Engineers puzzle over bridge collapse

http://news.bbc.co.uk/2/hi/americas/6927526.stm ^

[traumer]

Engineers are trying to understand what caused the catastrophic collapse of the bridge over the Mississippi river in Minnesota.

Resurfacing work was taking place, but the bridge was last inspected in 2006 and no significant structural problems were found.

Such complete bridge collapses are a very rare occurrence.

If they happen, it is either because the load is too heavy, or the connections between the bridge's structural elements are too weak, Keith Eaton, chief executive of the UK's Institution of Structural Engineers, told the BBC.

"The engineers will have to see where the collapse started. Clearly a failure occurred somewhere which imbalanced the whole thing," he said.

Speculation that hot weather contributed to the accident by weakening the concrete or expanding the steel framework was not a likely explanation, he added, as modern bridges are built to cope with extremes.

A crack in the steel making up the bridge's structure was the most likely explanation for the disaster, he said.

Corrosion

The I-35W highway bridge (Bridge 9340) was built using a framework of rafters, posts and struts - a structure known as a truss bridge.

In 2005, it was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.

It rated 50 on a scale of 100 for structural stability in that study, White House press secretary Tony Snow said.

About 140,000 cars are thought to have used the bridge every day, but a 2001 report by University of Minnesota's civil engineering department found traffic levels were below those the bridge was designed for.

See graphic of the bridge collapse

The report went on to express concerns that a single crack in the main truss could "theoretically" lead to the entire bridge's collapse.

However, it also said that even if there was a crack, the load could "theoretically" be redistributed along the steel trusses or the concrete deck of the bridge, keeping the bridge aloft.

It added that no fatigue cracking had occurred, and that the bridge "should not have any problems with fatigue cracking in the foreseeable future".

File photograph of the Minnesota bridge The bridge crossed the Mississippi River near downtown Minneapolis

The state need not "prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project".

The truss bridge was built in 1967, with eight lanes over a span of 581 meters (1,900ft). It had no piers in the water, allowing easy passage for river traffic.

While no longer the cutting edge of bridge design, truss bridges are relatively cheap to build, and were a very popular structural choice in the US in the 1960s and 1970s, Mr Eaton said.

They have a downside, however.

"They are made of lots of complex pieces of metal, interconnected bolts or rivets," Mr Eaton told the BBC.

"They have little corners between two pieces of steel where water can collect and cause corrosion."

Nesting pigeons could also be an issue.

"Their droppings are very corrosive, which can be a problem," he said.

[Lonesome in Massachussets]

I worked on VTC (Vessel Traffic Control) proposals and the Coast Guard RFPs made a fussy distinction between allision, the act of a ship hitting a stationary object and collision two moving ships meeting.

Prior to the Exxon Valdez disaster (more economic and political than environmental) Prince William sound was monitored but not controlled. Afterwards, there was an explosion in VTC at various U.S. ports.

[Eye of Unk]

I just heard on HN that there is a focus now upon the construction equipment and why the span shifted 80 feet.

[Bean Counter]

I was down to an event next to the Columbia River Crossing this morning, and had my digital camera handy, and snapped a good picture to share with you folks about why the Interstate Bridges on I-5 are so decrepit...



It was kind of misty this morning, so excuse the quality.

The problem with these bridges it twofold, the piers as I showed you with the bathymetric image of the scour on the riverbed, and the second is those towers.

Those are two of the four counterweight towers that guide enormous concrete and steel counterweights up and down when the drawspan is opened and closed. (That's right, drawspan....the last of it's kind in the nation on an Interstate Highway, and you can imagine what a bridge lift does to rush hour...) As you can see they are a steel truss design that are riveted and bolted together.

Those weights are stowed in the top of those towers, and you can see that it would not take much of an earthquake to set up a sway in those weights that could easily topple the towers, and drop those tons of concrete like guillotines through the bridge decks.

This region is one of the most seismically active areas in the United States, and there is an active volcano that has been continuously erupting for years, only 50 mile North of this spot.

Again, the Northbound span was built in 1917, and is rated 19.3 on a scale of 0-100. The Southbound span was built in 1965, and is rated 48.6 on the same scale. Both sit on 50 foot wooden telephone pole pilings.

Washington and Oregon are into the second year of a $65 Million Dollar study, about how best to attach a $4 Billion Dollar plus Loot Rail system onto the replacement bridge project. They estimate entering the Federal Funding process sometime after 2010 at the earliest, but Senator patty Murray has already told the Project to expect "Millions, not Billions" in federal funding.

[JGE]

Engineers are like everyone else, still learning.

