Bridge Collapse Theories, Anyone?

Question

My thought is that a large portion of road collapsed due to the resurfacing and damaged the bridge's support.

It's obvious the talking heads on TV have nothing of substance to add at this point, so I'm turning to the yahoo for some great expert and thinks-they're-an-expert opinions.

Answer 1

If you sift through the info you hear on TV, sometimes they will stumble on some useful information.

The Bridge Inpsection Report prepared for this structure as part of the FHWA Federal Bridge Re-Inspection Program, which began in 1979, indicated that there were cracks proprogating in some of the main load carrying members.

The DOT was faced with two (2) options: (1) they could arrest the proprogation by installing retrofit plates to provide a redundant load path for the fractured members or (2) inspect the structure on a shorter inspection cycle to see if the cracks were increasing. They chose to do the second until funding would be in-place to replace the structure.

The failure was most likely due to the proprogation of cracks in the main load carrying, fracture critical members. The result of failure of a fracture critical member is catastrophic failure.

The Tacoma Narrows structure failed because welded plate girders were used for a long slender span rather than open deck trusses which allow most of the wind to pass through. Catching the wind, the superstructure oscillated and found the natural harmonic frequency of the span and failed. An open deck truss bridge is not as susceptible to this type of failure. To my knowledge, no open deck truss spans have failed from harmonic oscilation.

While impact does affect the load carry ablility of a structure. The impact produced by traffic is substantially more than that generated by jack hammers.

Answer 2

No, it's unlikely that global warming had anything to do with it. Structural Deficiency is actually a pretty good statement of the cause, as the NTSB sees it in their preliminary examination. Structural deficiency is a scale (0-100) used to rate bridges and other structures. It combines two factors: issues or flaws in the original design, and damage, wear or weathering of the materials. This bridge had a structural deficiency rating of 50. It was in the 47th year of a 50 year design lifetime. In the USA, it was an average bridge. They will be examining the original design for "we wouldn't do it that way today" type issues, but most of that should already be known and included in it's rating. They'll also look at the past inspections, as a record of how it was aging and being maintained. Some cracks for example, would be expected over time. If and how they were dealt with alters the rating. They are also looking at the weather on a day by day basis since the last inspection. Weather has known effects in terms of expansion and contraction, etc. All this data together should give them a pretty good idea about whether the structural deficiency rating had changed since the last inspection. Because they have video tape of the collapse as it occurred, they already know where the first failure occurred, and how it progressed from there. I'm sure they'll be focusing on the key points. I heard a report that it may have had more weight than normal on it at the time, due to the construction equipment doing some repaving. They'll have a pretty clear picture once all that data is put together. I know the usual suspects on here were hee-hawing away when it happened, suggesting people would attribute it to global warming. My heart goes out to anyone in that community reading those posts. It shows their social skills are on par with their science. I haven't heard anyone other than those folks try to bring Global Warming into that tragedy. If some turn up, I'm sure it will be sources on the same level as our pet contrarians.

Answer 3

Does anyone know when the river dam to the west of the bridge was built or installed or modified? I know the bridge was built in the 60's but am wondering if the river control mechanism was installed after the bridge was built. This leads to questions about river flow at the bridge site and what design requirements were used to in assessing the needs for the bridge foundation.

Also, as stated by a few others here, the consequences of vibration, or more importantly resonance points, can not be understated...hit the exact resonance point and effects can increase quite dramatically. However, I did not think that the train was actually moving under the bridge at the time...no derailments.

Answer 4

Questions, questions but few answers. Engineering failures are very complex in nature and a select few can understand and fewer still can effectively explain the failure mechanics. The type of structure is extremely vulnerable to fatigue and the design methodology used at the time did not accommodate load redistribution. In a truss structure the members are designed to resist tension forces or compression forces. The truss apparently was locked due to bearing freeze. Meaning corrosion and debris rendered the rocker bearings inoperable. This put a great deal of unanticipated force in member not designed to resist such forces. The bridge adjusted to distributing the high stresses. When the contractor released the bridge expansion joints and possibly the rocker bearing, the structure had to redistribute the forces. This action most likely caused the bridge to shift and caused a non redundant member to buckle near the expansion device. After the first member fails in such a structure the structure experiences a progressive collapse due to insufficient redundancy built into design. This seems highly likely given the failure of the entire main span almost simultaneously.

