posted
I'm going to copy here some posts from the other thread.
I've watched that footage of the bridge falling over and over to try and understand what happened first. It's weird cause it almost seems like the main span let go on both ends at once. And it stayed so level as it fell, for the most part. It's as though whatever happened happened symmetrically on both sides of the bridge at once (widthwise), on both columns.
quote: Bridge 9340 consists of a deck truss and steel multi-girder approach spans built in 1967. The deck truss has a center span of 139 meters, north and south spans of 80.8 meters, and cantilever spans of 11.6 meters. The bridge was designed using the 1961 AASHO standard specifications. At that time, unconservative fatigue design provisions were used. AASHTO fatigue design rules were substantially improved as a result of research... in the 1970s.
The approach spans have exhibited several fatigue problems; primarily due to unanticipated out-of-plane distortion of the girders. Although fatigue cracking has not occurred in the deck truss, it has many poor fatigue details on the main truss and floor truss systems.
Stress ranges calculated using the lane load as live load are greater than fatigue thresholds for many of the details. The poor fatigue details in the deck truss include intermittent fillet welds, welded longitudinal stiffeners and welded attachments at diaphragms inside tension members. These details are classified as Category D and E with threshold stress ranges 48 and 31 MPa, respectively.
The design analysis using the AASHTO lane load in all lanes, shows design-live-load stress ranges in the truss members much higher than these thresholds. Design-live-load stress ranges were greatest, up to 138 MPa, in members that experience load reversal as trucks pass from the outside spans onto the center span. The predicted average life at that stress range is between 20,000 and 40,000 cycles. With 15,000 trucks per day crossing the bridge in each direction, these details should have cracked soon after opening if the stress ranges were really this high.
It seems like they're coming rather close to arguing that since it hasn't failed yet, it must actually be okay despite the fact that analysis shows it should have fallen a long time ago.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
When I read these parts of this civil engineering report, I'm just chilled. The thing is, engineers are asked to research and give opinions on things all the time, and of course we aren't perfect. But there IS a systematic bias toward telling whomever is paying the engineer to study something whatever it is they want to hear. We're trying hard not to let that happen all the time. That's what our professional ethics are about. But it's very hard not to let it creep in a little, subconsciously, because so many things are so very open to interpretation, and the client just could (and sometimes does) fire the engineer and get another one, if the engineer tells them too many things they don't want to hear. There are responsibilities going in the other direction too. The client should listen to what the engineer says and actually base their decisions on it, instead of brushing it off or pretending they didn't hear or understand it, or firing that firm and hiring someone else who gives them an answer they like better.
But I wonder if we don't need a different system altogether somehow. I wonder if we should change how things are done. Here's what the engineers said about that bridge a few years ago (2001), in the executive summary of their report (i.e. the only part decision-makers are going to read).
quote: Bridge 9340 is a deck truss with steel multi-girder approach spans built in 1967 across the Mississippi River just east of downtown Minneapolis. The approach spans have exhibited several fatigue problems; primarily due to unanticipated out-of-plane distortion of the girders. Although fatigue cracking has not occurred in the deck truss, it has many poor fatigue details on the main truss and floor truss systems. Concern about fatigue cracking in the deck truss is heightened by a lack of redundancy in the main truss system. The detailed fatigue assessment in this report shows that fatigue cracking of the deck truss is not likely. Therefore, replacement of this bridge, and the associated very high cost, may be deferred.
Okay, so obviously whatever warnings are contained in here should have (in retrospect) been very much more strongly worded, at the very least. What indications do we actually see in this report that their conclusions are not fully supported by the observations? They let a failure of the bridge to do what it was designed to do count as a GOOD thing instead of bad. Because the bearings were corroded and the supports weren't free to slide, the members were under less stress than the model predicted, and so they concluded we're SAFER. Instead, a failure should count as a failure! The bearings aren't moving so some member somewhere is under a lot higher compression than it was designed to take. Red flags should have been going up.
They did this same meta-idea with the Challenger O-rings, too. They took a failure (the O-Rings were burned through 1/3 of the way) and bizarrely reinterpreted it as having a safety factor of three. I'm not an expert in this field, and so I could be wrong, yet that seems wrongheaded to me, to credit a failure in the bridge to function as designed, as though it were a good thing instead of bad, as though it made us safer instead of throwing us into totally unknown and unanticipated territory.
