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|Publication Number: Date: Sept/Oct 1999|
Issue No: Vol. 63 No. 2
Date: Sept/Oct 1999
Skagway, a picturesque restored gold-rush town, is located in a spectacular fjord at the head of Lynn Canal, the northernmost end of Alaska's Inside Passage. The city of Skagway, responding to increasing visitor and tourist traffic, recently decided to improve access to the old abandoned ghost town of Dyea. Dyea is located about five kilometers from Skagway "as the crow flies," but it is 15 kilometers over a narrow gravel road that winds around two deep fjords.
Dyea and Skagway were the booming, thriving gateways to the Yukon gold fields at the beginning of the fabled gold rush in 1898. Thousands of stampeders crowded into Skagway and Dyea by ship from the "lower 48 [contiguous states]" There, they began the grueling trek up the infamous White Pass from Skagway or up the shorter, but still brutal, Chilkoot Pass from Dyea to Lake Bennett. At Lake Bennett, they built boats to float down the Yukon River to Dawson City, site of the fabulous gold strike. The Royal Canadian Mounted Police, stationed at the summit of Chilkoot Pass, would let no one into the Yukon (Canada) unless he could sustain himself with more than 900 kilograms (at least 2,000 pounds) of supplies - all of which had to be carried up the last steep slope of the pass, "The Golden Staircase," on men's backs, about 25 kilograms (50 to 55 pounds) at a time!
Skagway is still an active community and is visited by many passenger ships cruising the spectacular Inside Passage, a natural waterway extending more than 1,500 kilometers along the coasts of southeast Alaska and western British Columbia (Canada). Dyea, however, was abandoned. No structures remain in Dyea. Only stumps of wooden pilings show where piers and wharves once stood.
Nevertheless, the old Dyea cemetery is a popular tourist attraction, but the only public access to most of the Dyea Flats required fording Nelson Slough in an area of tidal influence. Dyea has one of the largest tidal ranges in the world, as much as 7.3 meters in six hours. The normal half meter of water at the Nelson Slough ford becomes about 1.2 meters deep on an extremely high tide, so access was problematical unless you were on a tall horse! Since the year-round-flowing Nelson Slough runs through the area, a bridge was needed to provide reliable access to Dyea Flats at all tidal stages. The bridge is located about a half kilometer below the ford crossing.
|This Alaska yellow cedar bridge crossing the Nelson Slough provides access to Dyea Flats at all tidal stages.|
Another interesting feature is that the Skagway-Dyea area is "glacially rebounding" at the rate of about 2.5 centimeters (cm) per year. This means that the bridge site is almost 2.5 meters higher than it was a century ago!
The U.S. Department of Agriculture (USDA) Forest Service administers the Wood in Transportation (WIT) cost-sharing demonstration grant program to promote cost-effective, structurally sound bridges, preferably using local timber as well as local businesses and employees. The city of Skagway received a grant from the Forest Service for assistance in design and construction of a timber bridge. The grant specified that the bridge must be constructed from untreated Alaska yellow cedar, a naturally decay-resistant wood species, using recently developed stressed-deck technology. For the project, the Forest Service provided monetary assistance and made Alaska yellow cedar trees available from Forest Service land.
In the spring of 1997, the city of Skagway retained Muchmore Engineering International of Juneau, Alaska, to design the bridge across Nelson Slough.
Criteria for bridge aspects relating to stress laminating were based on Chapter 9, "Design of Longitudinal Stress-Laminated Deck Superstructures" of Timber Bridges - Design, Construction, Inspection and Maintenance (USDA Forest Service Publication No. EM 7700-8, August 1992) and on Guide Specifications for the Design of Stress-Laminated Wood Decks , published by the American Association of State Highway and Transportation Officials (AASHTO). All other aspects of the bridge design were based on the Standard Specifications for Highway Bridges, also published by AASHTO (1996).
The bridge is designed for AASHTO HS20-44 loading - a bridge length of slightly more than 23 meters, a width of almost 5 meters outside-to-outside (4.36-meter roadway width), and a skew of 0 degrees. The bridge has three continuous spans of 7.62 meters each, center-to-center of bearing. The stress-laminated deck is about one-third meter deep and consists of 7.62-cm (3-inch) by 33.02-cm (13-in) rough-sawn Alaska yellow cedar laminations and is continuous for the full length of the bridge. Individual laminations are 4.9 meters in length with 3.65-meter, 2.4-meter, and 1.2-meter laminations staggered so that no joints are closer than 1.2 meters in adjacent lines of laminations (per AASHTO Guide specifications).
|The construction crew uses a 35-ton, all-terrain crane with a Vulcan #2 diesel pile hammer to drive the pilings.|
The stressing system is designed for 1.6-cm- (0.62-in-) diameter high-strength galvanized steel thread-bars, conforming to the requirements of ASTM A722 (American Society for Testing and Materials, 1988). The 5.5-meter-long stressing bars are spaced at 0.6-meter centers through holes drilled at mid-depth of the deck laminations with heavy galvanized steel bearing plates at each end. Prestressing tension is applied with a center-hole hydraulic jack, one rod at a time. Rods are sequentially tensioned several times until each rod is "squeezing" the laminations together with about 129 kilonewtons of force. The result is one huge slab of wood that is much like a big butcher block.
