OT: I-35W bridge collapse in Minneapolis

[Anonymous]

erikem,

I think your assessment is fairly accurate.  As you point out, the bridge design was relatively unique, pushing the envelop on the efficient use of materials, but thus lacking prudent reserve or redundancy.  I speculate that, for this design to be viable, it must be perfectly inspected and maintained, which did not happen.  So it appears that the design may have been structurally viable, but impractical, given the high probability of inadequate maintenance.  

I too have wondered about the structural effect of removing the concrete deck, which was underway at the time of the collapse.  I would assume that the structural performance of the bridge would be rated and stipulated completely independently from the deck, but I wonder about that.  If the concrete deck were cast in a way that it became one with the top chord of the truss structure, it would have added compression strength to the top chord, which would stiffen it against downward buckling of the truss.  If this were the case, removing the deck would reduce strength.  It would also remove loading, so it is hard to say how a reduction of strength and a reduction of load would tally up.

I can see this possible scenario:  The deck removal would reduce the load, partially offsetting a weakening effect, but leaving a net weakening of some amount.  That amount would be considered acceptable because it was not necessary to the bridge design capacity and its proper safety factor.  However, the bridge had already been weakened by a lack of maintenance, and had lost its safety factor or worse.  So the amount of weakening caused by removal of the deck made the difference between holding and failing.  In other words the strength added by the deck was not an essential part of the design, but became essential as the design failed over time due to structural decay.   

[erikem]

I too have wondered about the structural effect of removing the concrete deck, which was underway at the time of the collapse.  I would assume that the structural performance of the bridge would be rated and stipulated completely independently from the deck, but I wonder about that.  If the concrete deck were cast in a way that it became one with the top chord of the truss structure, it would have added compression strength to the top chord, which would stiffen it against downward buckling of the truss.  If this were the case, removing the deck would reduce strength.  It would also remove loading, so it is hard to say how a reduction of strength and a reduction of load would tally up.

The U of Minn study on the bridge specifically mentioned the strengthening effect of the concrete deck - probably should try looking it up and posting the URL.  You're right in that the concrete would be under compression and adding to the strength - heaven help us if it was under tension...

I'm looking forward to reading what Henry Petroski has to say about the collapse - though it might be a couple of years before enough data comes out of the failure analysis to paint a coherent picture.

- Erik

 

[Expresslane]

 

          Most trucks do not gross out at 80000. I have not seen any mention of the deicer they use on the roads and bridges. This stuff will rot steel in no time. I'm talking about the liquid deicer not the rock salt. Some bridges in the Twin Cities have this sprayed any time the weather looks bad. Could dip down and rot the bridge.

[alphas]

Don't forget the bridge had been repaved several times and was in the process again at the time of collapse.   Awhile ago I heard the most common cause of parking deck collapses is they get repaved various times and no one stops to calculate the stress caused by the increased weight.

[lattasnip9]

I live in a suburb of Minneapolis and I was watching it on the news about 30 minutes after it happened.  I believe the bridge overpassed a BN line.  My old man and I biked down a month after (to the day) and saw the devastation.  There is still a portopotty standing on the slanted deck leaning on the barrier.  The construction was minor resurfacing and supposedly had no influence on the collapse.  Thank god there was only 4 lanes of traffic instead of the 8 that are usually open due to the construction.  But man the piers on the north side had been partially lifted out of the water and it was a mess.  The one hopper had been crushed like a pop can.  Hope I cleared some of the confusion,

Robbie.

[Anonymous]

The people of Minnesota want a practical new bridge at the lowest cost.  The political class empowers themselves by spending our money, so naturally they want the most expensive bridge.  To make an expensive bridge, you design it with a lot of bells and whistles.  A good way to make bells and whistles expensive is to make them intangible and hard to measure.  And then when the three bids come in, you award the highest bidder because their price includes the best meeting of your intangible bells and whistles. 

But, apparently nobody told two of the lower bidders how this works, and they were actually trying to bid low so they could win.  However, the winning formula was to bid high and then sell your bid for intangible bells and whistles to the state with an intangible sales pitch. 

