Since 1994, PB has been conducting in-depth inspections of the mechanical and electrical systems of movable and fixed bridges under an on-call service contract with the Washington State Department of Transportation (WSDOT) and municipalities in Washington. Testing criteria are based on the International Electrical Testing Association (NETA) Maintenance Testing Specifications. All the electrical power and control systems need to be evaluated for compliance with the following standards:

• National Electrical Code (NEC)
• AASHTO Standard Specifications for Movable Highway Bridges
• AASHTO Movable Bridge Inspection, Evaluation, and Maintenance Manual (1998)
• United States Coast Guard 33CFR Part 118, Bridge Lighting and other Signals
• Manual on Uniform Traffic Control Devices (applicable sections for movable bridges), published by the Federal Highway Administration.

The inspections include visual and tactile inspection of the bridge electrical components and interviews with the electricians who service the bridge. Motor starters, motor brush compartments and limit switch enclosures are opened, parts are inspected for wear and signs of overheating and motor currents are measured. The service entrance conductors, motor feeders and submarine cable insulation is megger tested for resistance to ground. The following excerpts from recent inspection reports for movable bridges demonstrate the variety of bridges we have been inspecting in Washington.

First Avenue South Bridge

Figure 1: First Avenue South Bridge

The First Avenue South Bridge is a modified Chicago trunnion-type steel-truss double leaf bascule bridge that carries four lanes of traffic over the Duwamish River in Seattle (Figure 1 on the following page). It has an unusual tubular steel trunnion shaft that extends transversely between the bascule trusses. This shaft is supported by two trunnion bearing assemblies located outboard of each bascule truss.

Completed in 1996, the bridge is a modern twin structure to an existing bascule bridge. In-depth mechanical and electrical inspections of the bridge's operating systems performed in August 1998 showed that, overall, the bridge machinery and electrical equipment were in good to new condition. The hydraulic drive systems operate the bascule spans at relatively low pressures. Under normal operating conditions (low wind, all pumps on line), the bascule span moves freely and the hydraulic drives run smoothly. The hydraulic pump motors draw less than full load amperes. Only minor maintenance items were noted as needing attention.

Montlake Boulevard Bridge

Figure 2: Montlake Boulevard Bridge, Seattle, Washington

Erected in 1925, Montlake Bridge is a double leaf, trunnion bascule bridge that carries four lanes of traffic over the Lake Washington Ship Canal (Figure 2). Each bascule leaf is powered by two 65 horsepower split case 500-volt d.c. electric motors that are coupled to an open gear train. The current practice is to operate each leaf on a single motor. There are two brakes with solenoid type operators on each leaf.

An in-depth electrical inspection performed during 1996 showed that, overall, the bridge electrical system rated fair. Most of the equipment did not look its age, and with the then current levels of activity and maintenance, the bridge system would be operational with a reasonable amount of reliability for five or more years from the time of inspection.
It should be noted, however, that PB has since designed the bridge rehabilitation, which is under construction. When completed, Montlake Bridge will have new racks, new instrumentation, and a new control system. The existing motors will be refurbished and operated by new drive systems. The rehabilitation is intended to extend the operational life of the bridge by more than 25 years.

Figure 3: Evergreen Point Bridge, Seattle, Washington

Evergreen Point Bridge

The Evergreen Point Bridge is a floating concrete bridge that carries four lanes of SR 520 traffic over Lake Washington (Figure 3). It has two floating draw spans that retract into the flanking pontoons to create a 61-m (200-foot) opening for marine traffic. The flanking pontoons have a lift span that raises the roadway deck and machinery to pull the draw span under the raised deck. The bridge was opened to traffic in 1963.

In the mid 1980s, one of the two lift spans started to rise suddenly during regular traffic operations, blocking the four traffic lanes without any warning. Most drivers were able to stop. One car crashed into the partially raised span, however, and the driver was fatally injured.

