In the June issue if MR is a spread on a nice layout that includes a model of the Keddie Wye. In the posed photo, a passenger train is crossing one side trestle, while a large articulated locomotive sits square in the center of the other side trestle. It heads a freight train waiting to follow the other train..
I know the bridge structure is designed to handle the load of engines and trains, but would it be railroad practice to leave that heavy loco in the center of the trestle span? Or would they have made it stop short of the trestle itself to wait.
Or is this just not a concern?
Hi Enzoamps
I do believe railroad bridge engineers design them to not only withstand but exceed the heaviest possible load bearing situation they could be subjected to. There's a saying that floats around out there that the railroad over builds everything and probably holds a lot of truth to it.
Ed posted a picture a couple times of a bridge that was built around a bridge. Darndest thing I ever seen the first time I saw it and still like to see it. The old bridge's load capability was marginal after more modern hopper cars were made larger to carry heavier loads. It was more cost-effective to build around the old bridge to (Beef It Up) instead of tearing it down to build a new one.
Now and then you always hear about a highway bridge that collapsed. The 35W bridge collapsed up here, I think close to 10 years ago. The gauge of steel used to build that bridge was too thin and the gussets were rusted out more then would have passed inspection. It's no wonder that bridge held out for as long as it did, especially after they removed both outer lane decks at the same time while redecking it is when the collapse happened. They seriously compromised its structural Integrity by doing that, especially leaving it open to traffic during the process.
I remember driving over that bridge while they were reconstructing the deck and when the semis and cement trucks were stopping in traffic I'd feel it move. It was a rather unsettling feeling to say the least and it collapsed a few days later.
It's not often you hear about a railroad bridge collapsing unless having to do with a bad flood or a wicked storm.
There was a high steel trestle a tornado blew over that may have withstood it. Unfortunately inadequate anchors were used to secure it to the footings. That bridge failed from human error of using the wrong fastening hardware, not necessarily the bridge structure failing itself. Other railroad bridges have been washed out in severe flash floods.
I think it would be awfully rare you'ld hear about a modern railroad bridge structure failing after being subjected to an extreme load. I bet some of the old 1800's bridges did though.
TF
Thanks. I was just idly thinking. I know for example in some large airports, the roadway crosses under the taxiways or other aircraft ground path, and ther are signs for the pilots telling them not to park their planes over the roadway. Apparently the underpass is built to withstand a passing 747, but not happy if a 747 sits on top of it. SO by extension, I wondered...
Enzoamps Thanks. I was just idly thinking. I know for example in some large airports, the roadway crosses under the taxiways or other aircraft ground path, and ther are signs for the pilots telling them not to park their planes over the roadway. Apparently the underpass is built to withstand a passing 747, but not happy if a 747 sits on top of it. SO by extension, I wondered...
I'm somewhat surprised by this. Many years ago I managed an engineering office when we designed the taxiway bridge at Nashville's (BNA) over Donnelson Pike. The static load of a parked, or stopped, plane is significantly less than the design load which includes an additional "impact" load for the moving plane. The "No Parking" may have been more related to taxiway traffic patterns than bridge loads.
Likewise, the parked load of a large engine on the bridge is much less than design load for the moving locomotive.
Ray
As long as the bridge is rated for the load, there is no problem having two trains on a bridge at the same time. The dispatcher would never know where the train or engine was with regard to the bridge unless there was some drastic restriction and the signal system had the ability to hold a train off the affected area. The only way to regulate that would be to have some sort of special instruction.
I do know of some old 2 track bridges that would not hold the weight of a modern train on both tracks so the CTC system was set up to be able to stop a train short of the bridge to allow only one train at a time over the bridge.
One of those bridges was replaced with a modern bridge and the restriction was lifted, trains operate at will on either track.
Dave H. Painted side goes up. My website : wnbranch.com
As Ray points out about the airplanes, the dynamic load of a moving vehicle is much greater than its static load sitting still.
Even the floors in your house are designed taking that into account.
When new, most bridges are designed for loads somewhere between 5 and 10 times the expected loads, sometimes more.
This is done knowing that the structure will deteriorate, and that use may change in the form of higher loads.
The dynamic loads created by steam locomotives are pretty high, so I'm sure the Keddie wye can handle any parked locomotive.
By virtue of how much the steel rail can hold, railroad equipment is limited to about 80,000 lbs per axle. This spreads the load out pretty well on the average articulated loco from the golden age of big steam in the 30's and 40's.
In fact, the loco in question, a Western Pacific Challenger, only has an actual axle loading of about 60,000 lbs per axle, 10 axles total, 590,000 lbs, spread over a wheelbase length of 60'.
That is about 2,000 lbs per sq ft on the deck of the bridge. Not a lot for a steel bridge of that type. I don't have formulas handy, but a guess would venture that the dynamic load of the moving steam locmotive is at least 3-5 times that.
