Many of America's earliest roads were joined by
picturesque timber bridges, linking towns together and helping to form a
stronger nation of states. Historians tell us that American bridges in the
1700s and early 1800s were often built of wood.
Today, timber bridges are enjoying a resurgence because they
offer special benefits to designers and owners. They are considered
environmentally friendly because timber is our only renewable resource. It is
widely available, competitive to other materials, has a long life expectancy
when properly maintained and also is lightweight and compatable with the
natural landscape.
In addition, the increased strength of today's modern
glued laminated timbers helps to achieve longer spans and carry heavier loads
than conventional timber bridges in the past.
By the looks of it
The owners of Woods Valley, a northeast San Diego, Calif.,
gated community, needed to build three bridges to connect the sections of their
430-acre enclave. They could have specified steel or concrete. Instead,
developer Newland Communities and its contractor, engineer and bridge
fabricator elected to build the spans with glued laminated timber framing.
"We felt that the timber construction had the most
attractive appearance, competitive cost and fast construction, and also
harmonized with the rural, natural landscape for our 270 country homes,"
commented Jim Stewart of Newland Communities. "The timber spans were also
very cost competitive and arrived at the jobsite ready to be assembled."
The vehicle/pedestrian rail-type bridges were designed to enhance and protect
the fragile environment they span, he added. They also have the advantage of
reduced thermal expansion compared to concrete or steel, which reduces the
number of expansion joints.
The three timber bridges are called St. Andrews West, 116 ft
long (two 58-ft spans) and 24 ft wide with a pedestrian walkway; St. Andrews
East, 265 ft long (four 65-ft
spans) and 24 ft wide with a walkway; and the Hazeltine Bridge, 170 ft long
(three 56-ft spans) and 32 ft wide with no walkway.
The transverse bridge deck panels are 51/8 x 48-in.
glulam planks supported by glulam timbers that average 52 in. deep and from
51/8 in. to 83/4 in. wide. The glulam girders are pressure treated
prior to fabrication and are spaced 5 ft 6 in. on center supported by concrete
piers and abutments.
Jim Gay of contractor Hazard Construction said, "The
whole project went together nicely because it arrived by truck pre-cut and
pre-drilled ready for assembly from the Western Wood Structures plant in
Portland, Ore."
Rather than an asphalt-wearing surface on the bridge deck,
the Newland Community team specified 3/10-in. longitudinal glulam planks
as the traffic surface for the Woods Valley bridges.
The bridges were designed for an AASHTO Standard HS20-44
live load truck. Jim Frost, P.E., of Simon Wong Engineering in San Diego
said the high-strength glulams
made it possible to provide for standard highway live loads on the bridges with
spans up to 65 ft.
Preservatives = shelf life
Western Wood Structures in Tulatin, Ore., designed and
erected the three Wood Valley Ranch bridges and has a long history of
leadership in building glued laminated bridges across the U.S. They specialize
in pinned arch, side girders, bowstring and parallel-chord truss timber bridges
for vehicular and/or pedestrian traffic. Each bridge is completely fabricated
before pressure treatment.
Most treated wooden bridges require minimal maintenance in
order to achieve the desired life expectancy. The pressure treating process
provides a protective envelope for the wood that prohibits decay organisms and
insects from attacking the wood. The steel assemblies and connecting hardware
are galvanized to protect against rust and corrosion.
The easiest, most economical method of reducing the hazard
of decay in timber bridges is to control moisture. This means identifying the
source of wet timbers and taking corrective action to eliminate these sources.
The drainage patterns of the approach roadways should be routed so the water is
channeled away from the bridge. Debris and dirt, which can trap moisture next
to the wood, should be removed. This includes dirt at the girder bearings and
between deck panels. The bridge should be designed to provide air movement
around the timbers, allowing them to dry out between wettings and reducing
moisture content of the wood.
The bridge should be inspected annually for newly exposed
untreated wood due to splitting or checking. A field application of copper
napthananate or other approved preservatives can be brushed onto the exposed
wood. Care should be used to prevent unnecessary spilling of the preservative
on the ground or into the water below the bridge.
Most bridges produced by Western Wood Structures are
pressure-treated with an oil-borne preservative such as pentachlorolphenol.
This preservative will provide for a long service life. The color of the timber
bridge will fade to a driftwood gray color over the course of several years.
The effectiveness of the treatment continues even though the color of the wood
has faded, so it is not imperative that the wood be restained.
The bridge can be stained periodically to provide additional
protection from ultraviolet degradation and to keep the "new wood"
appearance. A high-quality wood preservative such as Sikkens Wood preservative
or Sikkens SRD exterior wood finish can be used. One disadvantage of using a
finish on treated wood is that the appearance of the finish may degrade over
time and the bridge will need to be refinished to keep up its appearance.
It is not uncommon for nuts to become loose on the bridge
after the wood members have had time to dry and shrink. All nuts should be
inspected after the first year of service and tightened as necessary. Tightening
should not cause the washers to damage wood fibers. The thread beyond the nuts
may be deformed by striking with a hammer to prevent the nuts from further
backing off.
