Bridge collapse in Minneapolis

[AHSPanther23]

^Tensile strength of concrete is about 1/10 of it's compressive strength, that is why reinforcing steel is placed in concrete...to take the tensile force in the tensile region of a member. From the fallen debris, it looks as if pre-stressed reinforcement (concrete sections) was used...however I am not sure about how far along design of such was in the 1960's compared to today.

 

Failure at the main connections by shear is very possible.....high strength bolts or rivets subjected to moisture could corrode, thus limiting thier effective cross section area and weakening the connection. The video very well indicates a connection could have failed in a vertical shear plane.

 

It would have to be better, or at least more well designed. The structure appears to have been constructed in a 1960's sense, cheaply.

[Sportz Addict]

From what I have heard the Brent Spence in Cincy is actually more dangerous than this bridge that collapsed. It's about 40 years old and they are saying it's one of the most dangerous bridges in America. CNN was in Cincy today doing a story on it. I take it very often.

[HHSDad]

From what I have heard the Brent Spence in Cincy is actually more dangerous than this bridge that collapsed. It's about 40 years old and they are saying it's one of the most dangerous bridges in America. CNN was in Cincy today doing a story on it. I take it very often.

It's also carrying almost twice as many cars per day as it was designed to carry.

[Son of Bear]

AHSPanther, trying to introduce new concepts into engineering practice can take years.......I was referring to reinforcement theories in concrete. Typical old school rebar is being replaced with prestressing of steel wire.......in fact UK has done extensive research in carbon fiber reinforcement in new bridges and in "beefing up" older ones as well. Carbon fiber fabric (with the threads in an axial direction) can have 4 to 5 times the tensile strength of A36 standard steel.

 

The KTC has tried this out on some smaller bridges in KY that were showing small cracking and age......the results are fantastic. This process is expensive, but could revolutionize bridge design in the future. Even though the research has shown great things, it could take years and years before anything like this goes from academia to being used in practice.

 

Before anyone says this thing was a piece of crap or was built terribly...remember that it has been standing for 40 years. Bridges are just like automobiles, they need proper maintainence or they are going to fall apart.

[Sportz Addict]

It's also carrying almost twice as many cars per day as it was designed to carry.

 

Yes sir, designed for 80K, it's taking on 150k. Also heard today that it will cost the city 1.5 billion...I believe I heard billion, to rebuild the BSB. Wouldn't start till 09 if they can do it that soon.

[Freebird23]

Saw the news footage of the bridge collapsing and it appears that the north and south piers collapsed at the same time. When I first saw the story hit the air waves, I thought it started on one end and rippled to the other. Not the case...

 

Also, there is thought that the train below could have caused a vibration that set the chain of events into motion. Continuous vibration from trains over time could have caused structural damage to the piers...

 

Will be interesting when the mystery is solved...

[Son of Bear]

^The bridge piers did not fail...they are standing and look unharmed other than the anchorage to the steel.

 

Watch the video of the event.....It was the main span of the tower superstructure that collapsed, the sections over the riverbank then fall in the aftermath.....

[Admiral]

My nephew and his girlfriend went across this bridge about 4 hours before it collapsed. Prayers for the families.

[AHSPanther23]

The link posted above screams connection failure to me. The main span which was nearly 500 feet between supports falls as one piece....therefore indicating that member buckling was not a primary cause.

 

The jackhammering and removing of pavement took some rigidity from the structure leaving it vulnerable to vibrations. That mixed with a load distribution of the bumper to bumper traffic in only a few lanes (or an odd load case) which the bridge was not designed for, could have overstressed a weak connection. Inspecting connections can be tough since most bolts (or maybe rivets in this case) wouldn't be fully exposed. One has to think that with the snow/ice that Minnesota gets, rock salt could have damaged exposed steel.

 

It will take the forensic engineers months, maybe years to figure out a "cause", which will still only be speculation. They are going to have to remove all this debris and rebuild the thing somewhere, which will be a nightmare since most of it is in the Mississippi River.

 

Son of Bear, how damaging is rock salt to concrete and steel on bridge spans?

[AHSPanther23]

http://www.msnbc.msn.com/id/20085333/?GT1=10252

 

Records show that Minnesota officials were warned as early as 1990 that the was structurally deficient, yet they relied on a strategy of patchwork fixes and stepped-up inspections. The bridge got the rating from the federal government, who cited significant corrosion in its bearings. The bridge is one of 77,000 bridges in that category nationwide, 1,160 in Minnesota alone.

 

Kind of makes you think about some of the busy bridges that are traveled in NKY everyday.

[nees1212]

Son of Bear, how damaging is rock salt to concrete and steel on bridge spans?

 

In 2001, this bridge had a de-icing system installed where it was automatically sprayed onto the driving surface. It was basically a sprinkler system to keep it from icing up.

[Son of Bear]

Son of Bear, how damaging is rock salt to concrete and steel on bridge spans?

 

If a concrete member (bridge deck, driveway, sidewalk, etc.) has even microscopic cracks in it, rock salt can penetrate into these and damage reinforcing steel, and weaken the member.

 

In reinforced concrete design, there are measures taken to help prevent this. One is clear cover of the steel by concrete, which is usually 3 inches. Another is epoxy coating rebar.....you may recognize these as the green peices of rebar you see sometimes.

 

Bottomline is concrete is going to crack, any SE will tell you that most sections are designed to crack a small amount. Most problems in concrete design stem from placement of the concrete by construction workers.........by adding wayyy too much water.........the more water used in concrete, the greater the risk for surface cracking and spalling.