how were bridges designed before, say 1850, sometimes way before, able to handle the weight of a ww2 tank?
Were they overbuilt by orders of magnitude, miracle of the arch, or something else?
In: Engineering
Modern engineers use calculations to optimise bridge designs so that they don’t use way more material than they need in order to make a bridge strong enough. This saves time and money.
We know how strong certain types of steel, cement, wire cables/ropes and other materials are because we (material scientists) have done lots of testing, by taking samples of materials and loading them in different ways until they break or deform past some limit after which we say they have failed.
Engineers calculate the expected loads on a bridge, and then the bridge is built with some safety factor. That means that if they expect that the heaviest load a bridge will ever see is a 100 ton train going over it, they might build it so that it can actually support a 500 ton train. If the bridge expected to see 100 ton loads and they only designed it for a 100 ton load, then a train that was very slightly overloaded and weighed 101 tons might make the bridge fail and collapse. Having a safety factor of 5-10x (in reality, safety factors may be different for the different materials / elements of a bridge e.g. 5 for the concrete and 8 for steel wire ropes) allows for things like an overloaded train, wear, corrosion, general aging of materials and things like that.
Before modern engineering and materials science existed, we couldn’t calculate the loads on bridges, and we didn’t know how strong materials were, plus metals and other materials (e.g. concrete/plaster/mortar etc.) were of poorer and less consistent quality. They couldn’t calculate how to build a bridge whilst also saving on materials reliably, so they just overbuilt them, sometimes by a lot. They might have had a safety factor of 100 or more, meaning that they could support 100x the load they really needed to.
Bridges from hundreds of years ago which weren’t overbuilt by enough wouldn’t have survived until now, so the ones which are left are generally the stronger ones. This is called “survivorship bias” that some other people have mentioned.
Bridges from today can still fail, but it’s usually through neglect (failing to maintain or repair bridges as the materials age), design faults, or unexpected loads like just recently when a boat crashed into a bridge pier in Baltimore.
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