frame" components bolted together.  I came up with a family that fits an "A frame" into the top half of a rhombus with no geometry in the lower half.  That geometry will be provided by the row below. 

This seems to work, but later on we get a slight anomaly when the family is loaded into the project.  You get an extra "phantom" row at the top.  Basically Revit has to chop edge panels in half (making them into triangles)  When it cuts the bottom off an A frame it is cutting "nothing" away.  This seems to cause confusion.  I should mention the floor faces.  For simplicity I start with 40 levels, all equally spaced, 4.2m apart. That will do for the first pass.  Most of these levels get a floor face, but there's a double height ground level and some plant floors near the top that don't follow the gherkin shape.

The next image illustrates the "second pass" as we build a bit more sophistication into the model.

Firstly, the skin is lofted in two parts: the "main body" and the "top cone". The grid is relaxed from 72 divisions to 36 in the top cone.  I guess this is to avoid very small panels at the top as the diameter gets smaller.  Secondly we need to tab-select individual components and modify them.  There is the very tedious task of swapping clear panels for black ones.  You can see that I am half way through this task in the image above.  The result is a spiral effect which hints at the presence of voids behind the glass.  Also some of the skin panels need to be deleted at the base to create an open "colonnade" at ground level. 

The spiral voids are very interesting.  Basically we have 9 "A frames" in our half-gherkin structural shell.  That's 20 degrees per frame.  And there are 4 floors per structural rhombus.  In other words the glazing panels are one storey high.  This is very clear in the picture above.  Divide 20 by 4 and you have a 5 degree rotation per floor level.  I'm talking about the black spiral bands. 

There are triangular voids that follow the spirals.  Use a shaft opening for the first one.  Create a radial array with 6 of these and group the whole thing.  Copy to clipboard and paste aligned to the next level.  Rotate this second group by 5 degrees.  Repeat the process.  Every sixth floor remains complete as a fire break.

To solve the "A frame" anomaly I switched the pattern type to half step.  This is like stretcher bond in brickwork.  That's a more accurate abstraction of the way the A frames work.  Each row is offset half a grid compared to the row below.

You also need to make a curtain panel family based on the same pattern.  Select the grid within the family template and change it to whichever pattern you need.  That's a step that people often miss when they first start making pattern-based panels.  You'll see that I've been tweaking the top cone: splicing in a zig-zag join, adding some bits and pieces like the glass dome at the top and the rails for the cleaning system. 

But I want to finish this first overview post with a snapshot of my "geometry analysis" page. Later on this will be available as a higher resolution pdf download.  There will also be a more detailed stepping through of the processes to build these analytical models of the Gherkin.  And of course the actual Revit model will become available. Give me time :-)