By: Dipal P. Vimawala, S.E., P.E.
The historic Wells Street Bridge is a double-deck, double-leaf, fixed-trunnion bascule bridge. It crosses the Chicago River and carries the double-track Chicago Transit Authority elevated railway on the upper level and vehicle, bicycle and pedestrian traffic on the lower level. The overall length of the main span is 345 ft and the overall width is 72 ft. The Wells Street Bridge was built in 1922 and is listed on the Illinois Historic Bridge Survey.
The bridge superstructure was in poor condition with a current Inventory Rating of HS 4.7 and Operating Rating of HS 7.9, with a posted load limit of 10 tons. The sufficiency rating for the structure was only 19.0. Reconstruction of this project was one of the top priorities for the Chicago Department of Transportation (CDOT), Division of Engineering, which owns and maintains the bridge. The bridge has been in operation far longer than the 50- to 60-year lifespan of a typical movable bridge. This longevity can be attributed to a combination of robust original construction and routine repairs.
Engineering
Although one of the key elements of this project is the structural design, equally important components of this project are mechanical and electrical engineering, bridge houses, counterweight design, staging to have the least impact to CTA services and coordination with various agencies as the project is located in the heart of downtown Chicago and is surrounded by major traffic corridors and businesses.
Dual truss girders, as main load-carrying members, support both level of framing supported by transverse floor beams spaced about 14 ft. At the lower level, metal grating is supported by longitudinal roadway stringers which span between transverse floor beams. Sidewalks are supported on brackets which are cantilevered from the exterior truss girders. They consist of similar steel grid deck; however, the grid was filled with concrete. At the upper level, the CTA structure is supported by longitudinal track stringers spanning between transverse floor beams. The rear-joint and center-joint assemblies are critical design elements where the movable and fixed parts come to gather. Two bridge houses are located along the northwest and southeast corners of the movable span.
For the main bridge structure, a major rehabilitation to restore the bridge to an adequate rating required replacement of a number of truss members along with the entire upper- and lower-level framing, floor system and lateral bracings. The “river arm” portion of the trusses from the cantilever tip of the truss to panel points 10-13 were entirely replaced. The remaining portion of the trusses received significant repairs to the bottom chord, verticals and select diagonal members. The center-break castings and rear brakes needed to be adjusted to accommodate the new floor system. The live-load bearing castings will be cleaned, repaired and reset. Steel components including the trunnion frames, anchor frames and machinery frames also will be repaired. The existing paint has extensive deterioration throughout and the entire bridge will receive a new coat.
Sidewalks are being replaced with fiberglass decking and existing railings will be replaced with new railings to match the original historic railings.
Since significant rehabilitation is to be performed to the structure, a detailed structural analysis was performed to determine the appropriate counterweight adjustments to bring the bridge to a “balanced” condition while keeping track of addition/deletion of weights of the structural components. STAAD-Pro was used to model the bridge superstructure. The upper-level structure supporting the CTA lines was designed using ASD methods in accordance with AREMA and CTA guidelines while the lower-level roadway structure was designed using AASHTO-LRFD specifications.
The bridge is equipped with four machinery drives—two machinery drives for each truss. The prime mover of each drive is a 100-hp 550 VDC mill-type electric motor. These are the original motors that were installed in the 1920s. Each machinery drive is equipped with a machinery brake and a motor brake. The machinery brake is a hydraulic thrustor operated by a wheel and shoe-type brake. The motor brake is a solenoid-operated wheel and shoe-type brake. The motor brake wheel, 22 in. in diameter, is directly connected to the electric motor shaft.
The bridge machinery drive motors will be removed for complete refurbishing. Refurbishing of the original motors was selected over complete motor replacement to retain the same geometric relationship between the motor-shaft centerline and the centerline of mating spur gears as they are mounted upon the existing machinery frame. Refurbishing can be readily performed in a motor rebuilding house and be tested to meet original torque-speed requirements. The project included the replacement of the existing solenoid-operated motor brakes with hydraulically operated motor brakes.
