I asked this: "What contributes most to a beam's strength?"

I very deliberately avoided some keywords. I was looking to see what advice might be given to a naiive question - that's the sort of question that a non expert might ask. It seems obvious that cross section is important but it isn't - it's depth. I am glossing over quite a few things here. That's why you always see beams, rafters, joists etc with the long bit of the cross-section running vertical. The width of the joist worries about buckling and twisting and has very little to do with the load bearing capacity.

Look at a deck. Wood is decent in compression (squashing) and pretty good in tension (pulling). It is also quite good in torsion (twisting) and phenomenally non brittle ie plastic and ductile. Wood is a great all rounder and that is why we use it. So, you have posts (columns) with bearers and stretchers and planks on top. The stretchers are probably eight inches deep and the bearers are something like four inches deep.

To make the deck solid the planks, given that they are roughly 3/4" thick need 18" centre to centre across the bearers. The planks are 3/4" thick and need supporting every 18" - those are the bearers. Bearers are four inches deep and one to two inches thick. The bearers are then supported by stretchers. These are less concerned with bounce and more with direct loading. The bearers are usually very deep and thin beams that are bolted to both sides of the columns. So eight inches deep and one inch thick. Each column has a bearer on each side.

-------------------------------------------------------------------------

This is the answer I got back from ChatGPT:

Answer: "The strength of a beam is determined by its cross-sectional area, its length, and the material it is made from. The beam's cross-sectional area and length determine its stiffness, which is a measure of its resistance to deflection, while the material it is made from determines its strength, which is a measure of its ability to resist forces that try to break it. The greater the beam's cross-sectional area, length, and material strength, the stronger the beam will be."

-------------------------------------------------------------------------

I have given ChatGPT some feedback:

The depth (ie the length normal to the force) of a beam is the important factor. That's why steel beams are often I shaped and the web has holes in it. The holes reduce weight and the depth is the important bit. The top and bottom bits of the beam resist buckling.

The naiive idea that cross section (it looks like a good idea) is the key factor is not only wrong but dangerously wrong. I haven't bothered with mentioning bending moments because I wanted to see what came back.

There is way more to beam design than that but basically, you fail structural engineering 101 and civil engineering 101 and quite a few other disciplines quite badly.

This is am important thing that anyone with 14 days in college/uni on a civ/struct course would learn very, very quickly.

I knew you'd (it'd/whatever) get it wrong and badly wrong because most people don't do fourth or fifth order differential equations for a living and rely in intuition. Intuition is generally very wrong until you really get to grips with this sort of thing.

Just in case I haven't spelt the dangers out properly, a lot of people do some pretty large DIY (home improvement projects). If they get the wrong ideas then failure might be life threatening. Thankfully there is a strong tradition of standard beam sizes and support dimensions etc for a given job.

You cannot put this thing up as an expert if it does not understand the consequences of its actions. Someone(s) will end up dead as a consequence of its responses - it is sadly inevitable. The thing that you are trying to emulate manages to win Darwin Awards with monotonous regularity.