What effect did welding have on the surrounding steel? Perhaps they quenched it and left it more brittle than the rest of the beam? The holes obviously provided an increased stress point, but I wouldn't think that the holes alone would weaken it a lot. They are about 1/3 of the way from the neutral axis toward the edges.
Has anyone here studied stress in beams? My intuition says that the flexing of the structure would tend to make somewhere near that point (just beyond but not at the end of the thicker support) most likely to form cracks, regardless of the extra holes.
The neutral axis is relative to bending stresses. This is a shear failure where the holes would matter and the neutral axis would be irrelevant.
Historic boundaries and conservation efforts.
The neutral axis is relative to bending stresses. This is a shear failure where the holes would matter and the neutral axis is irrelevant.
Historic boundaries and conservation efforts.
not my real name, post: 410719, member: 8199 wrote: This is a shear failure
I can't see that. The beam moved UP after it broke. It also had some tension as shown by the gap. Most of the stresses would be from supporting variable weight further away from the attachment, hence flexing.
Trusses are designed for axial loads. There should be no bending moments in the members.
Likewise there should be no shear stress in the members either.
It is a very peculiar failure. Experts are saying the holes are the reason for the failure and that
the holes are decades old.
They also say it will take a few weeks for an answer.
Historic boundaries and conservation efforts.
Virtually any kind of load CAN affect a member, but it depends on what is actually happening. In a perfect scenario, only certain things should happen. As we leave that perfect scenario, almost anything could happen.
There are statically determinate and statically indeterminate circumstances occurring once that perfect scenario occurs. They are attempting to determine what had to happen for the event to occur as it did. There may be a variety of possible reasons.
Take a bicycle wheel. Remove one spoke. What happens? Remove another spoke somewhere around the wheel or one right next to the first one. What happens? Remove a third one either at random or adjacent to the first two that were removed. What happens? How does the rim fail and why? Let's just say..........it's complicated..........beyond easy explanation.
A complex structure ( a bridge for example) has a huge number of things happening and changing every moment as the loading occurs and changes based on a vast array of potential reasons. There are static loads and there are dynamic loads. They all are subject to change every instant. That change is the culprit leading to failure. Identifying what one major event or millions of little events over the decades was the straw that broke the camel's back is tough to determine. Well, unless that one event was something like a bomb going off.
Friday update. Originally I was thought repairs could be made within two weeks, the bridge closure has now been determined to be indefinite.
At this time the fracture has been reinforced, bur repair plans are still ongoing. It is necessary to build 8 supporting towers under the damaged section of the bridge in order to jack it back to it's original position and support it during repairs.
Bridges move in the order of inches during a day and extreme heat increases the movement. The I-35W bridge in Minneapolis failed catastrophically on an extremely hot day, this bridge failed after cold weather.
Paul in PA
Thermal expansion/contraction may have accelerated failure, but didn't cause it.
I was once under a steel girder bridge while freeway traffic was steadily crossing it. You could see/feel the girders flexing as each truck crossed.
With an ADT of 15,000, it amazed me that something hadn't failed due to the cyclic loading/unloading of the structure over the years.
imaudigger, post: 411142, member: 7286 wrote: Thermal expansion/contraction may have accelerated failure, but didn't cause it.
I was once under a steel girder bridge while freeway traffic was steadily crossing it. You could see/feel the girders flexing as each truck crossed.
With an ADT of 15,000, it amazed me that something hadn't failed due to the cyclic loading/unloading of the structure over the years.
I was once young and foolish and attempted to set up an instrument on a bridge deck. It was a single span concrete deck with pre-stressed concrete beams in a rural area (low ADT). In my mind it should have been static enough to hold the point. Boy was I wrong. I can only imagine how much a large steel girder affair floats around under heavy loading.
ADT on the PA-NJ bridge is 41,000, or one vehicle every 2 seconds, 24/7/365.
Daily thermal expansion makes the actual repair more difficult. While an engineering student at Lehigh University, we got to visit the Commodore Barry Bridge South of Philly during construction, 1972. As I recall they had plans to be making the connection at the center of the river and anticipated the time it would take from early morning until the 6" gap was closed. That was when everything was new and essentially working as designed. It is not easy to figure out exactly what 61 year old bridge is doing.
Paul in P
Bill93, post: 410718, member: 87 wrote: What effect did welding have on the surrounding steel? Perhaps they quenched it and left it more brittle than the rest of the beam? The holes obviously provided an increased stress point, but I wouldn't think that the holes alone would weaken it a lot. They are about 1/3 of the way from the neutral axis toward the edges.
Has anyone here studied stress in beams? My intuition says that the flexing of the structure would tend to make somewhere near that point (just beyond but not at the end of the thicker support) most likely to form cracks, regardless of the extra holes.
You know, that does sound right. That would be the weakest point, right before the sister plates. It probably doesnt help any that there is the point they weakened the steel beam with the impurities form the weld job. And drawing a heavy current across the beam - depending on where they grounded - had to have changed the eleasticity of the beam in that area. What you said sounds right.
