: Design flaw took down I-35 bridge, not maintenance/age.

12-19-08, 07:02 AM
http://www.startribune.com/politics/state/34404129.html?elr=KArksi8cyaiUo8cyaiUiD3aPc:_Yyc:a UU

Its official now from the NTSB.

12-19-08, 08:21 AM
Nothing like parking the equivalent of a 747 on a bridge that was designed for a cessna.
Thanks for the link, my wife will be interested in this, she is a Civil (and Environmental) Engineer.

12-19-08, 11:26 AM
from an engineering standpoint, this is very unfortunate. I cant imagine forseeing that kind of tonnage focused on such a small point.


12-19-08, 12:05 PM
from an engineering standpoint, this is very unfortunate. I cant imagine forseeing that kind of tonnage focused on such a small point.


Exactly. They keep calling it underdesigned. I wouldn't call it underdesigned, but overloaded. Looks like shifting the blame from whoever allowed that much load on that bridge to the designers from 40 years ago.

12-19-08, 01:32 PM
Hmmmm... Interesting.

12-19-08, 07:16 PM
It was the pigeon poop, I tell ya!

Night Wolf
12-19-08, 07:42 PM
Several parts of that artical stick out to me:

287 tons of construction materials concentrated on the center span.

despite the unusual load, the underlying cause of the collapse was the gusset plates that were supposed to hold together the bridge's steel beams.

Some NTSB members expressed surprise that contractors and state officials had not considered whether the bridge could sustain the added weight of the construction equipment that was being stockpiled for a repaving project.

The use of quick-drying concrete required it to be mixed on site, which led workers to stage their equipment on the bridge, the bulk of it directly over the "U-10" gusset plate nodes that first failed.

"The collapse was the result of a serious design error," said Mark Bagnard, the NTSB's lead investigator on the 35W bridge collapse.

But even as the NTSB appears to be laying the blame on under-size gusset plates, some corrosion and pre-existing cracks were found in other parts of the structure.

Although the corrosion did not contribute to the collapse, "it is a source of concern," said Jim Wildey, an NTSB engineer who presented a structural analysis of the bridge's last moments.

Wildey said that the critical U-10 gusset plate nodes on the south span of the bridge that failed first showed no signs of corrosion. Rather, they showed signs of sudden fracture associated with excessive loads.

"Clearly all the metal was intact, and it was subjected to a very high load," Wildey said.

The heavy loads, Wildey and other NTSB investigators found, resulted from two previous bridge modifications in the 1970s and 1980s, as well as construction materials.

Sounds to me like the "serious error" is people not paying attention to the capacity of the bridge. The bridge was obviously way overloaded, especially with storing all the construction equipment there... it's hard to say that it was a flawed design 40yrs ago if it was not designed to hold that much weight.

12-21-08, 01:51 AM
If you look at the graphic part of that article, it shows that some of the gusset plates were 1/2" thick instead of 1" thick. I blame the design.

12-21-08, 08:06 AM
There is only one way that it would be considered a "design flaw".
That is if the bridge was designed to handle rolling 747 traffic.
If the bridge is that old and the engineers designed it for a certain reasonable load tolerance based on whatever was going on at the time it seems it was designed properly.

If the engineer had said it should handle that much weight so they figured it was safe to put 27 tons of construction equipment on it then it was indeed a design flaw.

It is all based on what the design intent of the bridge originally was.
Someone decided to put all that equipment on the bridge and they share a great deal of the responsibility.
I also saw photos of those plates buckling, that should have been a clue to someone.
I would assume the photos were taken before the collapse by some time, so with that kind of knowledge extra precautions should have been taken to prevent the overload condition.