[eastside]

Replacement bridge to be 10 lanes.  Two lanes to be dedicated for buses or light rail.  Described as a utilitarian design.  Story

[eolafan]

While in St. Paul this week I asked some folks what the status of the bridge replacement was and was told by all that the cleanup was virtually complete (this is supported by blhanel's photos) and that the design of the new bridge had been approved and contractor appointed for construction but that while contractors crews are in place and ready to go they are waiting for MN state and federal approvals before construction finally begins.  I was also told the new bridge will take approximately a year to build and that the contractor was offered a $12 million bonus for on time completion.  No doubt this will be the most inspected and monitored construction project in MN history (if not U.S. history).

[Anonymous]

Quote from the above linked article:

"Executives of the joint venture promised to erect a bridge with structural redundancies that would allow it to last 100 years, 60 more than its predecessor."

That sounds like it is comparing a bridge built to last 100 years to one built to last 40 years.  It is true that the old one lasted 40 years, but how long was it supposed to last according to its designers?  Certainly it was not built as a bridge with an intended lifespan of only 40 years. 

[spokyone]

Quote from the above linked article:

"Executives of the joint venture promised to erect a bridge with structural redundancies that would allow it to last 100 years, 60 more than its predecessor."

That sounds like it is comparing a bridge built to last 100 years to one built to last 40 years.  It is true that the old one lasted 40 years, but how long was it supposed to last according to its designers?  Certainly it was not built as a bridge with an intended lifespan of only 40 years. 

That beautiful cable stay bridge in Alton IL was designed to last 50 years. This according to PBS that filmed the construction for their program. IIRC it is because needs may change over time.

[Anonymous]

It is interesting that the contractors apparently submitted their bids base on their own bridge designs.  How can you compare bids when the item being bid is different with each bidder? 

In addition to the comparison of the actual price of each bid for the new bridge, the state used a technical score based upon criteria other than price.  Here are the criteria used by the state to score the bids:

Evaluation Criteria Utilized (Corresponded to Criteria Contained in the RFP)

• Quality (50 percent)
• Experience and authority of key individuals
• Extent of quality control/quality assurance
• Safety
• Measures to evaluate performance in construction
• Aesthetics/Visual Quality (20 percent)
• Visual enhancements to the structure
• Involvement of the public after letting
• Enhancements (15 percent)
• Geometric and structural enhancements
• Public Outreach/Involvement (15 percent)
• Impacts to the public
• Approach to communications

Note the item called, Public Outreach weighted at 15%.  I can understand how experience, quality control, and safety would be needed to build a bridge, but I do not understand why the state needs to pay the bridge builder for public outreach. 

Or in other words, how much public outreach does it take to screw in a light bulb?  

[Convicted One]

Quote from the above linked article:

"Executives of the joint venture promised to erect a bridge with structural redundancies that would allow it to last 100 years, 60 more than its predecessor."

That sounds like it is comparing a bridge built to last 100 years to one built to last 40 years.  It is true that the old one lasted 40 years, but how long was it supposed to last according to its designers?  Certainly it was not built as a bridge with an intended lifespan of only 40 years. 

 

I don't think that by saying the new bridge will be designed to last 100 years, they are trying to say the original was designed to last only 40. They are just pointing out that it did last ONLY 40.

Surely the catastrophic failure of the original was never planned as transpired

[Convicted One]

It is interesting that the contractors apparently submitted their bids base on their own bridge designs.  How can you compare bids when the item being bid is different with each bidder? 

In addition to the comparison of the actual price of each bid for the new bridge, the state used a technical score based upon criteria other than price.  Here are the criteria used by the state to score the bids:

Evaluation Criteria Utilized (Corresponded to Criteria Contained in the RFP)

• Quality (50 percent)
• Experience and authority of key individuals
• Extent of quality control/quality assurance
• Safety
• Measures to evaluate performance in construction
• Aesthetics/Visual Quality (20 percent)
• Visual enhancements to the structure
• Involvement of the public after letting
• Enhancements (15 percent)
• Geometric and structural enhancements
• Public Outreach/Involvement (15 percent)
• Impacts to the public
• Approach to communications

Note the item called, Public Outreach weighted at 15%.  I can understand how experience, quality control, and safety would be needed to build a bridge, but I do not understand why the state needs to pay the bridge builder for public outreach. 

