For an engineer, running calculations and numbers in your head is an occurrence of frequent timing. It is not our fault. The world is constantly presenting us with situations for number crunching, usually to be more efficient, make sure we are on time, or be content that the building won’t collapse.

Case in point – the elevator. I got on an elevator the other day and saw posted a sign that is visible in various forms on most elevators: Weight Limit 4000 lbs.

Am I safe in here? What if I am in here and a number of heavy people get on at the next floor before I have a chance to exit? So, my mind starts going: If heavy people, let’s say 250 pounds each, get on the elevator:

4000/250 = 16, so it would take 16 people each weighing, or averaging, 250 pounds to max out the elevator capacity. Though unlikely, can they even fit?

I estimated the elevator to be about 6 ft by 7 ft, or 42 square feet. This means each person would need to fit with:

42/16 = 2.625 square feet, or in a square with a side of 1.62 feet. That is 19.44 inches. This would be an extremely tight fit. I think my shoulders are about this width, and I thought about the stagger and organization of the squares. Fitting 16 people of that size in here would be difficult, not to mention the low odds of that many people of that size showing up at the same time. But not impossible.

I convinced myself that not only is the chance of that many big people wanting to enter at the same time very low, but, and here is the real comforting thought, they always throw in a good factor of safety on these things. Without looking this up on the internet, I was at ease riding the elevator. Until that delivery guy wheeled into the elevator a flatbed containing numerous boxes that may well be holding lead plates or gold bars, elements with very high specific gravities.

The calculations begin all over again…