This bridge has been under a microscope as far as the fatigue cracking goes. I will be very surprised if the forensic analysis shows that this was a fatigue induced failure.

On the other hand, I wonder about the corroded bearings that were noted as “frozen”. If one suddenly popped loose the result would be a very significant impact loading as the restraint conditions suddenly changed from a fixed to a pinned condition. The high strain rate would be similar to some of the seismic induced loads that showed that heavy welded connections can have sudden failures without prior damage.

While this is a tragedy for those killed or injured I am certain that we structural engineers will learn another brutal lesson about the true limitations of what we call the “state of the art”.

[Ronzo]

Some interesting pic's of the bridge (before the collapse) that I found on Flickr...(WARNING...BIG PHOTOS!)

One of the main pins that connects the bridge to the concrete piers.

This is a photo of the side of the bridge that fell first into the river...the bridge twisted itself off the piers on the one bank by the lock before falling. Notice the rusty drain pipe! This particular part of the bridge seems to be that which failed, if the post-collapse pic's are any indicator. If you look closely, you can see all sort of rust discoloration where the drain pipe and the pin are...lots of corrosion in that particular area, in a very, very critical part of the structure.

[Ronzo]

Some more interesting pic's of the bridge after the collapse that I found on Flickr...(WARNING...BIG PHOTOS!)

It seems that the bridge collapse began near where the nearest concrete pier is to the camera. Notice that the rocker plate does NOT seem to be on top of the pier any longer...but it's hard to tell. The bridge twisted while collapsing, twisting towards the camera.

Another photo of the two piers where the collapse seemed to originate. It is very, very difficult to find ANY good pictures of these two piers on the 'net.

These are the peirs on the other side, the ones that are visible in the collapse video. Notice how cleanly the bridge has broken away from the piers, and how the rocker plates are intact on top of the piers.

[Ronzo]

More interesting pic's...from Flickr and other sources...

Very intersting overhead view clearly showing how the bridge twisted while collapsing. It is also obvious from this photo that the point of collapse must have been very near the concrete pier near the bottom of the photo...the "intact" truss structure visible was resting on top of the pier near the top of the photo.

This picture show the little dump trucks that must have been used in the "resurfacing" project...however, those type of vehicles are normally used when you are hawling away lots and lots of broken concrete...they must have been making some very BIG holes in the deck of that bridge.

[Bean Counter]

Excellent analysis! Please keep it up, because you are doing quite well.

And I believe those little dump trucks are called “Georgia Buggies”...I’m not sure of the origination, but they are used to move concrete, debris, or just about anything else on a construction site. Just a small gas engine and a hydraulic dumper.

[Ronzo]

...I can't believe I found these on Flickr...

ACTUAL PHOTOS of the I-35 bridge resurfacing crews! Taken by a guy who crossed the bridge everyday. Most of these were taken only days before the collapse.

[Ronzo]

More pictures of the bridge resurfacing crews...(WARNING: LARGE PICTURES!)

This is a very interesting picture...a trench dug across the entire width of two lanes...seems to be a bit much for just resurfacting...but perhaps someone more knowledgable than me can explain what they are doing.

[SeaHawkFan]

Minneapolis PD needs a physical fitness program.

[Ronzo]

A "Georgia Buggie" in action on the I-35 Bridge -- seems to be dumping concrete:

[dc27]

I am anxious for “seconds to disaster” on this one....

A very good show

[Ronzo]

The guy who took all these constsruction worker photos is Thomas Boblett. You can look him up on Flickr. He's quite the photographer.

[cornfedcowboy]

The pillars were not in the water. Therefore, no scouring.

[cornfedcowboy]

Good point (sarc)
Please...... don’t tell me the govt. doesn’t have enough of my money.

[investigateworld]

RE: post 105

The top photo.

On the vertical member, above the gusset, there seems to be some kind of gauge?

[End_Clintonism_Now]

“Fingers” of missing rocker plate assembly visible near center right bottom of photo #2. Appears to have slid right off of concrete column. Excellent photo.

[evad]

..seems to be a bit much for just resurfacing.

Looks like some sort of gigantic expansion joint, but that's just a WAG.

[Bean Counter]

It appears to me that from the milling marks visible on the concret, the asphalt driving layer was mechanically removed, down to the actual bridge deck. In Post 110, that sure looks like bridge deck structural steel underneath that nest of rebar.

It also looks like the layer of conrete that’s left is qwfully thin for a roadbed. the slab I poured to store my firewood on is thicker than that!

Has anyone seen the second video of the collapse that the NTSB spoke about a couple of days ago? Supposed to be 3-4 seconds from a different perspective...