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

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

(OP)

I am part of community that has a private bridge that consists of two flat bed rail cars suspended from concrete abutments. The side of each car has a capacity of 150,000 lbf given on the side of the car. My question relates to the assumptions that were used to calculate this capacity. Was a uniform load assumed? Was some version of point loading assumed?

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

https://www.eng-tips.com/viewthread.cfm?qid=402999

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

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

Quick Simplified AnalysisAssume 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-kipsFor 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.

www.PeirceEngineering.com

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

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.

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.

www.SlideRuleEra.net

www.VacuumTubeEra.net

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. Asdavidbeachpointed 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.www.PeirceEngineering.com

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.

www.PeirceEngineering.com

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

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

https://www.up.com/customers/all/equipment/descrip...

___________________________________________________________

Specialized Flat

Commodities: Tractors, military vehicles, steel plate, steel beamsFeature 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.

www.PeirceEngineering.com

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.

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

nominalrating 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.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).

What else is on the side of the car? That might help identify what the cars where, and the useful dimensions thereof. The model railroad community has a huge assortment of useful information.

cool47- See the 141 page report "Use of Railroad Flat Cars for Low-Volume Road Bridges" prepared for the Iowa DOT. Here are the conclusions:www.SlideRuleEra.net

www.VacuumTubeEra.net

www.PeirceEngineering.com

...suggest reading the entire document...

www.SlideRuleEra.net

www.VacuumTubeEra.net