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Delivery of a Cat 3 check to verify the design of the gantry frame, lifting lugs, lifting arrangement and stability to facilitate lifting the 1100 Tonne Thames Tideway sewer cover slab into position.
Client: Sarens
Project Overview
The Thames Tideway cover slab at Abbey Mills was cast to the side of the access shaft and was moved 30 metres and lowered into place using a gantry frame on SPMTs.
Andun Engineering Consultants were contracted to provide the CAT 3 check on the lifting lugs, lifting gantry frame, SPMTs and on the overall stability of the system.
Strand jacks were mounted on top of the gantry as the lifting mechanism.
Our client was Sarens, and the lifting system used a combination of large proprietary box beams and specifically fabricated components that required additional calculations.
Cat 3 Check Overview
The elements of the check included:
Lifting Lugs – custom lifting lugs were designed to suit the structure and the strand jack. These were checked via a comprehensive analysis model.
Slab Loading – due to the size of the slab and the way it was being lifted, the loads applied to each position could vary massively (up to 2000kN from the static state) depending on the balance of load across the strand jacks. This required many load cases to ensure that the worst case combinations were analysed.
Deceleration Forces and Associated Loads – as the slab was being moved on the SPMT, calculations were considered for the deceleration forces and associated loads.
Complex Analysis Model – due to the size of the structure, depth of the sections and the use of the SPMTs, there were a number of different loads and load combinations being applied. Andun Engineering Consultants built a complex model to demonstrate the stresses within the members during the different stages of the lift. Prior to using the analysis output, we conducted a thorough review of the model’s deflections and stresses to ensure the model was working properly.
Gantry Frame Beams – the gantry consisted of large box girders and CHS columns. The components were either checked from first principles using the Eurocode approach, or by using data sheets where possible (though each of these required different load factors).
Torsional Checks – due to the size of the structural components and the lateral forces being applied at their top face, there was a significant torsional stress applied to some beams.
On this project, changes to equipment availability meant adjustments were required and these were handled quickly and effectively to move the project forwards.
AEC were able to review and provide feedback to the design team to ensure the design and associated checks were completed in a timely manner.
Following the completion of the check, AEC found no design issues. However, it is worth bearing in mind that this is not always the case.
An effective Cat 3 process includes excellent communication between the design team and the engineer performing the CAT 3 check. AEC provided a thorough DRN and a marked up drawing where required to highlight any discrepancies.
This forms the basis for feedback and adjustments to be given and made in a constructive and collaborative environment.
Project Outcome
This project attracted significant media attention, due to the scale of the project and the size of the sewer slab. You can see the lift in progress during the video below.