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Bridges have lifespans and need constant maintenance. One of my first tasks as an intern at the Army Corp Of Engineers was to create 6 inspection packets (copi

Bridges have lifespans and need constant maintenance. One of my first tasks as an intern at the Army Corp Of Engineers was to create 6 inspection packets (copies of every part) of the cape cod canal bridge, which was being inspected. I know the tappan-zee bridge was replaced because the old one was just too old (designed to last 50 years, it was replaced at over 60 years). https://en.wikipedia.org/wiki/Tappan_Zee_Bridge_(1955–2017)#Replacement_bridge https://en.wikipedia.org/wiki/Tappan_Zee_Bridge_(1955–2017)#... One of the reason I left the engineering profession is no one wants to pay for it so it gets "deferred". The US's bridges get a c+ according to the American Society Of Civil Engineers. https://www.infrastructurereportcard.org/cat-item/bridges/ https://www.infrastructurereportcard.org/cat-item/bridges/ I'm also a little curious why the cables were wrapped in concrete. Steel cables are common on bridges. stability side to side? Concrete is not great in tension (steel is very good in tension). As the article indicates, it was potentially hiding some of the cable corrosion, making the situation worse.

sharkmerry 8y ago
>> I'm also a little curious why the cables were wrapped in concrete. Exposed steel cables are common on bridges. stability side to side? Concrete is not great in tension (steel is very good in tension). As the article indicates, it was potentially hiding some of the cable corrosion, making the situation worse. the concrete was meant to be a protective sheath to stop cable corrosion. poor materials engineering at the time sounds like the issue >> He believed using the system would reduce the sway of the bridge. Structural engineers seemed to agree. But Mr. Morandi also believed that the concrete coating would protect the steel cables inside from the wear and tear of the elements. “Concrete structures seemed to be eternal,” Mr. Majowiecki said. “This was the mentality.” In that hope, he added, Mr. Morandi was greatly mistaken. The concrete of the day turned out to be highly vulnerable to degradation, worsened perhaps by salty air from the Mediterranean Sea and the harsh fumes from nearby factories. Cracks in the concrete shell let water in, and the steel began corroding almost as soon as the bridge was opened for traffic in 1967. But unlike bare cables, any corrosion was hidden deep inside, making it hard to detect."
- acomjean 8y ago
  Thanks, Cracks in the concrete usually mean bad concrete. In the sites I worked on we didn't use much concrete, but the few times we did, we had to do a onsite slump test and send cylinders off to the lab to testing after curing. If the mix is wrong it effects the strength significantly. One of the things about Civil Engineering as a field is the huge responsibility in designing things that when they work nobody notices.
  - deepsun 8y ago
    Same thing with 90% of software engineering. Your work is notices only when it fails or sucks.
  - wahern 8y ago
    Concrete has become ubiquitous, cheap, and increasingly poured by people (including professionals) who don't appreciate, let alone understand, the chemistry and art. Walking around San Francisco and other cities I see brand new concrete (sidewalks, buildings) with cracks adjacent to 30-year-old flawless concrete. Someone attempted to parge the concrete foundation of my house (before I bought it), and it simply popped off. Reason? Everybody has switched to impermeable cement (e.g. Portland, Quickcrete, etc) these days, but my foundation (like most from that era) used permeable cement. So moisture and salts travel through the foundation, reaches the parge coat, and the pressure pops it off. I didn't understand any of this (including what parge meant) but it only took a few hours of bored Googling one night, when I was contemplating redoing the parge coat myself, to begin