WSDOT called on PB to identify the cause of the malfunction. The power was disconnected while the control and power diagrams were studied. On the following weekend nights, the bridge was closed to traffic to allow testing and trouble shooting. We started the search with a good idea of what might be the cause, so it took just a few hours to identify the problem. It was the unfortunate coincidence of two short circuits in the ungrounded system that caused the lift motors to be energized. Additional safeguards were recommended by PB and implemented by WSDOT following this tragic incident. [Ed. note: For more information on problems that can arise with ungrounded systems, see "Hood Canal Bridge: A Study of the West Structure Control System" by Mark VanDeRee.]

A random electrical inspection performed in 1997 revealed that the west structure high voltage service entrance cable had failed during testing. The failure required replacement of the cable and some high voltage equipment. With the exception of the failed 4,160-volt cables on the west structure, the bridge electrical system rates good overall.

Figure 4: Hylebos Waterway Bridge Figure 5: Hylebos Waterway Bridge Drive Machinery (trunnion type)

Hylebos Waterway Bridge

Constructed in 1939, the Hylebos Waterway Bridge is a Chicago trunnion-type double leaf bascule that carriers two lanes of traffic over the Hylebos Waterway (Figure 4). The bridge was constructed in 1939. The majority of the bridge machinery is original, with the exception of the rack pinions and span lock drives that have been replaced. The bridge is opened more than 25 times each week and occasionally more than 50 times a week. We performed a mechanical and electrical inspection on the bridge's operating systems in July 1998 for the City of Tacoma.

Each bascule leaf is driven by two 60 horsepower electric drive motors. The gear train (Figure 5) consists of a primary enclosed gear differential speed reducer, two secondary enclosed gear speed reducers, two open spur gear reduction sets and two final drive racks and pinions. The enclosed speed reducers are joined by floating power transmission shafts and flexible couplings. The open spur gear sets are mounted on forged steel shafts. These shafts are supported by bronze bushing type sleeve bearings. There are two motor brakes on each drive system. The racks are mounted to the main truss members of the bascule spans. Each bascule span is supported by two simple trunnions. The trunnion bearings are the bronze bushing type.

While observing the bridge during several test openings, we saw that deficiencies in the control system did not allow it to operate properly. Movement of the spans was very erratic, and the operator was required to start and stop the bascule leaves frequently throughout the entire range of travel. Severe hard stops transmitted the inertia energy of the leaves through the span drive machinery, so after each stop the leaves oscillated, sending reversing loads through the gear drive. Operation of the bridge in this manner placed the drive machinery under undue stress and fatigue. We advised the city that this bridge will need extensive repairs, replacement of components and maintenance.

Figure 6: Thea Foss (City Waterway) Bridge, Tacoma, Washington

Thea Foss (City Waterway) Bridge

The Thea Foss Bridge, also known as the City Waterway Bridge, is a vertical lift bridge that carries four lanes of traffic and two wooden sidewalks over the Thea Foss Waterway in Tacoma (Figure 6). The lift span is powered by two 75 horsepower, wound rotor electric motors that are coupled to an open gear train. There are two brakes with thrustor operators.

The bridge was built in 1911. There have been many modifications since then including replacement of the drive motors, brakes and motor controls. Although no drawings or records could be found during the inspection, an instruction sheet dated August 1951 was found in the box of a spare contactor, suggesting that the work was done in the early 1950s.

Our in-depth electrical inspection performed in June 1996 showed that, in general, the electrical equipment with the motors and controls was in fair condition, but the original conduit and wiring was in poor condition. We determined that at the then present level of activity and maintenance, it would be possible to keep the movable span electrical system in operation with a reasonable amount of reliability for five or more years, but recommended replacing the entire electrical system after then.

The electrical service was in need of immediate attention because there were no disconnect switches or overcurrent protection. The vertical bus system needed to be replaced because of a significant amount of corrosion and wear. We also recommended that fences be installed around the transformers and buses to prevent unauthorized persons from coming in contact with live parts.

WSDOT has since proposed to remove the Thea Foss Bridge and route traffic over a recently completed fixed bridge, resulting in a detour of about one mile. The City of Tacoma and local business people prefer a more direct access between the downtown business district and the port area, so the city has hired PB to study the technical and economic feasibility of replacing the Thea Foss Bridge with a low-level bascule bridge.