During construction of the Quebec Bridge in 1907, the bridge' s designer, Theodore Cooper, received word that the suspended span being built out from the bridge' s cantilever was deflecting downward by a fraction of an inch  Before he could telegraph to freeze the project, the whole cantilever arm broke off and plunged, along with seven dozen workers, into the St  Lawrence River  It was the worst bridge construction disaster in history  As a direct result of the inquiry that followed, the engineering rules of thumb by which thousands of bridges had been built went down with the Quebec Bridge
Twentieth-century bridge engineers would thereafter depend on far more rigorous applications of mathematical analysis
Cooper's absence from the Quebec Bridge construction site resulted in the breaking off of the cantilever.