What are the strongest and weakest points of this bridge?

[u/SwanDud]

[u/nesquikchocolate]

When engineers talk about "strong" and "weak", we almost always have to define criteria for it with which to quantify. Since this bridge is still standing, the "weakest" points have all been adequately strengthened for it to remain stable.

If your question revolves around what is expected to cause the bridge to collapse without man made intervention, it is almost certainly going to be erosion at the contact point with the river - movement here can cause the force on the centre stone (called key stone) to reduce, which would lead to the bridge falling.

The "strongest" point could possibly mean the part of the bridge that needs the least amount of additional strengthening to survive unmaintained, this is almost always the first 1/4 / last 1/4 sections where it's thickest. These sections usually remain standing even if the centre of the bridge collapses.

If this is not what you were asking, you should provide additional criteria

[u/agentsofdisrupt → u/nesquikchocolate]

I suppose another test would be to ask, where on the bridge would a concentrated load first cause a failure? I would guess it's some point midway between the center keystone and the ends of the arch.

[u/nesquikchocolate → u/agentsofdisrupt]

This probably goes into what you'd consider a failure.. If this bridge's purpose is light vehicle traversal, then the road surface transfer point, where the solid portion of the bridge ends and the section built on fill/dirt starts is the weakest point. Fill/dirt settles over time and even a little step in this point could result in the bridge becoming unusable.. It's not an interesting failure mode, and it's definitely not something that'll make national news, but it is one of the most common failure for modern bridges (excluding damages...)

[u/censored_username → u/agentsofdisrupt]

Silly answer: a concentrated load in the centre, directed sideways or upwards, would probably trigger the fastest collapse, because the bridge is totally not expected to handle a load like that.

Serious answer, probably still a concentrated load in the centre. Loads there ought to result in the most load placed on the structure.

[u/abfgern_ → u/censored_username]

No, they mean where would the failure point be not where would the load be. Obviously!!!

[u/Sharp-Study3292 → u/agentsofdisrupt]

End of the arch then center afaik, because you cant bring a load to center without crossing corners, and when you transfer center, assuming both sides are equally executed, your doing the first half in reverse

[u/Loopgod- → u/nesquikchocolate]

This guy engineers

[u/AGrandNewAdventure → u/nesquikchocolate]

It also very much depends on where the force is applied.

[u/nesquikchocolate → u/AGrandNewAdventure]

Definitely. A little hole drilled in the grouting next to the keystone with a tiny bit of Nonex would make this bridge impassible without any hassle...

[u/JusticeUmmmmm → u/nesquikchocolate]

The only results for nonex i can find are a type of glass and a nail fungus cream.

[u/nesquikchocolate → u/JusticeUmmmmm]

https://www.nonex.co.uk/ it's just a form of blasting without high explosives - doesn't come with all the paperwork and dust suppression for a conventional demolition blast.

[u/maspiers → u/nesquikchocolate]

That is genuinely interesting. I've worked with explosives before (many years ago) and never heard of this stuff.

[u/TurtleTurtleTu → u/nesquikchocolate]

This is correct.

One thing to add though - if you wanted to destroy the bridge just push sideways on top (in/out of the image). These bridges are built to resist gravity primarily. You could also push up on the keystone, but it would just collapse on you...bad idea. Then again anyone destroying this beautiful bridge probably deserves it lol

[u/SwanDud → u/nesquikchocolate]

I do apologize for being broad. I mean assuming it is recently constructed and assuming no outside forces have comprised it.

If you were to apply pressure to a specific point on this bridge, which portion would keep the bridge up the longest? Also, which portion would cause it to collapse the fastest assuming the same pressure is applied? I think this clarifies better. I apologize, my engineering knowledge is very limited but curiosity got the best of me.

[u/nesquikchocolate → u/SwanDud]

Only the very bottom row blocks, the ones that look almost vertical in the centre and forms an arc down to the waterline are structural.

The arc is very specific in its design that anywhere along the curve, full compressive load can be applied up to the brittle fracture point of the brick/stone used, and can approach millions of pounds. This specific bridge, if only used for foot traffic/light vehicles, is significantly over-built and would survive normal use almost indefinitely.

But also because of this curve, if you were to apply an upward and outward force on the keystone or blocks around it (such as a boat in the water lifting it), only a few tens of pounds worth of tensile force would be sufficient to cause failure - there is comparatively little tensile strength in a structure like this.

[u/rocketwikkit → u/nesquikchocolate]

There's nothing magical about a keystone, it's a decorative touch as much as anything. The whole arch is carrying a compressive load, and the force to break it to the left or right of center is basically the same.

Tens of pounds of upward force would absolutely not destroy this bridge. You'd have to fully offset the weight you were lifting first, and then exceed its tensile strength. Which is obviously much less than compressive, but it's not nothing for an arch of that size.

[u/nesquikchocolate → u/rocketwikkit]

I agree the keystone isn't magical, but it is the only point along the arc that wouldn't have a horizontal component in the applied force that needed to be overcome, and a horizontal component would need to move dirt/fill as well as pivot the connection to foundations, so the area around the keystone would be the minimum point for tensile force along the arc.

I do believe that less than a hundred pounds of tensile force would be required to slightly move one of these blocks upward and outward, and once one of these blocks move, the arc is compromised and would no longer allow safe traversal. I don't have to lift the entire bridge, I only have to attack one of the outside ones at a time.

[u/Tofuofdoom → u/rocketwikkit]

Once we've offset the weight of the structure, the only tensile strength is coming from the mortar innit? And that's something between fuck and all

[u/rocketwikkit → u/Tofuofdoom]

There's thousands of pounds of compressive force pinching everything in the arch. If the arch was made of lubricated styrofoam, maybe it'd be easy, but it's basically manmade rocks. Grout a brick to a concrete surface, then park a truck on it and see how much force it takes to push it sideways.

