Prefabrication of Bridge Decks
The concourse bridge decks are made up of 3 – 4 spans of simply supported decks for each rail line. Each bridge deck consists of 29 plate girder rail bridge decks, each with six main girders braced together and tied at the ends with trimmer beams delivered and erected as pairs. Following installation the beams were mass filled with concrete and fitted with platforms, rail lines and canopies. The total tonnage supplied for the project was 3,600 tonnes.
The girders were of a plate configuration, the lengths (spans) required were fabricated and painted at Darlington in 2017 and no additional longitudinal splices were required. The steelwork was prepared using Cleveland Bridge profiling equipment, consisting of a T&I machine and saw and drill line, and with considerable shop welding.
After fabrication, all components were placed in pairs together for a trial assembly to ensure perfect fit and alignment. Upon completion of the trial erection, the deck was separated into component pairs ready for dispatch to London.
Installation of decks and canopy
For the installation of the decks and canopy the project was split into six phases between October 2013 and May 2017. Close co-ordination between Cleveland Bridge and the main contractor Costain was essential to ensure the steelwork could be delivered efficiently to site and erected using either cranes or SPMT systems.
Each stage of installation was carefully planned and exacting to ensure follow-on trades were not hampered. The possessions for working were Rules of the route (very short window when trains are not running) synchronised with very tight and restricted short possessions for delivery vehicle road closures. The entire project takes place in a busy city centre location with narrow streets through which to move delivery vehicles, large plant and equipment.
The size of the steel structures required large bearings. However, installation of the bridge bearings was complicated by the fact there was minimal space on the pier heads to allow both longitudinal and rotational construction movement needed for installation.
This was resolved by Cleveland Bridge’s suggested use of a tapered plate between the bearing and bridge steelwork to allow the bearing rotation. This was sufficient to counterbalance the deflection caused by concrete pouring.
Once the concrete was placed and cured and majority of dead load was applied, Cleveland Bridge jacked up each bearing and replaced the tapered plate with a parallel plate, thus restoring the rotational capacity of the bearing to take the imposed loads.
The lifting schemes for all steelwork installations were planned by Cleveland Bridge’s in-house project teams and included the innovative use of heavy capacity scissor lifts mounted on the top of SPMTs to solve access and installation problems.
Challenges and innovation
The main logistic challenge for the project was the severely restricted site access; a requirement to consider scheduling for follow-on trades; and the essential need to keep the station fully functional. The architect Grimshaw designed the station and complex staging process based on the concept of prefabrication and modular offsite construction, where components are preassembled before being delivered to site to a strict schedule and installed. This has reduced the pressure on the construction programme and again the use of steel was advantageous.
Andrew Byrne of Grimshaw explained:
“When dealing with a complex project such as London Bridge Station, the only way you can keep a handle on the job is to keep the pure conceptual ideas as pure and as clean and as simple as possible so the scheme can move forward with a single coherent vision.”
The last two stages, Deck B and A, were extremely complicated due to the lack of space and the vast number of contractors onsite. Innovation on this project took the form of swept path analysis to study the feasibility of site access and the displacements once onsite, as well as the use of scissor lift by SPMT.
Benefits and achievements
The London Bridge project faced the unique challenge of remaining open for the 50m passengers that use it each year throughout the five year redevelopment; to entirely rebuild the station while keeping it open was a bit like performing open heart surgery while the patient runs a marathon. Despite this challenge, the project met all major milestones, which was vital because missing any of the strict staging deadlines would have had a negative knock-on effect for the project, potentially costing the taxpayer millions. The location of the station, in busy central London, added to the challenge.
Keeping the station moving
Pre-fabrication was a major innovation that allowed the project to meet these critical milestones. A specific example of its successful use was over Christmas 2014, when the project used pre-fabrication to keep operational tracks open despite demolishing the historic masonry of the roof structure and constructing a full 50 linear metres of new modular platform and roof canopy. The surfacing was successfully completed, allowing the railway systems team to commission the new tracks and bring these into service for the public on 2 January 2015. The use of pre-fabrication enabled the team to successfully build the 50m platform fabric in less than 52 hours.
Furthermore, as the programme progressed, the cycle it took for the team to construct each of the bridge decks, platforms and so on has reduced; what was taking nine months to complete initially, was completed in six months by the final two years of the project.
Since the major completion of the station over the Christmas and New Year 2017/18 period and the opening of the new concourse and final five platforms on 2 January 2018, the station has received fantastic feedback from passengers, stakeholders and the Department for Transport. Much of the project’s success can be attributed to the use of pre-fabrication.
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