On the Norwich Northern Distributor Road we were asked to create a 3d model of Rackheath bridge in order to determine if the Client’s design was fit to be built.
By creating this model it became apparent that the reinforced concrete diaphragm beam at the abutments could not be constructed as designed by the client.
• Due to the length of the longitudinal bars it would be extremely difficult to install them between steel beams already installed
• Closed links would be impossible to install due to the intermediate bracing between the beams
• Stirrup reinforcement clashed with the intermediate bracing
The clash between the reinforcement and the bracing can be seen in the screenshot below
Solutions suggested to the client included
• Changing the longitudinal reinforcement bars so that the bars could be installed on the ground before the steel beams are lifted into place
• Changing the closed links to open U bars so that they could be fixed after the intermediate bracing
• Moving the stirrup reinforcement or move the intermediate bracing away from the reinforced concrete beam
These proposed changes would allow the majority of the reinforcement to be installed on the ground, reducing the inherent risk of installing reinforcement at height, during a rail possession at night.
The suggested solution to the longitudinal reinforcement bars problem is shown below
If Balfour Beatty had attempted to construct the bridge without the 3d modelling it would have been impossible to fix the reinforcement as designed.
This would have led to large costs due to wasted possessions, including costs for plant, people and materials, together with delays to the programme.
It has been calculated the costs for the aborted work could easily be in the order of £500,000
Other problems found
As part of this exercise dynamic cross sections were cut which revealed that there wasn’t enough level difference between the proposed road levels and the proposed beams to accommodate the road surfacing and the deck, as shown below
This problem was subsequently sent back to the road designers who had to raise the vertical alignment of the road to allow for the road pavement layers and the deck construction