Elaine Meinel Supkis
I have designed and built smaller structures but the only bridges I have built have been over small streams. But the elemental fundamentals remain the same. Namely, you have to consider the flow of water, freeze/thaw cycles and overhead weight. The longer the span, the more one has to consider weight. In this bridge collapse there is another story: tax cuts and putting off repairs. Our entire national infrastructure has been placed in jeopardy on the altar of tax cuts. The tax on gas hasn't been raised in many years even as the cost of construction and rehabilitation rises. Congress recently passed a big, fat spending bill for rebuilding our roads and bridges and all of this will be paid for by the government of China rather than a small hike in the gas tax.
In an editorial during last spring’s legislative session, the Mpls. Star-Tribune said Minnesota’s bridges were unsafe due to chronic underfunding of transportation during the last decade. The legislature passed a nickel a gallon gas tax increase to increase funding for transportation, including infrastructure. Gov. Pawlenty, who ran on the slogan “no new taxes”, vetoed the bill.
— Posted by T. Walters
As the US has misspent its resources building exurban or vacation houses instead of renewable energy systems and modern mass transit, we also stiffed paying for repairs, updating or rebuilding our extensive highway system of roads and bridges. The collapse of a major bridge during rush hour in Minnesota is a bellweather event. It presages what is our fates in the years to come as the tax cut mania coupled with the funny money mania leads to the collapse of our extended auto-based transportation systems.
I read today that Amtrak will offer Scotch and fine wine on their overnight trains. Whoopee. I suppose getting drunk while idling at various side rails next to stinking cattle yards will lure customers to our benighted train service. Just as the entire world has surpassed us with much more superior train service, the giant autobahn system started by Eisenhower is now going to go into collapse and we will have nothing at all except Boeing jets when it comes to cutting edge transportation technology.
I have this mania about engineering. I grew up on mountaintop observatories that were built to sustain high winds and other stresses and I got to see the blueprints being developed since my dad let us live in the AURA offices. And it fascinates me how they solved some awesomely difficult engineering problems for not just the massive mirror devices but the road building, etc. I saw Caterpillars crushed by landslides or falling boulders, I saw delivery trucks swept from the road by high winds. It all seems so tame today but back when I was a child, it was dangerous. In my grandaddy's day, building Mt. Palomar and Mount Wilson as well as the Lowell observatory in Arizona, was done with mules and Chinese laborers.
If you go to the New York Times web page, you can see the bridge collapse on film. I just got the video off of this web site, LiveLeak:
It obviously started at the near end of the bridge. The section of steel connected to the near shore piers suddenly collapsed straight downwards and since one end of the span was no longer supported by anything, the whole thing fell, all within less than 3 seconds. There are many people who believe the World Trade Center was deliberately pulled down and they can't understand the sheer weight of the cement pads of each floor could make the whole thing pancake at the speed of sound but here is yet another example of this: the weight of the concrete pads were similar to the WTC pads and like them, caused everything to fall very fast when only one side was compromised.
Cement is very heavy. This basic principle can't be emphisized enough. Anything with huge cement pads involved are prone to sudden collapses like the McAurthur Freeway in Oakland during a mid-range earthquake. I drove under that highway exactly once in 1969. I said, 'Never will I do that again!' as I got a good look at the supports holding up all that cement over my head. When that span collapsed, I thought we would re-examine all such systems. No chance of that! People need to be fooled into thinking all is well when all is rotted. Look at our economic system!
When I first drove my 55 Chevvy truck on the West Side Highway in Manhattan back in 1969, I yelled to my finance, 'This is insane! This is so rotted, it will collapse!' He thought I was exaggerating but it was lunacy, driving a truck on that roller-coaster road with steel plates as Mr. Fixits. I looked at the understructure later and was doubly upset. 'See all the rust? The rot? See how the steel is expanding due to flaking?' I said. I knew enough of welding to know, when steel looks thicker, this means it is weaker, not stronger. Parts of the West Side highway such as the huge steel medallions, would suddenly fall and I knew scavangers and artists who would collect these highway parts and resell them. Then, one day, the road bed itsel collapsed and people were killed.