The rear-lock mechanism consists of a pair of motors which drives the speed-reducing and load-transfer mechanism to a toggle linkage locking mechanism that locks into a receiving bracket mounted on the rear of the truss and resists live loads applied between the rear bridge leaf break and the trunnion. These rear locks are driven with a pair of 7.5-hp AC motors with integral motor brakes. These motors are connected to a common shaft which drives a crank shaft, linkage and toggle-type linkage assembly to insert an upper rocker casting beneath the heel lock shoe of the roadside of the bridge truss.
Center locks are provided at the top chord. There is one linkage on each of the top chords driven by a linkage powered from two 3-hp, 900-rpm motors. The entire center-lock mechanism will be removed and sent to a machine shop for refurbishing.
The power to operate the bridge is provided by two independent electrical sources. The south side of the bridge is fed from a three-phase, 480-volt AC Commonwealth Edison utility service feed. The north side of the bridge is fed from a 600-volt DC feed provided by the CTA.
The bridge’s south side feed was installed during the Lower Wacker construction project around 2000. The three-phase, 480-volt AC power is converted into 600-volt DC power by two rectifier transformers. The DC power is then distributed to the bridge motors via the DC controller and resistor banks. A 480-120/208V transformer was installed to provide power to the south leaf bridge equipment requiring AC power (such as lights, receptacles, gates, etc.) in the south tower as well as AC power to the north side via submarine cable. The CTA feed for the north leaf is being replaced with a three-phase, 480-volt AC feed from ComEd similar to the feed installed for the south leaf of the bridge.
Each bridge house has a controller which controls its associated bridge leaf. New control consoles will be installed. The bridge’s up/down direction and speed is controlled by a trolley-car-type control lever and a foot-pedal brake located on the floor of the tower. Equipment such as the trolley control lever, gauges and switches are from the original bridge construction. New submarine cables are being installed to power up the north side DC motor leafs as well as AC power and controls.
The bridge is listed on the National Register of Historic Structures. To that extent, a detailed historical research was conducted to identify critical historical features that must be restored to maintain the original characteristics of the bridge. Historical drawings, photographs and material used since construction of the bridge were reviewed in detail. The project has gone through extensive coordination with the Illinois Historic Preservation Agency (IHPA) to obtain their approval for the proposed repairs/modifications to the bridge structure. The following were the key outcomes of the historical review:
- The new railing should match the original cast-iron ornamental railings;
- New truss elements must match the original open-web, built-up lattice sections;
- Existing truss-type stringers at the upper level can be replaced with solid web-plate girders; and
- Existing rivets may be replaced with high-strength bolts.
The bridge houses will receive select repairs and upgrades to the interiors, roof, walls and windows.
Construction staging and coordination with CTA
To ensure a successful completion of the reconstruction of this bridge, an effective communication and coordination between CDOT and CTA was the key. CTA operates its two major service lines over the upper portion of the bascule bridge. The train operations are critical to commuters as well as tourists thus critical for businesses in downtown. During an early stage of the project, coordination and detailed discussions were conducted to determine allowable windows of track shut downs. These shut downs were necessary in order to replace a major portion of trusses and the entire upper-level framing carrying CTA tracks.
Only one bridge leaf was reconstructed at a time. This allowed accommodation of the river traffic below the bridge. The vehicular and pedestrian traffic from the lower level were safely detoured.
Replacement of truss river arms, entire upper-level framing and select lower-level framing to maintain the stability of the trusses was performed in two shutdowns. CTA allowed two windows of nine-day shutdowns, spaced about six weeks apart, for the replacement of the trusses. Construction of the project started in the fall of 2012. River arm trusses were procured and fabricated earlier under a separate contract to expedite the project schedule. The south leaf was replaced during the first nine-day shutdown, followed by the north leaf during the second nine-day shutdown in the spring of 2013.
The first shoring towers were installed under the counterweights and the estimated unbalanced load was jacked into the shores. Initial and interim balance checks were performed, including temporary loading of replaced spans, to maintain the balance. Existing trusses were removed at panel points 10 and 13 and lowered on the barges and moved out of the way. A “catcher beam” along the lower chord of the existing truss was installed to provide stability and partial support during fit up of the new truss. The new truss assembly was completely prefabricated, including all major components of the trusses, off-site and floated on a barge into the final position. New trusses were jacked into the final position using shoring towers on a barge and connected with existing trusses.
The project is anticipated to be completed by December 2013.
