"Christopher Tidy" wrote in message
...

I guess maybe the core was built on a steel hoop made from bent round bar,
or something like that? Not sure how you hold the back end positioned to
1/8" accuracy, though.

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Chaplets.

https://en.wikipedia.org/wiki/Core_(manufacturing)

This notorious collapse of a cast iron bridge brought attention to the
difficulties of sand casting and prompted improvements.
https://www.open.edu/openlearn/ocw/m...able=1&id=3524

Fortunately stronger rolled structural steel was just then becoming
available to displace wrought and cast iron.

Many defects were found in the column castings, including shifted cores that
left a thin wall on one side. The foundry supervisor couldn't be
interrogated as he had emigrated (fled?) to Australia. Railway management
had pushed the designer to cheapen, and thus weaken, this bridge compared to
his earlier projects.

Two cylinder steam locomotives are inherently unbalanced. The wheel
counterweights are sized to divert the vibration from horizontal which the
passengers would feel, to vertical which increases the pounding load on the
tracks and bridges. The effect increases with speed, and trains reportedly
ran over the Tay bridge much faster than they were supposed to.
https://en.wikipedia.org/wiki/Hammer_blow