Bill93, post: 410718, member: 87 wrote: What effect did welding have on the surrounding steel? Perhaps they quenched it and left it more brittle than the rest of the beam? The holes obviously provided an increased stress point, but I wouldn't think that the holes alone would weaken it a lot. They are about 1/3 of the way from the neutral axis toward the edges.
Has anyone here studied stress in beams? My intuition says that the flexing of the structure would tend to make somewhere near that point (just beyond but not at the end of the thicker support) most likely to form cracks, regardless of the extra holes.
Reply to Bill,
It would have been better to leave the holes unplugged, there is no practical way to plug weld holes deeper than diameter without leaving inclusions within. Worst yet these appear to only have been surface welded leaving an unfilled hole within.
To R.J.
The holes were the source. There is most likely another set of plug welded holes closer to the cover plate matching the outer rivet row, and any bending stress would have been greater there. However having structural experience at and for Bethlehem Steel I know that W14 sections being extra thick have greater built in internal stress from the rolling mill the closer one gets to the web. Customers paid extra money if need be to have Bethlehem Steel heat treat said beams to relieve those stresses. The fracture is concentrated in that higher stress area
Paul in PA
Paul, It looks obvious the weld hole area is the fault. It may be like Bill mentioned and the steel was changed by the welding. I'm not sure where I got the idea that current would change the steel temper.
R.J. Schneider, post: 411327, member: 409 wrote: Paul, It looks obvious the weld hole area is the fault. It may be like Bill mentioned and the steel was changed by the welding. I'm not sure where I got the idea that current would change the steel temper.
Current per se would not change the steel temper, however at the grounding point, an electrical arc could erode or deposit steel and that point could become a crack initiator.
Paul in PA
Another PA Turnpike bridge held together with duct tape and paper clips. Watched a chunk of deck the size of a hard hat come out today and almost send an unsuspecting driver to an early grave.
Crappy cell phone shot with bad glare. Sorry
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So America's infrastructure really is crumbling and in a sorry state of affairs...I don't see any steel missing in that picture. Was it part of the deck that fell out?
imaudigger, post: 411709, member: 7286 wrote: So America's infrastructure really is crumbling and in a sorry state of affairs...I don't see any steel missing in that picture. Was it part of the deck that fell out?
That's the pic I took from underneath. They reinforced underneath with wood. It's rotten. I should have taken more pics of the beam seats and pier caps. We were running a BM in underneath and kept hearing a lot of banging as cars traveled across. I walked up to inspect and watched as a rig kicked up a large chunk of the deck. The car behind didn't have enough time to react and hit it with the drivers front tire. The impact caused the vehicle launch onto two wheels losing control of the vehicle. They were lucky, didn't panic or swerve and were able to ride it out.
I walked over to see if they were OK. They were obviously shaken. It wasn't long that patrolman spotted and stopped. He took the report. I had quickly, in "Frogger" like fashion, removed the concrete from the cart way.
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Sounds like that bridge needs to have a reduced speed limit until they can get it replaced. I'm sure it is already down posted significantly.
Even simple bridge replacements take about 5 years. I cannot imagine how long a major structure project would take from inception to as-built...10 to 15 years?
imaudigger, post: 411722, member: 7286 wrote: Sounds like that bridge needs to have a reduced speed limit until they can get it replaced. I'm sure it is already down posted significantly.
Even simple bridge replacements take about 5 years. I cannot imagine how long a major structure project would take from inception to as-built...10 to 15 years?
Current posted speed limit is 70 MPH. We are currently running control and establishing the construction easements. This is going to be a 2 yr (2 season project) project. Traffic will stay in its current configuration while we build new structures on the outside. We'll layout for 2-12' lanes and 10' shoulder on both the East and Westbound lanes. That will be completed this year and traffic will be moved to the outside. Then the existing structure will be demolished,a new built and tied to the ones that were completed. This will occurs throughout next winter and the entire project completed by next year. Length of this one is just over 100' and width is 120'
Total length of if job is 7000' with a about 4000' of widening and full depth construction.
I used to do a lot of the preliminary surveys. A 5 year turnaround is average on most from preliminary, to design, to bid.
The rapid bridge replacement projects are similar but on crack. We've replaced 150' double span bridges in 10 weeks. It's ridiculous but I've seen them done. That's counting from the day the "road closed" signs go up until the day the road is reopened for traffic.
A normal single span is averaging 40-50 days to remove and replace. Penndot will regret these in 25 years IMVHO. Pa bridges are in bad shape and we have thousands that are deficient. The intent was to start with the worst and work their way through it. By the time most are completed the original rapid replacements will be failing again.
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joabmc, we probably do very similar types of work from the sounds of it.
I have worked on more than one bridge replacement project where the safety of the old structure was of serious concern by the time construction rolled around.
Many times the approach/departure profile grades are not the best and the decks take a pretty good beating due to fast heavy trucks bouncing as they cross.