[pfdzm]

Modern calculations are usually multiplied with several 'security factors'[0], anywhere between 1,1 and 1,5 as far as I can remember now (at least in Germany through a DIN [norm], and now Europe-wide via EN). These design factors come in at many different stages of the calculations, from materials to static load distribution (not sure if these are the correct terms in English)

[0] https://en.wikipedia.org/wiki/Factor_of_safety

[jaclaz]

Well, it is not like "earlier calculations" didn't take into account such safety factors, but the debate is not about existance of safety factors or their amplitude (1.3 to 1.5 has been used as far as I can remember in concrete constructions) but rather on redundancy.

To give a different example elevators/lift (those using rope cables) have usually cables that have a factor of safety 5, but they have nonetherless additionally at least one set of independent brakes.

Back to bridges and more generally reinforced/pre-stressed concrete structures, in my experience modern methods of calculation are more precise than old ones, and allow usually - given the same loads/hypothesis - to save (i.e. there is less rebar steel and cable steel) between 5% and 10% steel and/or concrete.

In practice with old methods of calculation there was a 1.05/1.10 "implied" and "hidden" additional safety factor.