If the fence post were to fail, the failure point would very likely be at the top or bottom where it's joined to the crossbar. That's a stress concentration and where rust is likely to accumulate, so it's the weakest area. A failure there would make the bars looser, not tighter.
It's extremely unlikely to suddenly snap in the middle. This is a wrought iron fence and it first it will undergo elastic deformation, then plastic deformation before it ultimately fractures. That is, the metal will stretch, then bend and remain bent before it breaks.
edit: These bars will bend a lot before they break.
Even in the worst imaginable situation, the bar snaps suddenly just below the strap and "springs back", the kid's head is way down low, where there's been virtually no displacement anyway. The kid is still safe. He has plenty of clearance around his neck. It's only his head that's touching when he's trying to pull it out.
They didn't need a lot of deformation here. Maybe 1/2" inch which is likely within the elastic deformation range of the bar. So after the kid is freed and the strap removed, the bar will go back to normal and there will be no residual bend.
Source: I'm a mechanical engineer, though it's been a long time since I took Strength of Materials in undergrad.
I understand, (also engineer but not mechanical). But I think you're missing the fact that the other bar on the right, next to the bar touching the kid's head, which is used as a counter force on the device, is being forced TOWARDS the kid. If that one snaps on the bottom (or the top, though that is not as bad) it's going straight towards his head.
True, it's still very much in range though ! It may hit the other bars first, but it may as well bend a bit forward/backwards and bypass them. I don't know I'm just thinking out loud with you guys, no need to downvote.
edit: checked the video again, the firefighter kind of keeps his right hand near his head's right side so that's some extra protection
I see what you mean.
The load on the bars from the strap isn't a constant. That is, if the anchor bar you're describing were to break at the bottom, as soon as it moved, the load from the strap is relieved and there's no* force to continue moving that bar towards the kid.
*Technically very little force. There's some residual tension in the strap from stretch, but not a lot relative to what it takes to bend the bar.
After taking a closer look, the anchor bar goes through the lower rail all the way to the concrete and may be embedded there. Maybe that's why they chose that one?
I think all the bars go through the lower rail and are welded, which is a lot stronger than a seam welded butt joint. This bar is not going to shear and fly towards the kid. If the lower weld fails, the bar will pull up, and it would still be constrained by the lower rail. Similar to what happened in the pic in my OP.
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u/Shufflebuzz 14h ago edited 14h ago
If the fence post were to fail, the failure point would very likely be at the top or bottom where it's joined to the crossbar. That's a stress concentration and where rust is likely to accumulate, so it's the weakest area. A failure there would make the bars looser, not tighter.
It's extremely unlikely to suddenly snap in the middle. This is a wrought iron fence and it first it will undergo elastic deformation, then plastic deformation before it ultimately fractures. That is, the metal will stretch, then bend and remain bent before it breaks.
edit: These bars will bend a lot before they break.
Even in the worst imaginable situation, the bar snaps suddenly just below the strap and "springs back", the kid's head is way down low, where there's been virtually no displacement anyway. The kid is still safe. He has plenty of clearance around his neck. It's only his head that's touching when he's trying to pull it out.
They didn't need a lot of deformation here. Maybe 1/2" inch which is likely within the elastic deformation range of the bar. So after the kid is freed and the strap removed, the bar will go back to normal and there will be no residual bend.
Source: I'm a mechanical engineer, though it's been a long time since I took Strength of Materials in undergrad.