Historic Structures

Description of Bridge Willamette River Swing Truss Railroad Bridge, Portland Oregon

The bridge's through Pennsylvania (Petit) type Parker Truss design utilizes riveted box girders, beams, and lattice struts, pinned panel point connections, and heavy wrought bar ties. Modjeski designed the draw span with ten uniform 24 feet, 6 inch width panels on each side of a 31 feet, 0 inch center tower, again going against prevailing bridge design philosophy which preferred varying panel widths. The bottom deck chord of each truss is straight. The top chords of the fixed side spans are segmentally curved with sloping portals. The draw span is similar but with six panel-straight sloping on each end and a five-panel hump over the center. The draw span is designed to function as a balanced cantilever span when open and as two simple spans when closed and locked to the draw rest supporting piers. It appears from observing the truss panels, that Modjeski adapted the K-truss principles patented from Stephen H. Long (1830) which use intermediate struts to stiffen the main diagonal struts, resulting in a K pattern within a panel.

The draw span's center tower panel is designed as a braced frame, three panels high. The center tower supports the cantilevered draw spans when open and is largely redundant, self support only when the draw is closed. The two simple spans, partially continuous, of the draws are designed to undergo a stress reversal in their top and bottom chords when changed from closed and end supported to open cantilevered positions. Inspection shows that certain struts and ties are designed to preserve their function during this reversal; a rod tie may not function as a strut. Portal, intermediate, and center cross bracing provides for lateral stability of the through truss assemblies.

The materials specified for the bridge, wrought iron and steel, cast iron, and cast steel, represented the state of steel construction technology at the time, in accordance with Association for American Steel Manufacturers for Structural Steel 1903 specifications, a precourser of the current American Institute of Steel Construction (AISC) specifications. The specifications are in general conformance with the 1912 American Bridge Company and American Society for Testing and Materials specifications A9 1901-09, as published in a Carnegie Steel Company 1917 manual. A minimum wrought steel elastic limit (yield) strength of 35,000 pounds per square inch was specified. The riveted connections in the assembly specified especially reamed and aligned holes during fabrication to attain maximum strength from the fastenings. Until it was demolished, the bridge remained fully operational and in excellent condition, without significant alterations since built, and in conformance with modern loading requirements.

Fabricated steel and iron work was to be cleaned and protected with one coat of red lead in oil primer paint except for certain finished surfaces where white lead and tallow or plain linseed oil coatings are specified. Modjeski states that the Willamette Eiver spans were finished after erection with two coats of "Nobrac" paint, an old brand name which appears to have been a special marine air corrosion resisting finish. The original color was probably a carbon or graphite black, as used on the other spans. When the finish colors were changed to the aluminum pigmented finish is not recorded, but presumably in the later 1930s or 1940s after aluminum pigmented industrial finishes became popular.

Bridge piers were built, using pneumatic caissons sunk to the river bottom. The main Willamette River piers are supported on concrete footings bearing on the coarse gravel and rock subgrade of the channel. Only the end abutment piers and north pier "A" have pile supports. Caissons were built of caulked timbers and closed with pressure hatches of iron. The Willamette caissons had to be placed from special barges from which they were lowered by long suspension screws, except for the draw span Pier III which had to be sunk with its barges because of its size. Plain concrete was placed pneumatically on the excavated subgrade within the caisson air chambers. Once the bottom was sealed, the caisson could be emptied of water and the remaining concrete and any ashlar facings placed until above water level. The draw span pier is constructed full height of plain mass concrete, octagonal in section, 47 feet, 8 inches across, with a two course ashlar coping. The draw span pier is almost totally hidden by the timber piling and sheathing of the draw protection, or shear fence.

The timber shear fence has been rebuilt and repaired many times. Its most recent reconstruction was after the 1978 collision of the M/V Marie Bakke with the fence and draw span. The records on the bridge show that some part of the piling and sheathing timbers are replaced several times a year because of damage by steamers or riverboats and their tows. Principal changes in the shear fence from its original configuration have been in removing the taper indicated on the Modjeski plan and in regular changes in the installation of navigation lights and safety markers associated with the draw. Originally, there was to have been a platform and capstan on the downstream works for the emergency manual operation of the draw, possible removed sometime in the early 1950s or before.

