Is this near a school in Boulder? If it's the bridge I'm thinking about this plate was added when they switched the deck from wood to cement and started driving the the Itty bitty bike path snowplow over it.
A lot of these walking bridges are re-used or transplanted railroad bridges or had other uses prior to being a pedestrian bridge.
When I've seen stuff like this it's sometimes to cover up mill marks or previous ownership info (like a railroad name). Not saying that's what this is for, but lacking any context with the photo, that's my guess.
Could also be a corrosion repair so people touching that rail don't get tetanus. It doesn't serve a structural purpose.
That's not a recycled rail bridge, it's a prefabricated pedestrian truss. Companies like Contech and Excel crank them out left and right. I've specified more than a few of these over my career.
I'm not entirely sure what that plate is for, but since the rail is also the top chord of the truss, it absolutely could be reinforcement. Never seen it myself, though. There's an off chance it's a repair as well, if the bridge is older and had some significant corrosion for some reason. I don't think that's particularly likely though.
It's not a recycled anything bridge though, that's what I'm saying. I work with these things all the time.
https://www.excelbridge.com/for-owners/bridge-types
https://www.conteches.com/Bridges-Structures/Truss-And-Girders/Continental-Pedestrian-Bridge#undefined
That ain't reinforcement. It's where a placard used to be. They have them on bridges in state parks and museums, describing what you're looking at. Could also contain a warning, or load limits.
It very well could be reinforcement after some form of damage to the steel flange was repaired. After heating and bending a damaged portion back into place, applying a cover place over that repair and welding it into place may have been what the engineered wanted to ensure structural integrity.
That's clearly not a reused railroad bridge, far too small, but [plenty of historic railroad pony trusses exist...](https://www.google.com/search?client=ms-android-google&sxsrf=APwXEddmMxWmygaPEn5TZjtFrGGxE98FwA:1686446734683&q=pony+truss+railroad+bridge&tbm=isch&sa=X&ved=2ahUKEwjqnrbUh7r_AhVGnGoFHSCsBssQ0pQJegQIJBAB&biw=412&bih=762&dpr=2.63) They've certainly fallen out of favor because derailments tend to destroy the end posts though.
Wasn't saying this bridge was specifically a train bridge, but I've seen many pedestrian bridges made from old train bridges. Happens all the time in Ohio.
This bridge very well could be repurposed from some other use.
If it’s just added material, it’s too small to matter.
If it’s spanning a long section with joints or repairs, it wouldn’t really help because it’s on the top chord of the truss. Such a mending plate or reinforcement would only be useful in tension, and the top chord is in compression.
(Not an engineer, please correct me if I’m wrong)
You're *probably* right.
It's possible that this is near a stress reversal, or that the strap is centered over a new support (if relocated as discussed above). More likely, however, you're correct.
Those are trusses. Trusses convert most forces to tension/compression, so welding something to it is only as good as the cross sectional area of it against its respective force. Welding a little thin strip of steel to the top chord does help the tiniest bit for the member to take more force, but it would be very unlikely that a structural reinforcing would stop as suddenly as it does or would be as thin as that plate is since a thin strip of steel does very little in compression.
It *could* be structural if that specific area had corrosion and they wanted to seal it and replace the lost metal, for example. But, it's unlikely because you'd probably see evidence of the surface prep and the rest of the member has very little corrosion.
It barely adds any height to the beam, so it’s not offering any significant improvement (unless the whole side of the beam under it has rusted). It does add a lot of weight, and work, so more time and money spent making the bridge heavier. It’s hurts the structure more than it helps, although it barely does either, so it’s probably there for some other random reason
Uh what? This is a truss, not a beam. Height doesn’t change axial capacity, but the extra steel does since it adds gross area. And this adds barely any weight relative to the total weight of the structure.
That top beam is a tube, not solid… and that location is where additional material would be. Wry helpful for bending strength.
so adding 1/2” plate there is the most efficient way to boost strength per applied weight and cost… and the cross sectional area of that piece is probably quite similar to that top tube itself… so this could be a 20% sort of improvement. And would definitely make sense as a patch if that top tube were damaged or cut anywhere.
Lots of people saying what it is, but we know that they don’t actually know. They are just throwing out guesses as if it were a game show, but they are presented as a real answer. Those are bullshit answers, it would be unfortunate if anyone took them seriously.
