How are rail cars load rated and what does that load rating mean when rail cars are used as bridges? 4

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The rail cars' rated capacity of 150,000# (150 kips) per each is about twice the GVW of any street legal semi-truck (72,000# to 80,000#). The capacity of the rail car is probably calculated for a relatively uniform load spread over the floor area of the car. Vehicles crossing your rail car bridge will not be uniform loads but, instead will be a series of axle loads moving along the rail car.

Quick Simplified Analysis
Assume 40' span between axles on the rail car. Assume a total car length of 60'. Assume each of the side by side rail cars carries one lane of traffic.
150 kips / 60' = 2.5 klf uniform load. Distributed load Moment < 0.125 x 2.5 x 40 x 40 = 500 ft-kips

For a moving 72,000# truck with 14' axle spacings crossing a 40' span, M = +450 ft-kips which is a little less critical than the moment from a uniform load.

The actual rail car length and axle spacing (span length) may be different than I assumed but I would expect the results to be similar for other reasonable dimensions.

PEinc...Thanks for the quick analysis....Lets say it is a cement truck with most of the weight over the rear axles. I assume in this case your analysis would say the bridge is overloaded?
Are you pretty sure about the uniform loading assumption for rating the capacity? This makes a big difference in how we might look at this.

Thanks again for your thoughts.

cool47 said:

...sure about the uniform loading assumption for rating the capacity?

Click to expand...

It has to be uniform loading (UDL). All of a rail car's axles are assumed to be equally loaded using Cooper loading assumptions. Only two ways to get all axles equal with a single load:
UDL or point load at the center of the rail car. A single point load is not realistic, consider load distribution in common rail cars such as those for transporting coal.

My mental picture from the OP description is a couple of old TOFC (trailer on flatcar) cars. They would have been around 89’ overall, don’t know the bolster spacing. Back in the day they’d load those by backing a tractor and trailer down a string of cars, secure it, drive the tractor off and repeat. Each car could have multiple trailers driven across it and then two trailers parked on it. If the abutments are under the bolsters the car isn’t seeing much it wasn’t originally designed for. Whether it still has its original design strength or not would be a different question.

Thank you all for the information and insights. It has been very helpful. I will go forward with the assumption that the loading was assumed to be uniformly distributed.

If only it was that simple. Unfortunately, load rating a bridge is not that easy. Truck wheel loads are not fully distributed across all the support beams under the rail car. The wheel fraction needs to be calculated for each support line (there were 3 on the rail car bridge rating I just completed), and the support lines typically have different capacities.

There's also the not so small matter of the beams typically being tapered to a shallower depth toward the toward the bolsters, which decreases the capacity more quickly than the moments decrease. The critical capacity of the rail car I rated was located in the tapered region. You'll need either a girder line analysis program, have to generate influence lines, or conservatively assume a stepped transition to the shallower section, to calculate the capacity.

Cool47, a cement concrete truck is a street legal vehicle with weight and axle loads less than what I used in my above, quickie analysis. A moving truck (i.e., a truck to move people's furniture and possessions) would likely be less critical than a concrete truck. As davidbeach pointed out, rail cars are often used to transport heavy vehicles, including military and construction vehicles and equipment. I can't imagine this residential property would need a bridge to carry more than a concrete or fire truck. Check the condition of the rail cars.

PEinc, what a railcar can carry with the axles supporting it, is considerably more than what it will carry supported at the ends. Yes, for residential vehicle traffic it would likely do fine, but the OP asked about load rating it, which requires an assessment of it's capacity in it's current situation, under loading that may or may not be centered on the car. Remember, there are 2 railcars; a truck crossing the bridge may have one wheel line (one side of multiple axles) in any position transversely on the bridge, including with one wheel line near the edge of deck, relying mostly on the smaller exterior stringer.

As I said before, there are no quick and easy ways to get an accurate estimate of the load carrying capacity of a railroad car used as a bridge. It must be analyzed based on the sizes and configurations of the stringers, the support conditions, and the distribution of the wheel loads to the individual stringers.

