Freezing Temperatures and Limited Space: Rehabilitation of the Hudson’s Hope Bridge
Approximately 7km south of the remote community of Hudson’s Hope in British Columbia in western Canada, there is a historic 207m span suspension bridge originally completed in 1965. 33 pairs of vertical cable hangers are attached to the two main suspension cables and support the 34 precast concrete deck units. The deck units are post-tensioned using 70 post-tensioning tendons.
After some regular inspections, the Ministry of Transportation & Infrastructure of British Columbia, MoT, decided to completely replace the existing post-tensioning tendons to further the life of this magnificent bridge. DSI Canada Civil was awarded the contract to supply the necessary post-tensioning system and special equipment.
The existing tendons were externally placed in the box sections of the precast units and anchored into a 12.19cm thick diaphragm wall at the ends. Access to the precast sections was only possible through the end diaphragm wall with an opening that was only 100cm wide and 75cm high.
The anchorages of the old post-tensioning system essentially consist of a plate with a conical hole, and a large wedge that is used to grip the cable. Behind the anchor plate is a 38mm Ø duct to allow for installation of the cables after erection of the precast deck units.
This section was ungrouted in the original construction, and the anchorage and wedge were not protected from corrosion. Since the anchorages are located directly below the finger joint expansion joints, over time, water and road salts penetrated the anchorages, causing extreme corrosion and the failure of some of the tendons.
Initially, the approach slabs were removed to allow access to the tendons. Afterwards, the tendons were destressed in a controlled manner, cut up and removed through the small access hatch.
Next, the new strands were winched into place, the end anchorages were installed and the wedges set for stressing. It was decided to winch the bundle of strands together directly from the strand packs as there was no area for the laydown of prefabricated tendons. Since there was no sheathing over the tendon length, they could not be pushed into place.
The existing, 2.86cm Ø tendons were each replaced with 3 galvanized 0.6" prestressing strands. The wedge plate anchorage had to be specially designed to fit with the existing anchorage wedge cavity. Due to the space constraints, the tendons were stressed using a monostrand jack. After the tendons had been stressed, a protective hood was installed along the face of the precast unit over the anchorages and then a concrete patch was poured in place to protect the anchorages. The duct behind the anchorage was also grouted in place to protect the new tendons from moisture.
In total, 70 new, Type 3-0.6", 235m long DYWIDAG Strand Tendons were used. This required a total of 55t of galvanized strand.
This project was very challenging due to its remote location and tight construction schedule as well as the inclement winter weather with freezing temperatures. Furthermore, the bridge had to remain in service during the complete course of work. Despite all difficulties, the General Contractor, Surespan, completed the work approximately 1 month ahead of schedule.