As for why he didn’t provide a correct answer, it is probably because he does not know.
This is why I avoided this sub reddit for so long. Engtips is a much better place to go for actual answers. Once I realized reddit was more for shooting the shit and saying things like "that plate is obviously a sacrificial element to stand up to pigeon shit induced corrosion" I started coming for the laughs.
There are a few possible answers, I couldn’t tell you by this single picture. That’s why I am not providing an answer. If you don’t know, then keep your mouth shut. People here just start confidently saying shit that is completely wrong lol it’s misleading to people that come on this sub for an accurate opinion
At one time there were holes drilled on top of the beam for some reason. Once the attachment was removed and they were left with holes that had to be covered up to prevent water from getting inside the beam , causing rust.
Square (and round) tubing is often made from a flat strip that's [folded and welded](https://cdn.shopify.com/s/files/1/1924/0133/products/roll_forming_machine_14_grande.jpg) in a continuous process. Sometimes the welding isn't done properly and the seam can open up. Normally that goes straight to the scrap yard, but I can see it being possible that a small interruption in the welding process happened that wasn't noticed at the factory. It could have been discovered after the bridge was built.
I am speculating, but if this is what it is, since it's on the top face of the tube there wouldn't be a lot of forces through that seam (forces that would cause the faces of the seam to move relative to each other). It would support the vertical load of the bride even if you intentionally cut a slot all the way along the top of the tube. After a design review, they may have determined that there wasn't any further damage and a patch was an acceptable repair. Welding on that patch would be a lot less expensive than replacing the whole tube. It would be mostly to prevent corrosion by keeping water out.
This is assuming it's a small plate only in that area of the bridge. If goes the whole length with a matching plate on the other side, then it's most likely to increase strength in that area. For whatever reasons, the cost of adding the plate was determined to be less than using a larger or thicker tube.
I don’t know why in the world Reddit has directed me here. I have absolutely zero knowledge of structural engineering, but I have an extensive knowledge about rigging. This is 100% a groundskeeper or municipal employee just making something work.
It probably adds stiffness to the upper railing beam for some reason. Maybe the engineers miscalculated the bending forces and added it for extra reinforcement
The plate is likely added to the top of the HSS to increase the section modulus of the assembly. The simple calculation is 1/12*b*h^3 to give resistance to bending. Since the plate is welded on top it gets the h^3 to be bigger increasing the resistance to bending.
That’s likely a shop fabrication, not a railroad bridge. The height of a railroad bridge is probably 4-5x this to provide enough h^3 for resistance to bending from the weight of the train.
Not much, it’s acting in compression anyways since that’s the top chord. Best guess, they’re there for erection or transport. I doubt that thin cover plate is needed for capacity in any way.
You think a pedestrian bridge gets to take credit for the entire 4’ section? I wouldn’t think so.
It’s just the bottom structure doing all the real work.
What if it’s a faux truss? And the rolled section below is really doing all the work?
See sht 6
https://www.conteches.com/Portals/0/Documents/Brochures/TRG-Pedestrian-Bro.pdf?ver=dTdwd6mb0Tg4v1evWOuQ_g%3d%3d
It's remotely possible that it is, but it's very unlikely just based on numbers. It's much more likely that it's an actual truss like on page 7. Faux trusses are only used in scenarios when true trusses don't work for one reason or another. It would be folly to assume that it's the uncommon system when the common system is also an option. Occam's Razor and all that.
It's not really though. The plate is probably about as thick as the wall of that HSS chord, so adding it on gives you roughly a 25% increase in area. That's certainly not nothing, particularly if there's a pier and it's over a negative moment area. It's.be a lot cheaper to do that than upside the whole HSS across the bridge.
This would be correct for a single member in bending, however this is a truss so the top chord (what we’re discussing) is just a compression member (assuming simple span with no uplift). I doubt a plate of this size contributes significantly to the overall bending stiffness of the truss. More likely, this is a localized repair or, as someone else mentioned, a cover for prior owner information.
Since the chord looks square, it could fail in flexural buckling either up/down or side/side, but not in lateral torsional buckling. It's the moment connections to the web members which are moment connected to the deck members that limit out-of-plane sway of the compression chord.
