Factor of safety in the way that most people understand it has gotten increasing difficult to tease out since the switch to load and resistance factor design (L

Factor of safety in the way that most people understand it has gotten increasing difficult to tease out since the switch to load and resistance factor design (LRFD) over allowable stress design (ASD) for most structures. I think timber is the only one which still defaults to ASD for permanent construction. But the short version of the story is that for LRFD there are two main factors which affect “factor of safety” in the traditional sense (eg - structural capacity / structural demand) which are load factors and resistance factors (as you’d guess). Load factors increase applied forces while resistance factors decrease assumed strengths. Load factors vary depending on the type of load but typically they’re between 1.2 to 1.6. Resistance factors vary based on failure mechanism under consideration and are between 0.6 to 0.9 for structural work. So the range is 1.2/0.9 to 1.6/0.6 or ~1.35 to ~2.75. There are secondary sources of additional factors of safety. For example, material properties assumed in design are the minimums that are allowed and so chances are you’ll get something on site that exceeds by 25-50% or so because nobody wants to deliver material that doesn’t meet the minimums and open themselves to claims liability. There’s also a redundancy factor which is touched on in the report where loads are increased if no redundant load path exists, for concrete there’s some additional caps/limitations on a couple values due to time effects like creep, etc. I would say most modern construction has an overall factor of safety of 3-5 with bleeding edge construction sitting at around 2 and public works construction (utilities or highways - things where architects aren’t involved) at over 5.