In physics forums there are often questions about the construction of machines or structures that remain unanswered due to this guideline (to be found in the global guidelines of the physics forums):

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But all you want to know is how much weight a beam can support or something like that. There are formulas for that?

There are. But the well-known equations give that Maximum value that the bar can possibly stop. In other words, if you have 1000. to get No In response, this means that it will certainly be impossible to cross that limit … but the value could be lower if things are not perfect. And they never are.

How much less? Here is the concept of Safety factor is presented.

Safety factor

The safety factor relates to how confident you are about the data and the mathematical models used:

  • Have the material properties been determined by actual tests on the material to be used or is it an average value from the Internet?
  • Was the mathematical model very detailed or was it a simplification?
  • Will the environment be stable or are temperature changes, corrosion, or other variables involved?

In mature projects, safety factors between 1.2 and 3 are regularly used. Yes, that means that our 1000. even in the toughest projects No Value would be at least 833. reduced No and maybe even as low as 333 No. In the case of heavy equipment or where the protection of human life is required, a safety factor of 10 is quite possible. This is for mature projects.

Public forums

But for a piece of text found on a public internet forum – where you are not sure – that’s a different story:

  • Is the project well described and is the description well understood by everyone?
  • Which material is used? Will there be a representation by one of the readers? (for example not all steels have the same properties)
  • Will anyone change plans or uses along the way?
  • Is the construction carried out professionally (e.g. welding quality or correct tightening of the screws)?
  • Are there any local codes that can be different for each reader?

With all of these unknowns, a safety factor of 10 is likely too optimistic to safely release such information publicly. This would result in ridiculously large structures or parts that are therefore completely useless to anyone.

Unfortunately, once a number is given by someone on a science forum, it seems more trustworthy than the traditional statement “Sure, it’s strong enough” often found in public forums. But the number is just as good as the information it is based on, so no more reliable than any testimony from a stranger on any forum.


The safety factors are based on empirical values. Once science spits out a number, experience with each individual case will determine how reliable that number is. Experience is much more important than science. Even an untrained builder can have enough experience to determine how strong a beam would be in a given situation.

The only thing science brings to the table is the evaluation of this maximum value, which is determined by mathematical models. Experience is required to get as close as possible to this maximum value. Engineering sciences combine science and experience. There are different engineering areas to cover different competencies.

Ask what then?

Would you like to build something but don’t know how? A public forum like Physics Forums can:

  • help you understand the theory … assuming you looked for it first;
  • help you locate the sources for the relevant information;
  • help you ask the right questions of the experts you consult.

In the end there is no easy way. If you don’t know how to make something safe:

  • You have to find someone who knows how;
  • or it will take a lot of work on your part to learn how to do it.


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