It was probably poor judgement on Norwich's part to use the tragedy in that way, however, I think we all agree that engineering is a life long learning process and everyone needs to have continuing education, but, yes it was poor judgement.
As a side note, isnt engineering sometimes about using your judgement??? I wonder what they teach there.......
I've only been (peripherally) involved in one truss bridge rehabilitation/widening. The widening of the bridge required the truss members to be checked in great detail. The software used to check the members didn't check the connections. The gussetts were probably checked elsewhere, but I never saw those calcs. I would be interested in hearing from other consultants who do rehab work. Are the gussets typically checked? Also, if the gussets are found to be deficient, how are they replaced?
I think the sense is that gussets are rarely (if ever) checked after the initial design because standard practice in the design industry has been that they are designed for more than the members they connect and therefore subsequent load checks are focused on the weakest link in the chain - i.e. the members.
Just heard a FHWA guy give a brief report on this the other day (after the press release came out).
Gussets were underdesigned (or there was a drafting error, which is my personal belief). The reason they were looking at them from the beginning was because those gussets (just inside the piers) were half the thickness of other gussets in similar locations (just outside the piers). (Again, to me that's such a *weird* design error that my vote is for drafting error.)
This is not the kind of thing an inspection would have caught, unless you hire the kind of inspectors who can do structural analysis on the fly. Corrosion is not thought to be a factor, nor is fatigue cracking.
Connection details were not examined when the bridge was analyzed in the past because, as someone else here pointed out, it's been assumed that the connections were designed to be at least as strong as the members. Lesson learned here is that's not a safe assumption--even if it was *designed* that way, who's to say it got built that way? I'd go a step further and say field-verify every dimension. I've seen "as-builts" that didn't even have the same number of spans as the real bridge.
The proverbial straw looks like it may have been the distribution (rather than the magnitude) of the construction loads that were on the bridge at the time.
A little off topic, but I saw a story about the 1906 SF earthquake and fire on History Channel. Just after the rebuild started very strong codes were passed to prohibit unreinforced masonry buildings and require specific construction methods. The City leaders wanted the rebuild to get underway and get completed quickly. the contractors complained that it took too long to rebuild with the new rules and they were given permission to return to the old ways of unreinforced masonry building. After several months the new rules were never followed and construction continued until the next big earthquake and massive loss of buildings.
Will the same thing happen with this bridge collapse? I read that the NTSB will tell each bridge owner to have every bridge re-analyzed and reported. The local news is reporting a proposed $0.42 increase in the Fed gas tax to pay for all the work. With gas already at $3/gal will the public be willing to pay the increase?
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I have been called "A storehouse of worthless information" many times.
I think you bring up and interesting point for discussion. Assuming that the distribution, not the magnitude of the CONSTRUCTION loads was a factor, if not THE factor, wouldnt this still put some liability on the contractor? As a designer, of buildings or bridges, we typically do not know the means and methods of construction, and as such, place limits on what can be done, at least on new projects. I would think that the contractor should have some culpability in this, given that the "undersized" gussets showed no signs of distress under full traffic loads. Dont get me wrong, I think the desingers should have checked these gussets as well, but it appears that no signs of distress were noted on these gussets during inspections.
Strguy11 - If the gusset plates were undersized in the first place, how can it be the contractor's fault? The contractor being on the bridge may have caused the undersized gussets to fail, but if the gussets had been designed properly in the first place, they might not have failed, even under the construction loading.
I would think that any responsible contractor would recognize that his work (removal, etc.) and the corresponding equipment and distributing of equipment and materials would put loads on the structure that are different than the original design.
I also think that the state recognizes the same possibility and knows more about the actual condition of the structure to be worked on or specified repairs to.
Both parties are obligated to makes sure that all appropriate measures and co-ordination of work will not cause a problem. That includes requiring a approvals for construction methods/sequences and submitting accurate proposals for progress including material storage and alterations to the the structure and the use.
Only an attorney can sort this out after the construction documents and compliance with them are verified.
Nutte, I agree.
I often aid contractors in bridge reconstruction, and I cannot count the times that I have compared the forces due to the constuction load to the forces induced by the design vehicle. It is a simple and fast way to make sure that the structure is not being over stressed.
in this case, the bridge was overstressed prior to construction. the pavement thickness had been increased almost 50% over the original design and the center concrete barrier had been added. This increased the dead load sitting on the deck by perhaps 50% over the original design. on top of that, several gusset plates were grossly undersized to even handle the original design loads. MnDOT did not find this in the inspections or bridge ratings. Hard to say that the contractor should have known better than the state highway engineer in this case.
