Any advice on making a cattle grid?

Useful link. CHS or circular hollow section is just a posh way of saying "tube".

It is "Moment of Inertia", aka "Second Moment of Area" don't blame me for whatever terminology structural engineers use :-)

...

I don't think the dog is supposed to get into the pit at any point.

If you dont mind your poles reaching the pit bottom after a twenty tonner has gone over.

High strain to failure with low elasticity means plastic deformation, You want springiness...

Don't be obtuse, you were complaining that scaffold poles were weak, and I was pointing out that you want scaffold tubes to be ductile. So yes, they are not an ideal choice for a cattle grid, you would be better to make these from a "structural steel" with a higher yield stress. But OTOH they are more readily available and affordable. If you go to a steel supplier looking for something with a yield strength of say 350MPa it's going to be more expensive than random steel scaffold tube (and of course there is a second hand trade in that too).

No, I wasn't complaining they were weak. The phrase was 'the worst grade s*it imaginable' . That is an observation, not a complaint.

You then interpreted that as weak. In the sense of low strain to failure. That is *your* invention, not mine.

I actually meant they are pretty damned soft and will bend under very low loads and *stay bent*.

It's the difference between say - piano wire - and a coat hangar.

Scaffold steel is of the coat hangar variety.

Fantastic for bending and staying bent, not much good for bending and springing back.

I think the correct adjective is 'ductile'.

An elastic band has a higher strain to failure than a steel fishing line leader. Whether that is a =good or bad quality depends...

You do know what 'strain' means don't you? - how much change in (length) there is under a given STRESS....

So yes,

I walked past a cattle grid today at the boundary of some National Trust land. It was made from steel tube of about the same diameter as scaffold pole - maybe slightly larger. The width was about 2.5m and it regularly has heavy trucks driving over it as it gives access to a building firm. There were four supports plus the frame at the sides. The supports were in pairs, each pair being roughly where the wheels of a truck would cross. I have seen dogs crossing this cattle grid, so its not much use for the OP, but it shows the considerable amount of support that its designer thought would be necessary.

John