4 ms·

>> 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

>> 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.
  - vlehto 8y ago
    Umm. Why would you want to parge coat that foundation? Did you try to do that on the outside or the inside? Do you have concrete slab against ground? Cellar? Or is there unused airspace between the floor and the ground?
    - wahern 8y ago
      My 1926 house, like most in SF, has an unreinforced strip foundation. (Concrete in my case, which supplanted masonry no later than 1900-1910, AFAIU.) The foundation is structurally fine (home inspection passed, earthquake retrofitters didn't seem bothered by it even though in at least one spot a installing foundation bolt blew through the concrete), but it's noticeably disintegrating. I presume the previous owner attempted a parge coat on the internal walls to hide this. My motives are both esthetic and practical. Practical because, theoretically, a proper parge coat can act as a sacrificial layer, slowing the disintegration. Whether a parge coat would actually extend the useful life of the foundation as practical matter is something only someone with real in-depth experience and knowledge of SF construction would be able to tell me. I just don't know how to find such a person. If you tour SF homes[1] you'll notice that most have remodeled the garage to include one or more bedrooms. The strip foundations are trapezoidal--slightly wider than the sill plate at the top (which is about a 1 or 2 feet above ground), expanding to much larger as it goes down 3 or 4 feet under the ground. Older remodels often parge and paint the foundation or box it behind wood molding; newer ones might actually hide it behind the refinished walls. But because of the awkward dimensions, whatever they do you can easily spot it if you pay attention. What I find crazy about this is that they're enclosing foundations which are (or will be) nearing their useful life. (SF homes are technically detached, but usually only have a few inches of gap at the most so the side walls are inaccessible.) Moreover, the refinishing is probably wrong. Either it'll pop off, or if they used a chemical binder they're (theoretically) accelerating the disintegration (the moisture and salts will create pressure behind the binder, pulling on the face of the older concrete). Likewise for impermeable[2] paint. Yet you won't even notice any of this because it'll be completely hidden. I'm happy that the home we found was (all things considered) both relatively well-maintained and unmolested. The garage was never refinished except for a parking slab poured circa 1970s. I'd like to keep it this way. [1] Which typically sit over a garage with a parking slab poured decades (or even a century) after the foundation. The garages were serendipitous. Most of the homes were built before cars were so common; what are now invariably garages were unfinished first floors intended to be used and finished (if at all) however the owner wanted--storage, workshop, in-law, extra living space, etc. When cars exploded most were turned into garages. Now in-laws and extra bedrooms are becoming much more common, either replacing the garage or reducing it to a size that would barely fit a lawn mower. Occasionally you'll find the odd house where there's still just sand at the first floor. [2] Permeability of paint is a whole 'nother can of worms. Like the foundation, the typical stucco finishes on homes on the west side of SF were designed to be semi-permeable. The external walls need to breath. But idiot painters (including whoever the previous homeowner hired) commonly use a low-permeability latex paint. (Latex comes in all kinds.) Moisture accumulates behind the stucco and the paint quickly bubbles and peels. In some cases (like ours) it can be bad enough to cause moisture damage inside the house because vapor barriers were either non-existent or primitive (our house was wrapped in tar paper), and in any event both the inner and outer walls were designed to be semi-permeable as compared to modern construction. Of course, ask most painters about these permeability issues and many or most wouldn't really understand what you were talking about, even in SF. Contractors who do a good job often do so because they've inherited a workable process (proper types of paint, etc), not because they understand why. But often times you still need that deeper knowledge to be able to debug or solve certain issues. Good luck finding somebody capable of doing that. I think that, like with modern network security, the saving grace is that most people end up dodging bullets by replacing whole systems before the consequences are realized.
kkylin 8y ago
What happened here seems to bear some similarity with Evans Hall (home to Math, Stats, & Econ, among other units) at UC Berkeley: https://en.wikipedia.org/wiki/Evans_Hall_(UC_Berkeley)#cite_note-no-3 https://en.wikipedia.org/wiki/Evans_Hall_(UC_Berkeley)#cite_...

