Load Transfer Quantification on the Cast Iron Underbridge
Due to its expertise in bridge testing and laser scanning services, DYWIDAG was contracted by one of Network Rail’s Tier 1 framework contractors for the route, to supply detailed information on the bridge’s behaviour.
The COL-94A is a skewed cast iron arch underbridge, located between Deansgate and Salford station in Manchester City Centre. The bridge itself consists of six cast iron bolted segmental arches spanning between bridge abutments and stone springing, with lateral bracing and cast-iron spandrels supporting desk plates. Span 94A crosses Castle Street, a minor road with a pedestrian walkway on one side.
The overall purpose was to gain a deeper understanding of its performance and subsequently develop a maintenance scheme for the asset. This was necessary as inspections carried out by Network Rail had already determined that a number of the ribs were starting to take load above their design level. Their purpose in the original design and installation was to act only as stabilisation to the bracing.
DYWIDAG was therefore tasked with designing, implementing, and providing results to quantify this live load transfer into these transverse ribs. Specifically, AmcoGiffen wanted to understand the axial load forces on 6no., ribs which traversed perpendicular to the bridge connecting the lateral bracing. Any system would need to be able to sample at a frequency high enough to enable a live load understanding and be able to publish a full report for use by the client. One of the challenges faced by DYWIDAG was the bridge carries the mainline into Manchester Piccadilly and therefore obtaining a possession to install train sensors was unlikely. As a result, site technicians would have to mark the data when trains were passing over.
With this challenge, DYWIDAG scoped and designed a series of foil strain gauges, which would be installed around each of the cross-bracing ribs. The set-up was designed to test the structure over a 24hr period, where representative locomotives were due to pass over. The sensors were cabled back to a locally installed comms unit where the data could be transferred to a supervised laptop set up. Installation of the system was carried out from street level, using DYWIDAG’s internal resource of engineers and technicians.
Prior to works commencing, DYWIDAG also carried out a laser scan of the span, in order to provide a complete ‘as built’ and updated reference for future use.
The solution was successful, with the testing programme enabling DYWIDAG to illustrate the level of load that was being transferred and referencing this to types of locomotives e.g. passenger trains or freight. DYWIDAG was also able to quantify the amount of load and how was this was dissipated across the different ribs. Overall, the data produced enabled Network Rail to feed into the larger design model for the bridge and appropriately control its management regime.
The dynamic testing programme was undertaken over 48hr period and was completed on-time and budget, and to the complete satisfaction of the client.