West Multnomah Falls Viaduct

West Multnomah Falls Viaduct

West Multnomah Falls Viaduct, ca. 1915

West Multnomah Falls Viaduct, ca. 1915


West Multnomah Falls Viaduct, ca. 1920

West Multnomah Falls Viaduct, ca. 1920


West Multnomah Falls Viaduct, July 1995

West Multnomah Falls Viaduct, July 1995

Location

Carrying the Historic Columbia River Highway along a narrow right-of-way between cliffs and railroad trackage west of Multnomah Falls, Multnomah County, Oregon, beginning at milepost 31,9.

Date of Construction

1914

Engineer

K. P. Billner, designing engineer, Oregon State Highway Department

Builder

Pacific Bridge Company, Portland

Owner

Oregon Department of Transportation

Present Use

Vehicular traffic

Significance

This 400′ half-viaduct was an engineer’s solution to aligning the Historic Columbia River Highway between a steep unstable slope and the Oregon Washington Railroad and Navigation Company main line right-of-way. It is a companion to the East Multnomah Falls Viaduct.

West Multnomah Falls Viaduct

From Crown Point Viaduct to Horsetail Falls Bridge, a distance of nearly eleven miles, the Historic Columbia River Highway passes one of the best collections of high waterfalls in North America. In the middle, the West and East Multnomah Falls viaducts run parallel to the Oregon Washington Railroad and Navigation Company (OWRN) line along the Columbia River, between the tracks and a steep mountainside. These viaducts carry the Historic Columbia River Highway east and west of Multnomah Falls, a 520* cataract which is one of the tallest on the continent, attracting visitors for centuries. Multnomah Falls was one of the destinations that Lancaster wished to include on his highway. Also, there were no realistic alternate alignments for the Historic Columbia River Highway in this section because the OWRN’s mainline ran through the narrow passageway between the mountains and the river.

Lancaster avoided marring the natural beauty of the landscape wherever possible. He often saw the best solution to creating satisfactory road alignments as simply constructing the road on fill behind solid dry masonry retaining walls. In creating the West and East Multnomah Falls viaducts he also faced the problem of bridging very steep and unstable rock slopes that were susceptible to slide action. Even minimal cutting and filling at the toe of these mountainsides, held together by underbrush and timber, might cause avalanches of rock and debris to cover the roadway and, probably more importantly, block the OWRN’s main line. Finally, the costs alone for excavating the toe of the mountainside were prohibitive, and authorization by the OWRN to carry materials across its main line to dump in the river was not possible. Lancaster’s solution to the problem was to employ viaducts, resting on unequal length columns, anchored to the slopes and nearly overhanging the OWRN right-of-way.

Design and Description

The West Multnomah Falls Viaduct is 400′ in length and consists of twenty 20’ reinforced-concrete slab spans. The deck is supported by two parallel rows of 16″-square columns, or bents, 17′-6″ apart. The corners are chamfered, both for aesthetic purposes, and to eliminate sharp corners prone to chipping. This shape also facilitated removal of the formwork. Roadway width is about 18′. The design engineer K. P. Billner included inclined struts between the footings of the inside and outside columns. He probably found firmer foundations for the outside bents than for the inside bents and saw a need to guard against settling of the upper columns and to achieve greater structural stability. With confidence he believed that they could “carry the weight of the structure.”

A theme carried throughout the Historic Columbia River Highway was the use of arches in concrete and masonry structures. Most of the larger bridges on the route take the arch form, and arched drainage openings were incorporated into the design of most masonry guard rails. On the West and East Multnomah Falls viaducts, along with the Oneonta Gorge Creek Bridge, the Horsetail Falls Bridge , and other structures on the Pacific Highway, designers used a delicate post and arch coursing, with a beveled cap for guard rails constructed of concrete and reinforcing steel, and arches constructed of plaster and hybrid steel lath. Each guard fence rested on a concrete curb. Finally, because the West Multnomah Falls Viaduct’s south elevation abutted the mountainside, the concrete railing was used only on the north elevation, paralleling the OWRN rail line, and a taller curb-like reinforced-concrete retaining wall formed the roadway barrier on the mountainside elevation. The cost for West Multnomah Falls Viaduct was $10,513.11.

Repair and Maintenance

Maintenance records at the Oregon Department of Transportation Bridge Section, located in Salem, do not exist for the West Multnomah Falls Viaduct. Nevertheless, it is known from newspaper accounts that in late November 1921 a strong winter storm covered the Columbia River Gorge with a heavy blanket of sleet, snow, and ice. Upon inspecting structures along the HCRH for damage in the aftermath of the storm, Lancaster noted that there was so much snow and ice piled upon the West and East Multnomah Falls viaducts that they were both in danger of collapse. Shortly, the Multnomah County Roadmaster and several engineers inspected the viaducts and found that several bents on the mountain side of the east viaduct had sunk into the unstable slope, causing some rotation in the structure. The west viaduct showed no sign of collapse. The Roadmaster contemplated sending out trucks and men with shovels to remove the snow, but by the time they reached the viaducts, the snow would have melted. As a more reasonable alternative he thought about ordering crews to put in place temporary jacks to shore up the deck.

It appears that the Roadmaster delayed action on the matter because several days later Lancaster became very irritated that nothing had been done. He pleaded with the Multnomah County Commissioners and officials of the OWRN that if crews did not install jacks in the failed sections of the east viaduct immediately, the structure might collapse, sending steel and concrete rubble on to the OWRN main line. It was to no avail, and the county commissioners decided to wait for the snow to melt. In 1922, the Oregon State Highway Department retrofitted the East Multnomah Falls Viaduct with additional columns and walls.

Maintenance activities on either viaduct since the 1920s are unknown. In the early 1990s, an Oregon Department of Transportation mason recast the concrete plaster outer guardrail on both viaducts as part of a long-term rehabilitation program for historic road resources along the highway.

Excerpted from Historic American Engineering Record, West Multnomah Falls Viaduct, HAER OR-36-G.

Historian: Robert W. Hadlow, Phd., September 1995.
Transmitted by: Lisa M. Pfueller, September, 1996.