Unforeseen expansion of commuter use of London's Docklands Light Railway led WS Atkins and Partners to reassess the fatigue performance of its bridges. These were of composite construction with concrete deck and steel plate girders. The reassessment showed that, due to increased train length and frequency, there was a need to augment the number of shear connectors between the decks and the top flanges of the girders - not an easy undertaking for an existing bridge.
One possible solution was to cut slots from the top of the deck down to the surface of the flange, weld on additional shear connectors and refill the slots with concrete. However, this would have meant closing the railway not only whilst the work was done but also for long enough afterwards for the concrete to cure.
The solution finally adopted was to drill holes up through the 35mm thick steel flanges and 125mm into the concrete. Spiral spring pins driven into the holes from underneath then provided the additional shear connection.
The advantage of these was that they had a diametral elasticity many times greater than a solid pin and were able to provide a tight fit without the precision machining which would be required if the pins were solid.
TWI came up with the spiral spring pin suggestion, but its main role was to carry out static and fatigue tests on specimens which modelled, at full thickness scale, short lengths of deck to flange connection.
The fact that the solution adopted enabled the strengthening work to be undertaken without interruption to traffic saved the operating company about £2M.