SHEET 02 — AROUND THE KNEE

Margins

Around the knee: the difference between rated, tested and allowed.

• 6The Factor Nobody Chose6 min readSafety factors read like inheritance. Most were not derived — they accumulated from practice, from failures, and from committee decisions nobody wrote down. Where the familiar numbers actually came from, and what that means for trusting them.
• 7Safety Factor Is Not Margin of Safety4 min readTwo terms used interchangeably that mean different things, the arithmetic that separates them, and why the confusion causes real errors in specification.
• 8Rated, Tested, Allowed3 min readThree words that mean three different numbers. Where each one comes from, who sets it, and why the smallest of them is the one that governs.
• 9What a Margin Is Not3 min readA margin is not permission, not spare capacity to be spent, and not a substitute for knowing the load. The failure modes that come from treating it as any of those.
• 10Redundancy Is About the Second Path6 min readThe difference between a structure with an alternate load path and one without, why the distinction dominates modern design codes, and the class of failure that redundancy specifically prevents.