This whole thing makes me really, really upset. Engineers have a sacred duty to protect people, so that they never have to think about crossing bridges or flying in planes or crossing the ocean depths in ships or whatever it may be. We have been tasked with the divine responsibility of making it as safe for people to do these things as though they were sitting beside the well in the cool of the evening. If we're going to fulfill this sacred charge, we have to develop and maintain systems of checks and balances, of reviews and counter-reviews, that will translate physical reality into models we use to decide how to act, where to spend money, what projects to undertake and which to defer. If we collectively let our wishes be the determining factor, instead of hard reality, then we are doomed to fail.
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -Richard P. Feynman, Challenger accident report.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
quote:The detailed fatigue assessment in this report shows that fatigue cracking of the deck truss is not likely.
That's a risk assessment: they believed the likelihood of failure was low enough that replacement was not immediately required. The fact that it did fail doesn't necessarily imply that they were wrong in their conclusion that it was unlikely to do so.
I agree with your broader point that the principal obligation of an engineer is to protect people, but at this point based on extremely limited information and a situation that is well outside my spheres of expertise I'm not willing to conclude that the engineers who assessed the bridge and compiled the report were biased, subconsicously or otherwise. I think that's premature.
This made me curious about the engineering code of ethics here in Ontario, though, so I went and read a bit more about it. Here's a summary:
quote:The Code of Ethics is a basic guide to professional conduct and imposes duties on the practising professional engineer, with respect to:
society; employers; clients; colleagues, including employees and subordinates; the engineering profession; and himself/herself.
Section 77 of Regulation 941 states that "it is the duty of a practitioner to the public, to the practitioner's employer, to the practitioner's clients, to other licensed engineers of the practitioner's profession, and to the practitioner to act at all times with,
i. fairness and loyalty to the practitioner's associates, employers, clients, subordinates and employees, ii. fidelity to public needs, iii. devotion to high ideals of personal honour and professional integrity, iv. knowledge of developments in the area of professional engineering relevant to any services that are undertaken, and v. competence in the performance of any professional engineering services that are undertaken."
Through the Code of Ethics, professional engineers have a clearly defined duty to society, which is to regard the duty to public welfare as paramount, above their duties to clients or employers. Their duty to employers involves acting as faithful agents or trustees, regarding client information as confidential and avoiding or disclosing conflicts of interest. Their duty to clients means that professional engineers have to disclose immediately any direct or indirect interest that might prejudice (or appear to prejudice) their professional judgement.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
Here's my vague theory from the other thread. I feel pretty sure whatever happened started at the south end and moved northward.
To change tacks completely, to take off my public policy hat and simply examine and make wild guesses about the physical system itself, about what happened and how.... I have a tentative theory.
After watching the collapse over and over on video, and studying the bridge structure model shown in the civil engineering report, and looking at the pictures of the collapsed debris, my tentative theory (highly speculative) is that there was buckling in the South approach span, or the south column substructure gave way allowing the south columns to topple, or maybe the superstructure above the south columns failed. I think whatever it was that started the whole chain of events, started back at the south column or pier. Then I think the main span was pulled southwards away from the North pier, causing the main span to drop neatly into the river. A few seconds later the North approach span, because it lost the counterbalancing pull from the main span after it fell, crumpled under the load. The north pier with concrete columns stayed intact.
If you look, the roadway surface is folded over the south pier with concrete columns, laying on top of it. The north pier with its concrete columns are the ones visible in the video, and they remain standing while the steel breaks up and falls off all around them. So it looks like the initiating event happened in the south span or over the south columns.
The structure is supposed to be pinned at one of the four support points, and free to roll on bearings at the other three points, in order to accommodate thermal expansion, and the small shape adjustments that come from squashing the elastic truss members under load. However, the bearings were corroded so they weren't completely free to move, as they should have been. Instead of flagging this as a serious failure of the bridge to be working as designed, though, the engineers inexplicably take comfort in this, since it means the steel members don't have to take nearly as much stress in tension as they would if the whole thing was flexing properly, as I noted in the post above.
But surely this has to mean (if it's designed to flex and it's not able to flex) that other members are experiencing far more compression than they were designed to feel. Steel isn't the best thing to use for compression members (the way I understand it, and civil engineers please correct me.) Concrete is far better in compression than steel.