And the bridge was likely designed for 10 times the dynamic load.
Other important fact - bridges are designed to move/flex. As long as they move as designed, that is a good thing. It means they are absorbing the dynamic loads safely.
But, if structures are compromised by age or repair methods, those dynamic loads can become harmonic loads that are destructive. Yes they do sometomes fail, and yes they are sometimes incorrectly designed, but overall it is rare.
The parked Challenger is perfectly safe.......
Sheldon
Could a steam engine on a bridge drop hot ash onto the ties and start a fire?
Thanks for the replies, so much for trying to be intuitive.
I recalled the airplane sign, it was at O'Hare. "aircraft, do not stop on bridge"
For anyone interested, here aer a couple people's thoughts.
https://www.quora.com/Why-there-is-notice-aircraft-do-not-stop-on-bridge-at-OHare-airport-runway-in-Chicago
MidlandMike Could a steam engine on a bridge drop hot ash onto the ties and start a fire?
Not dramaticly more so than when moving.
Seems to me some / many bridges have speed restrictions, so I would think a large engine going across a bridge would be more of a stress on the bridge than the engine sitting still on the bridge?
wjstixSeems to me some / many bridges have speed restrictions, so I would think a large engine going across a bridge would be more of a stress on the bridge than the engine sitting still on the bridge?
Correct a moving train has dynamic loads while an engine just sitting there has static loads. As the wheels push the engine forward, they also push on the bridge backwards. As the engine rolls across the loading changes and the forces on the bridge change as the wheels roll and change position on the bridge.
ATLANTIC CENTRAL MidlandMike Could a steam engine on a bridge drop hot ash onto the ties and start a fire? Not dramaticly more so than when moving. Sheldon
But stoping increases exposure time.
MidlandMike ATLANTIC CENTRAL MidlandMike Could a steam engine on a bridge drop hot ash onto the ties and start a fire? Not dramaticly more so than when moving. Sheldon But stoping increases exposure time.
Rolling makes a rolling wave of compression under and immediately in front of the locomotive, and just to its rear. Then come all those 80-100 ton cars in succession along the same route.
Rolling on a curved bridge imparts lateral forces at the same time.
selector MidlandMike ATLANTIC CENTRAL MidlandMike Could a steam engine on a bridge drop hot ash onto the ties and start a fire? Not dramaticly more so than when moving. Sheldon But stoping increases exposure time. Rolling makes a rolling wave of compression under and immediately in front of the locomotive, and just to its rear. Then come all those 80-100 ton cars in succession along the same route. Rolling on a curved bridge imparts lateral forces at the same time.
I realize that, but I'm not sure what that has to do with hot ash.
MidlandMikeCould a steam engine on a bridge drop hot ash onto the ties and start a fire?
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."
My understanding is that there are openings above the ash pans for combustion air intake.
With the train stopped, how much updraft over the ashpan would there be without the exaust steam draw?
MidlandMikeWith the train stopped, how much updraft over the ashpan would there be without the exaust steam draw?
Negligible. The fireman would determine just how much draft he needed by using the blower. Usually just enough to keep the heat and smoke from coming out the firedoor if the stop was going to be a while, say more then fifteen minutes.
If he needed to trim the fire or raise steam pressure by adding coal the blower would be increased proportionally.
Regards, Ed
Track fiddlerI do believe railroad bridge engineers design them to not only withstand but exceed the heaviest possible load bearing situation they could be subjected to. There's a saying that floats around out there that the railroad over builds everything and probably holds a lot of truth to it.
When the new Sunshine Skyway bridge was built in Saint Petersburg, I remember an interview with one of the engineers. I can't quote it exactly, but he said something like this.
The bridge is not designed to carry the maximum load of traffic on the bridge. The bridge is designed so that if there is a convention of heavy-haul truckers in Tampa, and they all go for a drive across the bridge, and then they all break down on the bridge, and then the bridge is hit by a tornado and a category 5 hurricane simultaneously while a cargo ship slams into it, it will not fail.
Pretty bold.
And now the bridge is obsolete and must be replaced, not even 20% of the way through its engineered lifespan.
Track fiddlerI remember driving over that bridge while they were reconstructing the deck and when the semis and cement trucks were stopping in traffic I'd feel it move.
The scariest highway bridge I have experienced is the Jonathan Moore Pike bridge on Interstate 65 in Columbus, Indiana.
It is supported by cantilever from a central steel arch. If you cross the bridge as a Semi enters from the other direction you can feel your car raise up and come back down. It is very uncomfortable.
-Kevin
Living the dream.
That bridge is a trip Kevin! That is the most elite example of cantilevered balanced beams I've ever seen in bridge design.
That engineer is definitely not your run of the mill type of human being but it's probable he's border lining the threshold equivalence of a mad scientist
At the very least beyond his days of proving you CAN put Square pegs through Round holes..... I think I like that guy