I have built an adjustable span two track Warren Truss Lego Train Bridge which has been tested to a maximum span of about 13.5 feet and a bridge abutment clearance of 42 bricks high, internal train clearance of 17 bricks .... The biggest challenge with bridges at extreme spans as the bridge becomes heavier -- bending and twisting /torque on the Lego parts is more pronounced. . To prevented twisting on short spans under 4 feet - technic bricks can be laminated by pinning them together side by side and then sandwich the technic bricks between classic lego plates. from the top and bottom. As the bridge span increased over 5 feet and got heavier - bending and torque forces get stronger - so to counter this a box frame using four connection points with a box side shorter than 1.5 feet is used to increase bridge frame rigidity - each point in this box has a connection along all the x y and z axis .. point are inter connected to each other by laminated beams that resist bending and twisting. San bridge supports and track at 13.5 feet long the bridge weighs about 20 pounds or so .. This bridge is now on its 5th version and is only used for Lego train shows - it will be 147 inches long at its next display at the 2017 Virgina Brickfair. My webpage for this bridge is here .. http://www.moc-pages.com/moc.php/389762
