LO#2 - About Bridges and Such Things

Some subjects in modeling  just lead into questionable areas and bridges is one that is sure to open a can of worms.  Here is a subject that is more than fully documented in printed material, internet blogs and historical records, yet so poorly understood by most of us designing, building and utilizing them.

Our club, Rail Roadrunners of Yuma, is building a 5'x10' layout that will require several of these mysterious devices of various types and sizes.  I, as a retired design engineer, felt most qualified to undertake this simple task and chose to begin with a curved section of the upper level where it passes over a curved section of the lower level at a fairly shallow angle.  We have all seen the beautiful curved wooden trestles built by men of old, but may be shocked to learn that there are none of today's materials.  If we move in to take a very close up view of a wooden trestle we will find that it is made up of many short, straight sections of about 13 to 16 feet between bents, the vertical legs that support the track-work.

While modern materials of steel and concrete allow us to make much longer spans than the wood beams between bents, the torsional stresses of a curved beam is still outside reliable railroad usage, although we see them used in highway designs.  

The curved bridge area over lower curved track roadbed is at the extreme right hand center of this picture.  The main frame is 1" x 4".  The four 8' and two 5' long members are one piece and the four 5' middle  members are in three pieces with all joints reinforced with a 2" x 2" x 4" block.  The cross braces are 1" x  2".  The lowest level is set at 3" above the main frame, minimum radius is 23" and maximum grade is 1.8%.  The sub-roadbed is 1/2" ply or scrap construction grade OSBwith a maximum span of 24".  All screwed and some glued.  After sanding the roadbed we will start laying cork roadbed to align turnout branches.  Ground contour will be formed with cardboard strips and plaster cloth or paper.