When I first started in the electrical trade, it was still predominately slide-rules. Young engineer would have some doubts about his calculations and add 5% to everything. Next engineer up the chain would check numbers, have some doubts and add another 5%. All along the line until the finished product was presented the magic 5% would be added at each step. that for the most part is why so many old things were so overbuilt; "just another 5% to be sure..."
Now, with computers, if a building is designed for a 6.0 earthquake it will stand one. However, give it a 6.00001 and that sucker is a heap of rubble. Computers do have the ability to cut costs with construction. Not sure if I'm comfortable with that.