Well I wonder sometimes. Like you don't build a building and then stress test it... so there must be some fields where simply adding the 30-50% margin for error is good enough.
Absolutely. Safety factors allow engineers to have good night sleep. ; )
Luckily in constructions the use of novel methodologies aren't that common like in other industries so previous small and sometimes big scale tests - also previous spectacular failures - predict future behaviour to a good extent with predictable risk factors, considered in safety factors. Also known things combined the known ways are preferred (or expects huge costs for your one off object). National building regulations on methodology encode some of these experiments (considering load, material and manufacturing uncertainties on top, also the time factor in the many many decades range) laying a legal foundation on what can and what cannot be done and expected, where the responsibilities start and end. Basically no need to test all new designs if proper - and legally approved - methodologies and procedures are used, rules and methodologies cautiously formed represent the validation there. (unusual shapes, materials, compositions, methodologies not covered by regulations might require much more scrutiny then, and potentially much higher safety factors too. ;-) )
