Jul 20, 20201 min
The output of a quick calculation is only as good as its input.
John Roebling, circa 1866 (Wikipedia)
Did you know that suspension bridges (a type of bridge in which the deck is hung below suspension cables) had been ripping themselves apart in the early 20th century?
According to Vic Vyssotsky,
"[It's].. an aerodynamic lift phenomenon, and to do a proper engineering calculation of the forces, which involve drastic nonlinearities, you have to use the mathematics and concepts of Kolmogorov to model the eddy spectrum. Nobody really knew how to do this correctly in detail until the 1950s or thereabouts."
But miraculously, the Brooklyn Bridge, a suspension bridge built around the same time, still exists. Why wasn't the Brooklyn Bridge tearing apart? This is because John Roebling, the engineer behind the Brooklyn Bridge had a sense of what he didn't know.
Roebling designed the stiffness of the truss on the Brooklyn Bridge roadway six times what a normal estimate based on known static and dynamic loads would have called for. And, he specified a network of diagonal stays running down to the roadway, to stiffen the entire bridge structure.
Roebling built a good bridge by employing a huge safety factor to compensate for his ignorance.
The lesson here? In estimating anything - size, cost, schedule, availability commitments - we ought to stay back from the objectives by a factor of 2, 4 or even 10 to compensate for our ignorance. De-rate your performance to compensate for your ignorance : don't place too much confidence in your calculations. We must remember that our output is as good as our input.