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I was sorry to hear of the death of one construction worker, and the injuries to four others. I am seeking some educated guesses about what went wrong with the two pedestrian bridges that collapsed on the Northern Wake Tech campus.
I saw the helicopter views from WRAL, but they don't tell much. The bridges looked almost flimsy, and I could not see what was supporting them. If I had to guess, I think they spanned too long an area for the supports. Anyone with other information?
"The 140-foot center span of the first bridge collapsed as a crew from Central Concrete of North Carolina was pouring the concrete deck, officials said."
Could be deficient design; could be deficient materials; could be errors in construction technique. But with two bridges down, it's no fluke.
For a deficient design to have escaped notice by the Professional Engineer who put his/her seal on the prints would be very unusual. I suspect an error in construction, but this will be thoroughly investigated.
Collapses like this are most often a disconnect between what needed to be built, and how to build it.
IE: the structural support to hold up the final concrete walkway is less than what is needed to hold up the walkway as it's concrete is being poured, PLUS the wood framing to hold in the concrete, plus the weight of the way of getting the concrete to the pour spot. Oh, and the dynamic loads (people walking, vibrations of machinery, etc0 also have to be considered.
The construction supports are quite a bit more complex, since they do NOT have the advantage of having in-place footings, aged (strong) concrete, etc.
I'm certain that the final design by the Structural PE (Professional Engineer), will be examined very closely, but I'm also sure the construction methods used by the contractor's structural people will be examined even closer.
NOTE: The second segment collapsing may or may not be for the same reason as the first. It's more likely to have dropped because it was depending on support provided by the first segment. (And those in charge didn't deem it a risk enough to need temporary bracing..
Could be deficient design; could be deficient materials; could be errors in construction technique. But with two bridges down, it's no fluke.
For a deficient design to have escaped notice by the Professional Engineer who put his/her seal on the prints would be very unusual. I suspect an error in construction, but this will be thoroughly investigated.
With two bridges collapsing, my first thought was a design error. One bridge could be poor construction technique, but two?
Just because a Professional Engineer puts a seal on blueprints does not mean it was well designed. I've worked construction and seen all sorts of stuff on blueprints that looked good on paper but didn't work in real life.
For example, concrete columns that call for so much rebar that it is impossible to tie and even if you could, would not allow enough room for concrete. Likely the amount of rebar satisfied some mathematical formula, but no one bothered to check if there was any room for the concrete. Back to the drawing board.
The Minneapolis bridge that collapsed in 2007:
"The NTSB cited a design flaw as the likely cause of the collapse, noting that too-thin gusset plate ripped along a line of rivets..."
"National Transportation Safety Board announced they had determined that the bridge's design specified steel gusset plates that were undersized and inadequate to support the intended load of the bridge..."
"review discovered that the original design process of the I-35W bridge led to a serious error in sizing some of the gusset plates in the main truss."
Collapses like this are most often a disconnect between what needed to be built, and how to build it.......
The construction supports are quite a bit more complex, since they do NOT have the advantage of having in-place footings, aged (strong) concrete, etc.
I noticed some unusual metal posts or triangles which seemed to be the structural supports, not the concrete columns I would expect. Some new design perhaps?
I am sure the full story will unfold in a few months.
I added a few of WRAL's pictures showing the laminated wood beams where they split, and the steel posts and tensioning cables underneath them.
Last edited by goldenage1; 11-15-2014 at 07:22 AM..
Kind of ironic that this happened at a technical college. I might think twice about sending my kids here to study engineering.
Right, because the kids were building the bridge?
Wake Tech is a public state funded college. Anything that is built with public funds must, by law, go to the lowest builder. Guess what happens when you go with the lowest bidder?
This is why these projects always run over time and over budget. When I lived in NY our village paid to have a beautiful comfort station built at the biggest, most-used park. Seriously could have probably been built in two weekends by your average DIYer. This thing took FOREVER to get built, because the original contractor (i.e. THE LOWEST BUILDER) ran out of funds and the village had to threaten to call the bond to get him to finish the place. Par for the course.
You'll see the same type of nonsense with the schools that are being built in Wake County with the bond money. I guarantee it.
I am sorry for the worker who was killed and a victim of all this nonsense but honestly, it could have been a huge tragedy if it happened 2 years from now during a time when many people were on the two bridges.
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