Bridge collapse in Minneapolis

[Ms Liberty]

They have down graded the death toll to 4, said they would update later.

[ram2003]

I'm sure at this point we've all imagined what it would be like to be on the collapsing bridge, but imagine being in one of the cars right before or right after it... they must feel touched by God.

[Qryche11]

I work with about 40 bank branches up there with my employer and I am praying none of my co-workers who live in the Twin Cities were on that bridge or have family that was involved. A very tragic thing to happen at such a busy time.

[NamecipS]

Horifying.....I have a fear of bridges.

 

Bridge collapsing while I'm on it is in my Top 5 of worst nightmares.

[HHSDad]

They have down graded the death toll to 4, said they would update later.

Correct. 4 confirmed dead, 78 injured, 20 to 30 missing. The rescue operation has been concluded. It is now a recovery operation.

 

One thing that was pointed out, and you can see from the pictures, is the span that collapsed did not have a center like the span next to it does. Both supports were on dry land.

[MsKitty]

http://www.cnn.com/video/#/video/us/2007/08/02/vosli.mn.i35w.bridge.collapse.side.view.cnn

[98NCCalum]

Did anyone else have a nightmare about falling last night? I did.

[MsKitty]

Did anyone else have a nightmare about falling last night? I did.

 

I kept dreaming about planes crashing.....

[HHSDad]

There are cars sitting upright on the collapsed span thats in the water. What a story those people will have to tell.

[Son of Bear]

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.

[STRIKE3]

This looks more like an "overpass", than what we commonly think of as a Bridge. Yes, it spans over a body of water but it doesn't have vertical surface support towers, which help absorb forced vibrations.

 

Forced vibrations and deflection, are a concern of any Bridge, overpass or deck, which will have frequent movement. Expansion occurs in warm weather and I'm only speculating here, but I believe the "pile driving" which was ongoing, helped to add vibration/deflection on the surface and thus, was carried down into the metal structure and resulting in a fatigue failure.

 

You'll see bridges, which have cables leading from Towers but not on this overpass. These cables help maintain lateral stability and help distribute the load capacity of the bridge. I'm not sure I see any damping either, under the structure on this overpass.

 

Tensile strength of concrete and asphalt, can be minimal when being stretched, vibrated and overloaded. Shear could have occurred, under one of the main support structures or as a result of the pile driving.

 

The AASHTO will examine and make a determination.

[Son of Bear]

^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.

[STRIKE3]

^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 their effective cross section area and weakening the connection. The video very well indicates a connection could have failed in a vertical shear plane.

Very true and concrete delamination and spalling, became common in many of the reinforced concrete decks, back in the '60's.

 

Salt, seawater, deicing materials and chlorides were instrumental in corrosive related concrete and structural steel failure. Often times, the steel within the concrete corrodes and begins to fail/weaken but it not viewable. The advent of structural rebar, being coated to minimize corrosion, is fairly commonplace now.

 

Various types of additives, into the concrete mix, can offer additional compressive strength and improve elasticity.

[Son of Bear]

^My guess is Type I cement was used for this bridge, which is basic concrete....I am really not sure about when admixtures started being used in practice. Reinforcing in the concrete was also probably not epoxy coated, like it is today in most bridge work.

 

As I referred to earlier, structural steel used in the 60's would be much more brittle then steel used today. Basically, older steel has a higher carbon content which makes it more like iron, and once fatigued it can break very sudden....almost like breaking a cracker or peanut brittle (hence the name). Modern steel is much more ductile and wouldn't "break", it would yeild and disfigure until it ultimately failed. I highly doubt high strength bolts were used in the connection either.....I think I saw where rivets were used as was common in this era.

 

Something that I have seen mentioned is pattern loading effects on this structure. This bridge was loaded very heavily from probably 7-9am and again from 5-7pm every single day for 40 years. It has been studied that material can lose some of it's strength when creep (basically age) and loading cycles like this take place. The loading and unloading can cause more harm then a full load at all times. Did anyone else hear reports that after a 2005 inspection that plans were in the works to replace the entire bridge? I think it was on CNN?

[STRIKE3]

I saw the footage with the School bus on the span and it catches your attention, how precious time and life, truly are.