Eastern Railroad Discussion > Train weight on bridges

I was thinking about the I-35W bridge collapse, and the continuing presence of Starucca Viaduct (c. 1845), Rockville Trestle (c 1900) etc.

Penny's Q-2 loco and tender weighed in at more than a million pounds, and the T-1 and J-1 were close behind. The C&O M-1 articulated came in at 1.2 million pounds. Add in a few 120 ton hopper cars and the weight would be far greater than any 18 wheeler or 10 of them on a bridge.

I would guess (but don't know) that the jointed rail would also put up a harmonic vibration that could do some damage, but Starucca, Nicholson, etc are still doing fine, 100 pus years after they went to work...

Stone bridges tend to last much longer...no girders to rust.

PittsburghMike Wrote:
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> Stone bridges tend to last much longer...no
> girders to rust.

Those old stone bridges such as Starucca were built as a series of arches. The stones in an arch are all in compression, making the bridge strong, since tensile forces are required to cause fracture. Ever noticed that some Roman aqueducts are still in use after 2000 years, as are many British Rail stone/brick bridges after well over a century? The life of any mortar between the stones/bricks pretty much determines the life of the bridge. However, corrosive effects of air pollution, combined with freeze/thaw cycles, can cause chemical decomposition of the mortar. And, under extremely corrosive conditions the stone itself can chemically degrade, leading to eventual crumbling of the stones.

no matter how heavy the old steam engines the weight on each axle was mostly below 20 ton per axle.
so yes they were a milliom pound but on 14 to 20 axles
Todays locomotie has a much heavier foot print with 436 000 lbs on 6 axles with axle loads of 33 ton per axle.

those must be really old steam engines you are talking about, as modern steamers such as N&W 611 and later Santa fe Northerns carried nearly 80,000 lbs per driving axle, C&O alleghenies were even higher at 86,000 lbs. Fact is though that 2 modern diesel engines weighing 450,000 lbs each, with a combined length of 150 feet would still not weigh more than a big boy at 1,000,000 lbs at 132' in length.

Your 39 ton Axle load would not even be suported by todays 141 Lbs per yard rail.
up till a few years ago max axle load was 25 for cars and 28 for engines.
Don't forget to spread the engine weight on steam engines over all axles including idlers and tender axles.

> till a few years ago max axle load was
> 25 [tons] for cars and 28 for engines.

"A few years" meaning ... a hundred or so?

> with 436 000 lbs on 6 axles with axle loads of 33
> ton per axle.

So, metric tons he apparently means ...

Well thats the standard in rest of world Axle load on rail is normaly given in Metric tons.
it is in Janes world railways anyway.

As an aside, axle limits and axle loading are not the same thing... axle loading is used to denote the max axle load a rail structure (be it a bridge, right-of-way, etc) can take, in the US, it's ~ 40 tons. Axle limit is the amount of weight an axle can apply to the rail, which in the end cannot exceed axle loading. Axle limits are determined by bearing size and wheel diameter.

For freight cars, there are seven AAR axle and bearing sizes in use... note all new 286K cars are required to use Class G, K or M axles. Class L is the new replacement for Class E, Class K is the new replacment for Class F, and Class M is the new replacement for Class G (though as noted, not required). All the newer axles/bearings are shorter to reduce flexing under load.

D 5.5” x 10” 44250 lbs per axle
E 6” x 11” 55000 lbs per axle
F 6.5” x 12” 66000 - 71500 lbs per axle
G 7” x 12” 78750 lbs per axle
K 6.5” x 9” 71500 lbs per axle
L 6” x 8” 55000lbs per axle
M 7” x 9” 78750 lbs per axle

Class B, 4.25" x 8" (26000 lbs)and Class C, 5" x 9" (35500 lbs) are older outdated classes, there was a Class A, but I don't know it's dimensions, just that it had a 16500 lb capacity back before 1961.


Freight car wheel sizes and their respective maximum capacities per axle:

28” 48750 lbs
33” 55000 lbs
36” 65750 lbs
38” 78750 lbs

Locomotives typically use Class GG bearings and 40" or larger wheels, and currently have a max capacity of 72000 lbs per axle.

One factor not yet mentioned is lateral loading on the bridge. With their very high TE ratings bridges can be damaged just by a locomotive pulling to hard on its train. The SL-SF bridge at Memphis had severe restrictions on power application with the arrival of SD45s, let alone more modern power.

Robbman,

I liked your summary and description of axle and wheel loads, I printed it for reference. In my RR career days I had to deal with this matter myself, not as an engineer becauise I am not, but as a liason on a major railroad trying to bring the matter of incresed axle loads to an understanding between operating and marketing types, and often having to explain the developemnt of heavier axle loads. My mentor was mostly Mike Pavlick of the then Transit America Co. Most railroads permit small overloads, beyond which there are severe tariff penalties to shippers who over load cars. This was due to the belief that the standards had some lattitude and that most scales were not perfectly accurate. Therefore, it makes me wonder why the 286,000GWR standard for the proper axles and wheels on a four axle car is 71500 pounds. Whay not make it an even 72000 lbs per axle (36 US tons)and 288.000 GWR for the car. It is such an even number to use. the old 263000 GWR for 100 ton cars was such an uneven number it drove layman (shippers and RR marketeers) crazy. that should ahve been 66,000 lbs per axle.