If the approach spans were showing out of plane distortion of girders, then maybe that's because they were close to buckling with the extra compression. That's what happens to a steel beam when you put it into more compression than it can take, it buckles. I'm trying to find now which support point was supposed to be fixed and which were designed to allow motion.
Again, just speculative theories, but I can't help poring over the evidence and wondering. Maybe I should go into forensic engineering. Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
I find forensic engineering really fascinating, in fact, and these cold impersonal facts are what teach us how to keep people safer, protect human life, and do real good in the world.
My hero is Richard P. Feynman, and though he was a physicist, not an engineer at all, and no aerospace specialist, he got right to the heart of the problem with the O-rings on the Challenger accident report, and his one-page conclusion of what went wrong is more cogent and accurate than the whole 14 volume set of the Rogers Commission report. I don't believe anyone ever read that. The real answers lay in Feynman's short addendum. He was awesome.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
Last post I will copy from the other thread is a list of possible contributing factors. These are things to be investigated and ruled out (or in) in coming up with the right answer.
One thing that seems sure to have been a factor is that it was heavily loaded, it was rush hour, with bumper to bumper cars and trucks across the whole bridge. Of course they were doing work on the bridge, so some of the lanes might have been closed. I see traffic barrels in some of the pictures. Perhaps there was even less total load at the time than usual during rush hour. Maybe the distribution of the load was different than usual, causing particularly high stress in some members, or something.
Another factor might have been that it was a hot day. If so then thermal expansion would have been at a maximum. In fact, weather observations for the city show there was a high of 92F that day. Warm, but not the hottest day of recent weeks. It was 98 on July 7th, for instance.
Another possible contributing factor could be the unique deicing system installed on the bridge. What if the chemical they sprayed for deicing had some unintended effect on the steel? That's something that would need to be examined, since it's not a commonly used system.
What other factors could there be?
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
quote:But remember, please, the Law by which we live: We are not built to comprehend a lie. We can neither love nor pity nor forgive. If you make a slip in handling us you die!
quote:Engineers have a sacred duty to protect people, so that they never have to think about crossing bridges or flying in planes or crossing the ocean depths in ships or whatever it may be. We have been tasked with the divine responsibility of making it as safe for people to do these things as though they were sitting beside the well in the cool of the evening. If we're going to fulfill this sacred charge, we have to develop and maintain systems of checks and balances, of reviews and counter-reviews, that will translate physical reality into models we use to decide how to act, where to spend money, what projects to undertake and which to defer.
Once again, what's so sacred about it? And who assigned the 'divine' portion of our responsibility? I know a lot of engineers who would be surprised to know they're working under a divine mandate.
Posts: 5462 | Registered: Apr 2005
| IP: Logged |
posted
Well, that's how I feel about it, and the iron ring ceremony and background puts it like that too, so I think a good many Canadian engineers feel the same way, at least. But I mean, don't you feel that way at least metaphorically? Even if you don't feel especially religious about it, don't you feel a duty and responsibility to public safety? Feel free to interpret it purely metaphorically, I mean.
Why do you do what you do? I don't want to put words in anyone's mouth. Tell us your motivations in being an engineer.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
As I said in the other thread, I think all engineers have a responsibility to the people who might use their creations. I don't think that responsibility is either sacred or divine. That doesn't make it any less important for me; it just makes it secular.
Posts: 5462 | Registered: Apr 2005
| IP: Logged |
posted
Primal Curve, no, but I own a large collection of guns, knives, and power tools, isn't that more or less the same idea? Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
El JT, I think that's the same thing I mean. Because I'm religious I put it like that, but it isn't meant to claim any special privilege or exclude nonreligious people. I mean it metaphorically for them and literally for me.
I actually (to digress to a philosophical thought before I get back to the engineering thoughts that I hope this thread will be about) think it's almost exactly the same idea or feeling for both religious and non-religious people. I think that almost all religious feelings and thoughts I have now were very nearly the same as I had back when I was an atheist, I just see them as part of a framework now whereas before they were disconnected and experiential, and so I assigned them less importance. They didn't form part of the central structure of my life and purpose of being back then. </religious ideas>
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
quote: Investigators from National Transportation Safety Board said yesterday that one end of the bridge fell sideways about 50 feet while the rest dropped straight down, a discovery they called ``a step forward'' in the investigation.
quote: Meanwhile, NTSB chairman Mark Rosenker said investigators were focusing on the southern end of the 1,907-foot-long steel truss bridge, which opened to traffic in 1967. A security camera that captured the collapse showed that the southern section "seemed to behave differently in the video and in the final way that it sat after the collapse," he said.
