Outside of a drought or miracle similar to the crossing of
the Red Sea, there is no way to make a dry crossing over a
body of water without a bridge. I desperately needed one to
cross the Class One stream on our property in the western
Cascades of Oregon. So I decided to build a log bridge, one
that would meet my needs without causing damage to the
environment.
The first thing I did was take a trip to the area State
Forest office and make a request for a bridge-building
permit. That was followed by an on-site inspection by the
local forester, who decided the stream banks were level
enough so that water drainage from a road wouldn’t flow
down onto a bridge and into the stream.
The permit explained the two best methods of moving logs
across a stream without harming the stream bed. The method I
selected, explained below, involves an elevated-cable
system, a truck winch, and an anchored-log tripod with a
cable pulley. This allowed me to haul logs across the
stream without touching the stream bed at all.
The other (more costly) method is to use a large backhoe to
lift the logs across the stream. According to the permit,
we were also responsible for reseeding those areas along
the roadway and bridge that were disrupted by our
machinery. The whole process was so simple and turned out
so well that I drew up some plans and diagrams to share
with you.
Bridge Construction
Begin by finding (or, if necessary, hiring) someone with a
portable saw mill to saw planks for the bridge deck. We had
a friend cut forty 3 x 12s, sixteen 2 x 12s, and seven 4 x
6s. Cut down and limb just enough cedar trees to make six 42′
span logs (which run across the stream) and two 12′ bunker
logs (which hold up the span logs).
Place the larger of the two bunker logs in a shallow trench
on the accessible side of the stream (the side you’re
standing on); you may have to dig a trench if a natural one
doesn’t exist. Level the top. Next, position long but
narrow logs across the stream, allowing the top ends to
rest on the bunker so that they form a ramp. Using the
ramp, roll the second bunker log to the far bank.
To make sure the bunker logs are level with each other, you
can create a water level: Take 50′ of plastic tubing and
stick one end through the bottom of a one-gallon container
filled halfway with water. Have one person hold the
container on the bunker log located on the accessible side
of the stream. Have another person hold the free end of the
tube on the bunker log, at the same height of the first
bunker log. If the level of water in the container is equal
to the water level at the other end of the tube, you know
your logs are level.
Next, attach the bunker logs to reinforcement bars so they
won’t be dislodged while you’re working. Place a 5/8″
reinforcement bar next to the bunker log and hammer it into
the ground. Then hammer a 20-penny nail halfway into the
log, bending it so that the nail makes a U-shaped curve
over the reinforcement bar and reenters the log. Do the
same for the other bunker log.
It’s now time to move the span logs. Haul the logs over the
top of the first bunker and into place by constructing the
following pulley system.
Attach a chain-log choker (which will wrap around the span
log) to a freewheeling pulley on a steel cable. This cable
should extend from the choker, across the stream, through a
pulley attached to an A-shaped log tripod, and back across the stream to a truck-mounted
winch. The log tripod will create a pulling point
approximately eight to 10′ above the stream bed, and the
free-wheeling pulley on the cable will hold the log upward
and horizontal over the stream. Arrange the span logs so
that each butt end (the bottom half of the log) sits next
to a top end. This will distribute strength, making the
deck more stable.
Next, mark the crowns in each log (the natural curvature
that runs along one side of the log) so that you can place
them facing upwards; this allows for maximum bridge
strength. Make a saddle cut at the butt ends of the outside span logs to fit
the bunker logs.
The best way to do this is to mark the cut on the span log
while it’s sitting on the bunker log with a piece of chalk
taped to a fork stick. Then level the top ends so they are
square with the butt ends. All four corners should now be
level.
Next, place one 3 x 12 deck plank at each end of the bridge
(so they are lying on top of and horizontal to the span
logs) and trim the tops of the logs so that the planks are
level and touching the top of each log (shims can be used
if tacked). To make sure that your end planks are parallel
to each other, form an `X’ with two pieces of string so
that it reaches across the entire bridge from corner to
corner. If strings don’t touch the center of the X, the end
planks aren’t parallel. If they touch lightly in the
center, the planks are parallel. If (or once) this is true,
secure the strings to each of the four corners.
Now stretch a piece of string tightly along each side of
the bridge so the pieces run across evenly. To give your
bridge extra strength in the middle–the center is a prime
spot for future sagging–place your center plank so that it
is 1/2″ higher than the side strings (and thus also the end
planks). Attach the strings to the center plank, and use
them as a guide for fitting the remaining deck plank, which
raises slightly the center of the deck.
As you’re positioning the deck planks, leave a 1/4″ gap
between them to allow for expansion. Rough-trim the log
tops to fit the planks by making a chain saw cut every two
inches to the necessary depth (an ax or hammer will help
you take the excess wood out). Attach the planks to the
logs below them using 12″ spikes.
Once the deck is complete, place pairs of 2 x 12s along the
full length of the bridge where tires will make contact,
and one 2 x 12 on each end to cover log ends. Last, attach
the 4 x 6s along the sides of the bridge to form the side
guards.
