Hi! We are in the design phase of an HO layout and, as of now, are planning to have the tracks cross some water (width, of which, has not been determined as yet). Question: Is there a “rule of thumb” to decide whether to use a girder bridge or a truss bridge? As always, many thanks.
Here is a simplistic answer…
Atlas girder bridges come in same length as a piece of sectional track for HO and N scale and look good for starters. I have seen prototype girder brideges, especially those built ca. 2000, somewhat longer.
I would think girder bridges look more appropriate for anything over 80 scale feet, but I don’t have a bridge engineering book in front of me.
Another consideration is clearance under the bridge, for roads or tracks, or for water-borne flood debris. Deck girder bridges provide least clearance, through girder bridges more and truss bridges more.
Is your water a navigable water way? If so then Coast Guard or Army Corps of Engineer specifications may come into play.
What are your options?
If you are crossing the Apple Creek a girder bridge is quite sufficient, even it it must be build in two (or more) sections and have a pier (or more) in the middle. Such a trestle over a swamp with a river at one end, if you must have a river is quite appropriate.
If you are crossing the Missouri River then you might emulate BNSF with a three section chord bridge with mid river piers.
If you are crossing the East River at Spuyten Duyvil where there are no tall banks on either side, you might want to consider a swing bridge. It would have girder bridges approaching it from both sides, with an upper level superstructure on a rotating pivot.
If you are crossing the East River in Lower Manhattan then you would need a tall Cantilever or suspension bridge such as the Williamsburg or the Manhattan Bridges. Even the Brooklyn Bridge was originally built with railroad tracks on it.
If you are crossing the Chicago River, then you will need a counter weighted Bascule Bridge.
If you are crossing the Hackensack River, then you will need a twin tower type lift bridge.
If you bit off more than you can chew, then you may need a dental bridge.
ROAR
Dunno about a rule of thumb. But up here there are (or were) plenty of truss bridges crossing not all that wide bits of water. For instance, there was a truss bridge across the Merrimack river at Concord. There still are truss bridges across the Connecticut river at Wells River. There is an old truss bridge across the Pemigewasset river at Sugar Hill. There are several truss bridges over the Ammonusuc river near Lisbon. All of these bridges only spanned a 100 feet or less. So if you want a truss bridge, go for it.
Leighant: Roar: David: A lot of great advice. Thanks.
PS ROAR…As a transplant from The Bronx, appreciate the NYC references.
Or a lift bridge like the one traversed by the PRR into Union Station.
Rich.
I checked Bridge and Trestle Handbook by Paul Malery.
For bridges built 1925-1955 a steel plate girder could be used for spans of 15’ to 200’. A steel truss for spans of 100’ to 700’ . This is for single spans. Where it is possible to construct intermediate piers, multiple spans could be used to cross wider obstacles. Bridges could also be mixed types. (For example, a couple girder spans, a longer truss span, then more girder spans.)
One, I believe somewhat oddball, bridge crosses Intertate 80 east in the Sierra Nevada Mountains near Auburn CA. It is a deck plate girder bridge built on top of a truss bridge.
ZImage21 by Donald Schmitt, on Flickr
I don’t know what the thinking was in earlier times, but generally today the railroads prefer deck bridges since they don’t cause restrictions on the dimensions of cars and loads.
Additionally, Kalmback has published a book, I believe it is titled “Bridges and Tressels”, I actually have a copy of it but I’'d have to find it and dig it out. Anyway, if you search on this website, you very likely will find it. I believe it is still in print and maybe even updated since I purchased my copy some ten years ago.I believe it will be very helpful for what you’re wanting to know as well as providing many different designs to go by for construction.
My “rule of thumb” was I built the bridge I wanted to have and assumed that someplace in the world there was a prototype, of should have been, but then I don’t model a specific era or place or even a real reality. The Kalmbach book refered to is wonderful for scratch building. My trestle was adapted from a couple of plans.
Very rough rule, not applicable to model railroads:
When designing a bridge, the cost of the spans should be approximately equal to the cost of the foundations (including piers where used.)
Of course, that assumes that there are no external forces at work. If there was no motorway under the truss in the picture above it would be a fill with a corrugated tin culvert at the bottom…
One thing to watch for is the use of a single, long, expensive span where it isn’t necessary. If a prototype line had to cross a shallow, not too wide stream with a local road on one side and a competing railroad on the other it wouldn’t build a 200 foot truss span. More likely it would bridge the road with a short deck girder, the stream with a longer deck girder and the railroad with a through girder. If the crossing was at some angle other than 90 degrees the abutments and piers would parallel the centerlines of road,stream and railroad and the bridges themselves would be skewed.
Prototype railroads don’t build bridges to have bridges. They build bridges to cross things that have to be bridged - as simply and as cheaply as possible.
Chuck (Modeling Central Japan in September, 1964 - with oxide-red deck girder bridges)
Girder bridges in the steam era were rarely more than about 100’ in length for each span. Longer crossings would use multiple spans with piers in the river. Today with better steel they are used up to about 150’ long spans. Once you get much longer a truss bridge becomes more economical.
As others have noted, the railways prefer a deck bridge, whether girder or truss, since it won’t create any clearance restriction to dimensional loads. But often that is not an option and a Through Truss or Through Plate Girder bridge is required to provide adequate clearance underneath. This might be for road traffic or shipping, or to provide a big enough opening to pass water in the “100-year” flood.
John
Thanks to all for your advice, pictures and information.