Dennis Wright's Blog

Well, in the attic which houses a permanent exhibition including this model of Casa Milà:

I was fascinated by what at first sight looked like an elaborate light fitting or design for a jelly-mould:

It hangs over a mirror so you can see the structure upside-down, the hanging chains becoming upright arches. It is a model for a structure supported by catenary arches.

Now I remember the catenary from my days as a Maths student. We were required to find the shape that a flexible string or chain takes up under gravity when the two end points are fixed. The problem is interesting because when the chain settles down to its final position we know all the tension in the chain is pointing along the interior of the chain itself, not outwards from it, otherwise the chain would be moving or bending, not at rest. In other words, gravity has solved the problem of balancing out all the forces which would make the chain bend or move, leaving only tangential tensile forces.

The clever bit comes when you turn the shape upside down and employ it for the design of supporting arch in a building. All the tensile forces are now forces of compression, but they still point along the interior of the arch. There are no forces tending to make the arch bow or twist so it can be much lighter for the same supporting capability, and you don’t need flying buttresses to stop the building collapsing outwards.

Gaudí doesn’t get the credit for being the first to use catenary arches in buildings but he is noted for using hanging models like the one pictured to help him with his architectural designs.

Catenary arches are used to help keep the attic space of Casa Milà itself as light and open as possible, as evidenced below. The suspended rope adds something of an ironic counterpoint.

In addition to the permanent exhibition there is a large temporary exhibition space on one of the lower floors. At the time we were there it accommodated an exhibition of paintings by the late Slovenian artist Zoran Mušič, entitled “From Dachau to Venice”. It was pretty grim and monochromatic, but very moving, and included pictures from the rather heavy series “We are not the last“. One gets the impression Zoran never got over his experiences at Dachau, even though he only died in 2005 at the age of 96, and that the subject matter kept resurfacing to haunt his work.