'Some people say post-tensioned structures with external tendons are easier to design than those with internal tendons, ' says structural engineer Atkins special structures group manager Chris Hendy. 'But they're wrong.
'There might appear to be more freedom in the location of tendons, but that's an illusion. By the time you've ensured a clear walkway through the box and access to the drainage, finding room for all the deviators and anchorages is far from easy.'
With the tendons closer together vertically the efficiency of the post-tensioning operation is inevitably reduced. And with no opportunity to spread the tendons across the deck cantilevers, a double layer of tendons was needed in the main balanced cantilevers, leading to some very complex anchorages.
As well as insisting on external rather than internal tendons, the Highways Agency also specified that the bridge should be safe under dead load with 25% of the tendons at any section ignored or two cables removed. It should also be possible for one cable to be removed and replaced with the bridge open to traffic. The design team had to maximise the efficiency of the prestress by all means available.
'For example, BS5400 only allows you to design for 70% of the ultimate tensile strength of the tendon after lock-off, ' says Hendy.
'By contrast, the new Eurocode allows 73.1% and the French code 80%. So we went for73.1%'.
A similar philosophy was applied to the tendons themselves, made up from 15.7mm strand. Ultimate tensile strength of such strands are limited to 1770MPa - but wire making technology has advanced since the standard was published.
A draft Eurocode limits 15.7mm strands to 1860MPa. By adopting such values, Hendy reports, a total saving of 700t of box girder reinforcement was achieved.
Sophisticated analysis was also needed on the massive anchor beams that transfer the compressive forces from the tendons into the cross section of the viaduct box girders. Measuring up to 2.5m wide and 1.5m deep, the most heavily loaded has a design load of no less than 10,000t. A finite element brick model was used to check the design - but buildability also had to be considered, Hendy points out.
'These beams had to be heavily reinforced. The reinforcement cages weighed up to 20t. It made sense to design them to be prefabricated at ground level and lifted into position by crane.'