I have an interesting idea. Back before calculators engineers often were less certain of figures than today, so it was not unsual to add 10% (or so) to most figures. To me (today) it seems that 10% is what causes a lot of old buildings to stay standing while a lot of the "enough to get by" buildings of today fail early. Just a thought.
Very good point; I can think of two examples from recent times:
(1) Some years ago, there was a proposal to reduce the width of nominal "2 by" lumber for wall studs from the present 1-1/2 inches to 1-1/4 inches, thereby cutting building costs. The idea was supported by engineering studies showing that the narrower studs would still have ample strength, given the conservative nature of code-prescribed wall construction.
But practical-minded reviewers noted that carpenters sometimes fail to hit current-size studs when nailing on sheathing, and could be expected to miss more frequently with the narrower lumber.
(2) After hurricane Andrew hit Florida, inspectors noted that a surprising amount of stapled-on roof sheathing had blown off. Theory and tests showed that the staples should have held as well as nails.
But they failed to take into account human laziness. The proper method of stapling was for the crown of the staple to be parallel to and centered on the the rafter, so that both legs of the staple would penetrate solid wood. However, this entailed holding the staple gun in an uncomfortable position, so some carpenters shot the staples perpendicular to the rafter. This practice greatly increased the likelihood that one leg of the staple would miss the rafter, thus reducing the holding power of that staple by half.
