Daniel and Joel come to play - Part 9
It’s been some time since Daniel and Joel came to visit. They have been busy at school, and were eager to enjoy their holiday. Daniel is now 11 and Joel has just turned 9. They brought along their sister Ariela aged 2 ˝ to play, and she started by fitting a screw driver into a screw slot – so I have high hopes for her. I had my computer all fired up and was ready with the latest edition of flight simulator, but they announced that they would like to build a bridge. This came as an immense and most pleasurable surprise as you will remember that at their last visit they had walked right past robots and clocks and all the Meccano parts – straight to the computer.
So we settled down at the computer and studied various types of bridges and compared one with another. They were quite taken with the Forth railway bridge and I was delighted as I had always had a love affair with this bridge having first seen it as a small boy. We discussed its construction, and I pointed out the cantilever and showed them where the columns of tension and compression were. I showed them a video from the internet of the terrible Tacoma bridge disaster, and told them about the Tay bridge and how no-one wanted to se a repeat of that - which was why the Forth bridge had been so over engineered. We found a very nice picture of the bridge on the internet and printed it in order to work out a scale. We decided that if we made the central structure 12 ˝” high, then the total length of the span would work out at two times 18 ˝ angle girders and we thought that these proportions would be suitable. So the boys had already had an interesting lesson in stresses and strains, and in proportions and scaling.
We set to work building the central tower structure, with Daniel taking one side and Joel building the mirror image on the other. This was soon completed and we worked out a way of joining the two sides together – the base being wider than the top. I pointed out that just as they had braced the sides according to the pictures, so the ends also needed bracing.
Now we started on the cantilever. It was easy to work out a length for the top girders, but not at all easy to build the curve for the lower ones. In addition, we had to build the correct number of bracing elements. As we progressed on one side, Joel carefully copied our design. Once completed, the four sections were attached to the central tower. We then built the train road bed and attached the cantilevers to support it and we could see with pride how the bridge was taking shape after four days of hard work. It was out intention to run an HO gauge electric train across the bridge when completed and the boys were eager to set the bridge up. The boys had thought of building ramps up to the bridge, but I pointed out that the river banks were on a level with the bridge and that dining room chairs could support the track and the bridge could stand on a low table with the end of the spans resting on the seats. This worked out very well, and the boys hurried to set the oval track up, connect the electric points and build a station which they named Queensferry. Soon the train was rushing over the bridge while Ariela showed a deft hand at attempts to derail it. You can see the set up in the picture.
Forth central bridge span in Meccano
Now of course we have a new project and that is to actually go and see the bridge – by train of course, stopping at York to visit the National Railway Museum. The boys don’t know it yet, but we are also going to buy tickets on the train that crosses ‘their’ bridge.
Daniel Joel and Ariela with Meccano Forth bridge and electric train