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Bridge Collapse
5

Bridge Collapse

RE: Bridge Collapse

We'll watch this unfold.  Montreals bridges and overpasses are very complex, convoluted, and in various states of deterioration.  All built of reinforced concrete some 30 years ago and subjected to a very harsh environment of chloride based deicers.  The Supreme Court of Canada ruled in the last several years that all construction in Quebec must be unionized to avoid the 'problems' with the construction industry of the past.  We drive through Montreal often and wonder about its infrastructure.  This problem will boil down to money and political will and lets hope finally will be put on the front burner.  Five people dead.

RE: Bridge Collapse

Connect2, Are you familar with this kind of bridge?  It appears to be a vaulted abutment that reaches out toward the roadway and has the concrete slab (or girders) bearing atop the abutment seat.  It looks like a single span.  

From the pictures I couldn't tell if the superstructure is concrete slab or slab on girder.

I'm very interested in learning more about this type of bridge.

Much Thanks

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

RE: Bridge Collapse

(OP)
From the Fox News article in my first post.

"The incident occurred before 1 p.m. (1700 GMT) One witness told TVA he noticed that the road sunk by 1 to 2 inches (2.5-5 centimeters) when the traveled over the overpass minutes earlier and called emergency dispatchers at 12:30 p.m. (1630 GMT)."

Was this a warning unheeded?

RE: Bridge Collapse

3
A road crew cleaned up some pieces of concrete from the roadway below a couple of hours before the collapse.

The bridge does seem to have a cantilevered bearing seat which is common in the area.  I think it's a two span bridge with simple spans.  A quick glimpse I got from a video seemed to show that the bearing seat and a large portion of the abutment face sheared off diagonally.  Some horizontal rebar are also seen extending from the top of the abutment indicating a possible lap/bond failure at the critical point.  The abutment would have been simply reinforced, not prestressed, so it would have cracked, perhaps at the back of the bearing seat, early on in order to mobilize the top horizontal steel.  This bridge would have also had an expansion joint directly above the bearing seat.  After 36 years (built around 1970) of deicing salt and a leaking joint seal (inevitable) any significant cracking at the top would have lead to likely corrosion of the top rebar.  A public inquiry has been called and it will interesting to see if an expanding crack at the back of the bearing seat and probably down the sides of the abutment have been overlooked during inspections over the years.

They've now closed a similar bridge and are looking at 45 more.  The Quebec construction industry had problems with organized crime about the time of construction.  We'll see if that played a part.

A local professor said that since the bridge survived 36 years it meant that the design/construction was OK and that it must have been simply poor maintenance.  While it's true that poor maintenance is a likely contributing factor even a compromised design or construction mistakes/shortcuts  can survive for a while until it's luck runs out due to it's reduced safety index.

In a way it's good that this didn't happen during construction with private contractors involved.  Legal suits involving private interests tend to result in publication bans on engineering reports as part of any settlement.  There have been many cases where the engineering profession can't learn from the failure due to these bans.

Sorry for the long post.

RE: Bridge Collapse

(OP)
I see a parallel between falling concrete and industry where a part keeps failing and is no problem as long as there is a spare in the parts bin. Here the there is no problem until the cleanup crew doesn't have any concrete to cleanup.

RE: Bridge Collapse

Some good pictures available at www.keystonebridge.ca.

To update my previous observations.  It is a single span bridge with cantilevered bearing seats and a suspended span.  Looks like a complete shear failure of one cantilevered end with a horizontal separation between the top of the slab and the top of the top rebar at the bearing seat (see 6th photo).  I think there was a lap failure in the top rebar.  The top rebar into the abutment isn't visible and is probably still embedded in the remnants of the top of the slab.  No apparent stagger on the lap.  Not much shear steel evident!

My previous comments about possible cracking at the top of the bearing seat don't seem to be correct.

RE: Bridge Collapse

Looks like a fairly standard 'half-joint' (or 'halving joint') bridge.  Britain has quite a few similar bridges on their older motorways that are 30-40 years old.

RE: Bridge Collapse

Excellent link, cooperDBM. I'd also agree that the steel looks light, although it's hard to tell from photos.
I'm curious about how bridge inspection works in Canada. I would assume that they have something similar to the NBIS in the US, which calls for inspections every 2 years. In the US, a lot (I'm guessing a majority) of bridge inspectors are non-engineers from various maintenance and construction backgrounds that have taken a few weeks of class for the job, and the rest engineers that have taken the same class. This varies from owner to owner, since even though NBIS is a federal program it is ultimately performed on the state and local level. Can anyone in Canada compare and contrast?

