How are weight limits on chairs and other things tested?
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By putting weights onto them.
For instance elevators need certain safety margins. You roughly know using mathematic formulas and such (simulations for instance.. don’t even need to do it IRL) how much that elevator wire and the chair leg can theoretically hold… material science, we know how certain materials behave in terms of load limits etc. We know when certain materials snap at xyz diameter.. how? We tried it… there is big data sheets for mostly any material you could ask for which one can put quite reliably into formulas and simulate stuff.
Then you just put weight on it to proove this hypothesis..again can be done virtually.
Then you take a good bit off that to be on the safe side.
Same goes for airport runways. Most planes can easily land on way less of a runway but we give extra safety margins just in case unlikely cases can happen.. this is quite a tangent but part of why the airbus a380 was inconvenient.. the thing was so large it regulary would need longer runways or less fuel to land safety according to regulations. Globally.. so it ever only flew to airports that can hold it safely….which limited it’s usefulness massively….not that it cannot do Land there given a pilot knows the machine…but we ultimately don’t wanna go beyond safety limits.
Ofc for the chair it is as simple as: the wood may not be as sturdy. After all we use averages. In 2 million chair legs there can be a flimsy one etc… for the plane it may be overshooting that runway.. for the elevator it is the cable snapping.
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Materials are produced in a specific way so they will have a certain amount of strength and toughness. When you buy a specific grade of steel, aluminium, or whatever, you know what it can do based on the specifications. Every batch of material that the steel mill produces gets tested via a small section to ensure it meets the intended specification.
When a product is designed, the designer knows (or more likely an engineer who checks his work) what it should be able to handle based on the material specs. In modern times you can do this all on a computer and use a special program to do tests on it. Finally you test it in real life by making one and putting weights on it. (Testing using overweight people would be an OSHA violation and difficult to repeat accurately)
Often times companies will utilize a series of incremental weights. Once the product is able to withstand a certain weight without compromising the structural integrity they move to the next weight.
They will also have these weights bearing down on the product for several hours or days evens to be sure the product design can withstand prolonged exposure or repeated pressure, aka lifting the weights, resting them on the product and lifting again etc.
Once the product has shown an inability to remain structurally sound that is now the weight capacity. Also often times they just get your mom to sit on it, much cheaper.
I work for a large furniture manufacturer in North America and am involved with the design and testing of various types of furniture daily. I can weigh in on the furniture side.
For a stationary piece like a sofa or chair (no recliners), there are a series of tests that stress the wood framing like pulling on the arm or back, or dropping weights into the seat, evaluating the stability, etc. These are all standardized with static and cyclic loads and times. There are various regulatory agencies that develop these methods like BIFMA, UL, CSA, etc. They explain exactly how to setup and perform each of the tests.
99.9% of the time, there is no FEA/Simulation performed on these units prior to building and testing them. At my company, the designers for these are not trained in a engineering background, and instead are equally as knowledgeable based on years of experience designing, building, and testing similar frames. It is faster for them to spend an hour building a new frame rather than performing a simulation on every joint. Most frame connections are reused from existing designs, with the exterior profile of the frame changing to represent a different shape.
Motion furniture is tested in a similar fashion but the motion mechanisms underneath would have a much more rigorous evaluation process prior to building and testing, including FEA/Simulation. The designers in this group are trained in an engineering background. To think of it in a time perspective, the designers for the framing might develop a new frame every few weeks, while the motion mechanism would be developed over 1-2 years.
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