×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Minnesota Bridge Findings
4

Minnesota Bridge Findings

RE: Minnesota Bridge Findings

(OP)
Gusset Plates found to be about half of the thickness that they should have been.

RE: Minnesota Bridge Findings

If you add load to a bridge, wouldn't you check everything, INCLUDING the gusset plates?  But connections typically have a larger factor of safety than the truss members themselves, so perhaps that is why they weren't checked.

DaveAtkins

RE: Minnesota Bridge Findings

The article was saying that they were under designed from the beginning (even ignoring the added load due to the renovations).

RE: Minnesota Bridge Findings

Quoting a portion of the article here...

"Transportation Secretary Mary Peters was expected to issue an advisory urging states to check the gusset plates when modifications are made to a bridge — such as changes to the weight of the bridge or adding a guardrail, said a federal official with knowledge of the plans. The official spoke on condition of anonymity because Peters had not yet made the announcement."

"Currently, such calculations are done for the entire bridge, but not down to the gusset plates, the official said."

So, it seems to be implied that modifications to the original design were made, but the gusset plates not rechecked.  For them to be undersized by 50% seems to be a pretty substantial "modification".  And the statement that it is still the practice not to recheck the plates really concerns me.

Mike McCann
McCann Engineering

RE: Minnesota Bridge Findings

"And the statement that it is still the practice not to recheck the plates really concerns me."

Me too.  It is NOT standard engineering practice to ignore connections.  Then again, it's a statement coming from some "official" speaking anonymously from stolen tidbits.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Minnesota Bridge Findings

In the bridge industry, especially at DOTs where knowledge and money are limited for bridges of these types, it is not common practice to check connections.  

This is not the standard that the rest of the industry is held to.  Bridge Building engineers typically are expected to check everything from load path to members to connections.

It is true that much of the software in use, BARS, Virtis etc do not provide detailed results to the connections but that shouldn't prevent someone from checking them.

Speaking of checking....the undersized plates should have been caught in the original design.

Regards,
Qshake
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

RE: Minnesota Bridge Findings

Should we short the stock shares of Jacobs Engineering?

RE: Minnesota Bridge Findings

jike,

Did you note that in your link (which was way back in August, 2007) that the NTSB was saying that the focus on gusset plates was "way overblown"?

RE: Minnesota Bridge Findings

(OP)
JAE:

Yeah, I noticed that too!

RE: Minnesota Bridge Findings

One would think that intuition alone would prevent someone from using plates that thin.

RE: Minnesota Bridge Findings

So much for NTSB reports regarding Aircraft incidents...

Or anything else for that matter.

Mike McCann
McCann Engineering

RE: Minnesota Bridge Findings

This really does not make sense. The bridge worked for 40 years. The construction loads to support the decking operation sould not have exceeded the deck capacity. There must have been a trigger. Corrosion or a frozen bearing must have started this. Once it fell, the plates would look  like spagehti no matter what caused it. It sounds like the Feds were hell bent on proving their boss's theory. I'm not buying it yet.

RE: Minnesota Bridge Findings

DRC1 - As I read the report, they're not saying the plates alone are the reason for the collapse, just that the undersize plates represent a design flaw that contributed to the collapse.  In the news conference the spokesman noted that they continue to look for the 'straw that broke the camel's back'.

Regards,
Qshake
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

RE: Minnesota Bridge Findings

The "Adequacy of the U10 & L11 Gusset Plate Designs for the Minnesota Bridge No. 9340 (I-35W over the Mississippi River" report available from the link below provides detailed analysis of the main truss gusset plates:

http://www.ntsb.gov/Recs/letters/2008/H08_1_Design_Adequacy_Report.pdf

It shows in great detail calculations and assumptions used to obtain D/C ratios for the main truss gusset plates.

ok, Structural Wiki > Gusset Plate

RE: Minnesota Bridge Findings

I'm not a bridge guy so forgive my ignorance, but were there 1/2" gussets on both sides of the flanges?  I can see the outer gusset plates but I wonder if they use another 2 on the inside of the flange that straddle the web.  

When I design a butt splice in a tower leg member, I put an angle on the inside and 2 gusset plates on the outside legs so the bolts are in double shear.  The total thickness of the inside splice angle and the outside gusset plates add up to more than the leg thickness so bearing is not a problem.

_____________________________________
I have been called "A storehouse of worthless information" many times.

RE: Minnesota Bridge Findings

When you look at the elevation of the truss in the report, it looks like there was a drafting error.  The gusset plates in the main span may have been transposed with those in the backspan at the two nodes in question.

RE: Minnesota Bridge Findings

I'm with Hokie in thinking it was a drafting error.

