You don't need precise numbers to figure out whether your bridge will stand. What you need is a calculation designed to be robust to the errors incurred in mea

You don't need precise numbers to figure out whether your bridge will stand. What you need is a calculation designed to be robust to the errors incurred in measurement and computation. The standard for floats guarantees you specific and precise error bounds that you can use to do an error analysis for your whole calculation. Most likely whatever engineering software you use to check your bridge design, will already have this error analysis baked in. If you introduce some arbitrary other errors, you'd have to redo you error analysis from scratch. And it might not even be tractable, depending on the errors introduced. (The standard floating point error guarantees are designed to behave reasonably well and easily predictably when combined into a larger calculation.)