Or in other words, how much public outreach does it take to screw in a light bulb?  

 

Just my opinion, but the 'outreach' comment looks like a cover for something along the lines of using minority owned contractors/sub-contractors... You know, making sure ALL the community feels involved? etc.

[Anonymous]

I don't think that by saying the new bridge will be designed to last 100 years, they are trying to say the original was designed to last only 40. They are just pointing out that it did last ONLY 40.

Surely the catastrophic failure of the original was never planned as transpired

Convicted One,

I understand and agree that that is what they were saying, but there are implications in the overtone.  The old bridge failed to meet its projected 50-year lifespan, which surely must have been proudly heralded when it was opened in 1967 just as the 100-year life is proclaimed about the new bridge today.  I would hate to think the citizens of Minnesota could be led to believe that the reason the bridge collapsed was that they bought a cheap, low quality bridge the last time. 

You may be right about the public outreach.  Public outreach may be a bridge to the community.  The more I read about this project, the more apparent it becomes that this is about so much more than just building a bridge.  There are statements to be made about the tragedy and loss, and also about the fabric of the riverfront.  But as a taxpayer, it would be nice to know the workings of MNDOT's so-called technical score that justified their awarding the contract to the contractor that bid $57 million higher than the next highest bidder.

 

 

[Convicted One]

.  But as a taxpayer, it would be nice to know the workings of MNDOT's so-called technical score that justified their awarding the contract to the contractor that bid $57 million higher than the next highest bidder.

 

I share COMPLETELY your sentiment there.  So the following should not be taken as an argument against your position, just stating facts.

At one time I was heavily involved in contract work with the General Services Administration. Where EVERYTHING is supposed to be bid competitively. Anyone who is qualified to bid the various projects (supposedly) being welcome to do so. Often the contracting officers know before the original solicitation for offers  is even made public, exactly who it is they intend to have do the work. The bidding process is (often) a mere formality to maintain a charade of protecting the public interest.

In such instances there are unlimited opportunities to make the prefered contractor somehow appear to be 'uniquely qualified' in order to steer the work towrds the intended awardee'.

It feels great when you know you are the contractor that they prefer to use anyway, and EXTREMELY frustrating in the instances where you figure out that they have a prefered source OTHER than you. Making you feel like you are beating your head against a wall even writing the bid.

Sometimes the rationale in determining a 'prefered' awardee is semi legitimate, maybe a contractor has an extensive track record of completeing the type project at hand in an expedient and trouble free (to the contracting officer/government) manner.

And sometimes the motives are LESS than honorable.  Hard telling which is which in the example at hand.

Funny that the government does not feel the need to be more forthright in explaning the conditions you mention. Usually when the government is at a loss for words, that runs up a red flag, fwiw

[Anonymous]

.  But as a taxpayer, it would be nice to know the workings of MNDOT's so-called technical score that justified their awarding the contract to the contractor that bid $57 million higher than the next highest bidder.

 

I share COMPLETELY your sentiment there.  So the following should not be taken as an argument against your position, just stating facts.

At one time I was heavily involved in contract work with the General Services Administration. Where EVERYTHING is supposed to be bid competitively. Anyone who is qualified to bid the various projects (supposedly) being welcome to do so. Often the contracting officers know before the original solicitation for offers  is even made public, exactly who it is they intend to have do the work. The bidding process is (often) a mere formality to maintain a charade of protecting the public interest.

In such instances there are unlimited opportunities to make the prefered contractor somehow appear to be 'uniquely qualified' in order to steer the work towrds the intended awardee'.

It feels great when you know you are the contractor that they prefer to use anyway, and EXTREMELY frustrating in the instances where you figure out that they have a prefered source OTHER than you. Making you feel like you are beating your head against a wall even writing the bid.

Sometimes the rationale in determining a 'prefered' awardee is semi legitimate, maybe a contractor has an extensive track record of completeing the type project at hand in an expedient and trouble free (to the contracting officer/government) manner.

And sometimes the motives are LESS than honorable.  Hard telling which is which in the example at hand.

