Structural engineer here: Buildings and bridges do not use 5, give us a little credit<p>We mostly assume that materials are 90% as strong as they say they are, that the building weighs 1.2 times as much as it really does, and that stuff in the building weighs 1.6 times as much as it really does. The total factor of safety is something like 1.5.<p>The reason that buildings don&#x27;t vibrate (noticably under service loads) is that people would get queasy, so we design things for stiffness as well as strength. Aircraft are more space and weight limited, and no one is sitting on the end of the wing, so the engineers mostly just care that everything will stay attached.