The sides of the bridge were cut and assembled in the shop.  While the parts are the
same, the right side is a mirror image of the left.  They were assembled from 2x4s
with 2x6s used for the end diagonals only.  The idea is for the bridge to hang from
the top rails while the diagonals provide end to end stiffing.  I used 3/8 inch steel
rods 4 feet long to hang four 3x4 lateral beams below the stringers.  The center 3x4
stringers are bolted to these lateral beams for strength.  Oversize washers and square
nuts were used for aesthetic effect. I used cut lengths of 3/8 all-thread rod for bolts
so that I could use square nuts on each end.   Lockwashers hold the nuts in place.  All
hardware on the trestle and bridge was painted black before installation for looks.  
The inside clearance is just under 4 feet.  The sides were spaced so that the four foot
decking would fit within the structure on top of the 2x4 sides.  Consequently, the top
beams of the support bents are wider – 5 feet – to support the bridge sides.  Notice
that the ends of the stringers are notched to provide a lapped joint with the next
stringer.  Also, a layer of  blue foundation foam keeps the wood bents from direct
contact with the concrete.  I believe that this design could easily be extended to 20
feet if desired.
Bridge construction:
Last update 01/21/07
North Pacific Coast Railroad
Bridge and Trestle Construction
Click on photo to enlarge.