It didn't surprise me even slightly. Everyone had to pretend no one could have foreseen this event! Ditto the McAurthur collapse. I foresaw them easily and it didn't take the powers of a psychic. Just taking a hammer and screw driver, I could prove these structures, all built during the 1960s-1970s were unstable. This was during the time the US first started pretending we were still an empire but the dollar was losing value so building things meant taking short cuts. Suddenly, it wasn't so important to double-build. Namely, to make something twice as strong as specifications. And designers thought, if they built things that had many intersecting parts, it didn't need hyper-support systems. But the downside of all this is, if just one bolt breaks, the whole thing collapses. We have seen such collapses over the years over and over again and this bridge collapse is a classic example.
I am grateful for the film of this latest disaster. It clearly shows the bridge suddenly shearing off at one shore and then the unsupported sections then fell instantly after. Looking at before and after pictures, it is obvious where the collapse began and why it happened: rust at the point of contact where the flying sectors were bolted to the piers on the side of the river.
From the Times:
His point: the concrete arches stand independently, but in a truss design, if one steel truss falls, they all do. “This poor design based primarily on cost considerations has been required all over this country in countless projects for the past 50 years,” he continues.
This is the standard bridge design and it was chosen because it was cheap. We want to drive our cars no matter what and if we can't afford proper bridges, we get cheap ones. And spans have life spans. They don't last forever. And cheap ones have shorter spans than well-built ones. The concept of 'if one truss fails, they all fail' should have forced engineers to reconsider the utility of these spans. If no earthquake or hurricane still causes an entire span to suddenly pancake, this is a warning sign that all similar structures can just as easily collapse and there are lots of similar structures equally old, all over our highway systems.
From the NYT blog:
Scores of ‘Deficient’ Bridges | 9:44 AM The Department of Transportation’s 2005 judgment that the bridge was “structurally deficient” has emerged as one of the most prominent signs of a missed signal of an impending disaster. But there are many, many more bridges with that rating, according to a 2006 count by The Federal Highway Administration. Minnesota alone has 1,135 bridges on the list of “Deficient Bridges,” and other states have thousands more.
Here in NY, they are rebuilding all the Depression Era bridges on small, back country roads in our county. The one connecting us to our relatives are being rebuilt totally this month so we have to make detours. New York has high gas taxes but then, we have road and bridge work going on! The example of NYC was from when the City was going bankrupt. When it went nearly bankrupt during President Ford's rule, all the systems collapsed, literally. Subways became very erratic. Huge holes opened up in road beds. We even had a contest for 'biggest hole in the road' which was won by Fort Greene residents who submitted this 10' deep hole that was a 6'x12' section of roadbed that collapsed and had a big Caddie with tailfins stuck in it for a YEAR! Then a dump truck fell into a hole that completely swallowed it up on Mermaid Ave in Coney Island. Such competition!
Well-designed, well-built bridges can last for many centuries. Here is the very first iron bridge, built in England.
Location: Shropshire, England
Completion Date: 1779
Length: 100 feet
Purpose: Roadway (original), Pedestrian (Today)
Materials: Cast iron
Longest Single Span: 100 feet
Engineer(s): Abraham Darby III, Thomas Farnolls Pritchard
Darby severely underestimated the cost to build the Iron Bridge. He remained in debt for the rest of his life.
In 1934, after years of repairs, the Iron Bridge was closed to vehicles and listed as an "Ancient Monument."
So many people gathered on the Iron Bridge in 1979 to celebrate its 200th birthday, pieces of the bridge actually broke off and dropped into the river. Today, no more than 200 people are allowed on the bridge at any time.
The Minnesota bridge didn't last 40 years. This one lasted nearly 200 years and still holds up even though it is pretty weak. I used to live next to the Brooklyn Bridge. It is over 100 years old and takes massive bumper to bumber traffic every day for most of its life!