I have to feel bad for the inspector that was sent to the bridge. I don't understand from the statement if he left his office 30 minutes before the collapse or if he was on site 30 minutes. If it is the former he would have probably arrived just in time to see the collapse. Even if it was the later, I think one would still be hard pressed to comprehend the problem and get a bridge closed in that much time unless it was pretty obvious.

RE: Bridge Collapse

It is my understanding that the bridge was inspected in 2005... Dik

RE: Bridge Collapse

I'm inclined to agree with cooperDBM. In 1984 I was inspecting four bridges in Staten Island NY - multiple simple spans, rolled beams, built in the 1960's - each pier consisted of four concrete columns with a concrete cap beam. The center portion of the cap was similar to the configuration of the Laval Bridge - supported by a cantilevered seat. All four cap beams were cracked above the seat to various degrees, with some being fully cracked, being held in place by friction or a miracle.

Our chief engineer believed it was a shear failure although we never were privy to the results of the owner's investigation. He also said it was "down and dirty" way to design a pier quickly.

RE: Bridge Collapse

(OP)
In the 4th picture in cooperDBM's post just right of center below the deck it looks like someone has grouted a crack.

RE: Bridge Collapse

Hardly ever hear a more subtle possible reason for the failure--poor inspection techniques (notice I didn't say 'poor inspectors'). I can only imagine if similar things were happening in the aircraft industry, what would happen if visual inspections were the SOP (standard operating procedure) but an aircraft crashed from damage caused, it was later determined, by defect that was not visible? Certainly NDIs (non destructive inspections) would be specified from then on (along with the usual visual inspections), would it not?

RE: Bridge Collapse

I do not think poor inspection was the reason for the collapse - it's combination of aging and design parameters of the bridge.
Halved seats were quite common in 70-ties and 80-ties. I have also design some of these.
It's my understanding that the code applicable at the time of design would allow for the design for shear by combination of concrete and steel capacity - while for example DIN, or PN (Polish Standard) disallowed such provisions, when the allowable shear was exceeded. This was based on the assumption, that stressed concrete would develop micro cracks, thereby loosing the capacity to carry the stresses.
In very light and short spans, the “square” seated joint could be acceptable by some, but my mentor always requested a 45 degree transition, and all shear to be carried by steel alone, with ample provision for corrosion, regardless of the concrete capacity.
Judging form the photographs it’s rather difficult to assess if the failure was related to the inadequate design. But the rebars look thin, there are not severely corroded, leading me to the conclusion that the issue of the shear design was the culprit.

RE: Bridge Collapse

"Sheikh added that there have been an estimated 34 bridge collapses in the United States, Canada, Europe, and Asia over the past 160 years."

34 collapses in 160 years seems QUITE low

RE: Bridge Collapse

I would have to think that the 34 collapses in 160 years is a misquote or typo. Perhaps 34 collapses resulting in fatalities? Maybe 160 collapses in the last 34 years?
If we just count all bridges on public roads with spans over 20' in the US, India, the UK, and Canada (i.e. where most of the people on this form are) we could name at least one collapse per year.

RE: Bridge Collapse

unclesyd:  I think the crack you're referring to is simply wet due to a transverse construction joint above it.  The white crack is filled with efflouresence (salt).

I think the shear failure started as a vertical separation of the top long. steel lap at the back of the bearing seat (along a horizontal plane).  Long. steel on the bearing side of the failure went with the falling concrete while the long. steel on the abutment side stayed embedded in the renmants of the topping.  Seems that there was no shear steel tied over the top of the top steel to prevent this separation.

If this is true it would have been quite sudden and prior inspections may not have seen much of a crack unless they could have seen the back face of the bearing seat (probably a hor. crack a couple of inches below the top of the deck).  The inspector who looked at the bridge immediately before the failure wouldn't have had a change to make that observation.  The span seems to be box beams placed side-by-side which wouldn't have allowed a view of that face.

Annual or biannual inspections in Ontario, and I believe Quebec, are carried out by trained technologists or maybe young engineers.  Experienced engineers would inspect the bridge if a structural evaluation was required, say for a rehab.  I don't think it mattered in this case since I think it was a sudden brittle failure due to bad design detailing.  If I'm right I'm amazed it lasted as long as it did.