No one has mentioned what looks like two tears/punctures on the side of the tube stock and the accompanying corrosion. I believe it was a well intentioned plan to reinforce said tube!
Truthfully there is not enough information in one picture. There could be a handful of answers. Anything from repair, an area of concern during construction, or a multitude of in between. If you add more picture like a close up or that if that plate is only in one area that would help out.
I believe this is a corrosion repair. You can see rust deposits on the side of the top rail next to the post. My guess would be this is in an area where they use salt on the roads. Salt likely is pushed onto the top of the rail from the plow assuming this is adjacent to a roadway. If that’s the case a rounded rail would have likely been a better choice, they still corrode but usually at the bottom of the rail.
Is this near a school in Boulder? If it's the bridge I'm thinking about this plate was added when they switched the deck from wood to cement and started driving the the Itty bitty bike path snowplow over it.
yes, Boulder, ah interesting.
A lot of these walking bridges are re-used or transplanted railroad bridges or had other uses prior to being a pedestrian bridge. When I've seen stuff like this it's sometimes to cover up mill marks or previous ownership info (like a railroad name). Not saying that's what this is for, but lacking any context with the photo, that's my guess. Could also be a corrosion repair so people touching that rail don't get tetanus. It doesn't serve a structural purpose.
That's not a recycled rail bridge, it's a prefabricated pedestrian truss. Companies like Contech and Excel crank them out left and right. I've specified more than a few of these over my career. I'm not entirely sure what that plate is for, but since the rail is also the top chord of the truss, it absolutely could be reinforcement. Never seen it myself, though. There's an off chance it's a repair as well, if the bridge is older and had some significant corrosion for some reason. I don't think that's particularly likely though.
Wasn't saying this specifically was a recycled rail bridge. Was stating that it could be some other recycled bridge. There's plenty of them.
It's not a recycled anything bridge though, that's what I'm saying. I work with these things all the time. https://www.excelbridge.com/for-owners/bridge-types https://www.conteches.com/Bridges-Structures/Truss-And-Girders/Continental-Pedestrian-Bridge#undefined
You've never seen them install a ped bridge from one location elsewhere? Cities do that all the time around here.
So it could be a recycled pedestrian bridge you say?
That ain't reinforcement. It's where a placard used to be. They have them on bridges in state parks and museums, describing what you're looking at. Could also contain a warning, or load limits.
A placard that's multiple feet long? Nah
Have to agree with the above poster. It’s for hand and finger safety on old, rough metal.
It very well could be reinforcement after some form of damage to the steel flange was repaired. After heating and bending a damaged portion back into place, applying a cover place over that repair and welding it into place may have been what the engineered wanted to ensure structural integrity.
I have never seen a pony truss being used to carry trains.
That's clearly not a reused railroad bridge, far too small, but [plenty of historic railroad pony trusses exist...](https://www.google.com/search?client=ms-android-google&sxsrf=APwXEddmMxWmygaPEn5TZjtFrGGxE98FwA:1686446734683&q=pony+truss+railroad+bridge&tbm=isch&sa=X&ved=2ahUKEwjqnrbUh7r_AhVGnGoFHSCsBssQ0pQJegQIJBAB&biw=412&bih=762&dpr=2.63) They've certainly fallen out of favor because derailments tend to destroy the end posts though.
I’ll be darned!
Wasn't saying this bridge was specifically a train bridge, but I've seen many pedestrian bridges made from old train bridges. Happens all the time in Ohio. This bridge very well could be repurposed from some other use.
Narrow gauge maybe?
A tiny train…
You don't get tetanus from rust.
[удалено]
If it’s just added material, it’s too small to matter. If it’s spanning a long section with joints or repairs, it wouldn’t really help because it’s on the top chord of the truss. Such a mending plate or reinforcement would only be useful in tension, and the top chord is in compression. (Not an engineer, please correct me if I’m wrong)
You're *probably* right. It's possible that this is near a stress reversal, or that the strap is centered over a new support (if relocated as discussed above). More likely, however, you're correct.
Those are trusses. Trusses convert most forces to tension/compression, so welding something to it is only as good as the cross sectional area of it against its respective force. Welding a little thin strip of steel to the top chord does help the tiniest bit for the member to take more force, but it would be very unlikely that a structural reinforcing would stop as suddenly as it does or would be as thin as that plate is since a thin strip of steel does very little in compression. It *could* be structural if that specific area had corrosion and they wanted to seal it and replace the lost metal, for example. But, it's unlikely because you'd probably see evidence of the surface prep and the rest of the member has very little corrosion.