HotRod10, yes, the rail car with axles can support more than a rail cars without axles and supported only at the car's two ends. However, we do not know any car dimensions, span lengths, number of lanes, etc. I was strictly comparing the bending moment due to the weight of a distributed, uniform load (on an assumed rail car length and bridge span length) to the moment on the same span length due to street legal trucks. The original question concerned distributed loading vs. point loading (which I assumed was meant to represent axle loads). Nothing was asked about a bridge rating. There was not enough information to give an exact answer for this particular bridge in an assumed residential area.

PEinc, I was adding a caution regarding your comment "rail cars are often used to transport heavy vehicles, including military and construction vehicles and equipment. I can't imagine this residential property would need a bridge to carry more than a concrete or fire truck." What a rail car can carry in its configuration as a rail car is not an indication of what it can carry as a bridge, for the reasons I mentioned above.

Also, load rating was mentioned (twice) in the title of the thread.

3DDave, that a popular one around here...

cool47,
From GOOGLE:
Plain Flatcar
Length of Car 60 and 89 ft.
Gross Weight on Rail Range from 250,000 to 286,000 lbs.
Load Limit Range from 147,000 to 202,000 lbs.
UP: Flatcars
___________________________________________________________
Specialized Flat
Commodities: Tractors, military vehicles, steel plate, steel beams
Feature Attribute
4 Channels Channels run the length of the car
Chain Tie Down Devices Chain tie down devices run along the channels on both sides of the car
Wood or Steel Floors Nail blocking and bracing
Length of Car 89 or 60 ft.
Gross Weight on Rail Range from 263,000 to 286,000 lbs.
Load Limit Range from 145,000 to 160,000 lbs.

I just coincidentally came across it while reading an article on the PPRUNE site about why the back-logged 737s are being outfitted with cinder blocks (or some such) instead of engines. Pretty funny for anyone not on the production management or sales teams.

PEinc, that's interesting info on a few modern railcars, but railcar bridges can utilize many different configurations from the last hundred years. The one I rated last month had support beams built up from plates and angles and was made in the 1920s (presumably from steel with a yield strength of 30ksi). It rated less than 13 tons, which means it would likely not carry a typical concrete truck carrying more than 5 or 6 yards of concrete.

I'm not trying to be difficult, but making assumptions about the capacity of a particular railcar based on general information, or making assumptions about the load distribution, is an ugly failure in the making.

All...I should provide some context for my initial question. The rail car bridges have been in place 20 plus years. They have had loads cross them estimated to be 80,000 lbf many times over that 20 years. Mostly concrete trucks. They may have been load rated but if so I am unaware of it. My goal is to understand how close we are (if at all) to failure.
How big an effort would it be to load rate these bridges?

"How big an effort would it be to load rate these bridges?"

To do it accurately, it will be a significant undertaking.

1) Measure the support beams (height, thickness, etc.) to determine the section modulus of beams. If the section modulus changes, calculate it at all critical points (i.e. beginning and end of tapered sections)

2) Determine the wheel fractions (WF) (how much of each wheel load could potentially be carried by each support beam) for each support beam. AASHTO has tables for wheel fractions in the Standard Specifications.

3) Apply the load equivalent to the portion of the wheel loads of the rating truck chosen (for HS20 truck, 16kip * WF for trailer and drive axles and 8kip * WF for front axle location) at the presumed axle spacings for the design truck (for the HS20 on simple span, 14' between axles) and move the truck across the bridge to find the maximum live load moments at the critical locations.

4) Calculate the moment capacity of each beam at the critical section using the assumed or known yield stress of the steel.

5) Subtract the dead load moment from the moment capacity to get the moment capacity available for live load. Divide the result by the moment produced by the design truck. This is the nominal rating factor. If the result is greater than one, the bridge can theoretically carry a truck heavier than the design truck (by a factor equal to the rating factor, i.e. a rating factor of 1.2 means it can carry a truck of the same configuration 20% heavier than the design truck). Again, theoretically, with no margin of safety. Typically, rating calculations include load factors to account for unknowns.

"what a railcar can carry with the axles supporting it, is considerably more than what it will carry supported at the ends"

Remember that, when configured as a rail car, the car body is supported in two places: the center pin (bolster) of each truck (assuming it's a 4 axle car).