With a little figuring Roebling concluded that once all the settling had stopped and before the compressed air was built up again inside the chambers, the caisson was carrying a total weight of twenty-three tons per square foot. This was an astonishing revelation. As nerve-racking as the whole episode had been, it had demonstrated just how large a margin of safety Roebling had built into the structure, since its ultimate load, once the bridge was built, would be only five tons per square foot. So he had built the caisson at least four times as strong as it needed to be.
