Example: An exercise bike that can support 250 lb. Does the engineer find a large enough weight that deforms the bike, then take a fraction of that as the amount it can support?
In: Engineering
On top of what everyone has said, we find (normally through modeling and simulation) what the weight would be in multiple configurations to induce “plastic” deformation, thats when the material starts behaving not like a rigid solid, but more like goop. We then design to a safety factor. Bikes for example, where material isn’t an incredible cost, maybe a safety factor of 5. The space shuttle had something like 1.1, where the vehicle could withstand 10% more than was ever expected.
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Civil-Structural Engineer here:
Structures are usually made out of materials we know the properties of very well, eg. concrete, steel, wood, aluminum, etc. Most of these materials have a point at which their deformations become permanent even after you take away the load. Picture a paperclip not returning to its original shape bc you bent it too far. This point is often used as a reference point to determine the “capacity” of the structure or material. Depending on the use-case of the structure and material, there will be safety factors, checks, and troubleshooting to make sure you never get close to that point.
It’s a lot of knowing maximum stress of a material (typically public information) and then using it’s ultimate stress (which is force by area) of materials, and then measuring area, apply area to application, carry some numbers, you did it!
For bridges, they drive bigger & bigger trucks over it until the bridge collapses. Then they rebuild the bridge and weigh the last truck. (Calvin’s dad)
actually, in the case of bridges, they build it, test until it breaks, then build an exact copy. just kidding. most cad software can run analysis for you, so they do that now.
Say you have to design a makeshift bridge over a gap with just a plank of wood (think of a pirate gangplank)
If the plank (bridge) has to support 250 lb you could analyze the structure and use maths to determine how thick the plank has to be in order to support the pressure of a 250 lb weight before permanently deforming (snapping in this case I guess).
And then you factor in a safety factor and pick a plank that’s thicker and will theoretically deform at that new bigger pressure.
Im honestly not sure what that means for the ‘technical’ weight the bike for example can support or how it’s marketed or labelled.
If you turned the question around and came upon a plank bridge already existing and had to calculate if it’s safe to cross or how much it could support and you’d come to the conclusion it can support 250 lb before deforming it would just be that :p
I dont know if that makes any sense, I struggled a lot trying to find the words to explain what I mean in that last paragraph but I hope you managed.
You can calculate the strength of the structure based on the material strength properties, shapes and configuration of the structural supports, and location of the load and then you apply a factor of safety to account for manufacturing defects and variances in material properties. If you calculate it can support 200 pounds with a factor of safety of 2 then you rate it for 100 pounds.
Aircraft structural engineer here.
Firstly, we have material properties. By performing special tests with specimens we could determine how much “force” will hold material regardless of part size and form.
Second, during hundreds of years of strength science humankind has developed various mathematical models, which helps us predict behaviour of various parts under various ideal loadings. So, we could determine how much deformation or force a particular structure will hold up to some failure point. With accuracy less than 1-2%, which is more than enough, because most materials will vary in properties for the same 1-2%.
Third, we could predict under which condition structure will be operated in real world. That really depends on the type of structure or machine, like for the usual chair we knew it should withstand a sitting person but somebody could put on it an anvil or something, which is hard to predict. But for an airplane we had determined the exact condition under which it will be operated. That allows us to make more accurate calculation and less safety margins, therefore lighter structure.
Having this knowledge we could predict behaviour of almost any structure in reality under ideal loading. And to account for some reality, like not an ideal load (not exactly like in a mathematical model), sometimes much greater applied load (anvil on chair) or some discrepancies during production we use “safety factor”. In other words, we just multiply predicted load conditions for some factor, which really depends on the type of machine or structure. So, for the usual chair we could easily make it 5 times heavier than it should to be by mathematical model with ideal load. But for an airplane (or any other structure where we must keep it as light as possible) we have much more accurate load conditions, therefore less safety margin.
So, in summary we have a lot of math models to predict loads and structure behaviour. Finite element modeling is good in concept, but in reality only really good engineers could use it properly and make safe structure.
Thanks for your attention, I will try to answer any questions)
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