Volume Six - Pre-Victorian to the Present Day - Select Aspects - Structural Analysis of the river Ely Subway by Rota Design Limited - 2015

The foregoing examples are basic failure modes which we have to consider when we design and verify the design of a 'structural' product which may have a service life of 30 or 50 years. Our river Ely subway has been there for over 115 years and when last seen by by Brian Colquhoun & Partners it wasn't in too bad a condition, witness the report and images taken at the time.

How did we analyse the subway. Simple, we used finite element analysis (FEA). Wot, Eh, Ugh!!! This is where the eyes glaze over, panic sets in. I've seen clients go comatose or if still conscious, climb out of the window and hide on the window ledge. But I don't care, I'm going to try to explain anyway!

So, having produced a 3D CAD model of the individual cast iron segments, we made an assembly of one ring using 7 segments with a key block at the very top dead centre just as those incredibly brave men did back in 1899 or thereabouts. We copied the rings a number of times in the lengthwise direction and then added the internal features including the nuts and bolts holding the structure together. The CAD model is output in a standard format to the FEA software package and this is where the fun begins.

We now have a digital virtual model of the subway. So we do our 'Spiderman' trick and cover it in a web or mesh. Where each bit of the mesh touches the next bit we have a point which we call a 'node'. The position of each of these nodes is known mathematically by the computer software in 3D space. There are 17,554 nodes in our simplified subway example. So now we can just throw the original 3D digital model away.

Now lets add the physical characteristics of cast iron and the 'boundary conditions' they are the restraints that keep it in the ground and held together in once piece. Add the 'load cases', those being soil and water load and the direction in which the forces act. Add gravity for planet Earth. Now ask the nice shiny new computer to do some sums; 103,788 complex equations to be precise.

Easy isn't it? Not so fast, remember the coke can that changed shape when we squeezed it? So now run the program again with the new deformed shape and then again, and again, step by step, until something goes twang or the subway shoots off into space because we forgot to add gravity again!