to wrap my head again things. My neighbor had his entire foundation replaced. They lifted the house and I guess they did a good job. (I wouldn't really know!) I was chatting with the supervisor and decided to ask him if it was worth properly parging my foundation, and what to look for to know whether I was getting the right kind of materials that will work with my foundation. He had NO CLUE what I was talking about. Maybe I didn't understand the lingo and had some facts wrong, but even if so I'm pretty sure someone knowledgable would have realized what I was trying to ask. But, nope. The guy is a professional who specializes in concrete--maybe not an engineer but someone laying foundations which need to be structurally safe--and just... clueless. All he knows is a particular process, not the whys, not the alternatives. And much like with consumables, the culture is to simply replace, not fix. It's cheaper to replace than fix because that's what the industry is optimized for. That's where we're at, today. Expertise is gone and expectations have suffered.
GuB-42 8y ago
Beside the (wrong) idea of protecting the cables from corrosion, the concrete, as you said, was there to improve stability. It is pre-stressed concrete, it is working in compression against the cable tension. The idea is that as the load increases, instead of pulling directly on the cables, it just decompresses the concrete. You can think of the cable as a spring, with the bridge suspended it. As you walk on the bridge, it bounces. To prevent the bouncing, you can put the spring under high tension into a rigid pipe, with the ends of the spring attached to the ends of the pipe. This way, as you walk on the bridge, the spring tension will stay constant, only the compression on the pipe will vary.
- jmts 8y ago
  It is pre-stressed concrete, it is working in compression against the cable tension. This seems strange to me. My understanding is that pre-stressed concrete is concrete with reinforcements inside that apply a compressive force on the concrete. As mentioned by GP, this is to mitigate the effect of tensile forces within the concrete since concrete performs poorly in tension. Are you suggesting that the cables within the concrete are not just stays, but also the component of the pre-stressed concrete applying a compressive force? Or is the reinforcement some other component? I would assume that if the cables themselves were applying compressive force to the concrete, then their use as stays releases some of this force since the stays are in tension. Since concrete itself performs poorly in tension, it's going to provide a negligible counter to reinforce the cables even if it is still compressed, since the compression forces are provided by the cables. It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place. Or is concrete in some compression able to respond better in tension by weight than say, a thicker cable at the same weight as the given cable and concrete combined?
  - deleted 8y ago
    [deleted]
  - vlehto 8y ago
    >It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place. I think you are correct there. But it looks like the stays were in separate tubes with little allowance around them to separate them from the pre-stressed concrete. That airspace around the stays then would prevent the concrete from providing any chemical protection against corrosion. And concrete as physical protection against rainwater is often only as good as the paint in the surface of the concrete. It's often way too porous even without cracks.
microcolonel 8y ago
> The US's bridges get a c+ according to the American Society Of Civil Engineers. Which bridges, in which states, are we talking about here? I feel that these ratings are tossed around a lot, but there are a lot of bridges which are fallow/virtually unused in the U.S. by the nature of its evolution. It is conceivable that the average bridge is worse than the average overall risk from bridges (i.e. rating multiplied by proportion of miles/minutes on each bridge).

