Clever Designing 160 years ago
One drowsy, summer’s day in the World Heritage Site, Derwent Valley Mills, an elegantly curved weir makes the river swish and some ripe old engineering appears.
Beside it, through the green canopy, slightly mysterious stone structures appear.
As you approach it, machinery begins to take a clearer form; but more of that in the next post.
The stone structures and the machinery can contain and control the river flow even when it is thunderous in full winter spate. This place is where the mastery of water power brought industrial scale manufacture in its youth. The first factories here needed what, in the terms of the early to mid 19th Century, was a lot of power. When the river was angry in those winters long ago a lot of clever, state-of-the-art engineering design was needed to avoid their destruction, let alone power extraction and control. There can be more momentum thundering by here, far inland, than you will see other than at a gale bound sea coast.
The factory manager sometimes needed to control water flow rates into his mill wheels; at other times needed to allow water to bypass the weir. Then they raised or lowered massive gates. When closed and water depth was thus high on the upstream side, , these gates needed to be able to resist, without bursting, the high pressures of the water. Some things can resist pressure but flex a lot while doing it; balloons and car tyres are examples. But, these gates had to be rigid. If they were not, the flexing would jam them in their closely-fitting slide-ways and they could not be made to move up or down when that was needed.
An ideal material for the faces of the gates wetted by the water was then timber; for the strength of its fibrous structure, its integrity in large sizes and also because it expands in water to make a good seal. Massive though the timber gate baulks were – even bigger than railway sleepers then or now – they would still deflect. So the design engineers set out to design a way of stiffening the timber beams: they used a design called a ‘composite deck’. This is like decking on some bridges where there is timber to walk on or drive a cart across and a steel structure of ties and struts below to stiffen it. Build this to the right size, turn it on its edge and, Olé, a water gate.
Now this was all being done by engineers, quite a long time ago. And here’s proof.
What? Ah yes, the gates themselves. Here’s one and we’re looking vertically down at the back of the gate at the stiffening structure.
Note here that the strut components, mounted at right angles on the backs of the timber baulks are cast iron [molten metal poured into shaped moulds] while, joining the top of the struts to each other and the to the gate frame at each end of the gate [out of shot in this image], the ties are wrought [forged at red heat by a blacksmith]. There are several identical [in this case], independent courses of struts and ties, above and below each other.
Each end of each tie is passed through to the water side of the gate and the nut is turned up tight to make the timber and the metal structure act as one.
I don’t know the name of the engineer who designed the gates and the stone structures. Whoever it was, he [it probably was a ‘he’ in those days] faced a similar conceptual struggle as does the design engineer today. Nowadays, she still has to look at a more or less blank page and produce ideas of a design that will fulfil the task specification. This designer did a good job with the technology to hand. Although the gates had to be of different sizes and so had to with stand different loads whilst doing the same job, he used a standard design concept. That concept used standard struts and ties in a determinate structure for each design.
With these simple designs of strut and ties, and depending on the depth of the water forcing the height of the gate, there are several levels of frame above each other. There are limits to the design in that those used here and shown above are determinate. By that I mean that, with these designs, it is relatively easy to calculate the loads that each component has to with stand. If you add any extra struts or ties and it is difficult to ensure that all are tight and if you do, then it is uncertain what load each is seeing. It is then called ‘indeterminate’.
This is not to say that this design did not have its shortcomings. For this, see the next post. Remember, few engineers of today will be credited with designs that are still doing a job after 160 years: I would be delighted if any of mine were.