Effects of extra long trains on track ?

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Effects of extra long trains on track ?
Posted by blue streak 1 on Monday, July 09, 2018 10:32 PM

We have note that if we take a small piece of steel and bend it many times that it will heat up and finally break.  However if we do the same number of bends but just  a few bends at a time the steel will not break unless doing a lot more bends.  Although not often we have noted that there has been several derailments due to rail breaks under the rear part of the derailed train and not under leading locos that weigh more than most cars.   

If the structure under a track has several  adjaecent cross tie sags would the rail more likely break with the constant bends and returns ?  It might be temperature sensors on rails at sags would provide answers ?  Anyone know if research has been done ?  Would this accelerate metal fatigue over say 2 - 500 axel trains ? Maybe at Pueblo ?  A  1000 axel train certainly has a lot of bends of a track .    Are we off base ?

What about bridges ?

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Posted by erikem on Monday, July 09, 2018 11:41 PM

When you bend a piece of steel to the point of it taking a permanent set, you have exceeded the elastic limits of the steel. When this happens, the steel will typically work harden and when stretched or compressed too much will start cracking. Rail and bridges are almost never loaded past their elastic limit in normal operation, but fatigue is thought to be caused by progression of micro-cracks from loading near the elastic limit (also known as the yield limit).

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Posted by Paul_D_North_Jr on Tuesday, July 10, 2018 8:05 PM

Bridges definitely have a limit on how many loads of varying weight they can carry - governed by fatigue as you surmise.  It depends on their 'history'.  The formulas are pretty complex.  Heavy unit trains are usually the worst, depending on the axle loading and spacing, and where you're looking at in the bridge. 

Track is much more complicated, but what Erik says is a good summary.

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Posted by bratkinson on Wednesday, July 11, 2018 8:30 PM

This discussion on bridge fatigue issues reminded me of when I was a contractor, developing software at an internationally known crane manufacturing company about 1982 or so.  In particular, the program was for their 'smaller' line of overhead traveling cranes that ride on a set of rails maybe 40-50 feet off the ground with the operator in a cab on the travelling boom to which the motorized pully lifting equipment is attached.  We've all seen pictures of such cranes at locomotive manufacturing and/or repair facilities that pick up an entire locomotive or heavy component and move it somewhere.

In particular, I was provided specs by the engineering department of how to create the program.  The 'user' input to the program was data regarding the height, weight, and load limits of the rail system on which the crane was to travel.  The distance between the rails (gauge) was critical as the further apart they are, the less load that can be picked up unless stronger bridge steel between the rails is used...up to a limit. 

Of course, the most 'major' criteria in designing the crane, whether done via slide rule, calculators, or computers, is what's the maximum weight to be picked up, how far apart are the rails, and how fast does it need to travel down the rails.  The program would then calculate a range of values for steel thickness, length, trusses if needed, etc, and provide a materials requirement list for each quality of steel needed (tensile strength/hardness).  The engineers would 'price it out' for each steel combination and ultimately get the crane built and installed for the customer.

One of the anecdotal stories the engineers told me was about a customer that had one of their cranes in their factory for over 50 years and wanted new one just like the old one for their new factory.  Of course, there was nothing common between the cranes as steel quality/strengths had been improved, and there was nothing 'common' between dimensions, weights, etc between the old and new cranes.  The new factory opened up with the new crane, and about 6 months later, the engineers were called to find out why the new crane had a very noticible 'sag' in the traveling boom.  The engineers triple and quadruple checked their designs for the crane along with the weight/speed/height requirements from the customer and everything was more than sufficient for the 150% load carrying factor designed into the crane.

So, they travelled to the new factory to see what was going on.  It turned out that like the old factory, they used the crane to pick up still red hot 5-10 ton metal castings and give it a couple of 'shakes' by releasing the hoist brake and then slamming on the brakes just before the casting hit the ground to shake loose any left over slag on the casting...just like they always did at the old factory for over 50 years!  

The difference was that in the 1920s when the original crane was built, metalurgy technology was still very limited as was their knowledge of tensile strengths of steel, etc.  Everything was done by slide rule calculations back then, too.  So, back then, stuff was designed for a 200% overload factor, but as they didn't know the limitations of the steel being used, it turned out that the cranes of that era really had close to a 300-400% overload capability!

