Retired Rail Car County Road Bridge

[Victrola1]

IOWA CITY — About midway between Iowa City and West Branch on Lower West Branch Road SE, just south of Interstate 80, a crew of bridge contractors carefully lowered three large sections of steel and wood into place.

Spanning Wapsinonoc Creek, the bridge is not a traditional structure of timber, steel or concrete, but rather three retired railcars placed side-by-side.....

“They’re stout enough to carry a couple of tanks on them, they’re not going to have any trouble at all carrying ordinary traffic on a gravel road in rural Iowa,” Bartels said. “We’re getting a tremendously good bridge out of this.”

http://www.thegazette.com/subject/news/government/a-new-kind-of-bridge-for-johnson-county-20170526

How well do such bridges stand up to overloaded grain trucks over time? 

[DSchmitt]

Legal US truck 40 tons

Max loaded grain truck typical 35 tons

Tank 60- 70 tons

Flat car load capacity 71-105 tons (limited by trucks, not the strength of the body) A 1999 study by the Iowa DOT concluded that not all flatcar designs are suitable for use as bridges, but flatcars of suitable design could have a capacity of close to 90 tons. They tested real in place bridges. A flatcar designed to haul tanks is probably a suitable design.

[PNWRMNM]

How well? Probaby well and long, BUT rail cars are supported at the trucks, NOT on the end as these car bodies will be as a bridge. Look at the picture and note the statement in the text about no intermediate support. The practical result is that the ends will be loaded far more heavily as a bridge of this configuration than they would be as a rail car or as a bridge supported at the car bolster. I hope they had a structural engineer run some calculations.

Mac

[DSchmitt]

Caltrans (California DOT) energency brigde system (89' flat cars) which has been used on freeways was tested with a static load of more than 123 tons

A 2007 Iowa DOT study concluded

"Single span RRFC bridges composed of two or three RRFCs are an economical solution to bridge replacement if the longitudinal connections and bridge span length are correctly engineered. • The longitudinal reinforced concrete beams connecting adjacent single span RRFCs, along with transverse timber planks on the bridge deck, effectively transfer the live load forces transversely across the bridge. • Single span RRFC bridges with little or no overhang at the abutments should be restricted to clear span lengths of less than 66 ft. Increasing the clear span lengths would increase the total maximum stresses beyond the allowable steel stress. • Multiple span bridges constructed using the 89-foot RRFCs are effective replacement bridges for use on low-volume roads where longer span lengths are required. • Strains and defl ections of the multiple span bridges tested in conjunction with this project were below the allowable limits as set forth by the AASHTO. • Preliminary investigation into the use of sheet pile abutments for multiple span RRFC bridges indicates that the use of such abutments is a viable alternative for low-volume roads for a compatible superstructure."

[mudchicken]

Have seen one flatcar become a turntable, two ex-DODX flats supporting gravel and cement trucks, and multiple applications of 89' TOFC cars...As long as you have decent approaches and low speed to minimize impact damage, the cars would be an improvement over what passes for many county road bridges.

[ACY Tom]

The article says the new bridge will have no weight limit. I'm no engineer, but this seems strange, especially considering the earlier observation that this bridge appears to be supported at its ends, and railroad cars are designed to be supported at the bolsters. 

Tom

[tree68]

ACY

The article says the new bridge will have no weight limit. I'm no engineer, but this seems strange, especially considering the earlier observation that this bridge appears to be supported at its ends, and railroad cars are designed to be supported at the bolsters. 

I would opine that the designation means that any truck legal on the road would be able to cross the bridge as opposed to an infinite weight limit.

We all cross bridges daily with no posted weight limit.   I'm sure they all have an actual weight limit, but it's a good deal more that they should have to carry with daily traffic.

