| The reason this works is that the safety margins accumulate. You have a safety margin on: * the loading on the bridge, which can be higher than the calculated amount. * the resistance of the materials used, that can be lower than the requested amount. * the quality of the soil, * the lifespan of the bridge, * ... So the loading can be 25% percent higher, the materials can be 25% worse, the soil can be 25% worse, the lifespan can be 25% higher, ... and at the end you get a bridge that would still work if everything fails with a margin of 25%. If the loads are 30% higher, the bridge might or might not fail. Who knows. Even though there are many other safety factors, and the bridge probably won't fail, it was not designed for that. There might be a critical part somewhere that has a resistance that's 25% worse than what it should be, and it therefore can only support a 25% overload, and with 30% that part might fail. Bridges are designed to fail very slowly, loudly, and visibly, to avoid loss of functionality (e.g. when a bridge start to fail, this becomes obvious, and you still have months or years to sanitize the bridge). I expect airplanes to have much tighter safety margins (<15% or <10%). The loads are more accurately known, the materials are higher quality, everything passes more extensive quality assurance tests, weight is much more important and companies are willing to pay prime prices on materials, manufacturing, etc. to reduce it, etc. Safety margins aren't chosen arbitrarily. The job of a safety margin is to quantify an uncertainty, and with enough money thrown at each one, most of the uncertainties can be quite precisely quantified. Then it's the job of the designer to say "This plane should be in service for 50 years", and from the uncertainties and statistical analysis you can compute the highest load that the plane will receive in those 50 years with a certain quantified margin of error, and continue the design using that. |