|
The development of these preservatives
ultimately became an important factor leading to the rebirth of interest
in timber bridges.
New popularity
 Bridge contractors and engineers are again using
wood in spite of the availability of concrete, stone, and steel. This
renewed interest in wood is a result of advances in technology, which
developed wood preservatives to extend the life of wood used in exposed
wood applications. Bridges capable of supporting heavy loads, such as
vehicular bridges, now provide an aesthetic choice for the architect,
engineer, or contractor, and assure a long bridge life because of these
treatments. Given the fact that treated wood requires little maintenance
or painting, the choice of wood becomes not only extremely
cost-effective, but also ecologically sound and enduringly beautiful.
Preservation treatments
Wood is inherently extremely durable when
protected from moisture. Evidence of this lies in the many covered
bridges built 150 years ago that have lasted because they are protected
from exposure to the elements. Today, wood preservatives extend the life
of wood used in exposed bridge applications, meaning that wood can be
protected from deterioration for a half century or longer.
Bridge Builders of Franklin, North Carolina, has
been in the business of constructing timber bridges for over 25 years,
and the company’s founder and CEO, Tim Kris, has used and advocated the
use of various treatments for his timber bridges since the inception of
the company. For instance, the preservative chromated copper arsenate
has been used for more than 50 years in a wide variety of wood products,
and has been found by the Environmental Protection Agency to have
benefits which permit the use of CCA for industrial applications —
bridge construction, utility poles, and pilings.
Kris points out that these changes gave rise to
three additional types of products marking the new generation of
preserved wood:
-
Ammoniacal copper quat.
-
Copper boron azole.
-
Copper azole.
These are marketed under such brands as ACQ
Preserve, Nature Wood, and Wolmanized Natural Select Wood. These
preservatives, approved by the EPA, extend the life of wood products
from just a few years to decades.
Enhanced applications
Beyond preservation, according to Kris,
technological advances in lamination have further increased the
suitability and performance of wood for modern vehicular bridge
applications. One of these is the development of glue-laminated timber.
This innovation expands the application of the use of wood for major
bridge construction.
Although tree diameter limits the size of sawn
lumber, glulam is manufactured by bonding sawn lumber laminations
together with waterproof structural adhesives, making the glulam members
available in limitless permutations of depth, width, and length. Because
it can be manufactured in a wide range of shapes, glulam enables broader
design parameters than sawn lumber and provides improved use of the
available timber resources. Glulam allows construction of stronger,
longer, and larger wood structural elements from smaller lumber
components.
Glulam allows creative options in circumstances
when high water, water flow, and environmental restrictions apply. An
engineer or designer could opt to use the glulam product as a clear span
to permit uninhibited water flow in flood-prone areas and so prevent
debris jams and possible water backup. The glulam or clear span can be
designed to attach to a typical trestle-type bridge in many
configurations. The advantage of this kind of bridge construction, Kris
says, is that the bridges can roll, bend, turn, and accommodate the EPA
or geographical dictates in environmentally sensitive wetlands, uplands,
and EPA-designated areas. Bridges are constructed around trees,
boulders, through wetland areas, and other environmental situations
where products like stone, steel, and concrete are limited.
Using wood
Wood is cost-competitive with other materials on
a first-cost basis and shows advantages in life-cycle cost comparisons.
Unlike other materials, bridges built of timber are not damaged by
continuous freezing and thawing, and are resistant to the harmful
effects of deicing agents, which in other materials may case
deterioration and/or pose safety hazards. Wood has always been a
renewable and recyclable material, and is aesthetically pleasing,
especially in natural settings.
Since the United States Department of
Agriculture’s Forest Service introduced the Timber Bridge Initiative,
the choice of wood, treatments, and design flexibility have expanded the
opportunity for a greater use of timber for bridges in various
applications demanding greater load-testing capability, width, length,
and beauty. The resurgence of the art of timber bridge construction
marries the use of timber, technology, and aesthetics to the first-line
environmentally friendly choice of building bridges with timber.
The rationale for using timber in building
bridges is based on cost-effectiveness, life, availability, application,
and aesthetics. Cost effectiveness is made possible by means of a number
of preservation products, which assure a long life to the final product.
Wood is naturally renewable and treatment options are plentiful. In
practical applications, wood has always been user friendly — easily
worked and an outlet for creativity for the builder.
Pat Friedlander is a freelance writer located in
Chicago.
|
Click here for copyright permissions!
Wood has always been one of the primary natural
resources used for bridge construction from ancient times through the
Middle Ages, and not surprisingly, it was used in the development of
American bridges.
