Using the Cantilever Construction Method in Environmentally Sensitive Area
Lower Screwtail Bridge with a total length of 329 m is located in Arizona above a 38m deep, rugged canyon in a very environmentally sensitive area, home to endangered wildlife and at the crossing of two historic paths. To protect this environmentally sensitive area, the four-span bridge was built using the free-cantilever method.
This project is part of a $ 30 million ADOT contract (Arizona Department of Transportation), budgeted for the construction of the Mesa Payson Highway (SR-87), 95 km northeast of Phoenix. Cantilevers on both sides of the pier consisted of 11 bridge segments, each 4.9 m long, 13.7 m wide, and 3 to 5 m deep.
Segmental construction started after the piers were ready to support loading. A single cell box DYWIDAG form traveller was placed on each side of the pier table. The dead load was supported by installing reinforcing bars and short post-tensioning tendons in the transverse and longitudinal directions. Once the concrete was poured and had reached the required strength, the post-tensioning tendons were stressed over the pier and the form travellers advanced forward to the next segment. Symmetrical positioning of the travellers is essential to maintain balance.
The project was unique due to the severe 4.5 % super elevation and the horizontal radius of only 366 m. Especially challenging for DSI and ADOT were contract special provisions that required meeting AASHTO recommendations for special anchorage (American Association of State Highway and Transportation Officials). Approved anchorage assembly required more local zone reinforcing and complicated concrete placement.
DYWIDAG bond head anchorages were placed as dead ends for the transverse post-tensioning. To expedite the construction cycle, stressing was conducted at 25 N/mm2 (3500 psi) without any reported problems.