How engineers built a record breaking £32m Meccano bridge over Clarendon Dock
08 December 2015
Image Courtesy of Jonny McKee Photography
The School of Planning, Architecture and Civil Engineering (SPACE) at Queen’s University Belfast designed and constructed the largest Meccano structure in the world during the course of 2014 and 2015.
The successful end product was a functioning cable stayed footbridge which was placed over the historic Clarendon Dock in Belfast by civil engineering contractor McLaughlin and Harvey. The project began in September 2014 as a design concept for third-year students within SPACE. The primary purpose was to challenge the students to create something inspirational for both themselves and others to observe.
Meccano was the material of choice as it was sufficiently strong and usable by the students with which to create a ‘real’ structure. The ambition to actually build this structure, if it was possible, required input from industry to guide both the design and construction works. Thus, from the outset AECOM and McLaughlin and Harvey provided free technical input to the process and gave the students exposure to the exacting requirements of leading private sector firms.
The visual and exciting nature of the project attracted interest from local schools and so during the course of construction 40 pupils were invited to the university to take part in the building process.
At project commencement the students developed design concepts for a bridge which could span across the river Lagan in Belfast made entirely from Meccano. The initial site chosen was close to Shaw’s bridge on the outskirts of Belfast, where the Lagan was 25m bank to bank.
The chosen design span of 28m was slightly larger to ensure the main structure did not encroach close to the river and potentially cause a collapse of the bank. The concepts began as simple engineering sketches, in which various structural forms were explored. From these sketches load paths were established and the forces within the main components were determined by simple static analysis.
The engineering properties of the Meccano components were determined by destructive testing of the materials in the laboratory. Flat strips of Meccano are remarkably strong with an ultimate failure load of approximately 7kN (700kg), although the material is somewhat more brittle than typical construction steels.
At this stage many of the concepts (flat truss configurations, below deck arches, suspension bridge) could be eliminated with relative ease for a range of reasons such as the magnitude of the forces, dynamic effects and, most significantly, practicality of building such a structure within a university context.
The remaining concepts were a tied arch and a cable stayed structure. Models at 1:10 scale were produced from laser cut mdf components. These models assisted in the final decision to attempt construction of a cable stayed bridge mainly on the basis of relative ease of assembly. Further analysis of a refined structure was carried out to verify the global forces, deflections and expected behaviour.
On the basis of this initial study it was determined that it would be possible to create a Meccano footbridge of the required length and within budget. The next phase in the project involved developing prototypes of the main sections of the bridge; the towers and the deck.
Due to the large number of components and the potential for a localised failure in the many thousands of components it was decided that the deck would be designed as a series of short trusses spanning from cable support points (a distance of 3.6m) rather than a continuous member with hogging at the cables. These short sections were load tested to 100kg to determine the load deflection response and recovery. The deflection was found to be 4mm giving span/deflection of 900 which was clearly better than the limiting ratio of 250.
Detailed design and construction of the tower components was developed in parallel with the deck. Three different cross-sectional configurations were developed for the columns ranging from relatively simple construction to intricate detailed assemblies.
The predicted capacities of each section was estimated using buckling theory. Spreadsheets were created to allow the second moment of area of the compound sections to be recalculated with ease as the geometry and designs were modified. Small sections of length 1.2m were assembled and tested to destruction to give confidence in the predicted behaviour.
The chosen structural form meant that should the towers of the bridge begin to fail no alternative load paths could be developed, unlike the deck. Thus, the tower sections of the bridge were deemed to be critical components, which should they fail would result in a complete collapse of the bridge.
As a result of this, in conjunction with the large number of structural components it was decided that a factor of safety of a minimum of five would be required.
The chosen configuration of the cross – section resulted in an even higher theoretical buckling capacity. The buckling length was assumed to be the full tower height, however it is most likely that that effective buckling length would have been reduced due to some later resistance offered by the interconnection of the towers, stability provided by the cables and deck and also the end conditions were not truly pinned!
The resultant design of the towers was thus robust and gave the team confidence that a structural failure would not occur despite not having had the luxury of testing a full tower. Spreadsheets were also created to allow rapid recalculation of forces as the geometry continued to be adjusted during the prototype and refinement stage.