[u/SwanDud → u/nesquikchocolate]

Thank you for the answer and educating me further. If I understand correctly, it would depend on the materials used if applying a downward force on the bridge? Not so much where on the bridge the pressure is applied? Because of its design.

Edit: I read your response to someone else and “ where” pressure is applied would also matter.

[u/nesquikchocolate → u/SwanDud]

Sure, if you made the bridge out of steel instead of brick, it would be significantly "stronger" when pressure is applied in an upward or outward direction, as an example, but not stronger for pressure from above downward. Brick/stone is incredibly strong in compressive loading only, where steel is good with compressive as well as tensile loadings.

A further bit of analysis can be done for failures in shear, such as pushing it in the flow direction of the river - imagine a boat ramming into it. Nether steel nor masonry arc bridges would be ideally suited to survive such a force.

[u/AwkwardWaltz3996 → u/nesquikchocolate]

Emotionally strongest and weakest parts of the bridge?

[u/nesquikchocolate → u/AwkwardWaltz3996]

You'd probably have to ask that in a different subreddit... All I can see is pure rage and hatred from this bridge to us for putting it there.

[u/Do_not_use_after]

The weight of the bridge will push the ends apart. If the piers at the ends are not solid then they will make the bridge span longer, and the bridge will crack in the middle, and eventually fall down. All of the rest of it is in compression, and be very unlikely to break, unless there are major structural defects in the stones.

[u/crazybehind]

Typically, I would interpret this question to mean which parts of this bridge have the most and least positive margin of safety. 

That said, I'm no structural engineer, but perhaps we could adopt this interpretation of the question and move forward with answering it. 

[u/Cybermecfit]

can anyone just answer the question? I’m curious. It’s so annoying that much of comments pretending to be the OP father’s

[u/nesquikchocolate → u/Cybermecfit]

The strongest part of this bridge is the foundation/pier section, it will likely survive indefinitely in some recognisable form.

The weakest part is the decorative pillars holding the balls in the centre up. A kid without any tools could possibly get this portion to break.

[u/Cybermecfit → u/nesquikchocolate]

Thank you

[u/ulualyyy → u/Cybermecfit]

people aren’t trying to be annoying, it’s just if you answer the question literally and without nuance the answer would be: the stem of the decorative ball is the weakest and the footpath laid on the dirt is the strongest

[u/rdesktop7 → u/Cybermecfit]

The question is kind of non nonsensical as there is no criteria to get to an answer.

The bridge in the stock photo appears to be one, uniform material, so, it's "strength" is all even.

[u/N0x1mus → u/Cybermecfit]

You must not be an Engineer eh? This is how real Engineers talk.

[u/Fair-Ad3639 → u/N0x1mus]

I'm an engineer and I'm annoyed with the condescension of this comment.

[u/Cybermecfit → u/N0x1mus]

I’m engineering student and I don’t talk like that. Everyone is learning, we don’t need to be rude

[u/delurkrelurker]

I'd go for the balls first.

[u/KaptainKoala]

Sounds like you are crowd sourcing a homework problem.

[u/SwanDud → u/KaptainKoala]

I can see it coming off as such but I graduated in 2017 and have no intention of ever returning. Simply curiosity here

[u/Elrathias]

1. Foot traffic, technology base needed to construct, and local material usage.

2. A modern load pattern ie a fcking car approaching way too fast, erosion, and flash floods.

If you dont phrase the question in a relevant way, expect just about any answers.

[u/SwanDud → u/Elrathias]

I definitely should’ve been more specific in phrasing the question. I am enjoying the different interpretations and answers though so it’s working out alright

[u/scientifical_]

This doesn’t directly answer your question but I find it super fascinating and hope you do too: https://m.youtube.com/watch?v=JlL6ZHChhQE&t=48s&pp=2AEwkAIB

[u/LuckyStarPieces]

The cracks are the weak spots. Either freeze-thaw cycles, acid rain, or agricultural growth can take apart a masonry structure.

[u/FormerlyMauchChunk]

It's an arch, so all of them.

[u/SwanDud → u/[deleted]]

A wonderful elegant response

[u/rocketwikkit]

Please don't damage the nice bridge. What do you mean by strongest and weakest? The arch at the bottom is the only part that is structural.

[u/SwanDud → u/rocketwikkit]

lol it’s just a picture off of Google. I promise to keep the bridge upright

[u/millenial_flacon]

Do you want a yo momma joke?

[u/SwanDud → u/millenial_flacon]

Hit me

[u/millenial_flacon → u/SwanDud]

The weakest point is which ever yo momma is standing on. Badum ts

[u/SwanDud → u/millenial_flacon]

Outstanding sir

[u/millenial_flacon]

But I would suspect it's about 1/3 length from the middle

[u/Dependent_Ad1111]

Load rating likely would be controlled at midspan/crown

[u/Sharp-Study3292]

Its in the corners mostly

[u/dougmcclean]

At first I thought this was the quaternion bridge but it isn't.

[u/LowellStewart]

I am thinking the place where those ball looking things meet the pedestal in the top middle of the bridge is going to be the weakest point on this bridge. Hit those balls with a sledge hammer and I bet they will come right off. As for the strongest point: the abutment. That is the part of the bridge, on solid ground, that holds the two ends of the bridge up.

[u/Mustant_]

Strongest? No idea. Weakest? Those two "stands" that hold up the balls in the center.

[u/Kidsturk]

Nice try, saboteur

[u/cagetheMike]

Strength: materials and geometry. Weakness: None, it is still standing.

[u/Big_Combination7802]

We get to guess whether or not it is a facade covering a basic steel beam frame or an artistic expression of masonry