 

[Paul_D_North_Jr]

A full analysis is too long for this space, but a couple observations can be made:  

• A load applied outwards of the bolster - between it and the coupler - will cause and be governed mainly by vertical shear stress in that segment, not bending.  The frame extension for the draft gear and coupler is usually massive enough to accept that.  
• Cars designed to have loads drive over that segment when being circus-loaded - i.e., the DODX cars, which would have tanks running over them - might be OK, too.  One thing that would have to be checked is load being applied in the reverse direction, and hence the stresses being reversed - i.e., the normal load would bend the outer ends down more than at the bolster, whereas in this configuration the bolster would be lower.  As such, the bending stress at the bolster would change from mainly tension in the top and compression in the bottom, to the opposite.  Some car structures might not have enough inherent strength or details to accomodate that.
• Depending on the distances between the tandem axles of the trucks being driven over the flatcar, there would likely be only one 20-ton set near the middle of the car at a time, whereas the car is likely designed to take loads in the 100 to 130 ton range.  Also, the impact factor - soft rubber tires vs. hard solid steel wheels - would be much more favorable.   
• In general, I'd be real surprised if a flatcar in good condition couldn't be repurposed - perhaps with some minor modifications, esp. at the end bearing points - to serve as a highway bridge.  They're just so darn heavily built and massive for much larger loads.        

- PDN. 

[cx500]

And with three flat cars side by side, I would expect the two road lanes will straddle the cars so each is only handling half the load most of the time.  There will, however, be some additional dead load as a more normal driving surface and handrails get added.

[Paul_D_North_Jr]

Also highly unlikely that the lane in each direction will both be occupied by a truck at the same time - so there'll be some reserve capacity from that as well.  Perhaps surprisingly to the non-engineers here, rarely is the loading condition based on a maximum load in each and every lane at the most stressed location.  Typically, there's a % reduction in the 2nd (and additional) loads, usually in the range of 20% to 30%. 

- PDN. 

[BaltACD]

Bridges involved in this story are not repurposed rail cars - they are full blow Interstate bridges on I-70.

When I was working, heading in a night I would see Westbound excessive dimension loads from the Port of Baltimore with State Police escorts - when they came to exits, the loads and their escorts would take the exit, depart the interstate and immediately take the on ramp to reenter the interstate.

I don't know if this was being done for traffic mitigation purposes or if the bridges lacked the structural integrety for the loads, but I observed this multiple times over my last 8 years of working.

[Paul_D_North_Jr]

You got it  - structural integrity (more accurately, capacity) is the likely reason. 

Likely there were also bridges on some of the ramps, but usually much shorter, so not as much of the load would be on them at a time.  Also, most such loads have an 'internal bridge' - the frame of the item or the trailer that's carrying it - which separates the loading points to achieve a wider spacing between, which helps reduce the stresses on the longer bridges especially. 

- PDN. 

[MidlandMike]

Could it be that the loads were too high to fit under the underpasses, and too wide to clear the side railings on overpasses?

[BaltACD]

MidlandMike

Could it be that the loads were too high to fit under the underpasses, and too wide to clear the side railings on overpasses?

Where I am talking about the Interstate has no height or width restrictions - the Interstate goes over the 4 divided lanes of the surface roadway.  The Interstate bridges have about a 10 foot passenger side shoulder and a 5 foot driver side shoulder in addition to the normal width travel lanes and the Interstate is 3 travel lanes in each direction.

[LNER4472]

BaltACD

Bridges involved in this story are not repurposed rail cars - they are full blow Interstate bridges on I-70.

Dunno what story you're discussing, but the illustrations in the OP's linked article CLEARLY show repurposed railroad car frames:

[BaltACD]

LNER4472

BaltACD

Bridges involved in this story are not repurposed rail cars - they are full blown Interstate bridges on I-70.

Dunno what story you're discussing, but the illustrations in the OP's linked article CLEARLY show repurposed railroad car frames:

 

Not talking about the OP's bridges at all as I stated.

[poneykeg]

Gosh my DAD back in the 50s bought a 50ft and a35ft flat bed frames from NKP in Frankfort, in had them trucked to the farm and they are still there over the open ditches but not used now because equipment has outgrown them in their width.