Voices of Science

Well yes it followed on from the Forth Bridge where we’d made major steps in reducing the weight of the long span suspension bridge relative to the American ones, but the biggest change was in the shape of the deck. The deck structure, instead of having a stiffening truss under the roadway, the Severn Bridge was constructed of a plated box section, trapezoidal, so that it was streamlined to the wind. And greatly reduce the force of the wind on the structure. And the other main change from the Forth Bridge was in the towers, where instead of having five – they were both steel, but instead of having five cells which we had the Forth towers’ legs, in the Severn Bridge there was just a single cell. The Germans had been building box girder bridges for some time, but this was the first time it was used on a suspension bridge. And of course the big thing, the big problem we had with suspension bridges was to make sure that they were stable in all wind conditions, including stability against flutter in very high winds. And we did show that the box was suitable for this, and on that basis we went ahead and built it. And since the Severn Bridge we built the Humber Bridge with a similar deck but then since the Humber Bridge three of the four longest spans in the world have all got this box deck structure. I think three – though I think all three of those they not only have the same box structure, they all picked the same depth that I'd picked for the Severn Bridge which was three metres deep, I did go up to four and a half metres for the Humber Bridge but maybe that was just me being a bit cautious.