In effect, the 'shake' technique created a very momentary apparent weight of well over 2-3 times of the casting being picked up.  It's comparable to a head on car accident where the 'crush' stresses far exceed the design limits of the car and it crumbles.  The result of using 'new' technology, lighter steel could be used, etc, and still have a 50% overload safety margin.  Unfortunately, the 'old' technology had roughly a 300% overload safety margin, which was being 'entered' every day.  They didn't tell me who picked up the tab for a new crane...

The same is true with railroad and highway bridges.  When they were built in the late 1800s/early 1900s, the engineers unknowingly were building them with exceedingly large overload handling factors.  The Smithfield Street Bridge in Pittsburgh PA comes to mind.  Built in 1883 and widened twice (per Wikipedia), it still has 4 lanes of traffic plus light rail street cars crossing it every day.  The 'built in' overload factor is a major factor in its longevity.  Compare that to various interstate highway bridge replacements in the past 20 years or so that I'm aware of that portions of the new bridge fell down in less than 5 years!  Minimal overload design factors and/or steel quality issues caused those failures.

Fortunately, the railroad bridges of yore were designed to handle the steam locomotives of the day, and then some.  How many diesel locomotives of today come anywhere near the weight of a fully loaded UP Big Boy, or a pair of N&W Y6Bs double headed?  In my opinon, bridges that could handle those locomotives will still be around 100 years from now with reasonable maintenance.

 

 

 

 

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Posted by BaltACD on Wednesday, July 11, 2018 8:42 PM

bratkinson

One of the anecdotal stories the engineers told me was about a customer that had one of their cranes in their factory for over 50 years and wanted new one just like the old one for their new factory.  Of course, there was nothing common between the cranes as steel quality/strengths had been improved, and there was nothing 'common' between dimensions, weights, etc between the old and new cranes.  The new factory opened up with the new crane, and about 6 months later, the engineers were called to find out why the new crane had a very noticible 'sag' in the traveling boom.  The engineers triple and quadruple checked their designs for the crane along with the weight/speed/height requirements from the customer and everything was more than sufficient for the 150% load carrying factor designed into the crane.

So, they travelled to the new factory to see what was going on.  It turned out that like the old factory, they used the crane to pick up still red hot 5-10 ton metal castings and give it a couple of 'shakes' by releasing the hoist brake and then slamming it on before the casting hit the ground to shake loose any left over slag on the casting...just like they always did at the old factory for over 50 years!  

The difference was that in the 1920s when the original crane was built, metalurgy technology was still very limited as was their knowledge of tensile strengths of steel, etc.  Everything was done by slide rule calculations back then, too.  So, back then, stuff was designed for a 200% overload factor, but as they didn't know the limitations of the steel being used, it turned out that the cranes of that era really had close to a 300-400% overload capability!

In effect, the 'shake' technique created a very momentary apparent weight of well over 2-3 times of the casting being picked up.  It's comparable to a head on car accident where the 'crush' stresses far exceed the design limits of the car and it crumbles.  The result of using 'new' technology, lighter steel could be used, etc, and still have a 50% overload safety margin.  Unfortunately, the 'old' technology had roughly a 300% overload safety margin, which was being 'entered' every day.  They didn't tell me who picked up the tab for a new crane...

The same is true with railroad and highway bridges.  When they were built in the late 1800s/early 1900s, the engineers unknowingly were building them with exceedingly large overload handling factors.  The Smithfield Street Bridge in Pittsburgh PA comes to mind.  Built in 1883 and widened twice (per Wikipedia), it still has 4 lanes of traffic plus light rail street cars crossing it every day.  The 'built in' overload factor is a major factor in its longevity.  Compare that to various interstate highway bridge replacements in the past 20 years or so that I'm aware of that portions of the new bridge fell down in less than 5 years!  Minimal overload design factors and/or steel quality issues caused those failures.

Fortunately, the railroad bridges of yore were designed to handle the steam locomotives of the day, and then some.  How many diesel locomotives of today come anywhere near the weight of a fully loaded UP Big Boy, or a pair of N&W Y6Bs double headed?  In my opinon, bridges that could handle those locomotives will still be around 100 years from now with reasonable maintenance.