The two piers are bents consisting of three pressure-treated Class A Douglas fir piles (ASTM D25-91) driven to a minimum bearing capacity of 18 metric tons with a 30.48-cm (12-in) by 30.48-cm cap and a 7.62-cm (3-in) by 33.02-cm (13-in) cross bracing. Because of environmental constraints in Alaska, the normal creosote or pentachlorophenol treatments are not allowed over streams in the state, so the piles are treated with copper naphthenate in heavy oil by Perma Post Products of Hillsboro, Ore.
Timber running planks were used because there are no paved roads in the immediate area.
The abutments are U-shaped retaining walls consisting of rock-filled gabion baskets, topped with 30.48-cm by 30.48-cm cedar sills to support the bridge ends. Curbs and rail posts, including approach rail posts, are also of Alaska yellow cedar. Bridge rail posts are spaced at 2.4-meter centers. Bridge railing is thrie-beam galvanized steel; approach railing is W-beam galvanized steel; and there is a galvanized transition rail between.
Important features of this stress-laminated timber bridge include:
|The tension rods were restressed in the spring of 1999, and no further stressing will be necessary|
The construction permit from the State Department of Fish and Game specified an in-stream construction window of June 1, 1998, to Aug. 7, 1998. Because the construction contract was not signed until late June, the piling had to be driven before any of the Alaska yellow cedar was delivered. Pile driving was completed by mid-July.
The city of Skagway procured part of the bridge materials prior to advertising the construction contract. This included galvanized hardware, stressing rods, gabion wire baskets, metal railing components, and treated Douglas fir piling. All of which were purchased from regular commercial suppliers. However, procuring the Alaska yellow cedar timbers for the bridge proved to be not so simple.
The city hired Whitestone Logging of Hoonah, Alaska, to cut the cedar trees on Forest Service land on Chichagof Island and to truck them to Icy Straits Lumber Co.'s mill for processing. Difficulty in finding a sufficient number of large cedar trees for bridge timbers delayed timber production. About two-thirds of the cedar was delivered to Skagway in late July, and the remainder was delivered in September.
The sawmill, located on Chichagof Island, is more than 160 kilometers by water route from Skagway. Because no overland roads connect any towns in southeast Alaska, the rough-cut timbers had to be shipped on trailers from the mill via state ferry to Juneau and then by commercial barge to Skagway.
Hamilton Construction Co. of Skagway was awarded the construction contract in late June 1998. The contract was to drive the piling, build the roadway approaches, fabricate the timber, and construct the bridge.
While the contractor was waiting for the cedar to arrive, he built the two pile bents and the gabion abutments. When the cedar arrived, it was fabricated and drilled. Temporary false-work bents were built in each span, and the deck laminations were assembled and then stressed together. The bridge went together very rapidly with no problems, and construction was completed in September.
The rods were restressed in the spring of 1999, and it is expected that no further stressing will be required.
The new bridge over the Nelson Slough is simple, strong, aesthetic, and aromatic.
"It is a terrific-looking bridge, smelling of fresh-cut cedar," said Skagway city manager Bob Ward. It's probably the best-smelling bridge in Alaska.
|This view of the underside of the bridge shows the rough-sawn, Alaska yellow cedar laminations.|
The bridge is strong enough to handle any vehicle legally allowed on the road and as many well-fed tourists as can fit on a bus.
Built exactly a century after Dyea sprang to life during the great Yukon Gold Rush, this "new technology" timber bridge will serve visitors to this historic area for many years to come.
Frank Muchmore is a bridge engineer for the Alaska Department of Transportation and Public Facilities, and he is also the owner and chief engineer of Muchmore Engineering International. Before returning to Alaska in 1993, he served as a structural engineer for the USDA Forest Service in Alaska and Montana for 29 years. During his tenure in the Forest Service, he helped to develop and validate the timber stressed-deck technology. He also developed the servicewide Oracle-based Bridge and Major Culvert Information System still used by the Forest Service. He has a bachelor's degree in civil engineering from Montana State University. Muchmore is a registered professional engineer in Alaska and Montana.