Funny that the government does not feel the need to be more forthright in explaning the conditions you mention. Usually when the government is at a loss for words, that runs up a red flag, fwiw

That sure sounds like what this I35W bid looks like.  There was a little hubub in the news right as the story revealed that the highest bidder won, but it quickly faded.  I can understand the state not stepping forth to expose the process behind the technical scoring.  It was challenged by the other two bidders, but apparently that has gone nowhere.  They claimed that they did not know that the bridge project included public outreach.  What is most disconcerting is the complete lack of curiousity on the part of the local press and TV news.

[Semper Vaporo]

If you will remember, some comedian said:

"Remember: Interstate bridges are built by the lowest bidder."

This alluding to the fact that they MIGHT not be safe.

Now, one has collapsed, adding credence to the quip.

Could the Authorities have awarded the bid to the lowest bidder this time, without public recrimination for doing so?

Would it be prudent for the news media to begin some journalistic investigation as to why it was not awarded to the lowest bidder?  What kind of headline would this be:

"The old bridge fell down, why aren't we building a cheaper one!"

 

[Anonymous]

If you will remember, some comedian said:

"Remember: Interstate bridges are built by the lowest bidder."

This alluding to the fact that they MIGHT not be safe.

Now, one has collapsed, adding credence to the quip.

Could the Authorities have awarded the bid to the lowest bidder this time, without public recrimination for doing so?

Would it be prudent for the news media to begin some journalistic investigation as to why it was not awarded to the lowest bidder?  What kind of headline would this be:

"The old bridge fell down, why aren't we building a cheaper one!"

 

I know the joke about getting something you want to be safe built by the lowest bidder. However, it seems to me that there should not be any connection between the bid price and the safety or quality of the bridge.  There would definitely be a correlation between the quality, strength, and durability of the bridge as those characteristics are embodied in the design.  If you design an inferior bridge, its performance will reflect that, but the performance should not be affected by the construction.  If the contract is properly enforced, you get the same bridge, no matter whether the bid is high or low.

While the bidders on the I35W bridge apparently were given latitude on the design and style of the structure, it is ultimately up to the state to design a bridge that won't fall down.  Ideally, they would then get multiple bids on that specific bridge, then consider price, delivery, and competence, and then select the best-suited bidder on that basis.   

Selecting a high or low bid would have no effect on the strength, durability, or longevity of the bridge, because those characteristics would be established by the design and specifications.  It would be up to the state to guarantee that the specifications have been met. 

Many politicians blamed the collapse on a lack of funds for proper maintenance, but of course this is nonsense.  A lack of funds cannot possibly be the reason for the collapse.  This is axiomatic because the state has a responsibility to know the bridge's condition, and maintain it in safe condition.  If they cannot perform either one of these tasks for any reason, they must close the bridge.  Letting the bridge collapse for lack of funding would be criminal.  Even MNDOT officials ran for the hills when they heard the reckless politicians blaming the collapse on Minnesotans not paying enough taxes. 

[eastside]

Today the NTSB is scheduled to give a news conference to announce its results concerning the Interstate 35W bridge collapse in Minneapolis.

Story

Early in the investigation of the Minneapolis case, the safety board identified the plate, called a gusset plate, as a possible cause.

[spokyone]

Today the NTSB is scheduled to give a news conference to announce its results concerning the Interstate 35W bridge collapse in Minneapolis.

Story

Early in the investigation of the Minneapolis case, the safety board identified the plate, called a gusset plate, as a possible cause.

Here is article about press conference.
http://www.startribune.com/local/13796646.html

[Anonymous]

It its interesting that the NTSB officials are using the design flaw to completely vindicate the inspection process, which previously had been suspected to have failed, thus causing the collapse.  Yet, at least as of this morning, they were stipulating that although they have confirmed the design flaw, they cannot yet say for sure that the design flaw caused the collapse.  By this evening, however, the thrust of all the news reporting seems to be saying that the bridge collapsed because of the design flaw.  I sure would like to hear what the designer has to say about the matter.  Do bridge designers just get to make mistakes without any consequences?

[solzrules]

It's called CYA, and many people are good at it.  My boss gives me a little tutorial on it every day.  It's kind of funny.

[Modelcar]

....And I believe they are indicating many other bridges scattered over the country have the same type gussets....If they are the culprit, I wonder if it is a straight forward matter of doubling the thickness of them by adding another layer on the original...If so they better get busy.  But how can they be sure that was the only problem.  Has maintenance been eliminated as an issue....?

