Thankyou sir.
Perhaps the concrete has been put in to alter the vibrational characteristics of the bridge. In a vibrating structure the natural frequency is proportional to both the stiffness of the structure and the inverse of the mass. so more mass would raise the natural frequencies, which would result in a less lively bridge since higher frequencies generally vibrate at a lower displacement.
Before somebody wails ‘but there aren’t any steam locomotives anymore’ in response to MC’s neatly accurate description of the Cooper’s loadings…
No, there aren’t – or precious few. But the loadings which the Cooper’s loadings represent are meant to be conservative (a bridge falling down can ruin your whole day), and have been found to account properly for impact and imbalance and other miscellanesou dynamic effects. The fact that they were originally derived for steam engines doesn’t affect their usefulness – or their relevance.
Ironically, a flyover bridge in Chicago (Junction of I294/394/94) collapse Friday evening killing an ironworker.
There were 6 - 40 ton I beams which were to be in place on a concrete pad. The last of the 6 beams was being lifted into place and the entire group fell.
The 6 beams are connected together and are wedged on top of the concrete pad which is about 27 feet high. The ends of the beams are about 60 feet in the air.
OOPS!
Making things even worse…it took over 24 hours to remove the ironworker.
Mark, your co workers description sounded very similar to what might have happened.
If anyone is interested, The Times, a Northwest Indiana newspaper has quite a few photographs and I am sure they are online at nwitimes.com
ed
ballast deck bridges are much easier to mantain. anytime you get any thermal displacement on a ballast bridge all you have to do is tamp the track. if there is no ballast on the bridge it is generally very difficult to do any maintance on the track. on a ballast bridge the replacement of ties are much easier.
thanks to all contributors to this thread - I have learned a LOT about RR bridges. (And to think there was not one mention of open access!)
dd
Mark, wow, that really takes me back to my college days…I pretty much aced steel design in school, but haven’t used it since…and its true, if you don’t use it, you lose it…though I still have my LRFD textbook and my AISC codes…
My first instinct was to talk about stiffness and lateral bracing, but you way more than I ever could have off the top of my head covered that subject.
But, I’m wondering, if it simply wasn’t an “ablative armor” to protect the bridge against collision damage in the event of a derailment?
eh, who knows. without an “as built” nobody will probably ever know.
Chern
The through deck girder bridge described above was designed about the time the science and understanding of bridge design amd modern materials science came into its own. Before the 1890’s bridge designs, lots of people were guessing with many tragic and humorous results that are well documented, especially on the eastern narrow gage lines. Many riveted construction Thru deck girder bridges have experienced failures in the floor system around the buttress bracing causing railroads to look for fatigue failures in old rivets since 1980. (newer welded bridges did not have the fatigue issues) Common sight in the “bad old days” was a 4-4-0 in some kind of no-win situation where a bridge under load had failed. Conversely many other bridges were indeterminate or horribly over designed wasting $$$ and material.
I would just like to say thanks to Mark W Hemphill for spending the time to write his long article about bridge design. I am an Electrical Engineer by profession but I really enjoyed learning about the facts in the posting and will try to remember even 10% of it !
David Starbuck
Malaysia
After viewing the picture, it looks like the center support was added. If so, load capacity is probally not thought of as a problem and somebody had “this great idea”. I’m wondering when the concrete was added. If fairly recent, it probally was just to contain ballast. The rust and corrosion of support members is a very big problem with this application.
I once had to prepare a set of drawings for individual bridge tie notches on a superelevated curved alingment over (of course) straight plate girders. A geometric nightmare!
EFS, P.E.
Ed2365 now has learned the “joys” of wedges, shims and dapped ties.
[quote]
QUOTE: Originally posted by TheAntiGates
Whelp, through a totally unplanned fluke, I was riding with a friend today, and got temporarily stranded about a block from this bridge, but of course had no camera with me (kicks self in head)… but decided to use the opportunity to get another look at the bridge. Some one here mentioned ‘plastering contractor’ and the thought got me to wondering if this stuff might have been plaster on lath, just for ballast control, and might be hollow inside. So, I fugured. I’d go give it a few good taps, just to apply a critical ear.
No such luck, beating on the surface with an available spike head produced some of the more solid sounding thuds I’ve heard, might just as well have been beating on solid concrete with a sledge hammer, no hollow sound at all.
But, surveying the curio with an eye more focused on proportion than on the recent, more superficial visit,…I noticed my original sketch had some proportional deficiency, which is potrayed more accurately, here:
Where the foot of the concrete filler is perhaps 16 inches across the base (the “A” dimension)
And, approx 6 feet top to bottom measuring from th
…I am one that cannot understand why the rail doesn’t eventually flip over on it’s side with conditions that exist as Gates discribes above…It is so common to see the spikes sticking up and not near the flange of the rail to hold it in place…
I had never seen the conditon before, “dynamically”…so, seeing it for the first time, so close, and with no place to really run to…was a rush.
I’ve seen sections where all the spikes were pulled up a good 2" or so, but with no train on them never really thought about how they got that way.
The live train explains everything.
After the train passed I went over to the worst sections, mostly in disbelief…just because so much of the spikes shank was exposed, And they were less than finger tight, not snug at all, just sitting in their sockets by gravity alone.
