Tawlks-Foster Suspension Bridge, 1995

Methow Valley Sport Trail Association, Okanogan County, WA

Use:  Light vehicle, pedestrian, equestrian, cyclist.

Span:  280'

Travel width:  11'

Towers:  38' tall, ACZA treated #1 Douglas Fir.

Anchors:  34 cy concrete vault deadmen w/ twin 1-1/2" anchor rods.

Mainlines:  Twin 1-3/8", ASTM 586, 1x55 Bridge Strand each side.

The Tawlks-Foster Suspension Bridge is a light-vehicle crossing of the Methow River near Mazama, WA. The design is based on a traditional suspension bridge concept with floor beam supported deck and lacking a stiffening truss. Twin mainlines were used to facilitate construction and the A-frame towers were chosen to increase tower stability during periods of heavy snow loading and operation of snow grooming equipment. Managed by the Methow Valley Sport Trail Association, the bridge is the primary crossing of the Methow River, linking a network of more than 200 miles of trails. Situated at the east-end of the North Cascade Highway in Washington State, there are year-round activities in the Methow. Throughout the year the bridge and trail system are open to hiking, cycling, and horseback riding, and in the winter all 200+ miles of the system are groomed for cross country skiing.

Construction began in late fall, with excavation and pouring of the main anchors and tower pylons ocurring in winter, during subzero temperatures. This was necessary because the high water table and porous glacial soils of the Methow Valley preclude deep excavations during any but the cold months of the year, when water levels fall from lack of snow melt runoff. Construction of the towers and mainspan resumed during the more seasonable spring and summer months.

This project was a collaborative effort between a number of partners, among them the Methow Valley Sport Trail Association, Methow Institute Foundation, U.S. Forest Service, Okanogan County, and the Interagency Committee on Recreation for the State of Washington. Funding for the project was limited, and Sahale worked closely with the clients to develop the most economical design possible. The end result we have come to refer to as the economy bridge: engineered and constructed at an astonishingly low cost of $60 per square foot of deck area. Future planned upgrades will bring the ultimate cost to under $100 per square foot. The photos on this page show the completed bridge prior to and during dynamic load testing with 7500 lb. vehicle. This test simulated the use of the bridge by a Pisten Bully, or snow groomer, for which the 11' wide travelway was designed.