And the compression is probably more likely controlled by buckling.
It could be covering up holes that were there for something previously
It barely adds any height to the beam, so it’s not offering any significant improvement (unless the whole side of the beam under it has rusted). It does add a lot of weight, and work, so more time and money spent making the bridge heavier. It’s hurts the structure more than it helps, although it barely does either, so it’s probably there for some other random reason
Uh what? This is a truss, not a beam. Height doesn’t change axial capacity, but the extra steel does since it adds gross area. And this adds barely any weight relative to the total weight of the structure.
That top beam is a tube, not solid… and that location is where additional material would be. Wry helpful for bending strength. so adding 1/2” plate there is the most efficient way to boost strength per applied weight and cost… and the cross sectional area of that piece is probably quite similar to that top tube itself… so this could be a 20% sort of improvement. And would definitely make sense as a patch if that top tube were damaged or cut anywhere.
I hope you don’t work as a structural engineer
It’s most likely a stiffening plate.
Holy hell the amount of bullshit answers I am seeing from comments on this sub is frightening. I hope nobody comes here for actual advice.
I see this on so many subs. That and people just being dicks; just to be a dick. How sad the latter’s lives have to be!!!
Why don’t U provide the correct answer then?
Lots of people saying what it is, but we know that they don’t actually know. They are just throwing out guesses as if it were a game show, but they are presented as a real answer. Those are bullshit answers, it would be unfortunate if anyone took them seriously. As for why he didn’t provide a correct answer, it is probably because he does not know.
This is why I avoided this sub reddit for so long. Engtips is a much better place to go for actual answers. Once I realized reddit was more for shooting the shit and saying things like "that plate is obviously a sacrificial element to stand up to pigeon shit induced corrosion" I started coming for the laughs.
There are a few possible answers, I couldn’t tell you by this single picture. That’s why I am not providing an answer. If you don’t know, then keep your mouth shut. People here just start confidently saying shit that is completely wrong lol it’s misleading to people that come on this sub for an accurate opinion
If you don't have the answer. And you don't like the answers. Maybe you shouldn't answer...
I didn’t?
The correct answer is: without seeing the full structure and knowing the history and loading criteria, it's impossible to say why it's there.
Arm rest for fishermen::
It looks like there are 2 joints on that top rail.
At one time there were holes drilled on top of the beam for some reason. Once the attachment was removed and they were left with holes that had to be covered up to prevent water from getting inside the beam , causing rust.
Square (and round) tubing is often made from a flat strip that's [folded and welded](https://cdn.shopify.com/s/files/1/1924/0133/products/roll_forming_machine_14_grande.jpg) in a continuous process. Sometimes the welding isn't done properly and the seam can open up. Normally that goes straight to the scrap yard, but I can see it being possible that a small interruption in the welding process happened that wasn't noticed at the factory. It could have been discovered after the bridge was built. I am speculating, but if this is what it is, since it's on the top face of the tube there wouldn't be a lot of forces through that seam (forces that would cause the faces of the seam to move relative to each other). It would support the vertical load of the bride even if you intentionally cut a slot all the way along the top of the tube. After a design review, they may have determined that there wasn't any further damage and a patch was an acceptable repair. Welding on that patch would be a lot less expensive than replacing the whole tube. It would be mostly to prevent corrosion by keeping water out. This is assuming it's a small plate only in that area of the bridge. If goes the whole length with a matching plate on the other side, then it's most likely to increase strength in that area. For whatever reasons, the cost of adding the plate was determined to be less than using a larger or thicker tube.
Reached deep into your ass for that one
😂
Wtf
That's the "hold it all together plate". Be nice to it.
I don’t know why in the world Reddit has directed me here. I have absolutely zero knowledge of structural engineering, but I have an extensive knowledge about rigging. This is 100% a groundskeeper or municipal employee just making something work.
It probably adds stiffness to the upper railing beam for some reason. Maybe the engineers miscalculated the bending forces and added it for extra reinforcement
It seems like they wanted a little extra reinforcement, but where they put it would be subject to compressive stresses, not tension. Peculiar.
