Gold-rush ghost town gets a new Alaska yellow cedar bridge - bridge gives access to Dyea, Alaska

Public Roads, Sept-Oct, 1999 by Frank W. Muchmore

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 coast 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.

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.

Design

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 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 centerhole 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.