building bridges

[Weetreefan <Weetreefan@aol.com>]

My son, for an eagle scout project, will be building a bridge across a stream
to extend a trail on Department of Natural Resources land.  The stream banks
have become too eroded for the horses to ford easily so entire loop of trail
has been unused for over two years.  he has the assistance of the DNR as well
as local riding groups.  These trails are considered multiuse but no motorized
vehicles are allowed and they are too narrow for carts.  We now need to come
up with the specs for width and weight holding capacity.  Does anyone have any
idea if there is a standard out there that we can present to the DNR as a
guidline.  Something that says "for safty purposes all bridges expected to be
crossed on horseback should be at least ______ feet wide and be able to
support ________pounds.  Both we and the DNR are at a loss for how much these
figures are.  I'm assuming the width will be between five nad six feet but the
weight part has me stumped.  I've ridden these trails and really hope to have
the whole section open again soon.  Thanx,  Judy.