If the standard for rail cars is 286,000 lbs, and most new locos are between 410,000-430,000lbs… Why doesn’t a string of locomotives overwiegh the railroad? Why can locomotives be over max weight? I know that they have 6 axles instead of 4, but by that logic why don’t we see more 6 axle railcars that are heavier? And the 6 axles still don’t solve loading problems on places like bridges…
Tooth,
Do the math. The rail car axle loading is 71,500#. The locos, at 420,000 pounds, are 71,000#, which is the identical axle loading.
There is little to no economic demand for cars of size and weight to require six alxle cars. That is why they are so rare.
Bridges are designed to “Cooper Loading” ratings expressed ENN where NN is a number. E65 is quite common. New construction for last 20 years or so has been E80. The number is basically the axle loading in thousands of pounds. The formula dates to about 1900 and is based on double headed 2-8-0 steam locomotives and their axle loading. This is a layman’s explanation, but is close enough for this purpose.
When cars started to gross out at 286,000 pounds the carriers asked the same question you are, “What about my E 65 and E55 bridges.” The bridge engineers pulled out their drawings and the old calculations, and ran their own figures and came to the conclusion that since older railroad bridges were massively over engineered with safey factors of 2.5 to 3, loading at E70 would not cause catastrophic failure, but could, perhaps, shorten the economic life of the bridges, particularly wooden trestles. These issues can be dealt with by routine inspection and maintenance, both of which the carriers do.
Mac
The locomotives are a little longer than the cars too, so for a 420,000 lb. 70-ft. long loco their average weight per ft. is in the range of 6,000 lbs. / ft., whereas a 286K car at 60’ is about 4,770 lbs. per ft. - not the 50 % more. Compare with a Cooper’s E-72 rating or standard, which is based on 7,200 lbs. per ft., so you can see there’s still a considerable margin.
Also depends on where in the bridge you’re looking at - the closer to the rail, the more sensitive the member is to the intensity of the individual axle loads. Further away in the main members and piers and foundations, not so much.
Also, the axle load limit x 4 or 6 axles ==> gross weight limit is based in part on effects on rail wear, not bridge loads.
- Paul North.
Paul, I recall you mentioning quite some time ago when this question came up that old bridges also had to contend with the forces generated by the side rod action on steam locomotives driving the whole train. Electric motors produce a continuous smooth force. This change from a pulsating to a smooth force also allowed an increase in total bridge capacity.
Bruce
In my library I have a a B&O manual from 1964 ‘Instructions Governing the Operation of Locomotives and Cars so far as Strength of Bridges is Concerned’; dated Jan. 1, 1964
The various line segments of each Division were identified for the maximum loads permitted. All Main Lines on the major routed were rated at 320…lines of secondary significance in many cases were rated for less capacity down to as low as 160 on some industrial spur tracks.
Heaviest locomotives on the roster at the time were some SD-9’s with a actual weight of 389,000 pounds - and their bridge rating in this manual was 230. The passenger E units with actual weights in the 315,000 range were rated at 180. Budd RDC cars were rated at 130. The GE 44 ton engines were rated at 110 - the lowest rating of any of the listed motive power.
Wrecking Cranes are a listed category - the heaviest of the cranes with a listed weight of 357,000 had a bridge rating of 290. Surprisingly, the 80,000 pound Scale Test cars are rated at 160.
I have no idea of the engineering principles that were followed in constructing these ratings, however, it does have the appearance of corresponding to how long and how many axles the actual weight gets distributed over.