Tilikum Crossing Transit Bridge. Portland, OR

Transcription

1 Tilikum Crossing Transit Bridge Portland, OR

2 Technical Information Size of Project: The Tilikum Crossing Transit Bridge is 1,720 ft. long and required 24,000 cy total concrete Strength Requirements: The contract required concrete strengths for individual elements: 4,000 psi for barriers and curbs, 5,000 psi for foundations and pylons, 8,000 psi for the deck/superstructure, and 4,000 psi for the light-weight light rail track slab Water/Cement Ratios: The contract required specific water-to-cement ratios for individual elements: 0.44 (barriers and curbs), (foundations and pylons), (superstructure), and 0.44 (light rail track slab) Unique or High Volume Admixtures: Admixtures for the mass concrete mixes included water reducers, stabilizers, and air entraining agents Unique Mix Designs: Because of the bridge length and slender superstructure, Kiewit elected to use a high performance fine aggregate concrete mix. This mix design ensured durability to stand the weight of traffic, structural integrity during seismic events, and longevity. Additionally, the concrete mix used around the light rail track was a concrete with light weight aggregate and fiberglass fibers.

3 Technical Information Tilikum Crossing Transit Bridge Other Special Technical Aspects: The bridge was designed with a thin deck to maximize both its visual appeal and vertical navigational clearance. Though a visually stunning bridge, its minimal, slender dimensions made every aspect of its construction challenging. For instance, the thin deck required seven to eight tendons per segment, not to the typical three or four tendons per segment, to support the weight of traffic. Additionally, each concrete segment and stay cable pitch was unique so every segment form and cable anchor had to be designed uniquely as well. Finally, cantilever construction had to be planned and performed meticulously to not overstress the deck, or to allow the cables to slip through the saddles while the segments were cast on opposite sides of the towers. Unique Specifications/Requirements : The bridge's unique design was selected by TriMet with input from a citizen's advisory committee. To ensure the final bridge met the conceived vision, the contract specified key geometrical and architectural elements including: a smooth all-white concrete finish, bridge deck width and geometry, tower geometry, joint locations, transit-way geometry, edge beam shape, barriers and handrails, column shapes and dimensions, and stay cable arrangement. In addition the mix for the pylons had to attain a strength of 3,200 psi in less than 18 hours so stressing could take place prior to form jumping.

4 Technical Information Placement Challenges or Unique Techniques: Several concrete challenges and techniques to overcome those challenges were used on this cast-in-place (CIP) segmental cable-stay bridge. In-water work windows drove foundation construction for the bridge, which were located 130 ft. offshore. Kiewit built trestles and a work platform to support cofferdam construction, shaft drilling, and shaft pours. After oscillating casings and installing rebar, crews poured 1,720 cy of concrete to build the shafts. Next, the footings required two-stage, 2,400-cy concrete pours. Rotating crews poured 1,800 cy continuously for over 24 hours. The pour was a logistical challenge as it required 180 truck loads of concrete arriving every 5 to 10 minutes. Crews then hand-finished the footing exterior from screed rails. Next, a 3-ft. concrete mat was poured on the footings and the 180-ft. tall CIP towers were built in ten 65-cy lifts using a custom-fabricated jump form. The two CIP pier tables were then constructed from the towers, each totaling 1,000 cy and poured in alternating 10-yd. sections to balance the structure. The CIP deck segments were then built using form travelers, one on each side of the tower, in alternating 16-ft. (115 cy) sections. The majority of the placements were considered mass concrete and required an extensive thermal cooling system. In addition, due to the high concentration of reinforcement with minimal clearance, slump and placement vibration was critical to achieving quality segments.

5 The Tilikum Crossing Transit Bridge is a multi-modal transit, segmental cable-stay bridge across the Willamette River that connects downtown Portland to Milwaukie. The bridge consists of two in-water piers at the towers, two landside piers, 3.5 miles of stay cable, and a 1,720-ft. bridge deck.

6 Originally round with eight shafts, the in-water footings were redesigned by the project team to be smaller ovals with only six shafts. Smaller footings reduced the foundation footprint, lowered seismic demands, and shortened foundation construction.

7 Each in-water tower footing required two-stage, 2,400-cy concrete pours. Rotating crews poured 1,800 cy continuously for over 24 hours. Nearly 180 truckloads of concrete were delivered to the site in 5 to 10 minute increments.

8 The bridge's 180-ft. tall pylons were cast in multiple lifts using jump forms. Saddles for permanent cables were embedded into the rebar cages of the top four lifts.

9 The pier tables were poured in two lifts using false work supported off the pile cap, eliminating the need for in-water false work piles.

10 Temporary precast pylon caps and cables were used to support the weight of construction equipment on the bridge deck. Once the deck was complete, permanent slanted pylon caps were poured and permanent cables installed.

11 Form travelers were used to cast the bridge's 78 deck segments, including closure pours, alternating sides to balance the structure. Deck erection occurred concurrently on the east and west foundations until they reached both landsides, and then closed mid-span over the river.

12 The roadway contains 1,720 ft. of embedded, direct-fixation light rail track and will carry Portland Streetcar.

13 The bridge design called for a smooth white concrete finish for the entire bridge, so special concrete mixes and finishing techniques were used. Great care was taken during forming to minimize patchwork so the aesthetic effect of the bridge could be realized.

14 A stunning "mood" lighting system changes colors based on the Willamette River's speed, height, and water temperature.