Fast Progress With Overbridge

Press, Volume CIV, Issue 30687, 1 March 1965, Page 1

Fast Progress With Overbridge

Fast progress was made during the week-end in laying the first pre-stressed concrete decking units o n Christchurch’s biggest pre-stressed concrete bridge—the 1000 ft overbridge at the Colombo streetMoorhouse avenue crossing.

Three of the 50ft bays of the bridge were decked with a total of about 41 prestressed decking units, each weighing about nine tons.

The units, each 50ft long, •re part of 170 being cast for the job at the Blenheim road

plant of Certified Concrete (Christchurch), Ltd. They are the biggest pre-stressed beams yet used for bridge construction in the Christchurch area. An interesting feature of the bridge will be that each half, extending a length of nearly 500 ft, will be strung together with hundreds of strands of ultra-strong hightensile steel cable threaded through ducts cast in the length of the decking beams. The beams will thus eventually be supported partly by the tension in these longitudinal stressing cables, with the weight being shared by the piers, which are built at approximately 53ft centres throughout the length of the structure.

The decking units, all of which are pre-stressed when pre-cast, can be stockpiled in advance for placing by crane. They, too, are supported by cables tensioned in the width of the bridge. Some of the pillars are built with about an inph space in the height of each to compensate for contraction and expansion in the length of the bridge. A system of steel reinforcing where the piers meet the foundation pads and at the caps of the piers also provides latitude for normal movement. The bottom of the decking is about 15ft above the roadway.

The decking provides a carriageway 50ft wide, and the traffic passage underneath

is also about the same width. Concrete coping at the edges of the bridge will have an iron railing about 2ft high. “Construction is going on with a pleasing minimum of interference with traffic, and completion of the bridge is expected by the middle of the year,” said a spokesman for Messrs Powell, Fenwick, and partners, the structural engineers for the bridge. He said that the laying of the pre-cast decking beams would be done mainly at week-ends to ensure that there was no obstruction to traffic during peak-traffic periods during the week. The general contractor for the bridge is Wilkins and Davies, Ltd.