Model Forum / General / Railroads / March 2005

snip

You may be combining two sets of terms - girder vs. truss, and deck vs.
through.  Girder bridges are simpler to build and maintain, but have are
limited in length between supports.  truss bridges can span much greater
distances, but are more complex and more costly.  For any given span and
load, a deck bridge will be lower cost than a through truss (the main bridge
elements are located closer together (under the rails or nearly so) so the
lateral  members can be smaller.  Also, the deck bridge does not impose any
clearance restrictions on the loads the RR cars can carry, while especial
through truss bridges and limit heights and widths of loads.  However, deck
bridges use height under the track, and thus require the tracks to be at a
greater height to provide clearance for the highway, railroad or waterway
passing beneath.  To lessen the grades for the approach to a bridge, you
often see deck bridge spans on the approach to the through span which
crosses over highway or RR or navigable channel of a waterway.

One of the best sources I've found for prototype practices in abridged terms
for a model RRers' use with getting too bogged down in civil engineering is
the NMRA Data Sheets.  Join the NMRA and get a copy of these valuable
documents.  See www.NMRA.org
Gary Q

A good source of information is a Yahoo Group devoted to the prototype your
interested in.  Just go to http://groups.yahoo.com/ and put it the prototype
you seek info on.  Then join the groups that fit and enjoy the drink from
the fire hose!

Peter

Thomas M. Sasala wrote:
*** My other question is, aside from this newsgroup and the variety of
BBs, are there any good websites or books that cover matching
prototypical behavior on a model RR? I goal is to keep the hobby fun and
not get bogged down with a lot of details, but I'm having a hard finding
basic information like the best height for a tunnel opening, width of
road ways, etc. Any suggestions would be great. Thanks.
-----------------------------------------------------
Two good books that will prove helpful:

"Track Planning for Realistic Model Railroading" by John Armstrong:

http://www.amazon.com/exec/obidos/ASIN/0890242275/002-5488552-9764854/billsrailr
oaempi/102-7028332-4471300

"Model Railroad Bridges and Trestles":

http://www.amazon.com/exec/obidos/ASIN/0890241287/104-3183206-0063943/billsrailr
oaempi/102-7028332-4471300

These books are up to 32% off list price and include free shipping on
orders over $25.

Bill
Bill's Railroad Empire
N Scale Model Railroad:
http://www.billsrailroad.net
Brief History of N Scale:
http://www.billsrailroad.net/history/n-scale
Model Railroad Bookstore:
http://www.billsrailroad.net/bookstore
Resources--Links to 1,000 sites:
http://www.billsrailroad.net/bills-favorite-links

Thomas M. Sasala <CyberReefGuru**Spam**Guard**@hotmail.com> wrote:

A truss bridge is one with intersecting members forming Xs and triangles.  
Girder or beam bridges just have solid sides.  So, I think you're really
asking why use a through bridge, where the train goes through it, rather
than a deck bridge, with the train running on top.

I'd say cost is the deciding factor.  A deck bridge takes less material,
since it doesn't have to go all the way around the train, so is preferred.
The bridge may have to clear things underneath, like roads or waterways
with boat traffic.  If clearance is an issue there will need to be a
through bridge.

A high bridge over a gorge would most likely be all deck bridge.  A bridge
over a river may have deck spans on the ends, but through spans over the
main channel.  Take a look at your bridges again to see if you can figure
out what is under the deck and through parts.  There should be some reason
for it.

Here's a site on building a truss bridge from file folder material that
you might find interesting:  http://bridgecontest.usma.edu/manual.htm

The NMRA website has info on clearances, that would give you dimensions
for a minimum tunnel portal and overhead bridge height.  I believe there
are engineering standards for roadway widths, but I don't know of any
websites with the info.  Think cheap.  Why would anyone build a tunnel or
bridge bigger than necessary?  

--  
Bill Kaiser
wkaiser@mtholyoke.edu

There are three ways to do a job: good, cheap, and quick.
You can have any two.
A good, cheap job won't be quick.
A good, quick job won't be cheap.
A cheap, quick job won't be good.