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
yes; many things are overbuilt, even on accident without knowing the future. Many stone bridges from colonial/victorian (and before) days could support a lot more weight than the average carriage or full farm wagon of the time…..now, did they build it knowing that in 100 years a metal tank would drive over it?? likely not.
then there is the reverse side of engineering; where the tank designers can say, these are the major bridges in the area and their dimensions…..so lets build tanks that arent wider than these bridges we’ll likely encounter
fun story: Joseph Bazalgrette was in charge of building london’s sewers………he basically said “lets only do this once and double the diameter of the pipes planned” this decision has allowed the sewers to last a lot longer than the original pipes would have allowed due to the increasing populatioj
as others have said, the bridges were overbuilt, though that might be selection bias at work (IE the less stable bridges have been removed by the ravages of time).
also, for a LOT of older bridges, it was common to build them at fordable stretches of river, because it was really difficult to build a bridge in deeper water with pre-industrial technology. The water around these old bridges was often shallow enough a tank could drive across, assuming suitable ingress/egress routes either side. What the bridges were vital for was the wheeled support and logistic elements of the army, that couldn’t just drive over the riverbed.
obviously, with larger rivers too deep to ford, the bridges over them tended to be more modern, or they used pontoon (floating) bridges to get the tanks over.
that said, the limits of infrastructure put a lot of constraints on tank design. stuff like the width of railway tunnels, the lifting capacity of dockyard cranes, or the weight limit of assault bridges were all factors that fed into tank designs. one of the issues with some of the late war German tanks with their very long, powerful main guns was they often struggled to move in some medieval street plans because their guns kept getting stuck in building, etc.
Anybody can build a bridge that stands for 100 years and can handle a 3 tons of weight. It takes engineering to build a bridge that can *just* handle 3 tons of weight and will last for *exactly* 100 years. Prior to optimal engineering people just had to build stronger than necessary because they couldn’t otherwise know it was *as* strong as necessary.
Survivorship bias. Before structural engineering was truly codified like it is today, you had a lot of variability in structures with a much larger range of over built and under built structures. The over built ones survive. The under built ones did not.
The Deutches Museum has an excellent exhibit showing the history of European bridge building
Yes they were mostly overbuilt. Keep in mind they didn’t have the same ability to calculate load bearing as now and it was an unbelievably amount of work to build these things before cranes and stuff were invented so they were built to be way stronger than they needed. The cost of one failing was massive.
It was also common for some rich dude to pay for the bridge as a vanity project or for philanthropic reasons. They’d want it to last well beyond thier own years and be big and impressive.
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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