sharkmerry 8y ago

>> I'm also a little curious why the cables were wrapped in concrete. Exposed steel cables are common on bridges. stability side to side? Concrete is not great in tension (steel is very good in tension). As the article indicates, it was potentially hiding some of the cable corrosion, making the situation worse. the concrete was meant to be a protective sheath to stop cable corrosion. poor materials engineering at the time sounds like the issue >> He believed using the system would reduce the sway of the bridge. Structural engineers seemed to agree. But Mr. Morandi also believed that the concrete coating would protect the steel cables inside from the wear and tear of the elements. “Concrete structures seemed to be eternal,” Mr. Majowiecki said. “This was the mentality.” In that hope, he added, Mr. Morandi was greatly mistaken. The concrete of the day turned out to be highly vulnerable to degradation, worsened perhaps by salty air from the Mediterranean Sea and the harsh fumes from nearby factories. Cracks in the concrete shell let water in, and the steel began corroding almost as soon as the bridge was opened for traffic in 1967. But unlike bare cables, any corrosion was hidden deep inside, making it hard to detect."

acomjean 8y ago
Thanks, Cracks in the concrete usually mean bad concrete. In the sites I worked on we didn't use much concrete, but the few times we did, we had to do a onsite slump test and send cylinders off to the lab to testing after curing. If the mix is wrong it effects the strength significantly. One of the things about Civil Engineering as a field is the huge responsibility in designing things that when they work nobody notices.
- deepsun 8y ago
  Same thing with 90% of software engineering. Your work is notices only when it fails or sucks.
- wahern 8y ago
  Concrete has become ubiquitous, cheap, and increasingly poured by people (including professionals) who don't appreciate, let alone understand, the chemistry and art. Walking around San Francisco and other cities I see brand new concrete (sidewalks, buildings) with cracks adjacent to 30-year-old flawless concrete. Someone attempted to parge the concrete foundation of my house (before I bought it), and it simply popped off. Reason? Everybody has switched to impermeable cement (e.g. Portland, Quickcrete, etc) these days, but my foundation (like most from that era) used permeable cement. So moisture and salts travel through the foundation, reaches the parge coat, and the pressure pops it off. I didn't understand any of this (including what parge meant) but it only took a few hours of bored Googling one night, when I was contemplating redoing the parge coat myself, to begin to wrap my head again things. My neighbor had his entire foundation replaced. They lifted the house and I guess they did a good job. (I wouldn't really know!) I was chatting with the supervisor and decided to ask him if it was worth properly parging my foundation, and what to look for to know whether I was getting the right kind of materials that will work with my foundation. He had NO CLUE what I was talking about. Maybe I didn't understand the lingo and had some facts wrong, but even if so I'm pretty sure someone knowledgable would have realized what I was trying to ask. But, nope. The guy is a professional who specializes in concrete--maybe not an engineer but someone laying foundations which need to be structurally safe--and just... clueless. All he knows is a particular process, not the whys, not the alternatives. And much like with consumables, the culture is to simply replace, not fix. It's cheaper to replace than fix because that's what the industry is optimized for. That's where we're at, today. Expertise is gone and expectations have suffered.

GuB-42 8y ago

Beside the (wrong) idea of protecting the cables from corrosion, the concrete, as you said, was there to improve stability. It is pre-stressed concrete, it is working in compression against the cable tension. The idea is that as the load increases, instead of pulling directly on the cables, it just decompresses the concrete. You can think of the cable as a spring, with the bridge suspended it. As you walk on the bridge, it bounces. To prevent the bouncing, you can put the spring under high tension into a rigid pipe, with the ends of the spring attached to the ends of the pipe. This way, as you walk on the bridge, the spring tension will stay constant, only the compression on the pipe will vary.

jmts 8y ago
It is pre-stressed concrete, it is working in compression against the cable tension. This seems strange to me. My understanding is that pre-stressed concrete is concrete with reinforcements inside that apply a compressive force on the concrete. As mentioned by GP, this is to mitigate the effect of tensile forces within the concrete since concrete performs poorly in tension. Are you suggesting that the cables within the concrete are not just stays, but also the component of the pre-stressed concrete applying a compressive force? Or is the reinforcement some other component? I would assume that if the cables themselves were applying compressive force to the concrete, then their use as stays releases some of this force since the stays are in tension. Since concrete itself performs poorly in tension, it's going to provide a negligible counter to reinforce the cables even if it is still compressed, since the compression forces are provided by the cables. It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place. Or is concrete in some compression able to respond better in tension by weight than say, a thicker cable at the same weight as the given cable and concrete combined?
- deleted 8y ago
  [deleted]
- vlehto 8y ago
  >It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place. I think you are correct there. But it looks like the stays were in separate tubes with little allowance around them to separate them from the pre-stressed concrete. That airspace around the stays then would prevent the concrete from providing any chemical protection against corrosion. And concrete as physical protection against rainwater is often only as good as the paint in the surface of the concrete. It's often way too porous even without cracks.

microcolonel 8y ago

> The US's bridges get a c+ according to the American Society Of Civil Engineers. Which bridges, in which states, are we talking about here? I feel that these ratings are tossed around a lot, but there are a lot of bridges which are fallow/virtually unused in the U.S. by the nature of its evolution. It is conceivable that the average bridge is worse than the average overall risk from bridges (i.e. rating multiplied by proportion of miles/minutes on each bridge).