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.
- vlehto 8y ago
  Umm. Why would you want to parge coat that foundation? Did you try to do that on the outside or the inside? Do you have concrete slab against ground? Cellar? Or is there unused airspace between the floor and the ground?
  - wahern 8y ago
    My 1926 house, like most in SF, has an unreinforced strip foundation. (Concrete in my case, which supplanted masonry no later than 1900-1910, AFAIU.) The foundation is structurally fine (home inspection passed, earthquake retrofitters didn't seem bothered by it even though in at least one spot a installing foundation bolt blew through the concrete), but it's noticeably disintegrating. I presume the previous owner attempted a parge coat on the internal walls to hide this. My motives are both esthetic and practical. Practical because, theoretically, a proper parge coat can act as a sacrificial layer, slowing the disintegration. Whether a parge coat would actually extend the useful life of the foundation as practical matter is something only someone with real in-depth experience and knowledge of SF construction would be able to tell me. I just don't know how to find such a person. If you tour SF homes[1] you'll notice that most have remodeled the garage to include one or more bedrooms. The strip foundations are trapezoidal--slightly wider than the sill plate at the top (which is about a 1 or 2 feet above ground), expanding to much larger as it goes down 3 or 4 feet under the ground. Older remodels often parge and paint the foundation or box it behind wood molding; newer ones might actually hide it behind the refinished walls. But because of the awkward dimensions, whatever they do you can easily spot it if you pay attention. What I find crazy about this is that they're enclosing foundations which are (or will be) nearing their useful life. (SF homes are technically detached, but usually only have a few inches of gap at the most so the side walls are inaccessible.) Moreover, the refinishing is probably wrong. Either it'll pop off, or if they used a chemical binder they're (theoretically) accelerating the disintegration (the moisture and salts will create pressure behind the binder, pulling on the face of the older concrete). Likewise for impermeable[2] paint. Yet you won't even notice any of this because it'll be completely hidden. I'm happy that the home we found was (all things considered) both relatively well-maintained and unmolested. The garage was never refinished except for a parking slab poured circa 1970s. I'd like to keep it this way. [1] Which typically sit over a garage with a parking slab poured decades (or even a century) after the foundation. The garages were serendipitous. Most of the homes were built before cars were so common; what are now invariably garages were unfinished first floors intended to be used and finished (if at all) however the owner wanted--storage, workshop, in-law, extra living space, etc. When cars exploded most were turned into garages. Now in-laws and extra bedrooms are becoming much more common, either replacing the garage or reducing it to a size that would barely fit a lawn mower. Occasionally you'll find the odd house where there's still just sand at the first floor. [2] Permeability of paint is a whole 'nother can of worms. Like the foundation, the typical stucco finishes on homes on the west side of SF were designed to be semi-permeable. The external walls need to breath. But idiot painters (including whoever the previous homeowner hired) commonly use a low-permeability latex paint. (Latex comes in all kinds.) Moisture accumulates behind the stucco and the paint quickly bubbles and peels. In some cases (like ours) it can be bad enough to cause moisture damage inside the house because vapor barriers were either non-existent or primitive (our house was wrapped in tar paper), and in any event both the inner and outer walls were designed to be semi-permeable as compared to modern construction. Of course, ask most painters about these permeability issues and many or most wouldn't really understand what you were talking about, even in SF. Contractors who do a good job often do so because they've inherited a workable process (proper types of paint, etc), not because they understand why. But often times you still need that deeper knowledge to be able to debug or solve certain issues. Good luck finding somebody capable of doing that. I think that, like with modern network security, the saving grace is that most people end up dodging bullets by replacing whole systems before the consequences are realized.

kkylin 8y ago

What happened here seems to bear some similarity with Evans Hall (home to Math, Stats, & Econ, among other units) at UC Berkeley: https://en.wikipedia.org/wiki/Evans_Hall_(UC_Berkeley)#cite_note-no-3 https://en.wikipedia.org/wiki/Evans_Hall_(UC_Berkeley)#cite_...