Using a factor of safety similar to the other connection plates, those few plates which were 1/2" were probably supposed to be 1 1/2"!
 

RE: Minnesota Bridge Findings

I've been watching the media reports about this, about the big dig collapse in Boston, the aftermath of Hurricane Katrina, and all the way back to the Hyatt Regency walkway collapse in '81.  

Based on all of these, my opinion is, if its the least bit technical, and you heard in the mainstream media, they probably have it wrong.  

I think I'm going to reserve judgement until the final report from some competent engineers is released.


regards,

chichuck

RE: Minnesota Bridge Findings

transmissiontowers,

I don't know the details of this bridge but here would typically only be a gusset on the other side of the chord, with nothing behind the diagonals' flanges or chord's webs, so that the gusset connections are in single shear.

Gussets are very susceptible to corrosion as moisture often collects at the joints in the debris (bird nests, etc).  If they're undersized so that stress levels are high, particularly accounting for the fatigue stress range portion, then you have a reduced allowance for corrosion pitting/loss.

RE: Minnesota Bridge Findings

I love how there is never enough money to spend on engineering, but somehow they find 500k in the coffers to spend on legal fees.

When are these people going to learn that engineering inspection actually saves money.

RE: Minnesota Bridge Findings

chichuck, just go to the link provided and within that pdf document there is another link that includes some of the calculations.  This isn't a media report but an NTSB report by engineers.

RE: Minnesota Bridge Findings

(OP)
I am with you csd72 on your comment.

When ever I go to a jobsite I can usually find a dozen or so structural problems. Often, I go within a day after a field person or an achitect has been there but yet they have not caught these issues! What happens if I don't go, is these problems get ignored.

Owners often don't want to pay for the service to have a structural engineer on site even if it is only periodic. They often claim that their field personal or the architect or a testing agency can do that job, which is often not true. There is nobody like the guy that did the design that is more familiar with all the aspects and what can and cannot be done.

RE: Minnesota Bridge Findings

Knowing how fatigue life goes down as stress level goes up and knowing that in some cases the stress was twice the allowable, it is hard to understand how the bridge remained standing for 40 years.

RE: Minnesota Bridge Findings

I noted the comment on the link

".. the plates had rusted away to half their original thickness..."

And they still say that corrosion was not an influentual factor?

RE: Minnesota Bridge Findings

eureka - The matter of it standing for so long, IMHO, is attributed to the fact that truss joints are designed for an envelope of member forces.  So unlike the truss joints in the old statics class, these joints may never even have equilibrium due to the envelope and based on the envelope concept you can see that it would be a magical day, indeed, if the same combination of loading actually occured on the bridge.

Another matter that I attribute the longevity to is ultimate strength.  The NTSB did an allowable stress design to show how they reconstructed the analysis to the practice in 1967 and then show the results compared to other joints in the same bridge.  however, the ultimate strength of the connection is not presented that I'm aware of.

Regards,
Qshake
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

RE: Minnesota Bridge Findings

Sounds to me like all they have proved is that it did not meet the code, this does not definately mean that it was the cause.

As other people have said, it sttod up for 40 years - why collapse now?

RE: Minnesota Bridge Findings

(OP)
It is sad to say, but anyone who even remotely had anything to do with this bridge will probably be dragged into this legal mess including the State DOT, inspection companies, construction companies, fabricators, engineering firms, etc. whether it was the original design, studies or subsequent renovation work.

As to why it collapsed 40 years later, the gussets may have been significantly undersized originally but as others have said corrosion and fatigue reduced their capacity over the years to the point that a "straw broke the camel's back". What the straw was is still up for speculation.

RE: Minnesota Bridge Findings

Yes,

Unfortunately, it is much easier to prove design flaws than it is to prove any other cause.

Once again, the bottomless legal coffers will be used to prosecute the underfunded engineers.

It would be one thing if this was used as a lesson on why not to cut costs on engineering. But sadly, once the lawyers are involved, it becomes a blame game, and no lessons are learnt, only compensations paid and scapegoats found.

RE: Minnesota Bridge Findings

As a bridge inspector, I can say this.  For a fracture critical bridge, such as this, the inspections must inspect each connection element (bolt, plate, weld) etc visually for signs of distress.  

Either this was missed during the inspections, or it showed no signs of distress.  If the plates were underdesigned from the beginning, I cant imagine that no signs of distress couldnt have been noted, since it stood for 40 years, and most likely saw more load than was on it that day it collapsed.  Something doesnt make sense.

