Bridge Collapse 5

[LittleWheels]

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.

 

[connect2]

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.

 

[LittleWheels]

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).

 

[precastengineer]

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?

 

[precastengineer]

New Updates:

http://www.ctv.ca/servlet/ArticleNe...12/montreal_bridge_061012/20061012?hub=Canada

 

[ozbridge]

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.

 

[connect2]

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?

 

[ozbridge]

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.

 

[Jordonlaw]

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 !

 

[ccw]

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?

 

[cooperDBM]

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

 

[cooperDBM]

Some recent articles from the Globe and Mail.

http://www.theglobeandmail.com/servlet/story/LAC.20061014.OVERPASSFRONT14/TPStory/?query=laval

http://www.theglobeandmail.com/servlet/story/LAC.20061014.OVERPASS14/TPStory/?query=laval

 

[unclesyd]

The fox is in the hen house!

 

[connect2]

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.