GRASPING THE GHERKIN

I'm grasping for a deeper understanding of the Swiss Re building, using Revit as an investigative tool. Construction photos from the net reveal a tubular steel framework beneath the cladding ... a series of A frame modules bolted together to form a triangular lattice.

I had a couple of tries at making this component.  First attempt was a 3-pick adaptive component.  Couldn't control the verticality of the central node.  Second time around I made the node using a conventional Generic Model template and embedded this in an adapative component, but no adaptive points. 

I just used the adpative template to access the conceptual massing tools.  I wanted a hybrid family: reference-plane geometry for the  central node, point-based freedom for the splayed legs.  The result is far from perfect, but it's good enough to take me forward as I journey into the heart and soul of the gherkin.

It would be possible to use hand-sketches and CAD diagrams for this kind of investigation, but there's nothing like a BIM model for  illuminating complex geometrical relationships.  Each tier of A frames has a slightly different tilt angle and base width.  Each floor slab is rotated 5 degrees and slightly larger than the last.  Even floor levels line up with the nodes, odd floor levels meet the A frames half way up the legs.

First thing I noticed was that the legs need more tilt than I expected.  You need about 400mm of tilt just to form a cylinder.  To understand this better I built a component that would stack up to form a cylinder.  Oddly enough this looks like a huge pineapple ring.  I seem to be trapped inside a world of fruit & vegetable metaphors.

This quickly confirmed that the upright triangles do indeed lean outwards.  However balance is maintained because the inverse triangles lean inwards by a similar amount.  And so the alternating triangles take you around the ring in facets. 

The component itself is another old-fashioned generic model, this time based on a blend.  18 sided polygons top and bottom, with a 10 degree rotation.  Had to to a bit of vertex editing to split the resulting parallelograms into triangles.  I added a circular void to represent the core.

To transform the cylinder into a gherkin (or a pineapple) we need parametric control over the size of the two polygons.  What's happening here is that I'm using a different method to arrive at the same shape that I created before using pattern based curtain panels.  Except that this time the panels are flat triangles (as opposed to curved rhomboids)

So we need to add two parameters to the pineapple ring family. I tried dimensioning from the origin (intersection of two reference planes), but when you flex this the results are unpredictable.  I could have swiched to point-based geometry again by using an adaptive component, but instead I created an array of reference lines at the origin and dimensioned from these to the perimeter of each polygon.  This simple trick prevents the angles from slipping out of alignment.

At this point it seemed useful to draw a plan diagram to summarise my understanding of gherkin geometry.  The circle is divided into 18 segments of 20 degrees each. Each pineapple ring represents 2 storeys and is rotated by 10 degrees so that triangles line up in pairs to form rhomboids.  So the floors rotate by 5 degrees causing the 6 triangular voids to spiral.  The six portions of useable office space are centred on the gap between pairs of successive openings in the floor slab, hence the 2.5 degree rotation.

Now the justification for this elaborate form is that you achieve clear floor space with a relatively lighweight structure.  It's a bit like those lightweight paper bins made from steel mesh, or think open weave fruit baskets, or maybe lobster traps.  The combination of curvature and triangulation makes for a very elegant self-bracing structure, more elegant I think than the Hearst Tower in New York, also by Foster.  And while we are making comparisons, I'm also reminded of a little glass pavilion by Bruno Taut.

But however clever the external shell, the column-free office space relies on conventional beams spanning from core to perimeter.  At maximum diameter the spans are around 15 metres, which is quite a stretch.  These beams are like spokes of a wheel, 36 spokes = 10 degree spacing.  The 5 degree rotatin of floor levels results in 2 basic conditions. At odd floor levels spokes hit on node points, so you also need a beam between nodes to carry every second spoke.  But at even floor levels each spoke coincides with the mid-point of a tubular leg.  One obvious point here is that this is not a pure structural form.  Space frames gain elegance and efficiency by treating nodes as hinges so that all the struts are in pure tension or compression.  But here we have loads at mid-span on both horizontal and diagonal members: a hybrid structural form.

The Floor Plate is another simple Generic Model family with a radius parameter that varies from floor to floor.  Once I added the beams to this and placed a camera looking up into the structure I had a view that seemed worth rendering.  What took me by surprise though was how much easier it is to read the rendered view, to grasp the spatial relationships.  Clearly there are subtle depth cues that the shaded view lacks and that our brains use to interpret the scene at a subconscious level.  Previousl I had thought of real-time rendering as a fancy marketing tool, but now I am thinking that it can really help us during design development.

I think that's enough for one post.  Lots more to learn though so I will return to the gherkin model another time.  One final render to conclude this episode.

A BIT OF LEVERAGE

I have this ongoing project called "the way we build".  It started with the "Let's Build Zimbabwe" textbook series that I worked on in the early 80s, targeted at secondary school students.  I thought there was potential to expand this and offer it to a much broader audience. 

I want to emphasize PROCESS: the activity of building.  This is often neglected in construction textbooks,  You get diagrams of Flemish Bond, and cross-sections through a finished window sills, but little sense of the physical activities that produce these end results. That's "the WAY we build."

Then there is "the way WE build."  We all build differently. It may be climate, available materials, personal prejudice, expression of identity ... Whether you analyse the pyramids or a mud hut, the technology and the politics are inseparable.

So this weekend I made a massing model of the Lever Building.  When I visited this about 3 years ago, I was astounded at the condition it was in, but later discovered it had undergone a major refurbishment some 10 years previously.  The building is almost exactly the same age as me and is a classic of its age.  Pioneering use of curtain walling and suspended cleaning cradle. Piloti enable a generous plaza to be donated to the public realm.  The main tower cantilevers some 3m beyond the column line to dramatise the separation of horizontal and vertical masses.

As always I gained new insights from the exercise and stumbled across new questions for further research.  Just where is (or was) the auditorium ?  Was it for staff or public use ? Does it still exist ?  I have a section drawing showing one basement level, is this parking ?  Which space was converted into a public restaurant in 2004?  I now have some of the answers but I'll come back to that another time.
Gordon Bunshaft was born in Buffalo, location of the Larkin Mail-Order Soap Company.  So he was almost the same age as another famous office building with social and environmental pretensions.  I read a couple of weeks ago that Foster was doing open plan offices before they became fashionable.  This may be so, but the soap companies were at it in the states 100 years ago. 

Apparently Larkin pioneered all kinds of marketing techniques too, but the supermarkets destroyed their business around the time Lever House was being built, and Buffalo had Wright's building demolished in punishment for unpaid taxes. 

Perhaps the folksy family marketing approach of Larkin was wiped out by the clinical strategies of modern multi-nationals like Unilever.  The Larkin Building belongs to the days of innocence before World War I, Lever House to the brave new world after World War II when everyone was eager to sweep away the mistakes of the past.

Returning to my Revit model, a few observations on techniques used, nothing original.  For the elevations I placed simple gradient jpegs directly in the view & cranked up the shadows to 75%.  Isolating the mass floor faces in a view can be very effective.  Here I selected individual faces and used overide by element to change the surface pattern to a different solid colour. This results in a very effective vertical zoning diagram.  I also rounded off areas to the nearest 10 sq m under Project Units.  Without this the schedule would display a misleading level of accuracy. 

This is a good example of a situation where it is useful to have 2 versions of the same schedule.  I just needed to summarise all the typical floors in a single line: easily achieved by un-ticking "itemize every instance" ... and then adding a comments field to give dummy labels to the floor levels. 

In the final version most of the information is hidden, so a working schedule is very useful to cross-check that all the data is correct. Simple, but effective.

It would be nice if there were a more elegant way to achieve this.  I am looking forward to the day when schedules are much more powerful and flexible.  They are, after all, the "I in BIM" and surely deserve a more prominent and profficient role in the process.

I will finish with some 2d Drafting in preparation for making the curtain wall. 



Aluminium curtain walls came in with the 70s.  This one is all steel, and rather than a clear separation between structural frame and cladding as we would expect today, there seems to be a gradation.  I have a very grainy construction photo showing horizontal angles and vertical channels following hard on the heels of the main frame. The angles also provide support for masonry infill (cinder blocks).  It's an interesting sequence of operations. 