RE: Minnesota Bridge Findings

Slightly off-topic, but I just received an e'mail solicitation (by way of eng-tips.com) for Norwich University which uses the I-35W bridge collapse as the basis for it's pitch of a Masters of Engineering.  Does anyone else find this to be in bad taste?  I mean, to say that if people take their online courses, collapses would be prevented strikes me as crass.  Below is the opening paragraph:
"Dear Structural Engineer,

Although catastrophic and heartbreaking, the August 2007 bridge collapse in Minnesota may not have been a surprise to many civil engineers. It was only a matter of time before a disastrous event such as this put our country’s aging infrastructure under intense scrutiny. Since this devastating event, the need for more experienced civil engineers, like you, has risen. Norwich University’s Master of Civil Engineering program has been designed to upgrade your skills and help you take control of similar challenges."

Castigliano
Bridge Engineer

RE: Minnesota Bridge Findings

Castigliano - I agree....yuck.

RE: Minnesota Bridge Findings

The bridge had been modified several times in the past 10 or 20 years.

As I recall, there were some wider walkways and new crashrails that could have increased the dead loads.

It was never a "bouncy" bridge in the times when I was stopped on it and seemed to have a good reasonably good alignemnt through the years.

I suspect (without looking at the deatails, but looking up at it from a boat), that it will end up being a combination of thin gussets, age and unbalanced load conditions (during traffic changes and construction progress) when it all comes down to it.

RE: Minnesota Bridge Findings

Why would a designer use  gusset plates to join beams that had total thickness less than the thickness of the flanges of those beams?  

RE: Minnesota Bridge Findings

quote:
"the need for more experienced civil engineers"

not the need for more educated engineers!

RE: Minnesota Bridge Findings

A couple of things to get off my chest:

1.) It will be the deck repair contractor who gets stuck with this - it is always the contractor who takes the fall. Thats my bet

2.) What really amazed me was that the bridge was designed in '67, and the comps can't be found (not reall shocking) The bridge was significantly modified in '98 and acording to the Feds the designers only checked a few critcal membersand if they passed, the modifcations were deemed safe. This appearently is viewed as standard practice and they are okay with this. A.) If we couldn't find the design comps and no one anayzed the strucure, how did anyone know what the critical members were? B.) Since '67 codes and allowable loads have changed significantly which may have made a non critcal member critcal. C. The last inspection report noted significant deteroration of the strucure. I find it hard to believe that there was no deterioration of the structure from 67 to 98 then rapid deterioration from '98 to 06. Certianaly the deteriorationmust have reduced capacity of some elements.
Appearently the feds recognize more should have been done and are urging DOT's to check gussets in future designs, but don't see this design practice as abnormal.
3.) Speaking of deterioration, the last inspection report should there were significant problems that needed to be addressed. The state opted to extend the life of the bridge 15 years with more frequent inspections. I,m sorry, but if you are ignoring the current reports, why bother with more reports?
The failure was not a tragedy, it was a governmental policy.

RE: Minnesota Bridge Findings

As I read the report, the Sverdrup calculations have been located and assessed, but they did not include calculations for the gusset plates in question.  Therefore, it is not known whether or not the design intent was correctly shown on the drawings.  To me, production of the drawings and checking thereof is most likely when the errors were made.

The truss member forces were shown in the Sverdrup calculations and on the drawings, and those forces agree with the analysis of NTSB consultants, so the original forces were used to check the gussets.

It is still unknown, or at least unreported, if these gussets were the elements that failed first.  I think the early release of this section of the investigation is just to give us all something to chew on.

RE: Minnesota Bridge Findings

hokie66 - just to add to your last comment, I think the news conference was helpful in that common practice should really be to check all structural elements when modifying a structure and hopefully that is what the industry will take away right now.  

On a couple of points by DRC1 - I think it amazing to find calculations from 67....in fact, I think the business standard is 7 or 10 years and out the door they go.  For someone to offer the calcs up shows a willingness to help.  Hell, anyone could have started throwing work into the shredder when the news it the stand.

I defintely agree with DRC1 comments on code changes, there was a significant code change for steel in 1989 AASHTO.  At present, I'm not aware of any mandate that when upgrading bridges for new loads, all member stresses and member proportions must now meet the new code.  I do think there are building codes that spell this out.  I could be wrong though, would like to hear others comments on that.

I also agree with the deterioration comment and Strguy11 about inspection.

Regards,
Qshake
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

RE: Minnesota Bridge Findings

Castigliano -

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.......

RE: Minnesota Bridge Findings

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?

RE: Minnesota Bridge Findings

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.

RE: Minnesota Bridge Findings

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.

Sounds pretty plausible to me.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Minnesota Bridge Findings

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?

_____________________________________
I have been called "A storehouse of worthless information" many times.

RE: Minnesota Bridge Findings

HgTX -

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.