If this was so grossly undersized, and was overstressed due to the live loads, shouldnt this have been picked up and noticed during the inspections? Part of the inspection criteria for a fracture critical bridge is to check every connection for signs of distress. I cant imagine that there would not have been any signs of distress/yielding if the gussets were so undersized. How can this be explained?
I think this is a good case for engineers doing the inspections. A decent structural engineer would pick up the fact that two similarly loaded gussets were completely different sizes.
I have seen a number of fundamental issues with buildings that have been passed by local authority inspectors including things like masonry supported on timber beams.
If the checks for the construction loading made an incorrect assumption, the contractor/engineer making those assumptions should have some liability as well as the original design engineer. It just appears to me that the contractor placed a load on the structure that was different than the design loads, that should have been checked completely, which, may have found the gussets were undersized for the construction loads.
Depending on the size of the engineering herd at an agency or company, the engineers who specialize in construction and other field operations may not be the same group who sit behind their desks and design. I don't think there's a guarantee that just any engineer would necessarily have noticed the problem, especially when their focus in a FC inspection is cracking. I don't know that I, not usually a designer, would have noticed a too-thin plate if I were crawling over the bridge looking for cracks.
On the other hand, a post-erection sanity-check inspection of the entire bridge for design flaws might be prudent.
Even today, detailing errors happen all the time. In my limited design experience, every numeral on the plans is double-checked, but a big stack of shop drawings based on those plans gets perfunctory attention. Until we go to 3D modelling of bridges with the same set of numbers going from design all the way to shop and erection drawings, we are going to see errors because some human has to look at a piece of paper and type a number into a program.
Very often such errors caught because things just won't fit. Sometimes they're caught because something just doesn't look right to someone. Surely sometimes they're not caught at all.
The current system does not make time or resources available to check every number on the shop drawings the way every number is checked in design (and I couldn't say whether checking every number in design is a universal practice).
There are a lot of things that the current system does not make sufficient time or resources available for. We're always expected to do more with less.
I was referring more to the design and construction stage rather than the routine inspections.
I can understand the philosophy of the modification engineer - at some level you just have to assume that the origininal engineer did their job right, otherwise you would basically be rechecking the entire bridge.
Personally, I wouldnt feel right unless I had also checked the critical connections - regardles of the standard practice.
Interesting to hear your take on the shop drawing thing. Where I come from, it is sometimes the opposite - if anything is ommited e.t.c. "it will be picked up at shop drawing stage'.
When I hear of the checks and measures that are done in accountancy, where there are no lives at stake, it really highlights how poor our level of quality checks are in the construction industry.
If something's omitted such that the bridge can't be detailed without it, it'll get picked up in the shop drawing stage. If it's just plain wrong, but buildable that way, not so much.
Your office may vary. If you're the ones checking the shop drawings, you'll know how well you can rely on the shop drawing check to plug the holes left in design.
I've never checked shop drawings, so I don't have a full understanding of what they do or don't look at, but I know that there is definitely not enough time to look at everything. I suspect some fabricators of taking advantage of that--sneaking in something convenient to them, and then saying, "Hey, it was on the drawings you approved." Nothing life-threatening, one isolated violation hardly ever is, but all in all less than we were paying for.
I think its crazy that there are people who only crunch the numbers but dont check shop drawings or go out on site. These should all be the same person/team so that it can be ensured that design intent is met.
It also means that you learn from your detailing mistakes, if what you specify cannot be built, then you know it
With us the people who do the design also check the shop drawings. But there's not a lot of time available for shop drawings (and even so, turnaround is still considered a problem).
But they don't go on site. It's not encouraged. During my design stint, my colleague and I wanted to go visit the bridge we were about to widen, and our manager demanded justification. We were so shocked that we almost couldn't come up with a verbal justification, since it was so self-evident to us that we needed to go out there and look at the real bridge. But it's not just where I work, though; I know a lot of consultants who don't get out into the world much either. You get the design out the door, you get the shop drawings out the door, and then you never think about it again till your opinion as EOR is solicited on some minor issue or other. It's not really a cradle-to-grave industry.
For the most part, though, it does work. Most of the engineering issues involved in erection are the contractor's responsibility, not the designer's. Having a design engineer on-site for every single erection on the off-chance that some egregious design error made it all the way through into the bridge--and on the further finite chance that said engineer will happen to notice said error while standing on the sidelines staying out of the way of the cranes--may not be the most efficient use of resources, even within the category of safety expenditures.