"It appears that it shifted approximately 50 feet to the east," Rosenker said, while "the rest of the bridge appears to have collapsed in place." He said investigators would analyze design factors that could account for such a shift. He said they believe the shift occurred as the bridge was falling, not that it caused the collapse.
posted
I think this is cool, a live view of the scene from MNDOT. You have to keep refreshing it. I've added it to the first post for ease of reference.
quote: ELEM No.... ELEMENT NAME........... ENV... INSP. DATE... QUANTITY... QTY CS 1...QTY CS 2... QTY CS 3...QTY CS 4...QTY CS 5 205.............CONCRETE COLUMN ...... 2 .... 06-15-2006 ........ 52 EA.......... 49............. 3............... 0............... 0 ............. N/A .................................................................... 06-10-2005....... 52 EA.......... 49............ 3 ................0 ..............0 ..............N/A 321 .............CONC APPROACH SLAB.. 2 .... 06-15-2006 ......... 4 EA........... 0............ 4................ 0.............. 0 ............... N/A .................................................................... 06-10-2005......... 4 EA........... 0........... 4 .................0 ..............0 ...............N/A Notes: [1991] All 4 approach panels have transverse cracks. Notes: [1969] Pier 9: east column damaged by train derailment (minor scrapes & spalls). [1993] Pier 7: west column has a vertical crack. [2000] Pier 11: west column has a minor spall. [1996] Pier 1 has tipped slightly northward. Likely related to hinge failure in span 2 (south abutment bearings are in full contraction).
This seems highly relevant. It's from the 2006 inspection report posted here. I added the dots in the table entry above to try and get the formatting to be understandable. I don't know what all the columns mean. They must divide the bridge into 4 sections and the numbers indicated how many of the elements described fall in each section, maybe.
But the important part is that the column (I think it's the south pier) is tipping slightly northwards. The bearings are at their limit of travel, so the forces don't have anywhere to go. They're beginning to tip the column, is the inference I make. Also, the transverse cracks in the concrete approach panels seem relevant. I'll try to find a good bridge structure diagram to point all this out on. It all comes back to the bearings not being able to move.
posted
I know I'm reading into this, but the phrasing "More than 70,000 bridges across the country are rated structurally deficient like the I-35W bridge, and engineers estimate repairing them all would take at least a generation and cost more than $188billion." makes it sound as if the task is nearly impossibly difficult and unrealisticly expensive.
$188billion is only slightly more than the cost of 2years of the IraqWar, or rather the direct deficit funding for the money being spent in the US to supply the USmilitary in Iraq. Extra items not covered in the military budget raises direct spending to over $100billion per year. The US can't end the war AND cut off the deficit spending; not without causing a deep recession. Due to the multiplier*effect, cutting that $100billion per year from federal spending would cause a decrease of $427billion per year in the overall economy. So if the US should pull out of Iraq, rebuilding the infrastructure could help save the US economy from going into withdrawal shock.
Okay, we've taken care of the "where's the money going to come from" problem.
Most of the aging infrastructure was built within 25years after WWII. If they could build that infrastructure with a population less-than-1/2 growing to ~2/3rds the size of the present population and a proportionally even smaller economy, we should be able to repair or replace the structurally deficient portion of that infrastructure both faster and more easily than the original builders.
* Simplisticly, worker spending multiplies total economic activity by a factor approximately equal to the average wage of an employee within an industry divided by minimum wage. I used the ratio of the last years in which I could easily find both the average salary of a defense worker and the average salary of a full-time minimum wage worker. Given the increased spread between average salary and minimum wage since 1993, the actual multiplier effect is probably higher.
posted
Sounds like I'm getting into the right career at the right time (Civil Engineering).
(Note that this is a direct response to aspectre's post, not to the overall point of the thread as the latter would be a cold and heartless thing to say in response to an accident responsible for several deaths and double digit injuries).
posted
A little, but not enough to think anything beyond what you've gone into. The biggest thing that caught my eye was the lack of redundency. Of course that's not the cause of failure, that's the reason the cause of failure worked.
posted
I'm sort of wishing I were civil now. Do you think I chose the wrong discipline? I'm totally fascinated by infrastructure, disaster preparedness and relief, water and wastewater treatment, communication networks, electric power generation and distribution, roads and bridges, public health and medicine, nutrition and agriculture, oh and animal husbandry and animal welfare. Isn't that stuff mostly civil? What courses are in your civil curriculum that I wouldn't have had?