On June 12, 1806, John A. Roebling, civil engineer and designer of the Brooklyn Bridge, was born in Muehlhausen, Prussia. The Brooklyn Bridge, Roebling's greatest achievement, spans the East River to connect Manhattan with Brooklyn. For nearly a decade after its completion, the bridge, with a main span of 1595 feet, was the longest suspension bridge in the world. Steel wire cable, invented and manufactured by Roebling, made the structure possible.
It isn't just massively utilitarian, it is one of the most beautiful bridges on earth. Generations of artists and photographers have adored this most beautiful of spans. Never have I feared, crossing it! I have biked over it, jogged over it, marched in parades over it, watched fireworks from it, and in general, loved it with all my heart. It is the Queen of all Bridges in America.
In 1869, Roebling died from tetanus he had contracted in an accident on the bridge pilings. From 1870 on, Roebling's son and partner, Washington A. Roebling, supervised construction. A series of mishaps, including an explosion, fire, and contractor fraud hampered completion of the project.
Pneumatic caissons sunk to a depth of forty-four feet on the Brooklyn side, and seventy-eight feet on the Manhattan side provided dry space for workers to dig footings for the bridge's foundation. Alas, working in the caisson often brought on "the bends" -- a serious medical condition caused by moving too quickly from a high-pressure atmosphere to a low-pressure atmosphere. Washington Roebling was among many workers permanently impaired by the then little understood "caisson disease."
And it was very difficult to build. Many new technologies were created in order to build this bridge. New manufacturing techniques, new equipment, new challenges. Both the father and the son sacrificed themselves building this bridge. And both are held in honor and awe ever since.
Here is a story about Minnesota winning an award for highway design just a few years ago:
Category 2—The Rural Highway
I-35 Interchange at Medford, Minnesota, Minnesota Department of Transportation (Mn/DOT)
Faced with an inadequate 1958 diamond interchange and two-lane crossover bridge on Interstate 35 at Medford, Mn/DOT chose the innovative solution of installing two roundabouts on either end of the bridge to better handle the traffic and improve safety. Partnering with the community resulted in a unique gateway into Medford, with such architectural details as a red brick-patterned concrete surfacing treatment to enhance the visual appeal of the roundabouts.
Contact: Scott Robinson, Mn/DOT, 651-284-3783.
There are plenty of intelligent people working for highway systems and designers are not always wrong. But the problem is, we can't rest on the labors of the past. The period of poor highway design haunts us today and we can't ignore this nor can we paint over this!
"The bridge's deck truss system has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and the floor truss system," said a report conducted for the Minnesota Department of Transportation in 2001.
Note the obvious rust where the girders tie into the piers that are in the river. Here is yet another picture that shows clearly the piers were not undermined nor did they fail. The steel structure tying the main fram to the flying parts collapsed very suddenly right where the steel struts tie into the piers.
In other words: this collapse was totally AVOIDABLE. If the lower sections of the steel struts were re-installed or strengthened, no one would have died.
Here is the west bank of the Mississippi showing how the girders didn't separate but fell as a unit. Note the piers at this end were intact, too. This is a pure steel failure event. And steel rusts and the rust can't just be painted over, the weakened steel needs lots of jointure work and welding.
Jeanne Aamodt, a spokeswoman for the Minnesota Department of Transportation, told the Star Tribune, a Minneapolis newspaper, that the department was aware of the 2005 assessment, which gave the bridge a score of four on a scale of zero to nine.
A bridge receives a four when there is "advanced section loss, deterioration, spalling or scour." Spalling is a term used to describe cracking, chipping, crumbling or fraying, while scour is a term used for erosion caused by flowing water.
I think some people need to be arrested. We give autorities autority so they can protect us. And if they are brain dead like Greenspan and Bernanke, then they should resign. And people who are supposed to protect us and don't like Bush and Cheney on 9/11 as well as the Pentagon, should be arrested. They ignore obvious dangers because they get richer or more powerful if they ignore things! The Republican running Minnesota wanted the popularity of cheap gas so he gambled with people's lives. Arrest him!
"I am totally puzzled as to why both ends of the bridge would come down all at once. When my colleague tested it, it was very low stress," said Ted Galambos, a University of Minnesota engineering professor. "I don't think it was overload, so it could have been either some fatigue, failure or some sudden buckling that would cause the failure."