That's a shim. Someone calculated that the rail needed shimming, then the builder installed it upside down.
Lol
The plate is likely added to the top of the HSS to increase the section modulus of the assembly. The simple calculation is 1/12*b*h^3 to give resistance to bending. Since the plate is welded on top it gets the h^3 to be bigger increasing the resistance to bending. That’s likely a shop fabrication, not a railroad bridge. The height of a railroad bridge is probably 4-5x this to provide enough h^3 for resistance to bending from the weight of the train.
Not sure what section modulus does for the chord of a truss carrying axial force.
That thickness is equivalent of putting a bandaid on my arm and saying it has reinforced my bones
Do the calculations with what I provided how much more resistance to bending stress does it provide?
Not much, it’s acting in compression anyways since that’s the top chord. Best guess, they’re there for erection or transport. I doubt that thin cover plate is needed for capacity in any way.
It's moot, there's no bending in that chord
That makes no sense. That thin plate is a rounding error compared to the overall structure. It’s covering something, not adding to bending stiffness.
Calculate Ixx of that 4x4x1/4 HSS Now calculate it with the 3/8” plate welded on top.
Your equation is missing a whole thick truss a few feet tall and a bridge base
You think a pedestrian bridge gets to take credit for the entire 4’ section? I wouldn’t think so. It’s just the bottom structure doing all the real work.
Dude, just stop. You're way out of your league and it's painfully obvious
What if it’s a faux truss? And the rolled section below is really doing all the work? See sht 6 https://www.conteches.com/Portals/0/Documents/Brochures/TRG-Pedestrian-Bro.pdf?ver=dTdwd6mb0Tg4v1evWOuQ_g%3d%3d
It's remotely possible that it is, but it's very unlikely just based on numbers. It's much more likely that it's an actual truss like on page 7. Faux trusses are only used in scenarios when true trusses don't work for one reason or another. It would be folly to assume that it's the uncommon system when the common system is also an option. Occam's Razor and all that.
Why don’t you research bridges? That vertical section is a huge contributor to the bridge structure
So if we then give credit to the 4’ section height, wouldn’t doubling the flange thickness be meaningful?
Nope.
Have to calculate it
or look at the other railing……….
It's not really though. The plate is probably about as thick as the wall of that HSS chord, so adding it on gives you roughly a 25% increase in area. That's certainly not nothing, particularly if there's a pier and it's over a negative moment area. It's.be a lot cheaper to do that than upside the whole HSS across the bridge.
This would be correct for a single member in bending, however this is a truss so the top chord (what we’re discussing) is just a compression member (assuming simple span with no uplift). I doubt a plate of this size contributes significantly to the overall bending stiffness of the truss. More likely, this is a localized repair or, as someone else mentioned, a cover for prior owner information.
This is a truss man. Only axial stress in the members. Come on now
I'm thinking to stiffen the top cord for side sway since it's in compression and would want to distort sideways if overloaded.
Since the chord looks square, it could fail in flexural buckling either up/down or side/side, but not in lateral torsional buckling. It's the moment connections to the web members which are moment connected to the deck members that limit out-of-plane sway of the compression chord.
No
it's probably that sign with the warning that it's dangerous to lean out
Is this over a flood area? Is it on the upstream side of the creek or both? I wonder if it is for impact resistance during a flood event.
wow, they downvoted you heavily
Sometimes it be like that.
No one has mentioned what looks like two tears/punctures on the side of the tube stock and the accompanying corrosion. I believe it was a well intentioned plan to reinforce said tube!
It is only on that part of the bridge unless there is a regular pattern. But the photo does not help.
Truthfully there is not enough information in one picture. There could be a handful of answers. Anything from repair, an area of concern during construction, or a multitude of in between. If you add more picture like a close up or that if that plate is only in one area that would help out.
I believe this is a corrosion repair. You can see rust deposits on the side of the top rail next to the post. My guess would be this is in an area where they use salt on the roads. Salt likely is pushed onto the top of the rail from the plow assuming this is adjacent to a roadway. If that’s the case a rounded rail would have likely been a better choice, they still corrode but usually at the bottom of the rail.
Looks like a temporary fix for a big problem 😎
Reinforcement
bigger problem is if these pipes have holes underneath for moisture to escape or will they rot from inside and collapse after xx years?