RE: Minnesota Bridge Findings

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.

RE: Minnesota Bridge Findings

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.

RE: Minnesota Bridge Findings

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.

RE: Minnesota Bridge Findings

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.

RE: Minnesota Bridge Findings

CVG -

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?

RE: Minnesota Bridge Findings

According to the forensics, there wouldn't have been signs of distress at the time of the last FC inspection.  It was an abrupt failure.

Checks for construction loading included the assumption that the weak link was the member, not the connection.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Minnesota Bridge Findings

HgTx,

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.

RE: Minnesota Bridge Findings

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.

RE: Minnesota Bridge Findings

csd72--

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.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Minnesota Bridge Findings

HgTx,

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.

RE: Minnesota Bridge Findings

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.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Minnesota Bridge Findings

HgTx,

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

RE: Minnesota Bridge Findings

(OP)
csd72:

I am wholeheartedly in agreement!

RE: Minnesota Bridge Findings

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.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Minnesota Bridge Findings

Hgtx

Thats a shame some one would want to know why you would want to look at one of your projects. I used to work for a consultant before going over to the dark side. However one of the advatages of contracting is nobdy asks how many hours did you bill this week.
I gree the system does work, howeverthe contractor focus on putting the structure together, somehow. In staged construction, which is the most common, there is a lot of locked in errection stress that I do not think are accounted for in design.
Further, bad details construction wise are repeated for years because no one sees the work in the field. I have encouraged our DOT, to no avail, to have the designers visit the jobsites periodically so they can get some feed back. They have not done it -0 again can't justify the cost to the project.
Hope you got your approval for you visit.

RE: Minnesota Bridge Findings

Just confirms what I always knew since my student days.  Connections are the most vexing of all the design, the weakest link in the whole structure.

RE: Minnesota Bridge Findings

When I first read about the plates being undersized it seemed credible as someone pointed out that it was drafting error 1 1/2 vs 1/2. However, going through the calculations 1 1/2 would be too much; 3/4" would've been OK. It could still be a drafting error: The designer's intent might have been A514 ( a bit of overkill) in lieu of the A441 but without the calculations we'll never know.

Maybe the design wasn't checked; maybe the design was done by an inexperienced engineer; there are so many what ifs. I worked for a firm (now defunct) that designed a number of long-span trusses; we would design and detail connections simultaneously. The reason being if something didn't fit it was easier to revise a design in progress. I looked at some plans prepared by my former firm, including one bridge designed a few years after 35W, and compared plate sizes for similar loads, we never used a 1/2" plate for a main truss connection.

As far as finding the error in the shop drawing review, let's face it, shop drawings are usually checked by someone at the bottom of the food chain. Would we expect someone at that level to know 1/2" was undersized?

I've also performed about two dozen inspections of long-span trusses and to be honest, we're not looking to see if a plate is the right size. Granted, during bridge inspection, we look to see if something jumps out at us but whether a plate should be 1/2" or 3/4" is not going to be noticed. It's not as if someone put a 1/2" plate where a 2" plate is needed.

Anyway, I still think there's more to the collapse than an undersized plate. The stress was above the allowable but below the yield point. Let's wait for the report.

RE: Minnesota Bridge Findings

I'm just wondering if the bolt holes in the gusset plates were punched full-size.  It could help explain why they cracked.

RE: Minnesota Bridge Findings

I suspect that a lot of people haven't realised that the gussets are also acting as splices in the chords.

Looking at the published calcs, we see that the compression diagonal at U10 is designed to carry 2288kips at less than 22ksi giving a sectional area of at least 104 sq.inch. Scaled flange width is about 30in, so probably about inch & half thick. It seems hard to imagine why nobody thought it odd that the gusset thickness was only a third of the connected flange thickness. Maybe they did but decided that other people would know better.

If we play around with the calculation results for gusset U8 (above pier) we could conclude that a 5/8in gusset plate will just about do on a joint where the tension diagonals are carrying at least 2000kips. Follows that the calcs given are insufficent on their own and that additional requirements need to be met.

Personally believe there should be a code rule giving a minimum gusset thickness relative to connected plates or flanges.

RE: Minnesota Bridge Findings

Personally believe there should be a code rule giving a minimum gusset thickness relative to connected plates or flanges.

That knee jerk response that would hurt more than it would help.  I do these sorts of gusseted connections all the time for buildings.  Sometimes you have a member that is much heavier than the other members at a joint.  It may be at a low stress at this location.  Why should the gusset plates be penalized because one of the members has thicker flanges than the others?

This bridge failure was preceded by a design error.  There's no need to dumb down all design because of this mistake.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources