There is an inlet to a spring
fed pond that I thought would be
nicely highlighted by a bridge. I wanted to use natural materials
and we were developing a bamboo collection on the property.
The clients had seen a bamboo bridge in a magazine article that caught
their interest. So we set out to
make one.
The bamboo bridge spans 17 feet. The
timber bamboo I found varies from 3"
down to 1.5" in thickness. I cut 3 year old canes and seasoned
them in a shady garage for 3 months. The younger bamboo canes
I cut were much weaker and seemed to have a lot less actual
"wood". So I had to throw the younger material
away. I took bundles of 3 bamboo sticks and
lashed them together. There are three of these bundles that run the
length of the bamboo bridge. Then I laid 3-4 foot lengths of bamboo as
planking. I left a little bit of bamboo wood beyond each joint where
I made a cut, as that
material beyond the joints tends to split readily. I made a
footing for either end of the bridge out of treated landscape
timbers. This has stabilized the ends of the bridge. And it
allows the bridge to slide a bit when people walk on it. When I
first set the bridge in place I realized it was still a little
too bouncy. I lashed additional runners onto the top. This
stiffened the bridge enough that it holds 250 lbs
quite well. I eventually put a handrail up for safety. I have
repeatedly painted the upper deck with linseed oil. As it aged the linseed oil
blackened the bamboo and the string. For the string I used nylon
sting that is usually used for string-lines on construction sites.
The string tended to fray which made the knotting a little difficult but
with the linseed oil the string has stiffened and none of the knots have
loosened. It has now been 6 years since the bridge was
constructed. All of the pieces are still intact and looking strong.
The
bamboo bridge has
required some regular maintenance but this has not been excessively time
consuming. The cord has darkened considerably with each
successive oiling of the bridge. I have been coating the bamboo
bridge with
linseed oil about
every 3 months to keep the bamboo from splitting. Cracks open up
during the summer months but quickly closes again as it rains, or as the
oil soaks into the bamboo. My respect for bamboo as a building
material continues to grow as this bridge keeps being servicable.