RE: Bridge Collapse

Looking at the 1st photo on www.keystonebridge.ca at the twin bridge still standing (right side of the photo) there does appear to be a diagonal to horizontal crack in the side face of the cantilever near the bearing seat.  There are a couple of streams of efflouresence at that crack.  The surviving cantilever of the failed bridge (photo 2 & 4) also shows a similar horizontal crack (with eff.) near the top at the back of the bearing seat where the separation seems to have started.

RE: Bridge Collapse

I dunno; if an inspection 9 months ago doesn't tell you what you need to know to avoid a collapse (and fatalities) today, then I still think you have an inspection problem. Almost any design, no matter how poor, can be made safe with adequate inspections. The aircraft industry assumes that there will be problems with design, so that redundancy is designed into the system at the beginning, but proper inspections are still needed to avoid aircraft crashes caused by unanticipated problems.

RE: Bridge Collapse

Inspection won't help you with sudden brittle failure.

Hg

Eng-Tips policies:  FAQ731-376

RE: Bridge Collapse

There was a news article this AM that they had ruled out corrosion problems and were checking reinforcing to see if it had been installed as spec'd.  Can't verify this.

Dik

RE: Bridge Collapse

Sudden brittle failure? Caused by what? Again, if this were an airplane, and a failure mode could not be inspected out, so to speak (that is, inspections catch indicators of impending failure before catastrophic failure occurs), then the structure or system would have to be retired before the failure mode could kick in. If this isn't how the road systems are maintained and inspected, then I just started feeling a whole lot unsafer than I have felt in the past.

RE: Bridge Collapse

All I'm saying is that in-service inspection in itself is no substitute for proper design and construction (and inspection during construction).  If ductile failure was not a consideration in design, or if construction was shoddy and led to a situation that is prone to some kind of sudden brittle failure that doesn't give much external warning before it happens (which is kinda what sudden brittle failure is), then periodic inspections, especially those based on the assumption that everything in construction went as it should, are not going to help you.

Hg

Eng-Tips policies:  FAQ731-376

RE: Bridge Collapse

(OP)
HgTX,
If you had inspected the bridge and it checked OK according to the inspection parameters and as you were walking back to your truck and turned your ankle on a chunk of concrete and at the same time a chunk fell on your head, thank goodness for hard hats, wouldn't one think something is amiss.

 

RE: Bridge Collapse

I was speaking more in a general sense.  I'm not saying that there WEREN'T signs in this case that something was wrong (and they did realize there was enough wrong to send someone out there).  But the blanket statement that any competent routine inspection program will prevent any kind of failure (barring getting hit by an outside actor of some kind) is just not true.  Bad materials and bad workmanship can lead to a situation that deteriorates very quickly over a timeframe shorter than the design parameters suggest the inspection interval should be.

Hg

Eng-Tips policies:  FAQ731-376

RE: Bridge Collapse

I would wonder if any unusual heavy
equipment passed over this bridge in
the last month.  Did the overhanging
walkways put too much tensile stress
on the reinforcing bars?

RE: Bridge Collapse

This is certainly a very interesting discussion!

The bridge was 30 years old, right? I would think normally a bad design would be evident much sooner than 30 years. The fact that the bridge was erect for 30 years (assuming no problems encountered during that time) would suggest that the bridge was properly designed and constructed.

The statement was made by 'dik' that corrosion was ruled out (of course given the recent history of early news reports on various sensational cases in the States, I would tend to 'trust but verify' all such reports). After 30 years, some kind of age degradation is inevitable of course, the question is did the age degradation cause the failure?

RE: Bridge Collapse

I did a quick google on fatigue failure  of reinforced concrete. There are a number of papers/studies on this failure mode therefore it cannot necessarily be assumed that the bridge was properly designed and/or constructed because it stood for 35 years.
Failure due to inadequate design or construction doesn't mean that it didn't meet the required standards back in 1970.

RE: Bridge Collapse

One of the reports mentioned that they've started using huge quantities of salt on the roads in that area in recent years.

I'm guessing that chloride corrosion was not given a lot of consideration when that bridge was designed, because nobody bothered to salt the roads then.  When I lived near that latitude decades ago, salt was used only to keep the sandpiles from freezing solid in their shelters.  Salt is just an expensive abrasive when the temperature is below 0F all the time.

Sand was applied after each snowfall to provide traction, with maybe a little extra salt in the spring, to help break up the accumulated foot of ice on which we had been driving all winter.

Maybe global warming made it less futile to salt the roads, and hence catalyzed increased salt usage, and eventual failure of bridges not designed to withstand salting.  Speaking of which, are _any_ steel- reinforced concrete bridges actually designed to deal with heavily salted decks?