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
That looks interesting. I spent some time reading the course descriptions. Maybe I'll get a masters in Civil Engineering from UAB. My company will pay up to $5k a year in tuition and fees. I miss going to school.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
quote:Engineers have a sacred duty to protect people, so that they never have to think about crossing bridges or flying in planes or crossing the ocean depths in ships or whatever it may be.
Yeah, that's kind of why I didn't become an engineer, and what kind of scared me out of math and science of any kind as an adolescent.
I have to say that bridge didn't look very sturdy to begin with.
posted
Does anyone know details about the bearings? I'm zooming in on the bearing failure being the root cause of the whole thing.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
From what I have seen, the bearings were frozen, which was actually reducing the amount of strain that bridge was carrying. If the bearings had been working it would have collapsed even sooner, as the stress levels tehy found SHOULD have been 10 times (or more) greater than what teh bridge was meant to bear when it was built. When a trck would move from the inner lanes to an outer one the stress levels were MUCH higher than it was meant to take, but the bearings being frozen meant the stress wasn't distributed over the weak parts the way it would have been.
That is all second hand information at this point, though. I suck at math in a big way, so engineering was NEVER a possible career foe me. Posts: 15082 | Registered: Jul 2001
| IP: Logged |
posted
Kwea, the problem with frozen bearings is that they're there for a purpose. Even if that reduces the stress in some members, that means those stresses had to have been transferred somewhere else, somewhere that isn't designed to take them.
So I'm trying to find information about the bearings themselves, how they are designed, how they are supposed to work, and which places have bearings and which is fixed. I think that would tell us a lot about how they were supposed to work and what happened when they quit working.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
quote:Originally posted by Kwea: From what I have seen, the bearings were frozen, which was actually reducing the amount of strain that bridge was carrying. If the bearings had been working it would have collapsed even sooner, as the stress levels tehy found SHOULD have been 10 times (or more) greater than what teh bridge was meant to bear when it was built. When a trck would move from the inner lanes to an outer one the stress levels were MUCH higher than it was meant to take, but the bearings being frozen meant the stress wasn't distributed over the weak parts the way it would have been.
That is all second hand information at this point, though. I suck at math in a big way, so engineering was NEVER a possible career foe me.
I'm going to guess you were slightly tired when you posted this.
Posts: 1594 | Registered: Apr 2006
| IP: Logged |
quote: Three of the four piers supporting the river crossing have two huge geared rollernest bearing assemblies while the second pier from the north is a fixed connection.
posted
So here is a summary of the relevant data that I see.
1. In 1995 it was noted in the inspection report that all hinge bearings on Span 2 were locked in full expansion.
2. In 1996 it was noted that Pier 1 had tipped slightly northward, which was likely related to the hinge failure in span 2 (with south abutment bearings in full contraction).
3. In 2000 numerous fatigue cracks were found in the approach spans.
4. In 2001 all these things were again noted in the civil engineering report, along with the unanticipated out-of-plane distortion of girders and concern for lack of redundancy in the main truss system. A computer model was developed which showed that the stresses could be as much as 138 MPa in some members, with a corresponding predicted average life of 20,000 to 40,000 cycles. With 15,000 trucks per day crossing the bridge in each direction, these should have cracked soon after the bridge opened, if the stresses were really that high.
So they put instruments on the bridge (strain gages) and monitored them. They decided the actual stress on the bridge was less, because it's usually mostly cars, and also because the slabs and floor trusses reinforce one another, and because the bearings weren't moving as designed. Apparently, nobody ever took it to the next step and asked if the bearings weren't moving, then where were the forces going that the bearings were intended to relieve. Obviously if a bridge functions better without bearings, it would have been designed without them. The bearings obviously are designed to perform a needed function.
Because of the failure of the bearings, the south pier was tipped slightly northward. What happened next?
From the incomplete information I can find, some failure occurred at the south end of the bridge. Then the lack of redundancy kicked in and the whole thing fell.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
I found a list of steel deck truss bridges in the U.S. which shows this bridge, number 9340, in MN.