Fabrication phase and trial build
With the detailed design complete and prototype subassemblies constructed the project team moved forward with the full bridge construction phase. The full bridge was comprised of approximately 11,000 standard Meccano members and 70,000 nuts bolts and washers.
Despite this complexity and size the experience of manufacturing the prototypes allowed an accurate time schedule to be established for the manufacturing of the main components. The schedule deemed that manufacturing would be completed within six weeks or by the end of June 2015 allowing time for other studies and examinations!
The bridge deck was manufactured in eight sections 3.6m long weighing 40kg each in the Light Structures laboratory within the university. Each completed unit was then transported to the Heavy structures laboratory and gradually linked with each subsequent section. The tower components were assembled in small 3m lengths for ease of handling, ability to have multiple teams producing the sections, before full assembly within the university Heavy Structures laboratory.
By mid-June 2015 50 per cent of the components were fully fabricated and a trial assembly of half the bridge was carried out within the Heavy Structures laboratory. This was a significant milestone as it verified that the subassemblies fitted together correctly and significantly that the bridge would function as designed.
The half bridge was load tested to 100kg at the tower and the central position and it performed as expected. The successful completion of this stage was significant as it gave confidence in the overall design, build and functioning of the bridge. The load test on the half bridge was more rigorous than a full bridge, and the entire load was transmitted through one half of the bridge; at full scale it would be shared. The other 50 per cent of the bridge was fabricated slightly ahead of schedule as the project team became more practiced and familiar with the assembly.
Following the successful testing regime on half of the bridge in Queens’s laboratory, a larger space was required to complete a trial erection of the structure in its entirety. Space was limited within the university so the bridge sections were spit into six components (four deck sections and the two towers) and transported to an industrial warehouse at McLaughlin and Harvey’s head office complex in Newtownabbey, Co Antrim.
Within the large warehouse the four sections of the bridge deck were assembled on temporary supports, before the towers were positioned and lifted vertically into place using chain blocks and suitable lifting tackle. Once the towers were in position they were temporarily stabilised with guy ropes until the Meccano cable stays were installed, with the help of scaffold towers, from the towers to the deck. With some apprehension, the supports under the deck were removed, the guy ropes removed from the tower, leaving the Meccano bridge successfully acting as it was designed to do. Plywood deck panels were fabricated to serve as a walking platform to allow pedestrians to cross the bridge.
The construction phase
With the success and confidence obtained from the trial erection it was time to begin the final planning for the public unveiling and world record attempt, scheduled for the end of September 2015. The involvement of McLaughlin and Harvey, with its vast experience of planning and carrying out multidisciplinary civil engineering projects of varying size and complexity, proved vital at this stage. It took a lead on the project working closely with Queens University to finalise site selection, obtain relevant consents and applications and to develop the best installation method on a site outside of the controlled environments of the laboratory and warehouse.
The original plan was that the Meccano bridge would be designed and constructed to cross the river Lagan at Shaws bridge in south Belfast. However, as the project developed and public interest grew this became an unsuitable location as it would have required preparatory works for access, crane bases and bridge footings; furthermore, it was not easily publically accessible.
Following further discussion and option review the most suitable location was identified as being the Clarendon dock in the heartland of Belfast’s historic docklands. Through McLaughlin and Harvey’s close working relationship with Belfast Harbour Commissioners permission was granted to install the Meccano bridge across the dock and hold a public event. As the existing dock and its surrounding areas were listed, consents were obtained from the Historic Environment Division of the Department of the Environment, after demonstrating that the erection of the bridge would in no way damage or compromise the integrity of the dock or artefacts.
The site was particularly suited as it was easily publically accessible, the width of the dock suited the designed bridge span and minimal preparatory works were required.
Bridge installation options
The next challenge was to determine the best method of erecting the bridge across the dock with two options considered:
- Install a temporary deck across the dock using a crane, erect the bridge on the deck lifting the towers into position with a crane, before removing the crash deck, leaving the bridge spanning across the dock. The main problem with this solution was the removal of the crash deck once the bridge was erected.