What? Didn't this guy ever play with bridge building as a child? I did! It is simple! How can one end of a bridge with only TWO PAIRS OF PIERS hold up if one side collapses! Of course, the other side would go! And if one side is weak from rust, you can bet your booties, the other side will be equally weak! DUH. There is no middle span so of course, the bridge will fall, simultaneously. Unlike an arch bridge which is quite intact and much older right next door! And the Romans who 'discovered' the arch should be thanked for this great mental revolution! Arches are good!
Bridge deficiencies are summarized as "structurally deficient" and "functionally obsolete," according to the Federal Highway Administration.
A bridge is tagged structurally deficient when significant bridge elements have deteriorated and the bridge's load-carrying capacity is reduced, according to the highway administration.
A bridge is dubbed functionally obsolete when the bridge does not meet current design standards.
Neither label indicates a bridge is unsafe for travel, the highway administration said.
As of 2003, there were about 160,570 bridges deemed structurally deficient or functionally obsolete, according to the American Society of Civil Engineers. The number represented 27.1 percent of the nation's bridges.
So, if a bridge is bad, this is OK with the highway administrators? GADS. Talk about insane. Obsolete bridges are dangerous. We have very old bridges up my way and they have warning signs as to weight and mass! Indeed, all the bridges here have weight warnings on them.
The idea then took hold of a bridge in which all the forces could be determined by the principles of statics, so they would not be altered by small inaccuracies of construction, or by changes in temperature or settlement of abutments. In a truss bridge, this meant a span supported at the ends, with members pinned together so they could rotate at least a little at the joints. The number of members meeting at a joint had to be small enough that the forces in each could be uniquely determined. There is a relatively small number of truss designs that satisfy this requirement.
The most popular design was the Pratt truss, which could be used in spans up to several hundred feet. As shown in the Figure, it consists of an upper chord, in compression, and a lower chord, in tension, connected by vertical and diagonal members. The loads w are applied to the truss at the panel joints, and the reactions R are applied at the ends. The principal job of the vertical posts is to keep the chords apart and brace them. The end posts carry only tension, but the others are designed as compression members. The diagonal members resist the shearing forces between the chords that arise when the loads tend to cause the centre of the span to sink. In the centre panel, there are diagonals in each direction, although only one direction is in tension at any one time, the other being slack. The reason is that a moving load is not applied evenly across the bridge, and as it moves one set or the other of the diagonals will find itself in tension. These counters are generally used in one or more of the central panels.
These trusses are just fine...so long as the steel is good. And this is where inspectors matter. Obviously, when they saw the metal was rusted and in poor condition, they should have had the authority to close the road. But they couldn't. And so people died. Everyone passes the buck but the matter stands: that bridge was judged as in poor condition before the collapse but this judgement was ignored by authorities. Finger pointing here is necessary. Just like on 9/11. They kept telling us to not point fingers. Well, point, point, point!
The Pratt truss proved thoroughly reliable, never providing any surprises and capable of confident design. It is, however, not the most economical solution. Most of its dead load is in the middle of the span, and as the span increases it becomes increasingly more expensive to support. The depth of the truss increases with the span, which makes the members longer and more subject to buckling. There are modifications of the Pratt truss for longer spans that involve more bracing and other measures. It was usually more economical to break the bridge up into multiple spans supported on piers. The bridge is observed to be 'thinnest' at the piers, and 'thickest' between them. A more economical truss is designed like a continuous beam, which removes the joint at the pier, and allows the truss to bend over the pier. Now the bridge is thickest over the pier, with less material in mid-span. The ultimate is something like the Forth Bridge, with giant cantilevers over the piers, connected by light spans between the ends of the cantilevers. A continuous beam is not statically determinate, and the stresses depend on how much the members stretch. Nevertheless, the longest bridges are all of this type, since it is very advantageous.
And weak if one ignores the condition of the constituent parts. And this is the weakness of truss design: one part becomes weak, the whole edifice can collapse.