I'm not sure what "heat resistant glass" means.  In the 70s, when I was a bricklayer in the UK, "U" values were just starting to be taken seriously.  I would like to know the spec of the glass used in the refurbishment.  It doesn't look thick enough to be double-glazing, but is said to meet modern thermal standards.

GHERKIN TAKE 1

The mind is a strange thing. It loves to wander randomly making unexpected associations and diversions.  Perhaps this is the key to the emergence of artistic expression often dated to around 40 thousand years ago.  Whenever I'm sitting at my laptop working away on some urgent deadline, part of my brain is wandering off on it's own.  Seems like it takes the adrenalin shot associated with serious physical danger to focus our attention narrowly on the task at hand.

So despite having several more urgent tasks on my list, I found myself modelling the Gherkin this weekend.  They used to say that if your want to understand something, draw it.  There is freehand drawing, T-square drawing, CAD drawing and BIM drawing.  These days I find the best way to get deep inside a building's soul is to make a Revit model.  For 30 St Mary Axe I set up 40 levels with a typical floor to floor of 4.2m  I set the first floor at 8.7m on the basis that the crossing point of the diagonals is about a foot (300mm) below floor level.  These are all "intelligent guesses" but I don't think they are far off.

I had a source saying that the diameter at ground level is 50m, increasing to a maximum of 57m at level 17.  So I went into in-place mass mode and placed these 2 circles using reference lines.  Looking carefully at the finished form you can see that the radius gets progressively smaller over the last few floors, so I placed 3 more circles towards the top, selected all 5 and hit "Create form".
Because they are Reference Lines, the circles still exist inside the form, you can select them and change their offset levels and radii.  So I did just that, and tweaked the form till it looked right. 

Next stage is to select the surface and divide it.  There are going to be 2 masses, one slightly smaller one for the large white structural grid, and the bigger one for the glazed skin which is framed in black mullions.  Both of them are Rhomboid (Diamonds) and the structural grid is 4 times as big as the skin.
The structural grid is 11 diamonds high (4 storeys each), with 18 diamonds around the circumference (20 degrees each)  So the glazing grid is 44 diamonds high (including a double height ground floor and triple height top) with 72 modules around the full circle )5 degrees each.

In Revit, the Rhomboid grid is derived from 4 squares, so you need to double up all the numbers. But the mass comes in two halves, so the U/V grid numbers end up being 22/18 and 88/72.  I did it by trial and error.

Now we need a curtain panel.  The structural members are also diamond shaped in section, so I made a profile for this.  Host it on a point in the Rhomboid template, make form and you have a sweep. 

I decided to use the same family for the glazing so I added a glass surface with a visibility control and linked the profile dimensions to parameters in the panel family. 

Once this was loaded into the project and applied to the mass, a couple of problems cropped up.  The sharp corners were sticking out at every joint right at the top everything was distorted and spiky.  I just deleted the top 2 rows on the basis that the structure changes up there in real life. For the other problem I added a void in the family to cut the corners off.  There's still a bit of overlap at the join, but I decided to move on.

For the glazing I made a second mass with circles offset by half a metre.  By the time I had this kitted out with panels everything had slowed down.  No problem with normal working, but as soon as you change any parameters affecting the curtain panels, be prepared to make a cup of tea.

Next complaint is to do with changing individual curtain panels.  Had to delete a whole bunch at the bottom to make the entrance etc.  There seems to be no alternative to selecting one by one. With the delayed response it was taking me 5 or 6 seconds to delete each panel.  Not good.  Ideally I would have selected hundreds of panels going up in spirals and changed them to a darker coloured glass, but I didn't have the patience.  There has to be a better way.

I started to put in floors and explore the geometry of these spirals.  There are light wells/ventilation shafts behind the dark glass: triangular voids that rotate by 5 degrees (one glazing module) from one floor to the next.  Some of these are 2 storeys high and some link 6 storeys.  I need more time to work this out in the model.

Did some trial renders on the entrance and compared the result with a picture I took 2 years ago.  In the end I couldn't resist compiling a render, a shaded view and the photo into a composite image.  The live photo is only used around the entrance area.  It adds the subtlety of the internal lighting and the realism of the street lights and the taxi.  I masked out the structural frame and the glazing, so this is all from Revit, but the dark glass is faked by drawing a selection around those areas and adjusting levels.