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How the beams were prestressed

Design and fabrication of the simply supported bridge beam structures was by local firm Tindall. It is understood that design input came from an outside engineering firm during construction of the structures.

Each beam web, approximately 600mm deep, contains one row of 11, 12mm diameter steel prestressing strands. On such a long span beam, these were cast into the web so that they are at the top over the supports but concentrated and tightly packed together towards the bottom of the web at mid section.

It is understood that this was achieved by forcing the strands to the bottom of the web with a special rod thrust down through the top of the deck during casting. This conically shaped steel rod has a slot at one end to gather the strands together and would have been held in position with a hydraulic ram during prestressing.

Once the beam cured the rod would have been removed and the resulting hole grouted to complete the deck.

However, in such a vital midspan location, ensuring that the grout properly sealed the repair without leaving a water path to the steel would have been difficult. It is understood that an epoxy grout was only used in some locations.

Diagonally braced torsion stiffeners have also been attached to the midspan beam webs of both the bridges to resist twisting in the web. On the collapsed structure these angle section struts appear to have been attached with epoxy at midspan.

However, the neighbouring bridge, which is wider, has solid diaphragms between the webs at midspan attached with bolts drilled straight through the web and, it is assumed, through the prestressing strands cluster.

Spans are also tied together longitudinally with dowels over the supports to resist earthquake forces and prevent the beams falling off their supports.

No surfacing is added to the structure, with pedestrians simply walking on the top of the beam flanges.

Joints between the beams down the centre of the deck and at the supports are filled with mastic. A stainless steel handrail system is bolted to the outer flanges of the deck beams.

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