Part and parcel to engineering to price without understanding the reality of how the product is actually used.

Bridges constructed today will not have the longevity of those constructed 100+ years ago.  100 years ago bridges were constructed to withstand the recipricating impacts of steam engines - that coupled with the limited understanding of exact material strengths meant that 100 years ago bridges were constructed much stronger, for a margin of safety, than they are being constructed today.

         

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Posted by Paul_D_North_Jr on Sunday, July 15, 2018 10:08 AM

Tensile strength is the most important design component, but not the only one by far.  It always enlightening to young engineers when they see that a structure they designed is OK for strength but they forgot to check for deflection (sag) . . . Oops - Sign 

Railroad bridge design these days still uses the Cooper E-series loading system - a pair of 2-8-0s with tenders of various weights, followed by a continuous load of X thousand lbs. per linear foot.  Plus, an alternative load of like 4 ea. 100,000 lb. closely spaced axles - think of a heavy-load flatcar, or the ends of 2 ballasted C-C locomotives close together (I know, that's 6 axles, not 4, but it's the same principle - still 400K lbs. along about 18 ft. of track, if I remember right).  That, and a more sophisticated - and very complicated - approach to fatigue management.  So the greater 'robustness' is designed in these days, instead of happening as an unintentional by-product of lack of detailed knowledge.  Plus, earthquake resistance is now designed in as well.  

Notably, almost all railroad bridges are simple-span designs, not continuous.  That won't mean much to those are are not structural engineers, but it kind of guarantees that some corners can't be cut to save some steel or cost somewhere.  

- PDN.  

"This Fascinating Railroad Business" (title of 1943 book by Robert Selph Henry of the AAR)
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Posted by mudchicken on Sunday, July 15, 2018 11:06 AM

Back to the OP, simplifying things: It's not the length - it's the number of cycles compounded by the weight.

As PDN noted as well, it's darn complicated. And "foundation" is more than rails and bridges. Dozens and dozens of things have an effect on track modulus.

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Posted by tree68 on Sunday, July 15, 2018 12:20 PM

Thinking about this - the idea that the length of the trains is a factor (as opposed to cycles and weight) would seem to imply that the track can "recover" between trains.

Reading the posts of our resident experts leads me to believe that's not the case.

If you bend a paper clip a bunch of times, but stop before it breaks, lay it down for a while, then start bending it again, it'll break at the same number of flexes as if you hadn't stopped.

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Posted by RDG467 on Sunday, July 15, 2018 1:48 PM

Blue Streak, the TTCI in Pueblo uses the Facility for Accerated Service Testing (FAST) loops to evaluate track components for fatigue. www.aar.com. 

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Posted by erikem on Sunday, July 15, 2018 2:09 PM

tree68

If you bend a paper clip a bunch of times, but stop before it breaks, lay it down for a while, then start bending it again, it'll break at the same number of flexes as if you hadn't stopped.

The number of times that you can bend a paper clip can vary a huge amount between bending within the elastic limit and bending while exceeding the elastic limit. The elastic limit is the point where bending beyond results in a permanent change in shape. Other than the contact surface of the rail, I don't see anything in the track structure that will approach the elastic limit.

Keep in mind that "fatigue" usually applies to a wear-out mechanism for cyclic loading below the elastic limit. The theory is that such loading will generate micro-cracks that eventually lead to some cracks growing to the point that the piece fails.

As for the original question on length of trains causing problems, the most likely source of problems would be tractive forces from longer trains, with the worst case being an emergency braking application.

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Posted by VOLKER LANDWEHR on Sunday, July 15, 2018 3:06 PM

erikem
Keep in mind that "fatigue" usually applies to a wear-out mechanism for cyclic loading below the elastic limit. The theory is that such loading will generate micro-cracks that eventually lead to some cracks growing to the point that the piece fails.

The dimensioning for fatigue aims on preventing this scenario by limiting the allowable stresses. How much depends on e.g. number of load cycles, tension or compression or alternating.

When ties loose their footing and don't support the rails anymore the fatigue is accelerated. The stresses will most likely stay within the elastic limits but the higher maximal stresses will reduce the number of allowed load cycles.
Regards, Volker

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