OT: I-35W bridge collapse in Minneapolis

[spokyone]

Just doing a quick math based on guesses. It looks like 2000 tons of extra weight due to paving, jersey type center  barriers, and curb height increases. The original design  had enough safety margin without the extra weight. DOT may not have hired an engineering firm to look closely at the design limitations when these modifications were approved. Did someone just assume there was adequate safety margins? I believe NTSB will look at that also.

[Anonymous]

http://www.designnews.com/article/CA6466909.html

Here is an interesting article from last fall that discusses the rust-frozen roller expansion bearings, which impeded expansion / contraction movement of the truss.  It says that prohibition of this movement caused the truss to deflect out of plane, and speculates as to the amount of extra stress this loading caused, and whether it pushed the bridge past its design limit.  At one point, last fall, I heard that this condition of locked expansion bearings had caused the force of expansion / contraction to tip one of the piers out of plumb.  However, that is not confirmed by this article.  The following is from the article:

 

Seventeen years ago, the federal government first said the bridge was structurally deficient, citing significant corrosion in its bearings. Seven years later, significant out-of-plane distortion was noted in the main trusses connected to cross girders due to resistance to motion at the connection cross bearings, according to a report released by the University of Minnesota civil engineering department. The cracks in the girders were drilled to prevent further propagation. Support struts were also added to the cross girder.

Bridge bearings, such as the multiple roller bearings used in the I-35W bridge, are used to transfer loads from the deck to the superstructure as weight burdens vary or weather conditions change. Their primary purpose is to redistribute major longitudinal shifts in load.

The basic design of the bridge, however, may have made it less able to withstand the pressure of the distortions caused by the corroded roller bearing plates.

[squeeze]

The sad part is that there are 547 bridges that are similar to this bridge across America. I'll bet each state is scrambling to find out how there's are constructed. Here in Pa. we have 54 that are of the same build. Kinda makes you wonder.

[Mr_Ash]

The sad part is that there are 547 bridges that are similar to this bridge across America. I'll bet each state is scrambling to find out how there's are constructed. Here in Pa. we have 54 that are of the same build. Kinda makes you wonder.

Wonder things like what? If the bridge your driving across could suddonly fall out from under you leaving you suspended in midair with sign reading "Help!"

[Krazykat112079]

Do bridge designers just get to make mistakes without any consequences?

The answer is no.  There is almost guaranteed a Professional Engineer's seal affixed to the plans for that bridge who is accountable for design errors.  But, there really needs to be a thorough investigation.  It is entirely possible that the wrong pieces were installed in places that no one caught.  People tend to assume that the proper thickness widget or length whatcha-ma-callit was supplied and installed.  It could be a design error, supplier error, contractor error, or even inspector error.

Kind of makes you wonder, though, when engineers (all kinds) make mistakes, lots of people may die, but athletes and other entertainers get paid the big money.

[Anonymous]

Do bridge designers just get to make mistakes without any consequences?

The answer is no.  There is almost guaranteed a Professional Engineer's seal affixed to the plans for that bridge who is accountable for design errors.  But, there really needs to be a thorough investigation.  It is entirely possible that the wrong pieces were installed in places that no one caught.  People tend to assume that the proper thickness widget or length whatcha-ma-callit was supplied and installed.  It could be a design error, supplier error, contractor error, or even inspector error.

Well that would be my take on it too.  But as I listen to all the local news, I detect virtually zero curiosity about the accountability for the design flaw.  The general thrust seems to be that the bridge fell down, and it is nobody's fault because the design occurred so long ago.  It makes you wonder how many other Minnesota bridges have lost all accountability for their designs because their designers went out of business, sold out, retired, or died.

I wonder why the NTSB, after lecturing us all fall about not jumping to conclusions, would come out now, nine months before their final report, and tell us they stongly suspect that a design flaw was at fault, but they are not sure. 

[SactoGuy188]

The sad part is that there are 547 bridges that are similar to this bridge across America. I'll bet each state is scrambling to find out how there's are constructed. Here in Pa. we have 54 that are of the same build. Kinda makes you wonder.

The problem with that I-35W bridge was that if you've seen the pre-collapse pictures, the bridge sported a very spindly structure with lots of exposed structural steel and only four relatively small concrete pillars to hold up the bridge--it was a disaster just waiting to happen. The new bridge now under construction is built from reinforced concrete designed with withstand the considerable temperature changes of weather in Minnesota.

 

[Anonymous]

It is interesting that the NTSB said in effect that they did not intend to point fingers of blame for the collapse, and then proceeded to point the finger of blame at the designers.  But apparently, if the designer no longer exists, there is no finger of blame.

Here is an interesting article that seems to be going against the popular orthodoxy that the question of what caused the collapse is settled.

http://www.twincities.com/ci_7992046?source=rss&nclick_check=1

It mentions that there was considerable corrosion on the very plates that were made too thin by the flawed design.  The corrosion made them even thinner.  Here is a quote from the article:

Jim Schwebel, a lawyer who represents the surviving relatives of three victims, said those issues are worthy of follow-up.

"If investigators have any of (the gusset plates) that were compromised by corrosion, you've got to put that into the computer model," Schwebel said. "Do they have a right to make that assumption (that corrosion wasn't a factor) if they see that the plates are undersized? And if they don't see that, are they blind?"

[Krazykat112079]

It is interesting that the NTSB said in effect that they did not intend to point fingers of blame for the collapse, and then proceeded to point the finger of blame at the designers.  But apparently, if the designer no longer exists, there is no finger of blame.

Here is an interesting article that seems to be going against the popular orthodoxy that the question of what caused the collapse is settled.

http://www.twincities.com/ci_7992046?source=rss&nclick_check=1

It mentions that there was considerable corrosion on the very plates that were made too thin by the flawed design.  The corrosion made them even thinner.  Here is a quote from the article:

Jim Schwebel, a lawyer who represents the surviving relatives of three victims, said those issues are worthy of follow-up.

"If investigators have any of (the gusset plates) that were compromised by corrosion, you've got to put that into the computer model," Schwebel said. "Do they have a right to make that assumption (that corrosion wasn't a factor) if they see that the plates are undersized? And if they don't see that, are they blind?"

Corrosion over the design lifespan of the bridge is usually a factor used in the design of engineering components.  The thicker the piece is, the longer it will last against corrosion.  That is probably the reason they claim it is a design flaw.

I guess I just don't see where the lawyer is going with this one.  Since he is representing victims, I assume that he esentially is looking for ultimate blame to be placed.

The quote I find interesting is this:

"But Rosenker said the NTSB investigation has found no evidence that cracking, corrosion or other wear “played any role in the collapse of the bridge.” "

I just cannot fathom this statement from an engineering standpoint.  Surely the load on this bridge was not high enough to meet the yield stress of the original design, since that is pretty much the only way for the bridge to fail if no wear played a role.