[Anonymous]

...Has maintenance been eliminated as an issue....?

 

I think that is the official burning desire.

[Mr_Ash]

I think the real blame should fall on Gravity, ofcorse gravity cant be fired or sued so who ever designed the bridge should get introuble because if they didnt build it then it wouldnt have fallen so its gotta be there fault

[Anonymous]

I think the real blame should fall on Gravity, ofcorse gravity cant be fired or sued so who ever designed the bridge should get introuble because if they didnt build it then it wouldnt have fallen so its gotta be there fault

Gravity is probably an explanation that most Minnesotans would accept.

[spokyone]

Here is article about press conference.
http://www.startribune.com/local/13796646.html

On the 2nd page of this link, they talk about center barrier, higher outside walls and 2" thicker paving. Can anyone guess how much extra weight that would be? They also mentiontioned 300 tons of equipment. But that would only be 8 loaded semi trucks.

[Railway Man]

There's some leaping to conclusions and conspiracy theory here.

The NTSB has not determined a cause for the collapse.  All that it said today is that 8 of 112 gusset plates of the main trusses were significantly undersized for the load the design called for them to carry.  The gussets have not been determined to be a cause of the collapse or even a contributor to the collapse; all is known is that one of the gussets (identified as U10) appears to have failed early in the collapse event.

The NTSB released this preliminary finding in order to alert owners of similar bridges to a risk factor.  This is normal and conservative.

Quote: "The Safety Board has issued this recommendation, at this time, to ensure that the original design calculations for other bridges of this type have been made correctly, before any planned modification or operational changes are accomplished affecting such bridges and before any additional stresses are placed on them," said NTSB Chairman Mark V. Rosenker.

The NTSB report is found here:  http://www.ntsb.gov/Pressrel/2008/080115.html as is a link to an FHWA interim report.  If you want to skip the math just read the introduction and look at figures 5-6-7-8, then read the Interpretation of Results.  That's depressing enough, I think.

Weights of materials: 

Plain reinforced concrete, 150 lb./cf

Asphalt paving, 120-140 lb./cf

The deck supported by the bridge main truss was 1,064 feet long x 104' wide curb face to curb face.  I have no idea of the dimensions of the curbs, center divider, etc., which are important.

A question is posed about liability.  That would depend upon applicable law, which might only be the State of Minnesota.  Generally ordinary negligence in professional services is a civil matter and can result in payment of damages, civil fines, and loss of professional license for the Engineer of Record, assuming no intervening Statute of Limitations.  Gross negligence -- a willful and reckless disregard of safety -- can result in criminal penalty or punitive damages, depending upon applicable law.  Given that the bridge was designed more than 40 years ago, the parties to the design are likely deceased.

RWM 

[erikem]

Stepping way out on a limb... (or was that thin ice?) My background is in EE not CE, so take what I write with a large grain of salt.

From what I have read so far, this looks like a case of ordinary negligence for the bridge designers and builders, after all the bridge did stay up for 40 years. What looks to me as the critical design flaw was a lack of redundancy in the structure - that is failure of a single component could lead to failure of the whole structure. This point was driven home by the failure of the Silver Bridge in Ohio/WV in December 1967, also about 40 years after construction. If I recall correctly, design work was already underway on the I-35W bridge before the collapse of the Silver Bridge. Incidentally, FAA regs on large aircraft design  require the airframe to have enough structural redundancy to survive failure of a major member, e.g. a wing spar.

As for the crowd responsible for maintaining the bridge, it remains to be seen if the point of failure was something that could be detected by inspection.

[Anonymous]

Just to clarify, I did hear the NTSB say they had not concluded that the design flaw caused the collapse.  However, most if not all of the news during the last 24 hours unequivocally declared that the design flaw was the cause of the collapse.  Even listening to the NTSB press conference, one would have to work hard parsing words in order to conclude that the NTSB was not announcing that the design flaw was the cause of the collapse. 

And they certainly did rule out the possibility of the collapse being caused by a lack of inspection and/or maintenance.  I don't understand how a potential cause can be ruled out while the actual cause is still unknown.