Me thinks you just discovered how heavy trains really are!
Go back and look at the tie plates, and you will notice the plate has a “groove” in it…corresponding with the foot of the rail…as long as the spikes hold the tie plate in place, and don’t allow side to side movement, the pumping is pretty much not a issue, the weight of the cars holds it all in place.
Add in gauge rods, and as long as the pumping doesn’t exceed a set amount, it will work just fine.
Scary to stand next to, but still usable track.
Now, if left alone for a really extended period of time, eventually you will fatigue the rail to the point it breaks…but as long as it stays in gauge you are ok.
Think about it, simple spikes driven into wood ties, and they can be removed by a single person with a spike puller, yet it holds the track in place.
No ring shanks on the spikes, they are smooth sided…look at the length of the spike, after the amount of thickness of the tie plate is subtracted; there is only 3 to 4 inches of spike actually in the tie…
It’s all a balancing act, as long as you can keep the ties in place with the ballast, and keep the track in gauge, it’s really hard to roll rail over on its side, that’s a lot of weight pushing it straight down, and it takes a lot of force to make a railcar want to move sideways.
And rail is strong, really strong along it’s narrow axis.
Take a wooden 2x4, place it wide side down between two fixed points, and you can make it flex in the middle…take the same 2x4 and hold one end, have a friend try and twist, or rack it, you probably cant.
Support that same 2x4 on its narrow side between two fixed points, and it can support several times more weight than if it was wide side down.
Now, swap the 2x4 for a section of rail!
Its pretty tough stuff.
Most track failure derailments are not caused by broken or rolled rail, it’s because the track was allowed to
…But Ed…Isn’t it true the force from the weight of the train is not 100% straight down for vaious reasons…Curves for one…and isn’t the shape of the wheel surface that rides on the rail head somewhat tapered to “center” the running location, etc…In other words putting a bit of side load on the rail position…I understand modern heavy rate rail is and must be strong it just is hard to imagine say…a 200 ton engine running over a condition of rail jumping up and down pulling the spikes to the point of being not even finger tight in the ties, plus if on a curve the additional force on the outside rail…{in some cases}…It’s just hard to see how it continues to stay in place and not eventually become weak and roll…I know it must work as you state it sure does, it’s just hard to imagine how it does…
Because there is play in the gauge…as long as it stays within that limit.
The foot of the rail is wide, and because the rail is really a long single piece, even with the joints, and joint bars, you have to roll a great part of it at once to make it fail.
Add in the downward force, it gets pretty tough to make it roll…now; it will slide if the tie plates are not held in place, and yes, in curves, the centrifugal force can move it some, but with mudchickens adding super elevation, the force is still mostly down, not side to side.
Done correctly, the wheel balances on the rail, even in curves, the flange on the wheel has a lot more sideways play that one imagines.
Think about this, the flange on a wheel is only 1 inch deep, so something has to “hold” the cars to the rails…
Even the small switch engines I work with are heavy…254000 lbs…or 127 tons.
You can, if the brakes are off, make it roll forwards or backwards with a simple pry bar and leverage, but try to pry it sideways!
Stop one on a switch, and try and line the switch, you can’t, even though the switch plates are greased, and you have the leverage of the switch handle and the gears in the switch body….it just weighs too much even though the contact points between the rail and the wheels are around the size of a quarter on each wheel.
Inertia and momentum try to make the car go straight, and, when the curve is done right, elevated correctly, the “straight” path is actually the direction of the curve…All of us have driven on a well done highway, with a properly elevated curve, and noticed we don’t really have to steer our car through the curve, we don’t have to move steering wheel that much, the auto just tracks right dead on through the turn…same forces work on railcars, they “track” through curves, and almost all of the forces are still downward.
Ed
I discovered SEVERAL things today.[8)]
Some trains are REALLY heavy, even “old” diesels run mighty quiet whene they are coasting, and “nearly abandoned” lines can still throw a train at you when you least expect it.
I like to think of myself as a “safe” person. And, I’ve been trying to persuade myself that had the bridge not been a former 2 track line with one removed, and been a line notorious for atrophy, I never would have had such a lackadaisical regard for what might be coming down the rails.
But, truth be told, when i looked up and saw that headlight, maybe 400-500 feet away and headed my way at about 10 mph…there was a certain muscle that tightened up enough to probably bend a spike or two. Being at the center of a bridge span didn’t help much… Had just a couple things been different, I’d had some serious problems. (single track bridge, open frame construction, etc)
I sure could have out run the train for more than the length of the bridge…on level ground, but, on ballast? or on ties? good question, maybe, maybe not.
But, taking stock of the situation, I had all the room i really needed, and figured the best thing to do was to make sure the engineer knew i wasn;t going to flak out. So, shoulder blades against the outside girder, and an alert smile seemed like the best bet to convey my will to live.
Once I saw the rails bouncing up and down like that, it was a sight to behold, the funny thing being is that I was still very much a captive of the bridge, and at first contemplated working my way away from the loose rail section, till I noticed that the joints in either direction were bowing just as bad. So I just stuck it out.
I agree with you that the weight must play a factor in holding things in place,
I can’t remember when I haven’t seen bowing behavior, in any track…
Chern