The turntable and draw operating machinery is outlined in Modjeski's report. The published Modjeski report has a single drawing (LIV) of a half-section through the center which shows the drum and track but none of the turning machinery. Modjeski describes the machinery and operation:
"The turntable is part rim—and part center-bearing, the load being distributed in the proportion of five-sixths to the rim, and one-sixth to the center. In view of its large proportions, and great frequency of operation, more than ordinary care was exercised in the design of the operating mechanism. The navigation is open the year around; there is no period of closed navigation in which to make repairs. To provide for possible derangement of parts of the swinging machinery, the main pinions and all gears, shafts, etc., were made strong enough to operate the draw at reduced speed, by one pinion only, and by one motor on its overload capacity. The general arrangement of the swinging machinery is the same as in the Vancouver draw (many parts are interchangeable), excepting that all the details are much heavier and entirely of steel. There are two motors of seventy horsepower rated capacity, capable of a short time overload three times greater. As a matter of fact, the friction of the various parts of the turntable proved to be less than the one assumed in the calculations, and the draw operated the ninety degrees in 1-1/4 minutes. The gear ratio was subsequently reduced and slow speed gears added for emergency. As in the Vancouver draw, there are two independent sources of power provided. The gasoline engine is 165 horsepower, four cylinder vertical type. It is directly connected with the generator, and is intended to act as auxiliary power, the current from outside being used to operate the bridge under all ordinary circumstances. A third, or emergency motor, has been placed in position, and may be quickly connected to the gears. As an additional precaution, a hand-turning device has been installed, consisting of capstans placed on the protection, and cables which can be attached to the drum when needed. It is estimated that ten men can swing the draw by hand ninety degrees in twenty minutes."

American Bridge Company of New York contracted the fabrication of the bridge superstructure including the draw span turntable. Otto Gas Engine Works, Chicago, was the contractor for all turning machinery on the Willamette River draw. As originally built, the machinery is primary electrically-powered and -controlled The present turning machinery appears to conform to the original configuration as described by Modjeski and indicated on the drawings, which includes two primary drive motors, south Motor No. 1 and north Motor No. 2, and the auxiliary motor in the center of the turntable. Motors number 1 and 2 are connected to the turning gear train by a main drive pinion gear. Immediately adjacent to the two motors is an air-brake to slow or stop the turning motion. Most of the turning machinery parts are interchangeable between the Vancouver-Portland draws.

Turning Machinery

Only the auxiliary motor has a data and serial number plate attached. This motor, which appears identical to the two main motors, is a Westinghouse 75-horsepower, 550-volt/3-phase/60-cycle induction motor. There were no records of motor replacement, and the existing motors are presumed to be the original ones.

The auxiliary motor is direct connected to a longitudinal drive shaft which extends to the same gear trains driven by the two main motors. At the main drive gears, there is a sliding drive pinion gear at each end of the auxiliary drive shaft which is manually engaged with the drive gear when necessary. This motor is normally out of service and is not connected to the normal operating control circuit and electric power. Its operation must be manually engaged and switched at the main load panel within the drum area.

As originally built, the bridge was supplied with commercial electric power for its operation. Emergency on-site power was generated by a gasoline engine-generator set installed within the Bridgetender's house, up in the center tower. According to Modjeski's specifications, the generator set consisted of a four-cylinder gasoline engine with its supporting fuel and starting equipment and its direct connected dynamo and belt-driven exciter units. This installation is seen in the construction drawings. Burlington Northern records indicate that this emergency power generation equipment was removed in November 1963, because of new provisions for electric service from two sources, at the same time as the installation of electric heating to replace the former oil heater. Electric power now comes from both ends of the bridge, normally Portland General Electric Company with backup from Pacific Power & light Company. Electric service to the bridge is by wires on poles. Prom the service transformers, the electric service runs along the fixed spans in a conduit to the draw rest piers, where it goes to the draw pier in a submarine cable. Power from the fixed pier is conducted to the rotating draw span through a set of collector rings and contacts around the center pivot post, under the machine room. The main power panels and disconnects are located right above at the center of the machinery room inside the drum.

