This is the story of how an award-winning building that was an architectural and engineering triumph very nearly ended up as a not-at-all-award-winning heap of rubble but was saved by some last minute testing.
This is the Queen’s building at Emmanuel College, Cambridge. It’s a pretty cool building and an even cooler piece of engineering. It’s made of the same limestone (from the same quarry) that the rest of the college was built from some 400 years earlier, but where the original buildings have the massive metre thick walls required when working with such a weak stone, the Queen’s building doesn’t. The reason it doesn’t just collapse is hinted at by the little port holes that you can see dotting the building. Limestone, like concrete is weak under shear or tension forces, but strong under compression. If you looked inside one of those port-holes you’d see the ends of steel rods running down the centre of each stone column, pre-compressing it, similar to pre-stressed concrete.
Being a brand new construction technique, a bunch of bright people did careful calculations to work out what pressures the stone would be under, what the stresses and strains would be, and modelled everything to make very sure that the stresses on the building were well within the tolerances required – all good stuff. Work started.
Enter, Prof. Chris Burgoyne, a fellow of Emmanuel College, and structural engineer, who worked through the plans and calculations and said, “That’s great in theory – but we need to test that.” Chris had spent much of his life trying to remove over-confidence in models from his engineering students and had an engineering lab just down the road, so despite reassurances that it would all be just fine, insisted on the testing.
This is one of the test columns in a load press in the Cambridge University Engineering Lab. If you look closely, you can see the cracks as it comes apart at significantly lower than the safe load required. This posed a bit of a problem, as the building was already partially built!
So what had happened?
The models were right, in as much as they were correctly modelling things. The pre-tension technique is sound. The building stands Today as a quiet triumph of ingenuity and engineering brilliance. However, three things were causing the columns to crumble – none of which had been identified in the original models but all of which became obvious in testing.
- The modern mortar mix used, when placed on the aborbent limestone, set very hard, very quickly, so instead of the load carried uniformly across the surface, the surface was rough causing a relatively small number of contact points, which were acting as pressure points to start fractures.
- The blocks that made up the columns had cramps (basically big steel staples) holding them in place. These too were acting as pressure points.
- Some of the limestone blocks had the bedding planes (the lines where layers of sand were laid on top of each other before being squished by Geology) aligned vertically. Limestone can take much higher pressure when the bedding planes are horizontal – think standing on a stack of paper laid horizontally or vertically!
Today, in addition to the tensioning rods originally planned, the Queen’s building has some additional unusual features which you can’t see. It is built with very thin Roman-style lime cement (which sets slower and more putty like and so doesn’t cause fracture points). There are no staples holding the column blocks together, and the bedding planes in the sandstone blocks are all very carefully aligned. In addition to being a triumph of architecture and engineering, it is also a triumph of testing.
 Technically it isn’t. It’s the second of several smaller columns built and tested after the first column failed spectacularly. However, most of my photos of this testing were very under-exposed and rubbish. I was 13 at the time – I’d been allowed in as I was keen on becoming a structural engineer and Chris was a colleague of my dad’s and friend of the family. I didn’t end up as a structural engineer, but did take to heart the importance of good testing!
Edit: Updated with feedback from Chris Burgoyne, who corrected a few minor details and was also kind enough to provide the copy of the paper included above.