The basis for ‘strength of materials’ (the engineering discipline that deals with “load something can bear”) is in empirical testing, meaning, someone somewhere built a test bench to measure what happens when a load is placed on a something.
They document their findings, share it with someone else (usually far away) who repeat the test and either confirmed the result or noted a deficiency.
With enough confirmed data-points, we can make broad inferences about ranges of loads on a single something, or about a specific load on ranges of somethings…
This helps engineers choose which materials to use for specific loads/applications.
In a parallel process, physicists use the same data to derive formulae that generalise behaviour of specific materials, but we’re not very good with that yet, and we’re still discovering new combinations of materials almost daily, seeking lighter, stronger, cheaper and more reliable materials all the time. Recently we have also added “environmentally friendly”, “humanely sourced” and “carbon neutral” to the criteria – this removed a whole bunch of otherwise useful materials, like asbestos and teflon, from our “toolbox”
We also have regulations/guidelines in the industries we operate – these usually dictate safety factors, deflection limits, environmental considerations, fatigue cycles and functional lifespans – a suspension bridge design that is deemed safe in Germany might never be passable in Netherlands – as an example.
A lot is “trial and error” (loading until it breaks), taking note, seeing if it repeats with another of that thing multiple times, and then making an assumption that under certain conditions it will be the same in the future if the levels repeat.
If you mean in general construction, there are tables for the size required for different spans and different uses (e.g., studs v. headers v. joists), so a lot of times, it’s more lookup than calculation.
For custom jobs, the materials are so standardized and their properties so well know that there are formulas to calculate beam size.
OP, I don’t know why you’re getting all these replies that fail to answer your question. What you are asking is essentially the job of a mechanical (and in a more limited, dirt-eating capacity, civil) engineer. I happen to be a mechanical engineer, so I can answer this for you (I’ll do my best to ELI5)
Figure out the size and geometry of the load(s) (weight, how it’s spread, pulling/pushing/twisting, etc). We usually employ what is called a free-body diagram to represent this. We count support loads (like the ground) here too.
Determine the geometry and material properties of your load-bearing part (stiffness, strength, dimensions)
Analyze how the load-bearing part wants to deform because of the load. There are many ways to do this. Hooke’s Law is the most traditional, but the most common method today is via software (called Finite Element Analysis). The exact techniques you use depend on how your system looks and what you want to achieve.
Work backwards from your analysis to figure how much load equals the maximum deformation of your part before failure (however you define failure, usually yield).
Comments
The basis for ‘strength of materials’ (the engineering discipline that deals with “load something can bear”) is in empirical testing, meaning, someone somewhere built a test bench to measure what happens when a load is placed on a something.
They document their findings, share it with someone else (usually far away) who repeat the test and either confirmed the result or noted a deficiency.
With enough confirmed data-points, we can make broad inferences about ranges of loads on a single something, or about a specific load on ranges of somethings…
This helps engineers choose which materials to use for specific loads/applications.
In a parallel process, physicists use the same data to derive formulae that generalise behaviour of specific materials, but we’re not very good with that yet, and we’re still discovering new combinations of materials almost daily, seeking lighter, stronger, cheaper and more reliable materials all the time. Recently we have also added “environmentally friendly”, “humanely sourced” and “carbon neutral” to the criteria – this removed a whole bunch of otherwise useful materials, like asbestos and teflon, from our “toolbox”
We also have regulations/guidelines in the industries we operate – these usually dictate safety factors, deflection limits, environmental considerations, fatigue cycles and functional lifespans – a suspension bridge design that is deemed safe in Germany might never be passable in Netherlands – as an example.
A lot is “trial and error” (loading until it breaks), taking note, seeing if it repeats with another of that thing multiple times, and then making an assumption that under certain conditions it will be the same in the future if the levels repeat.
If you mean in general construction, there are tables for the size required for different spans and different uses (e.g., studs v. headers v. joists), so a lot of times, it’s more lookup than calculation.
For custom jobs, the materials are so standardized and their properties so well know that there are formulas to calculate beam size.
Where’s Calvin’s dad when you need him?
OP, I don’t know why you’re getting all these replies that fail to answer your question. What you are asking is essentially the job of a mechanical (and in a more limited, dirt-eating capacity, civil) engineer. I happen to be a mechanical engineer, so I can answer this for you (I’ll do my best to ELI5)