Do you have any clues as to why the AAR arrives at such uneven standards?

Okay, this thread was titled "train weight on bridges", not "train axle loads". Robbman did post an excellent summary of requirements for various axle loads. But for bridge rating, axle loads are only one part of the story. The *spacing* of the axles is also important. 35-foot ore jennies impose a more concentrated load than longer cars with the same axle load.

The industry standard is Cooper's E-rating, which really does (still) look at the load imposed by two coupled Mikado-type steam locomotives followed by a train of 27-ton axle load cars. One of the reasons there are so many old bridges still carrying mainline loads is that Cooper's load of two coupled steam locomotives was pretty severe. As noted in earlier threads, their drivers imposed heavier loads, more closely spaced, than modern diesels.

In any event, the more or less standard rating for modern bridge design is Cooper E-80, which will handle 39-ton axle loads without difficulty. But there are still some E-60 and E-65 bridges out there. All railroads keep careful track of such things, and I've reviewed bridge ratings as part of several studies of the economic viability of increased axle loads. In fact, I'll take partial credit for moving the industry from a 263,000 lb. to a 286,000 lb. standard.

And by the way, 263K ("100 ton") cars have been in common use since the late 1960s. Before that time, a 210,000 lb. weight limit was usual ("70 ton"). The reason I put quotes around the tonnage is that it is *net* capacity. A hopper design of the 1970s weighed around 60,000 lbs. and thus could carry 101.5 tons of whatever. The new 286K cars are lighter relative to their carrying capacity. Some new aluminum coal gon designs weigh only 42,000 lbs., meaning they can carry 122 tons of lading -- which is why the industry has moved to 286K.

Why not go higher? Well, as Robbman notes, you need bigger wheels and bearings, larger axles, higher capacity brakes, etc. While a 286K car can use essentially the same components as a 263K car (with some upgrades) the heavier components actually result in both a higher price per unit of capacity and a poorer net-to-tare ratio for a 315K car than a 286K car. When you couple that with an approximate 40% increase in track maintenance cost vs. 263K (the 286K cost increase is less than 20%), it becomes obvious why you only see 39-ton axles on stack cars (and they aren't often loaded to that level, either).

rresor Wrote:
> ...the load imposed by
> two coupled Mikado-type steam
> locomotives followed by ...

Consolidations, isn't it? 2-8-0s?

rresor Wrote:
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> Okay, this thread was titled "train weight on
> bridges", not "train axle loads". Robbman did
> post an excellent summary of requirements for
> various axle loads. But for bridge rating, axle
> loads are only one part of the story. The
> *spacing* of the axles is also important.



I also forgot to mention truck (bogie) axle spacing, which goes along with rresors write-up on axle spacing.

For two-axle trucks:

5' 6" (220001 lbs - 263K)
5' 8" (286K)
5' 10" (286001 lbs to 315K)


AAR raised axle limits on all axles in 1963, for Class F bearings, loading went from 251K to 263K, which then allowed most cars to carry 100T.


tp117, I suspect much of the reason for the odd loadings is a result of the bearing itself. These bearings are used in a variety of industrial applications, not just railroads. Search Timken AP Bearing or similar and see what comes up...

Robbman Wrote:
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> I also forgot to mention truck (bogie) axle
> spacing, which goes along with rresors write-up on
> axle spacing.
>
> For two-axle trucks:
>
> 5' 6" (220001 lbs - 263K)
> 5' 8" (286K)
> 5' 10" (286001 lbs to 315K)

You sure about that? I was under the impression that 100 ton cars (not necessarily
>286K) used 70" spacing. In fact, on many CSX cars, you'll see the number 70 stenciled
on 100 ton sideframes. 100 ton trucks also have larger bearings blocks that the roller
bearing rides on, 6"x12" than a 70 ton truck (5.5"x11" I think)

rresor says it well, but other factors also come into play. In addition to the standard Cooper loading, that specification for bridge design also imposed a very high impact factor that was associated with the piston action of a steam locomotive. Cooper E-60 was the base standard. Two locomotives with 60,000 lbs on the drive axles of a a double header followed by a uniform loading of 6,000 pounds per track foot. E-80 and even E-100 have been used in some designs. These simply scale up the E-60 loading pattern. Impact loads are added for superstructures as a percentage of the basic loading. As the members are sized things get scaled up to the next largest size (plate thickness, rolled member size, etc.). Then the connections are designed based on the capacity of the members being connected, not the lesser member design load.

Another factor is the bridge itself. Key is the length of the spans. Most Railroad bridges are simple spans. The length of span limits how much of the total train load is on that bridge element. The position of the load also effects how the bridge responds. A very short bridge may be most effected by the wheel loads from one end each of two coupled cars. A long span bridge may have no problem with one very heavy car in the middle of an otherwise light train, But the shorter local members of that bridge would need to be looked at carefully.

This is why every railroad has a special desk usually in the bridge department that evaluates and aids in routing all High, Wide and Heavy loads. This routing is based on a "rating" of the bridge in its present condition rather than the loads it was originally designed to carry. Very often the rating is much higher than the design. Thank the impact factor for some of that.

Ron Zimmer