The other interstate bridges on the list are
I-5 NB (northbound) in CA at SR99. I-5 in CA across the Sacramento River. I-680 in CA at UP BNSF Amtrak Suisun BA (whatever that means). I guess it's a railroad bridge. I-95 NB in CT over the Thames Rv RR local roads. I-95 SB in CT over the Thames Rv local roads & CVRR. I-70 WB and EB in KA over the Kansas River 3 RR 5 St. I-75 in KY over KY 2328 and KY river. I-95 in MD over the Susquehanna River PA RR. I-90 in MA over combined US20 and Westfield R&CSX. I-25 NBL and SBL in NM over Nogal Canyon. I-90 in OH over Cuy. Riv. Valley RTA45. I-90 in OH over Grand River @ MP 209.5 I-84 in OR over the Columbia River. I-5 in OR (5 times) over Umpqua corp RR stuff. I-84 in OR over the John Day river. I-5 in OR over the Willamette River. I-40 in TN over the French Broad River. I-5 in WA over Lake Wash Ship Canal. I-68 in WV over MGL CO 857 & cheat lake. I-79 in WV over CR73/7;45/9;Mong R & 2 RR. I-77 NB and SB in WV over the Bluestone river. I-64 in WV over glade creek.
I think that's all the interstate ones. There are also piles on state highways and county roads and so on. Probably best to check your state on the comprehensive list.
The U.S. Department of Transportation is calling for immediate inspection of all such bridges. Here's the bulletin.Posts: 6246 | Registered: Aug 2004
| IP: Logged |
quote: Hinge Joint (12 ft. south of Pier #2): There is an open finger deck joint above. There is severe corrosion and debris on the hinge assemblies and beam ends. The hinge assemblies (particularly SBL) are expanded beyond tolerance (the sliding plates have extended 1-3/4" beyond the base plates). At Beam #5, the sliding plate is tipped (falling off the base plate), preventing the joint from opening. Several of the beam ends are contacting - The top flange of beams #2,3,4,7,8,12,& 13, and the webs of Beams #4,5,& 7. All of beam 1 is in contact at the hinge. (See pictures #1 & #2).
Here the relevant thing seems to me to be that the sliding plate is falling off the base plate. Again, I would think such a failure would be setting off alarm bells.
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
Here's a description of rollernest bearings. I wanted to know what the darn things were and here is some descriptive text, but no picture.
quote: A roller bearing consists of a horizontal steel cylinder that “rolls” between the sole plate and masonry plate as the superstructure expands and contracts. The bearing may have a single or multiple rollers (“rollernest bearing”). Lateral restraint may be provided by pintles (on the top & bottom of the roller), or keeper bars attached the ends of the rollers.
posted
From the same document as the previous post, I discovered the meanings of CS1, CS2, etc. in the inspection report. They stand for Condition State 1, Condition State 2, etc. and they represent increasingly worse condition as the number gets higher. Condition states for various elements are defined in the inspection manual. For bearings, they are as follows:
quote: Condition State 1: Expansion bearing is in good condition and is functioning as intended. Bearing alignment is within design limits and is appropriate for the current temperature. Bearing assembly is relatively free of debris (no restriction of movement). Paint system (if present) may have some deterioration - corrosion may be present, but there is no significant section loss. Lubrication system (if any) is functioning properly. All bearing components (sliding plates, rockers, rollers, pins, etc.) are intact and properly positioned. Lateral guide/restraint system (or uplift restraint system, if present) is in good condition. Anchor bolts are bearing seat are sound (there is no loss of bearing area).
Condition State 2: Expansion bearing has moderate deterioration - bearing function may be slightly restricted (cleaning, painting, or lubrication may be recommended). Bearing alignment may be at or near the design limits (or inappropriate for the current temperature), but is still tolerable. Bearing assembly may have extensive corrosion (section loss may be present), or may be covered with debris. Lubrication system may have failed. Primary bearing components (sliding plates, rockers, rollers, pins, etc.) may be moderately worn or slightly out of alignment. Secondary bearing components (cotter pins, etc.) may be loose or missing. The lateral guide/restraint system (guide tabs, keeper bars, pintles, pin caps, etc.) may be moderately worn or slightly out of alignment (there may be minor binding). Uplift restraint system (if present) may have moderate deterioration, but is still functioning as intended. Anchor bolts may be corroded or bent, but remain intact. The bearing seat may have moderate deterioration (there may be a slight loss of bearing area).
