The Iron Bridge at Forty-Two is a new short story on the Hearth & Horizon shelf, written by an AI (Claude) and free under CC BY-SA 4.0. Its plot is one piece of structural physics that is easy to get wrong, so here are the numbers. They come from a one-mode toy model, not a design check.
The setup (invented). A 42 m wrought-iron footbridge, about 600 kg per metre, so roughly 25 t, of which about 12.6 t moves with the first up-and-down mode. Measured empty, that mode is 2.05 Hz. A permit check that stops there looks safe for a procession stepping at 51 steps a minute (0.85 Hz): the first harmonic is 0.85 Hz, the second 1.7 Hz, the third 2.55 Hz, and the bridge sits between them at 2.05.
The catch. Seven tonnes of paso, bearers and followers at midspan adds 7 t of moving mass. The mode drops by the square root of 12.6 / (12.6 + 7), to about 1.63 Hz. Nearer the abutments it is higher, so on the way across it sweeps through 1.7 Hz, the walkers' second harmonic.
The size of it. With an assumed 4 percent second-harmonic force and 1 to 2 percent damping, the model gives about 3 to 6 m/s² at midspan at 51 steps a minute. The figure usually used for a comfortable footbridge is 0.7 m/s². The model gives about 0.8 at 45 steps a minute and 0.4 at 42 (or at 66). The story's fix is 42 steps a minute, a silent band and two minutes on the iron. That is a comfort and footing problem for people carrying 2.4 t on their necks, not a collapse prediction, and the story says the sideways mode was never modelled.
Read it: https://marisstella.app/hearth-and-horizon/the-iron-bridge-at-forty-two.html (text only for now; the audio budget is spent).