# Prototype Girder Bridge Support



## Empire Builder 76 (Feb 11, 2016)

My c. 1947-55 layout (GN in Montana) has a 50-foot long dry creek bed and a through girder bridge in need of a supporting structure.

In the prototype, would a girder bridge ever be supported across its entire length by a trestle only or, as I've seen in a few photos, by stone or concrete piers with a small wooden trestle anchored in the ground at each end?

Thanks!


----------



## mesenteria (Oct 29, 2015)

It depends. Span, materials, intended supporting capability (weight limit)...a girder should be able to support itself across spans of near 40 feet, but it must also support at least a 100 ton steamer, 70 ton tender, and several cars of varying weights, usually only one and a half cars at a time though.

As a general rule, the web should be no less than 1/15th of the span. If you take a 4' web of 1" high tensil steel, it should be safe to span near to 60', but it still depends on the material, construction, and intended capacity midway between supports. A simple girder with no diaphragms, plates, braces, might be severely restricted in capacity. Add the various types of knee bracing and boxes, etc, and you might get up to 250 tons.

Micro Engineering makes a combination girder bridge kit for HO scale that is 150' long if I recall. There are four girders in total. I think their lengths come to about 37' each.

Might not be easy to tell, but these spans are very close to 40':










Here is the Micro Engineering bridge:


----------



## Nikola (Jun 11, 2012)

Empire Builder 76 said:


> My c. 1947-55 layout has a 40-foot long dry creek bed and a through girder bridge in need of a supporting structure.
> 
> In the prototype, would a girder bridge ever be supported across its entire length by a trestle only or, as I've seen in a few photos, by stone or concrete piers with a small wooden trestle anchored in the ground at each end?
> 
> Thanks!


Yes, there are dozens of them on the LIRR in NY (but concrete only). The span is usually not more than three wide lanes, but over the Long Island Expressway (I495) around exit 36 they widened the girders in place without tearing them down when they widened the road from three lanes plus shoulders to five lanes plus shoulders. Maybe a Google view and you can see it. Also look at County Road 97 (Nicolls Road) crossing the Port Jefferson line.

I am sure there are plenty of examples as well with wooden bents on a concrete or stone pier supporting the girders.


----------



## Empire Builder 76 (Feb 11, 2016)

mesenteria said:


> It depends. Span, materials, intended supporting capability (weight limit)...a girder should be able to support itself across spans of near 40 feet, but it must also support at least a 100 ton steamer, 70 ton tender, and several cars of varying weights, usually only one and a half cars at a time though.
> 
> As a general rule, the web should be no less than 1/15th of the span. If you take a 4' web of 1" high tensil steel, it should be safe to span near to 60', but it still depends on the material, construction, and intended capacity midway between supports. A simple girder with no diaphragms, plates, braces, might be severely restricted in capacity. Add the various types of knee bracing and boxes, etc, and you might get up to 250 tons.
> 
> ...


Ah, a bit of technical education here is most helpful!  Tell me what the "web" is on a bridge. I didn't see that term in the bridge book below.

Model Railroader's book, "Bridges and Trestles," with its load charts and such gives a good overview of "E" load capacities and design differences. For my transition era, E-60-65 loads (60-65K pounds/axle carrying capacity) apparently were the standard. And Wood's book, "The Great Northern Railway," tells me that my Alco FA-1 comes in a 251K pounds, or 125 tons. It doesn't list, however, the Mikado 2-8-0 I run, perhaps because GN didn't use it.

I bought ME's double track through girder bridge with little consideration at the time how it would be supported across that fifty foot chasm. I do like the look of wood bents and trestles and have even seen a photo or two of a girder bridge supported by wooden bents (with concrete or stone abutments). 

But in my ignorance, the gist of my question is if a girder bridge on top of a trestle would be, in effect, redundant? In would seem so. Still, the bridge requires some kind of supporting structure even for its short fifty foot span. Your thoughts?


----------



## mesenteria (Oct 29, 2015)

All reasonable questions. If you look at an I-beam from the end, there are two parallel flanges separated by a web. Just like the capital letter "I". On many girder bridges meant to support heavy loads over a broad span, there are knee braces every five or six feet, often abutted and bolted or riveted together. As you may recall from the ME version, there are also cross-bracing frames meant to tie two parallel beams together.


A girder on top of bent frames would be redundant...UNLESS...the load requirements were raised and/or the condition of the beams had deteriorated to an extent that they no longer supported the loads themselves, but could have an extended life of maybe 10-15 years if supported midway by a pair of bent frames bolted together. But girders are designed to cross substantial spans and would normally not be cantilevered. They would be suspended between two abutments and normally ride on bridge shoes.

http://pghbridges.com/basics.htm


----------



## Empire Builder 76 (Feb 11, 2016)

mesenteria said:


> All reasonable questions. If you look at an I-beam from the end, there are two parallel flanges separated by a web. Just like the capital letter "I". On many girder bridges meant to support heavy loads over a broad span, there are knee braces every five or six feet, often abutted and bolted or riveted together. As you may recall from the ME version, there are also cross-bracing frames meant to tie two parallel beams together.
> 
> 
> A girder on top of bent frames would be redundant...UNLESS...the load requirements were raised and/or the condition of the beams had deteriorated to an extent that they no longer supported the loads themselves, but could have an extended life of maybe 10-15 years if supported midway by a pair of bent frames bolted together. But girders are designed to cross substantial spans and would normally not be cantilevered. They would be suspended between two abutments and normally ride on bridge shoes.
> ...


Thanks for the eduction and the interesting website!


----------



## traction fan (Oct 5, 2014)

*Micro Engineering trestle*



Empire Builder 76 said:


> Thanks for the eduction and the interesting website!


Empire Builder;

Deck girder bridges supported by steel towers were common on most railroads with wide gaps to cross. I model Milwaukee Road in N-scale, but I have seen plenty of photos of GN and NP trains crossing the same type of trestle. The photo below shows a Micro Engineering trestle that I kitbashed. The Milwaukee did not use horizontal bracing on their towers as most railroads did. They used diagonal bracing only, so I cut the horizontal ones off. The towers are also cut down HO-scale models, and the deck girder bridges at the top are N-scale kits. The N-scale trestle kit did not have the lacing on the box girders that I wanted, hence the cut down HO.

good luck;

Traction Fan:smilie_daumenpos:


----------



## Empire Builder 76 (Feb 11, 2016)

Traction Fan,

Thanks for the great photo! This link shows the ME bridge I have. I like the look of a trestle or bents as supporting structures.

https://www.modeltrainstuff.com/mic...21-50-double-track-through-girder-bridge-kit/

Does your bridge have abutments at the ends or do the towers run the full length? What are the overhead wires for?


----------



## CTValleyRR (Jul 26, 2014)

Empire Builder 76 said:


> Traction Fan,
> 
> Thanks for the great photo! This link shows the ME bridge I have. I like the look of a trestle or bents as supporting structures.
> 
> ...


All bridges have abutments at each end. It's what supports the end of the bridge. You are certainly under no obligation to duplicate this on your own layout if you don't want to, but since you seem to want to make it look right, it needs abutments.


----------