Condition State 3: Expansion bearing has severe deterioration, and is no longer functioning as intended (repair or replacement may be necessary). Bearing alignment may be beyond design limits. Bearing mechanism may be frozen (seized) or severely restricted due to corrosion or debris. Primary bearing components (sliding plates, rockers, rollers, pins, etc.) may severe section loss, wear, or misalignment - they may have jammed, come loose or otherwise failed. The lateral guide/restraint system (guide tabs, keeper bars, pintles, or pin caps) may have sheared off, bound, or otherwise failed. Uplift restraint system may have failed. Anchor bolts may have failed. Bearing seat may have severe deterioration (there may be significant loss of bearing area) - supplemental supports or load restrictions may be warranted.
The 2006 inspection report shows 6 of the expansion bearings are in Condition State 3, with 44 in CS2. Posts: 6246 | Registered: Aug 2004
| IP: Logged |
posted
I'd like to read the bridge inspection reports for the bridges I cross daily. That's my next research project, but it will have to wait, since it's late and I have to go to work tomorrow.
This is so fascinating to me! I want to track this down and nail it!
Posts: 6246 | Registered: Aug 2004
| IP: Logged |
quote:Originally posted by Tatiana: This whole thing makes me really, really upset. Engineers have a sacred duty to protect people, so that they never have to think about crossing bridges or flying in planes or crossing the ocean depths in ships or whatever it may be. We have been tasked with the divine responsibility of making it as safe for people to do these things as though they were sitting beside the well in the cool of the evening.
Tatiana, I am impressed with your dedication as an engineer.
I do think engineers have a duty to the public, but I do not think that duty is exclusive to engineers. I am sure this is not what you were saying, or even meaning to imply, but I think it is worth pointing out.
All people who work in professions, or have roles, in which the less than satisfactory performance of those professions or roles could mean harm to, or loss of, life, have a clear duty to perform those professions and roles to the best of their abilities. An engineer's duty is no more than, and no less than, the duty of doctors, bus drivers, fire fighters, school teachers, nurses, ambulance officers, pilots, mechanics, maintanence officials, police, security officers, politicians, life savers, child care workers, parents, road workers, judges, scout leaders, lawyers, road users and so on.
I'm sure there are many I haven't included.
Posts: 4393 | Registered: Aug 2003
| IP: Logged |
posted
Or a mother, because as we all know, there are no real consequences when a mother does a crappy job.
Posts: 16551 | Registered: Feb 2003
| IP: Logged |
I think a sense of purpose is great. While my job is useful, if I do it badly, people will think dark, angry thoughts, but no one will die.
Posts: 1753 | Registered: Aug 2002
| IP: Logged |
posted
Tatiana, there are a couple of things that I haven't seen mentioned.
1) Some reports said there was a train going by on the east (south?) bank.
2) The road work on the top may have been using vibrational tools that aren't necessarily reccommended for use on bridges.
The impropmptu analysis from my civil engineering connections says that they don't believe that this was a pure "fatigue" failure. Normally when bridges fall, there is a warning, with moaning and groaning as the thing fatigues and dies, like Galloping Gertie, they knew it was going to fall long before it came apart.
The middle section going straight down is much more like the collapses you see during earthquakes, when a harmonic frequency is attained that causes a "resonance" collapse.
It is possible that the jackhammers used during the resurfacing project earlier in the day, pluse the rush hour weight, water current, etc. shifted the resonance frequency of the structure, and that the train going through was the trigger so that the reasonance frequency was achieved and sudden collapse occurred.
Of course the standard current civil engineering knowledge could be wrong and this bridge could be an educational experience that middle sections can drop out suddenly with out prior moaning and groaning "fatigue" warnings.
But it appears to be more of a "reasonance" collapse situation than anything else, from the early visuals on the collapse.
posted
Wow, resonance collapse. Was Orrin Boyle accounted for at the time?
Well, apparently I don't know the guy's name, but the guy with the Sonic Death Ray in Atlas Shrugged.
Posts: 11017 | Registered: Apr 2003
| IP: Logged |
Printer-friendly view of this topic
![[Smile]](smile.gif)
![[Wink]](wink.gif)
![[Frown]](frown.gif)