Answer 5

Bridges or any other man made structure undergoes degradation. Oxidation is the natures life saving and life ending mechanism.

Concrete is nothing but a mixture of stone sand and cement. This mixture always traps air bubbles which allow air to pass by. That pores are enough for water, moisture, chemicals to penetrate the cement layer during every season change. Nothing happens to the concrete except losing little bit of bonding strenght here and there. Inside resides the steel that is the real sufferer.

Steel corrodes exactly where the concrete is weak. Corrosion leaves empty space between steel and concrete. The structure starts moving, that movement is propagated to adjacent rigid areas also to make them flexible. The process is further catalysed by the minor or major jerks and the inaudible frquencies from hundreds of our modern drivers during breaking and speeding.

The strength of the whole bridge is challenged by some weak areas. That shows the result one fine morning.

Remeber, even during military parade soldiers are supposed to break the march while crossing the bridge.

It is a simple precaution taken hundreds of years to avoid the least possible damage to the structure.

Answer 6

My guess is rust.

There was an overpass collapse in CT a number of years ago. I think some were killed. Anyway the TV stations were playing it out for all it was worth and one of them played the angle, "Are other bridges safe?" So they went around with cameras and inspectors and showed some scary results.

They found some steel which had lost half its dimension due to rust. Naturally this could lead to failure.

On newer bridges I would suspect they could be built with modern anti-rust methods. I don't know this for sure. But on older bridges rust was dealt with by scraping, wire brushing, prime and paint. I think this is a continous process on large bridges like the Golden Gate Bridge. But on smaller bridges, I suspect this may be neglected. At least some of the pictures they showed proved neglect on some bridges.

As I said this is just some guess work on my part based on some investigative reporting I saw on TV once.

Answer 7

A physics professor has sent this idea out:

To whom it may concern:

I will get right to the point. I think I know what caused the Interstate 35W bridge to collapse...

Resonance.

This is not unprecedented. In 1940 the Tacoma Narrows Bridge collapsed for a physically similar reason though it was a different type. Please see the movie at this link.

http://youtube.com/watch?v=P0Fi1VcbpAI

The fault lies with the design in which regions of support or flexing joints are evenly spaced instead of staggered.

This is consistent with the simultaneous breaking into equal sections, and with eyewitness reports of vibrations, and up and down movements. All of these relate to oscillations of standing wave patterns. One witness identified the vibration as driven by a jackhammer. Resonance can amplify modest vibrations.

Sincerely,

Scott Boman
Physics Professor
MCC and WCCCD

Answer 8

First suspect is the riverbed itself. There is too much undisclosed about the entire event to form a reasonable hypothesis. Seismic forces cannot be ruled out.
Of course, there is the possibility of meddling by mischievous supernatural beings, and even saboteurs from the future. Whatever the truth is may be kept sealed by the government for a long time, then de-classified and released as nothing at all new, like the JFK thing.

Maybe all the people who know anything about it will die mysteriously or otherwise disappear off the face of the earth.

Answer 9

If I had to guess, the simple answer is that the heat caused it. With cool temperatures at night (helped by cooling effect of river) and blistering temperatures at mid day, I'm sure that there was significant thermal expansion and contraction happening here. The construction work may have contributed to the collapse by debris, of the road surface, being trapped in the expansion joints. If the debris was pushed in at the peak of the contraction (cooling cycle) and blocked the movement of the expansion process, the structure could heave upward enough to separate the bridge from the foundation, and then collapse. Only one section of the bridge needed to have this failure to cause the progressive collapse that the rest of the bridge experienced.

Answer 10

Well there was a jack hammer going a train going thru cars on it and all that vibration at once prolly hit at the right time and collapsed the bridge yes i live in MN and was listining what happend they didnt say anything but a few things