Mike Halloran
Pembroke Pines, FL, USA

RE: Bridge Collapse

It wasn't lack of maintenance that killed this bridge it was lack of ties.

The product was defective from day due to bad design/construction. The little bit of corrosion was simply the straw that broke the camels back.

RE: Bridge Collapse

I am amazed at the conclusions some get from these photos.  While I only looked at the photos for a few minutes on the web, I could not draw conclusions except one; that I thought the drop in span detail and design philosophy was por for this very reason, lack of redundancy.  That does not mean the design didn't meet the code or that the consrtuction didn't meet the specs.  I also suspect some degree of corrosion contributed to the problem but the bars I could see looked very clean.

As I understand it, shear reinforcement provisions in the codes have been increased since the fifties but I don't know a better date on that change.  This is one of the problems I see in our industry (transportation infrastructure) that rehabilitation of existing structures is rarely undertaken to address code modifications.  That is a major discussion in itself.  Also, I believe bridges are rated based on their moment capacity alone.  Shear is not part of the rating.  This should be appropriate for 95% or more of the structures in the inventory but I am sure some, as possibly this one, are controled by their shear capacity.

I hope we can find a specific cause that can be addressed in the future.

RE: Bridge Collapse

This goes a little off topic, but Mike H., in answer to your question about designing for being pounded by salt, "sort of." In my experience, in Pennsylvania, most mild steel above the beam seat, and anything below that we really like or won't ever see again (like the back steel in the stem of a cantilevered abutment) is epoxy coated. Obviously, there are other corrosion protection systems, including using non-corroding bar in the first place, but I've only seen that a couple of times. I've never really seen a good answer on just how protective the usual epoxy coating is. I have, however, seen bars in a parapet that had been hit but never repaired, thus exposing the bar. The green shell of epoxy looked great, but contained nothing but rust. Don't know if that was a fluke or what.

RE: Bridge Collapse

Sorry to double post, but Dinosaur:
Right, this is going to take a lot more analysis than can be had from a few pictures. Hopefully this all comes out in a form that we can all use on our own designs. It's fun to discuss now, though.
Shear is conisdered, and I find that shear often controls the ratings of concrete bridges. (PS and RC)
I will admit that we can be negligent on keeping a current rating on the substructures, which the failed portion of the subject bridge could be called, depending on your point of view.
I would also say that you have plenty of company (including mine, for what that's worth) on your dim view of drop in spans.

RE: Bridge Collapse

The area of concrete usable for shear strength is only that portion below the location of loading.  Some configuration of straps and hoops that transfer the load to the top would have to be present to use the concrete above the beam seat for shear calculations.

RE: Bridge Collapse

Dinosaur, in some countries (but perhaps not in North America) both moment and shear are checked to get a load rating for a bridge.  The similar bridges I assessed in Britain were generally shear limited.

RE: Bridge Collapse

For anybody that doesn't know for twenty or more years in Canada we haven't mixed our salt with anything, it's applied straight up in granular form directly as a road de-icer, and the bridges are deiced more often than the roads.  We've been through about a decade of climatic changes here, hovering often just below the freezing temperature.
It would be my opinion (at this tme) that the failure is cracking deterioration due to corrosion, time dependant cracking of concrete and at the end of the day poor inspection and maintenance.  Chunks of concrete falling of Montreal bridges is a fairly common occurance.  We will see that a number of bridges in Montreal will be closed, already I know of three since the collapse.  If you've driven in Montreal this is a problem for the 'public'.
The bridge remained safe and serviceable for 36 years.  We now know alot more about time dependant effects on concrete, corrossion, we certainly have the technology to inspect, and we know the construction sector in Montreal at the time was circumspect.  The government needs to put this on the front burner.

RE: Bridge Collapse

JEmH, continuing off-topic, the (few) examples of epoxy coated reo that I've come across have experienced severe localised corrosion, usually at coating defects (eg coating holidays or installation damage).

RE: Bridge Collapse

I have one question: where was the chunk of concrete found right before the collapse? Below the joint or near the abutment?

If it was found below the joint, then the support for the cantilever end was likely the first failed part. Corrosion could play a major role here. The cantilever part sat on the joint and it was a simple support beam until the moment that the support failed suddenly. If you look at the second and forth photo of http://www.keystonebridge.ca/, you can see the support for the cantilever was sheared off. Now the beam became suddenly a purely cantilever. The abutment side incurred much bigger moment and much bigger shear for which the beam was not designed. As a consequence, the bridge had a brittle failure because of one or both of the two reasons: 1) shear failure, 2) reinforcement details designed according the old code were not ductile as required by the current code. (I am not sure about the code development history.)

