real world considerations for model bridges

In the short time since I’ve joined these forums, I received an outpouring of support and help from those of you out there. I love reading all of the threads but rarely ever feel like I would anything positive to add to the discussion, as I’m learning all of this myself still. That said I still want to contribute what I can. So here it is! After finishing school I went to work for a design firm where I’ve had the opportunity to design numerous bridges, and many of them over railroads. Thus I thought I might provide some incite on what really happens for those you who want to model realistic bridges, but are not using photo of a specific prototype.

The basics
For starters, I acknowledge that many of the things that will be mentioned here, conflict with what most people will want to include in a layout. To follow every bit of information I proved you will have to be very dedicated to “making it real”.
-Bridges, either supporting the track or a roadway above, are almost always built with construction costs as the driving factor. So what materials you have available, local construction practice, and complete lack of concern for aesthetics are usually driving factors. That’s right, pretty to look at usually costs money (there are exceptions with some trusses), and we all know the mighty dollar has a way of dictating the way things pan out.
- Railroads have a way of doing things and they generally will not budge. They want to see X, Y and Z and they will not accept anything other than X, Y and Z. Railroads have prior right to most land they own, when they abandon a track they almost never relinquish the land, even if the track is completely removed.

Track Geometry, Stationing and Alignment.
-Although bridges look good on a layout, they are not used in real life unless absolutely needed. You will not see a river crossing then another a few hundred yards downstream unless the track had a VERY specific reason for going there. The curved track, additional land or other factors are usually cheaper then the construction of 2 river crossings. Even though railroads are experts at acquiring and retaining land (and are occasionally considered bullies), even they can’t get around the environmental impact studies and the related governmental review processes associated with them. These things traditionally do not go swiftly.
-The shortest distance between two points is a straight line, and bridges are no exception. The goal is generally to cross an obstacle as quickly and efficiently as possible. This means no curves, no skew (track 90 degrees to road/waterway), and as few piers in the water as you can reasonably get away with. There would need to be a very legitimate reason for one of these to occur. Examples may be protected lands the railroad can not acquire, a specific orientation of piers so that shipping traffic can negotiate turns or currents, or I’ve been told of instances where a small isolated portion of a river was home to an endangered species of mussel.
-Piers in the water will almost never be located in the middle of a channel, unless it is a wide waterway with a fairly flat bottom. Also Bridges will not be placed near curves in waterways if possible. Both arrangements increase the risk of scour (gradual degradation of the channel bottom in the area directly adjacent to the pier). If these locations can not be avoided, and the channel can not be spanned entirely, Pile or drilled shaft bents will be preferred over large piers based on footings.

Picking a Material
- Materials are largely regionally specific. With lots of factors weighing into them. In the low country of SC for instance, concrete is king. We have numerous granite quarry’s, sand mines, cement plants, lime deposits..... Everything you need to quickly and cheaply make concrete. Additionally we have a milder climate that is favorable for concrete.
- Timber. Common at one point in time, is rarely (if ever) used today. If you choose an era where timber makes sense there are a few things to keep in mind. Bents will be spaced at about 10ft on center (max) and the beams will be at least 12 to 18 inches deep with decking of 10in – 12in. Timber piles will be 12 in. in diameter and expect to see 4 to 6 per bent. I’ve seen some truly amazing trestle type timber bridges spanning gorges in some MRR’s. Present day these are impractical but I have no experience with them and would have to let you go else where to learn more about them.
- Steel. A very common material, and a must if your doing a curved structure. Expect to see steel girders range from 18” deep for a 15 ft span to as much a 8 ft deep for 75-100 spans. Any thing larger than this begins to get in the realm of “why not just design a truss”. Trusses are common on MRR’s (even in locations where they would probably never actually be used) and are very pleasing to look at. Lots of information on trusses exists and I’ve seen many quality models advertised in the magazines and the emails I receive. If your bridge is more than 25 ft or so above the ground line, it’s a pretty safe bet the substructures will be concrete piers and not piles.
- Concrete. The other common material, but rarely ever used on curves. Also don’t expect to get the same kind of spans out of concrete as you do steel. 140ft span would be about the max for a street crossing your trains, and then expect the girders to be 6 ft deep. For your tracks, a good limit would probably be the 50’ mark for a max span, again with that same 6 ft depth. Concrete at this length will almost always be prestressed and that means sections made and shipped to the bridge site. So it would be unrealistic to model a “flat slab” style structure. Flat slabs would probably not exceed 20 ft and you could expect them to be about 24” deep.

Protection and clearance.
-When modeling a street crossing about your track, keep in mind that the railroad will have very specific requirements on what can and can not be near their lines. No bent should be within 25ft of an existing track without a protective wall. If a wall is provided it will be about 4ft thick and 10ft tall as a typical. Even with the wall, no obstruction should be within12 ft or so of a track centerline.
-It is VERY common for the RR to require that clearance be provided under the bridge for a “future” track. Another unpopular idea for a layout as space is usually at a premium.
-RR’s generally are not big fans of digging/excavating near lines. If you have a 10 ft deep hole, and ran a 2:1 slope from the edge (20ft). That point should not be within 12 ft of any track centerline. If that’s the case you may want to consider modeling a shoring wall in the hole to stabilize the surrounding grade.... that is realistic and should be easy enough.
-One final though is slope paving. Bridges crossing a track collect water which will typically run to the end of the bridge then down the slope towards track. To protect the line and its roadbed, RR’s will require slope paving adjacent to and under the vehicular bridge. This keeps the soil from eroding away. Additionally ditches would be required to parallel the track to handle the runoff.

Hope this gives anyone planning to model a bridge in the future something to think about. I cant wait to actually get my hands dirty on my own layout soon so I can maybe contribute something a little more meaningful! Sorry for the typo’s I’m sure its full of them.
For starters, I acknowledge that many of the things that will be mentioned here, conflict with what most people will want to include in a layout. To follow every bit of information I proved you will have to be very dedicated to “making it real”.
Wow! a course on bridge construction, it's most welcome Kev. While it's true that a lot of model builders may not include everything in their model, knowing the facts of how it should be, certainly helps when "fudging things" so the model has a realistic look when finished.
Thanks for your input.
Cheers Willis
Kev, thank you for a wealth of information on bridges. It is a plus to know the practices of prototypical design on anything to help us achieve realistic modeling. Yes, we do have many restrictions built into our layouts, but having benchmarks, such as you provided, can at least get us close.:)
Bridge information

Thank you Kev!!!

And I didn't design any bridge in your layout....:eek: :eek: ( if I know that before...;) lol )
Now I have to take a beer and....;)

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