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Wayne Hale's NASA Blog: Factors of Safety

Status Report From: NASA Blogs
Posted: Thursday, December 18, 2008

Old joke: "You see the glass as half empty, I see the glass as half full, but an engineer sees the same glass and says 'it is overdesigned for the amount of fluid it holds.'"

When an engineer starts out to build something, one of the first questions to be answered is how much load must it carry in normal service? The next question is similar: hom much load must it carry at maximum? An engineer can study those questions deeply or very superficially, but having a credible answer is a vital step in at the start of a design process.

Here is an example. If you design and build a step ladder which just barely holds your weight without breaking, what will happens after the holidays when your weight may be somewhat more than it was before you eat Aunt Martha's Christmas dinner? You really don't want to throw out your stepladder in January and build a new one do you? Obviously you would should build a stepladder that can hold just a little bit more. Don't forget what might happen if you loan your stepladder to your coach-potato neighbor who weighs a lot more than you do? Can you say lawsuit?

So how do you determine what your stepladder should hold? Do you find out who is the heaviest person in the world and make sure it will hold that person? Probably not. Better, pick a reasonable number that covers, say, 95% of all folks, design the ladder to that limit and put a safety sticker on the side listing the weight limit. Yep, that is how most things are constructed.

But that is not all. Once you determine normal or even the maximum load it is a wise and good practice to include a "factor of safety". That means that you build your stepladder stronger than it needs to be. This helps with the idiots that don't read the safety sticker; it also helps protect for some wear and tear, and it also can protect if the actual construction of your stepladder falls somewhat short of what you intended. So you might build your stepladder with a FS of 2. That would cover 95% of all folks with plenty of margin for foolish people that try to accompany their friend climbing the ladder; or when your ladder has been in service for 25 years (like mine), or when your carpenter buddy builds the stepladder with 1/4" screws rather than

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