Drive power is transmitted from the motors, normally both in operation, through opposite sets of drive gears providing five reduction steps. The horizontal motor drives are changed to vertical drives by means of a cluster of bevel equaliser gears. The two drive trains on each side are required for backup and to provide for simultaneous rotation of pairs of bull shafts in the same direction. Through this gear train, each motor turns two vertical bull shafts under each draw span. At the bottom of each bull shaft, the bull shaft pinion turns on the bull ring gear and rotates the superstructure about its center. Machinery was overhauled before the Marie Bakke accident in 1957.

As previously stated, all cables, pulleys, and the capstan for emergency manual draw operations were removed. Only the shackle links on the base of the drum remain from the manual operation system. These links proved useful when temporary operation of the draw without its machinery was needed following the Marie Bakke accident; and the draw was turned by attached cables pulled by tug boats.

According to the older operators, the bridge's machinery was formerly lubricated by manually turning the hand-filled grease cups at each fitting. Gears were always hand lubricated with grease applied with wooden paddles. Now, all bearings are lubricated with hand-operated pressure guns and Alemite type fittings. The center pivot is provided with gravity oil cup. Lubricants both protect bearing and meshing surfaces from wear and protect them from rusting. Most of the machinery was originally exposed to the weather, only nominally sheltered under the bridge deck. The two drive motors and brakes and the auxiliary motor are now sheltered within wood frame and plywood sheathed huts built inside the drum's machinery room.

The swing span's end lock machinery is comprised of the end lock and support pier jack drive and linkage and the rail lock mechanisms. The machinery is motor-driven through a gear train located under the bridge deck between the tracks. The final drive is a quadrant gear which rotates a crank shaft with counter-weighted crank arms on each end outside the tracks, in line with the truss panels. A crank arm on each side pulls to unlock or pushes to lock the articulated end lock and jack linkages contained within the end post structure and which swing from a top mounted pin. When closed and locked, this linkage makes a rigid braced column of support from the top pin down to the support stool installed on top of the pier and locks the spans in alignment.

Accessory to the end lock machinery, the rail locks are driven in or withdrawn by a motor-driven crank, aligning the rail ends between the spans and making or breaking the signal connections (shore boxes) between the spans. The sequence of operations which lock and unlock and swing the draw are regulated by mechanical interlocks under the control handles of the operator's console and by signal lights on the control and signal panel in the Bridgetender's house. The Bridgetender manually resets the train signals before and after swinging the draw.

The bridge deck is built of preservative-treated wood ties laid normal to the longitudinal deck beams. Between the tracks, there is a plank walkway. Refuge and maintenance platforms are located at the center of the draw and toward each end outside the tracks. Originally, railroad signal and telegraph wires were carried across the span on the upstream side on bracket-mounted crossarms and glass insulators. The deck construction continues through all fixed spans on either end.

The Bridgetender's house is a wood frame structure, supported by the center tower above the train level. It was extensively rehabilitated and altered in 1963 when its heating was modernized and the standby generator system removed. There is a wood perimeter deck with T&G flooring. The exterior is painted sheet metal clad. Replacement aluminum casement sash has been installed. The interior ceiling has been lowered. Interior finishes are painted plywood and wood trim, resilient flooring, predominantly railroad maintenance blue-gray and aluminum colors. The hip roof is painted standing seam metal. The house is in generally good condition. The exterior deck shows some deterioration. Sheet metal trim on the house, its rain gutters, and the weatherproofing of the roof and floor spaces are in poor condition.

The access stairway on the downstream side of the bridge appears substantially original except for normal replacement of the wooden step treads. There are steel ladders up to the top center of the tower and up the end portals for access to and maintenance of the bridge's marker lights.