[lazide]

Interestingly, the design of beams and the like (used for columns) still means tensile strength is the limiting factor, not compressive strength.

For compressive strength alone to matter, you’d essentially need a solid steel column - which is so prohibitively heavy, it would often end up collapsing under it’s own weight (in tensile failure, most likely) before it got up to a useful height.

[max51]

I'm a structural engineer, and this is incorrect. For structural steel (e.g., A992, 350W, etc.), the tensile strength is the same as the compressive strength. That's why we only need one value for both.

And yes, when we design beams, we check the stresses in both tension and compression. For a symmetrical shape like an I-shaped beam, both verification will be the exact same formula so we don't need to calculate both explicitly.

For columns, we do check the compressive stress, and it is the controlling failure mode for short columns. Long, slender columns will buckle before the compressive stress exceeds the allowable limit.

[lazide]

It depends on the geometric stability of the column shape, which is why I said what I said. The reality is that compressive stress limits are much lower in steel than tensile, as you’re noting.

They are only the same in an abstract theoretical sense, not an actual one.

And bucking within the column (and/or the resistance too it) is primarily resisted by tensile strength.

[max51]

>It depends on the geometric stability of the column shape, which is why I said what I said.

What you said was like 99% completely false.

>that compressive stress limits are much lower in steel than tensile

No they are not.

> as you’re noting

I never acknowledge that. Your lack of understanding in what column buckling is doesn't change anything. Unironically, you should ask chatgpt, it can probably give you a high level explanation that is more compatible with your understanding of physics than what I can give you.

Buckling is a stability problem, not a resistance problem. The compressive stress resistance is literally not part of the formulas that we use to verify it. The column could have 100 MPa, 350 MPA, or 359918 MPa compressive resistance and it would change nothing when it comes to buckling. Only the elastic modulus and the physical shape of the column (length, inertia, etc.) is relevant for buckling verification.

>And bucking within the column (and/or the resistance too it) is primarily resisted by tensile strength.

Than please explain to me why we don't need the tensile (or compressive) resistance of the material to know the buckling resistance of the column. I really want to hear that one.

[lazide]

Ah engineers who do not understand the fundamentals of the values they are using. I expect this will piss you off, but I expect it is important you know.

The easiest way to visualize Young’s modulus is typically using a stress/strain curve from a tensile force test, but can also be visualized by a compressive force test.

Young’s modulus is the elastic region that is ‘under’ both curves, since it’s a value for how much something can safely bend* and return to it’s original shape without permanent damage, and bending causes both compressive and tensile forces in a material.

The lowest value of the two upper limits - of course - will set the upper elastic limit!

Specifically, to a very simple approximation, the ‘inside’ of a bent beam will be limited by compressive strength, the ‘outside’ by tensile. The overall beams strength will be whatever the lowest of the two values is, as that is when the beam will fail somewhere, hence, that what you want to use in civil engineering, eh?

*at a first approximation. It is of course much more complex than that.

For a column, the limit is the lowest of the ability of the column to prevent deflection (tensile) and bear the load without ‘pancaking’ (compressive strength). Pancaking is resisted by the full cross sectional area of the column, while deflection is generally only resisted by (to a rough approximation - greatly depending on the actual geometry, as I noted in my prior comment!) half the thickness of the column, so tensile strength of the material is usually the limiting factor for most simple columns of non-trivial length, where unsupported column deflection is the dominant failure mode.

Notably, this is why concrete often uses rebar in civil engineering, as concrete has trash tensile strength on it’s own and requires massive volumes to have sufficient tensile strength to form acceptable beams or unsupported columns, or use only very limiting forms to ensure only compressive forces actually occur. Unreinforced Concrete’s Young’s modulus is trash for this reason.

Which for civil engineering is the best bet!

Also, add a good safety factor on top due to all the other materials fundamentals that they apparently don’t teach in civil engineering school?

Which hey, I get, because they’d be too overwhelming eh? And they’re generally drowned in the noise at that scale anyway.

Depending on the sub discipline, mech-e, aero-e, etc. will of course have to know these things at a finer level of detail.

Someone computing fuselage thickness, designing an engine connecting rod or turbine blade, etc. needs to know what is going on at a much finer level eh?

Those folks will also have to quantify the type of failure modes they expect (fatigue limit, tensile failure, compressive failure, wear, etc.) and design around it.

Young’s modulus isn’t fully useless in those usages, but more specific values tend to be far more useful, as Young’s modulus is fundamentally the lowest value of a mix of material properties.

[max51]

omfg, I was replying to an LLM

[lazide]

Hardly, just someone who actually knows what is going on.

But then, you’d know that if you looked at my profile eh?

And if you actually knew what you were talking about. Care to quantify your ‘99%’ wrong comment?

[lnenad]

Yeah, nothing exists in a vacuum :), even for concrete columns rebar is added.