If it was found near abutment, it is difficult to understand. It could mean that the concrete crushed due to some reason. From the photos we see not much of reinforcements near the abutment. So it was not likely due to over reinforce if that is the case.


I found almost no top reinforcements on the face of abutment. Is this normal? Anybody can give an insight idea?
ponder

RE: Bridge Collapse

Whenever there is a disaster, whether you are down here in Australia, in North America or anywhere else you have a lot of people jumping to all sorts of conclusions which is a pity because aspertions are cast on many innocent people.

From what I can see from photos and (to some extent) guess from comments, I would say the most likely scenario would be:

primary cause was the reo (reinforcing steel) near top of cantilever and extending back over the pier was not properly tied in with the reo around the seat for the suspended span. This reo is to carry tensile forces on the top surface of the cantiver and down to the seat. If this is the case then there has been a fundamental design failure (including failure to check design) OR a failure to comply with the design (with constructor and inspection both guilty) during construction. Would appear that a more-or-less horizontal crack (possible initially from drying shrinkage) has formed on the vertical face between the two sets of reo and has propagated as one would expect along the plane of the lower (seating) reo then from the end of the reo diagonally down to the top of the pier.  

Other issues of spalling, salt, corrosion, etc would be incidental in this case. It is amazing that this disaster took so long to happen.

RE: Bridge Collapse

yes a very sudden collapse, after 36 years in service.  the type of failure in design or construction you describe ozbridge, one would think would have occurred much sooner?

RE: Bridge Collapse

cooperDBM (oct 3) has said much the same as I have. Regardless of details, the failure appears to have been that of an unreinforced area of concrete being subject to significant tensile forces.
In many (most) design situations, concrete is assumed to have no ability to carry tension but in reality it can usually carry at least 5 megapascals IN THE ABSENCE OF CRACKS. So with quite a bit of luck we had concrete carrying quite a bit of tension for many years - then the luck ran out.
Maybe it was corrosion of reo that triggered the collapse by increasing the tensile forces in the concrete (due to expansion of rust) but the fact remains that there does not appear to be steel available to carry the forces.
Maybe spalling led to redistributions of forces in the failure zone but it still comes back to bad reinforcing.

RE: Bridge Collapse

ozbridge,

You say "Whenever there is a disaster, whether you are down here in Australia, in North America or anywhere else you have a lot of people jumping to all sorts of conclusions which is a pity because aspertions are cast on many innocent people."

and then proceed to so exactly that yourself !

RE: Bridge Collapse

It seems to me that, after standing in service for 35 years, an explanation of failure that does not have a time dependent component will not be acceptable.  

The time dependent component hopefully will be something more than "... then the luck ran out...".  What about movement of soil bearing the footers?  Was sufficient care taken in design and construction to assure minimal or at least symmetric foundation shifts?

Were any unusual loads applied to the bridge during the past year, any history of large oversize loads taken across the span recently, any history of flooding or freeze-thaw heaving?

RE: Bridge Collapse

My comment, "survive for a while until it's luck runs", was of course not meant to be a detailed and final assessment of the final trigger leading to failure.  I was actually commenting on a reduced safety index which itself represents the wide variability and randomness of the structure's loading and strength.

Let's keep in mind that all this forum has for evidence is the posted photos and some unreliable media reports.  As I've said the photos do seem to give good evidence, in my hypothesis, of the mode of failure (shear) and probable reason (lack of shear steel) but they provide no evidence for the trigger that caused the failure.

If my hypothesis is about right then the question "what kept it standing for 36 years" is the big unknown.  We won't learn that from the evidence presented in this forum.

Lastly, keep in mind that in structural engineering we use a lower bound (or low percentile) approach to predict strength from a sometimes wide distribution of actual strength.  A structure can have a lot of reserve and redundant strength that we don't count on.

DBM

RE: Bridge Collapse

(OP)
The fox is in the hen house!

RE: Bridge Collapse

Well the two globe and mail articles certainly describe the Quebec construction industry and politics at the time, and probably to this day, well enough.  It will take political will and billions of tax payers dollars to repair these bridges.  We do have the assesment knowledge, monitoring capabilities, and repair technologies.  These disasters need not happen regardless of conjecture on failure mechanism.  Quebec shouldn't be appointing a commission of political hacks to investigate, they should be hiring a team(s) of independant private structural consultants to immediately review and make recommendations for all its Montreal bridges.  Then the fox will be truely in the hen house.

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