- The second option was to build the bridge, on a level surface adjacent to the deck, in its entirety, then using a spreader frame lift the complete bridge into position using a mobile crane. The challenge with this option was in lifting such a light structure without putting unanticipated load paths into it that would cause it to buckle.
Following detailed review of the pros and cons of each option a decision was made to go with the second. This option was taken forward with more detailed planning and a methodology for lifting the bridge established. This involved lifting the tower in its constructed state from the base of the columns on the two towers.
This was done using a modified lifting frame and wire ropes passing down through the towers to specially fabricated steel plates with lifting eyes at the base of each tower column. Spacers were used to ensure that the wire rope was central to each tower column allowing the bridge to be lifted in such a way as to allow the loads passing through the bridge to be the same as the loads acting under its normal function. This option avoided the need to work over water and provided a spectacular view of the bridge lift.
McLaughlin and Harvey carried out site set up prior to the commencement of works, ensuring welfare facilities and site boundaries were clearly established to separate the public from the working areas.
As the dock width at ground level was 15.4m and the towers of the bridge 14.4m temporary platforms were installed on the docks top four steps to accommodate the towers.
The bridge was constructed alongside the dock before being lifted into place via the bespoke lifting frame. Great care was taken in the attachment of the lifting frame to the predetermined positions on the towers and the slings were adjusted to the correct length to ensure an even distribution of load as determined by the temporary works design.
Once the final checks were complete the bridge was lifted 100mm off the ground to confirm the bridge was behaving as expected. Following final checks the bridge was installed onto the temporary supports with no issues. It was removed following the reverse of this procedure.
To enable people to cross the bridge plywood and Perspex deck panels were placed to form a walking surface. There were installed prior to the lift providing a measure of increased rigidity to the deck structure with perspex selected to allow users to view the structure below their feet. To ensure stability in the event of high winds guy ropes were tied to the towers and secured on concrete blocks.
While the project team were confident in the integrity of the structure to perform as designed it was important to ensure the safety of those crossing the bridge, particularly with no handrail, in the event of collapse or it becoming unstable. All users wore harnesses and were clipped onto an inertia reel mounted on the crane.
The culmination of the entire project focused on the first public crossing of the bridge which occurred on Saturday September 19, 2015. The visual nature of the project and scale of the bridge resulted in significant media interest during the site installation phase leading up to the public event.
This event was a celebration of the achievement of the students and the entire project team. The event had a family friendly atmosphere, with food, refreshments, music and Meccano kits being distributed. It is estimated that more than 1,000 members of the public visited the site during the day promoting interest in engineering.
Professor Trevor Whittaker and Professor Patrick Johnston, from QUB, welcomed the audience and the bridge was officially opened by Meccano’s Meccanoid robot. Prof Whittaker explained the functioning aspects of the bridge as he walked across it. The bridge was then confirmed by the Guinness adjudicator to be the ‘World’s Largest Meccano Structure’.
The staff and students then all had an opportunity to cross the bridge. The scale of the media presence to cover this project was not anticipated. The bridge was covered directly by four national television companies in the UK and Ireland (BBC, Sky, ITV and RTE) with the footage then disseminated worldwide.
Within three days of completion the story of the bridge had been told in 25 countries worldwide and had featured in 493 news stories, radio and television. The project team were delighted with this level of interest from the general public. With the huge engineering skills shortage looming within the built environment they hope this enthusiasm will translate into more young people entering this vital sector of industry. The project team are now exploring opportunities for a suitable home for the bridge and looking forward to their next challenge.
Department of Culture, Arts and Leisure
Queen’s Annual Fund
McLaughlin and Harvey
Advanced NI Scaffolding
Quigley Crane Hirehttp://www.engineersjournal.ie/2015/12/08/25822/http://www.engineersjournal.ie/wp-content/uploads/2015/12/Meccana-Feature-Image-1024x684.jpghttp://www.engineersjournal.ie/wp-content/uploads/2015/12/Meccana-Feature-Image-300x300.jpgMechbridges,Northern Ireland,Queen's University Belfast