East Multnomah Falls Viaduct

East Multnomah Falls Viaduct

East Multnomah Falls Viaduct, June 1994

East Multnomah Falls Viaduct, June 1994

Location

Carrying the Historic Columbia River Highway between cliffs and the railroad right-of-way east of Multnomah Falls, Multnomah County, Oregon, beginning at milepost 32,3.

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 860′ 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 West Multnomah Falls Viaduct

East Multnomah Falls Viaduct

From Crown Point Viaduct to Horsetail Falls Bridge, a distance of nearly eleven miles, the Historic Columbia River Highway passes by 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 and the Columbia River, between the tracks and a steep mountainside. They carry the Historic Columbia River Highway east and west of Multnomah Falls, a 620′ 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. Furthermore, there were no realistic alternate alignments for the HCRH 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 East Multnomah Falls Viaduct is 860′ in length and originally consisted of forty-three 20′ reinforced-concrete slab spans. The deck was supported by two parallel rows of 16″-square columns, or bents, 17′-6″. The corners are chamfered, both for aesthetic purposes, and to eliminate sharp corners prone to chipping. This shape also facilitated removing the formwork. Roadway width is about 18′. The design engineer K. P. Billner included inclined struts between the footings of the inside and outside piers because he 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. It consisted of a beveled cap of reinforced-concrete and plaster arches reinforced with hybrid lath and separated at regular intervals with concrete posts. The entire guard fence rested on a concrete curb. Finally, because the East 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 East Multnomah Falls Viaduct cost $22,520.83.

Repair and Maintenance

Maintenance records at the Oregon Department of Transportation Bridge Section, located in Salem, do not exist for the East 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, Samuel Lancaster noted that there was so much snow and ice piled on 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 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 that 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.

Several alterations were made to the east viaduct after the 1921 storm. Conde B. McCullough’s Bridge Department in the Salem offices of the Oregon State Highway Department created a design calling for sets of 10″ x 10″ intermediate posts placed mid-span and resting on concrete footings that straddled the inclined struts between existing bents. It also designed replacement 12″ X 12″ reinforced-concrete columns for those that failed. Finally, the designers created wall systems running transversely at the midpoint of each girder span from north elevation to south elevation, under the deck for added structural stability. These consisted of three 12″ x 12″ columns equally spaced and infilled with concrete walls reinforced with \ bars placed in a grid pattern at 4′-0″ centers. The bases of all footings measured 3′- 0″ X 3 ‘-0″.

Since the 1920s, maintenance activities on either the east or west viaduct is unknown. Both are very narrow for modern road standards and have received many scrapes and gouges from automobiles with large overhanging mirrors. 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 Historic Columbia River Highway.

Excerpted from Historic American Engineering Record, East Multnomah Falls Viaduct, HAER OR-36-J.

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