Glued-together beauty

Dec. 30, 2003

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.

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.

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