Caroline Cavanagh discusses aspects of the N22 Macroom bypass project, where the longest prestressed bridge beams in the UK or Ireland have been installed. In Part II, the logistical planning to get the 50m beams to site will be examined; and also a number of other culvert structures which are being built and which include crossings over the Sullane and Bohill rivers; the benefits of choosing an offsite precast solution; and the environmental considerations linked to the project. 

Part I outlined an overview of the project while also looking at Structure 26: the Laney River bridge; its beam design details; and the concrete mix designs used in the building. 

 

Logistics

Manufacturing and delivering the beams to Cork was a major logistical exercise. "We always said that we could design, manufacture and transport 50m beams, but up to now we never had a client brave enough," says Cavanagh.

"Our senior engineer, Eamon Stack, who developed the now industry standard W beams in 2005, worked very closely with Barry Transportation, TYPSA and our Clients Jons Cradock JV on the beam design; over several months, the calculations and designs came together until Transport Infrastructure Ireland (TII) was comfortable with the proposed beam performance.”

The beams were cast on one of Banagher’s 200m-long W bridge beam lines, utilising on-site gantry cranes to lift the 155 tonne beams. Transport of the beams required Garda-escorted overnight journeys from Banagher, Co Offaly, to Macroom, Co Cork, the 190km journey which usually takes more than 2.5 hours took more than 12 hours with these abnormal loads.

Transported in pairs by Whitten Road Haulage the beams travelled the first 40km of their journey on the first night, leaving Banagher late each evening in order to minimise disruption to other road users and local communities.

Beams the same length as an Olympic swimming pool

Manoeuvring prestressed beams the same length as an Olympic swimming pool through the neighbouring town of Birr was a serious challenge; given their length, the beams had to be reversed into a local car park, driven to another location, be turned and brought back through the town and navigated with incredible accuracy by the bogey team through the 'pinch point' in the town centre under the scrutiny of the crowds that had gathered.

The beams passed through the Jack Lynch Tunnel in Cork with a mere 200mm of headroom to spare; precise swept path analysis and precision control was required to navigate this section of the journey to site.

Another challenge was delivering the beams the final 400m from the existing N22 to the crane pad, which necessitated reversing them over the existing stone bridge with just 100mm to spare and negotiating a tight bend to get them into position. Again, with sufficient preparation and analysis, this aspect of the works was delivered efficiently.

Longest span prestressed concrete bridge Ireland and UK design.

Cross-section of the 155-tonne W19 beams.

Offloaded without the use of a crane

The W19 beams were offloaded directly onto trestles without the use of a crane. This offloading system is especially useful for this type of project where the logistics of moving 50m beams takes several days.

Along with the added benefit of reducing craneage on site, the trestles also allow for additional works such as the edge protection and working platform to be installed on the external beams prior to lifting.

The permanent formwork, FRC panels were also preinstalled prior to lifting to reduce the requirement for additional working over the River Laney once the beams were in position.

The beams were then lifted into place in December 2020 over two days, once again reducing craneage requirements on site. The final lift, including beam weight, lifting equipment, FRC, access walkways and edge protection resulted in the overall weight of the beams of more than 175T. These beams were placed into the final position using a 600T crawler crane.

Precast: An off-site solution

The benefits of choosing an offsite precast solution were manifold for this project: by removing expensive and time-consuming onsite steel fabrication works, it greatly accelerated the project programme and reduced costs, not to mention the environmental benefits of eliminating shipment of steel from Spain. In this instance, using an offsite precast solution enhanced site safety and eliminated ongoing maintenance often encountered with steel structures.

Another major advantage of using the prestressed W-beams was that, once lifted into position, they were immediately stable and provided a safe deck from which to work from – very important as they were installed over water and the safety of construction workers was a paramount consideration.

By manufacturing the beams offsite in a safe and controlled environment within Covid-19 working guidelines it further enhanced the overall safety of the project by reducing teams onsite.

Offsite precast benefits for infrastructure projects:

• Enhanced durability;
• Optimised production/fabrication process;
• Cost savings (steel, craneage, crews, transport);
• Reduced impact on the environment compared with a steel structure;
• Quick and safe installation;
• Reduced site programme.

Not only was precast concrete the most economical solution in this instance, it was also chosen for its strength, durability, stability, flexibility, achievable finishes and the ability to conform to a strict programme schedule. This structure despite Covid-19 lockdowns was completed on time and on budget. 

These record-breaking 50m beams demonstrate offsite engineering at its finest – it has proven that lengths of 50m can be achieved in prestressed precast, that boundaries should be challenged and that offsite should strive for constant improvement and innovative solutions.

These 50m beams have smashed records and expectations, they have changed the landscape for what can be achieved with offsite manufactured precast concrete for the infrastructure sector – especially critical in these times of change and great infrastructure investment. The lessons learnt from designing, manufacturing and transporting these epic 50m beams, the longest beams ever made in the UK and Ireland, now offers civil contractors a viable offsite precast solution for long-spanning structures.

Sustainability and environmental considerations

As with all sections of the project, we implemented and maintained all environmental constraints and control measures to protect the adjacent ecology. The River Laney passed within 5m of the construction works, so a triple line of silt fence was installed to ensure isolation of clean water flows.

A section 50 application was submitted in consultation with the OPW. The choice of a single span structure removed the necessity for any in-stream works.

The entire worksite was prone to flooding and consideration was given to storage of material and plant. A regime of upstream and downstream water monitoring was implemented to ensure water quality was not disrupted. A leave strip was established adjacent to the bridge crossing in consultation with the on-site ecologist.

Grey to green

In this instance the River Laney bridge’s requirement to span 50m meant that it was originally designed in the traditional method – steel. Although steel is a fantastic product for long spans, in this instance it would have required importation from Spain, increasing the programme and costs, not to mention several weeks of labour-intensive onsite works and an increased carbon footprint.

These 50m-long beams were designed to Eurocode 2, had 255 tonnes of prestressing force. The improved efficiency of the way prestressing design is dealt with in EC2 compared to the older BS 5400 design allowed an 11% increase in span for only a 13% increase in prestress.

Had the Laney River bridge 50m design been carried out to BS 5400, the increase in prestress would have had to be 24% more than the LSRR S06 45m span design and 10% more than what is required by EC2 to span 50m. The migration to Eurocodes II resulted in a 9% weight saving, that’s 14 tonnes of concrete and steel per beam, an overall saving of almost 100 tonnes.

The concrete structure as it stands today is robust and provides lasting transport resilience. Concrete was the natural choice ensuring that this transport link can withstand environmental effects and climate change with minimal maintenance. 

Sullane River bridge

While the 50m beams were in the limelight, Banagher Precast Concrete were also manufacturing the second-longest bridge prestressed concrete beams made in the UK and Ireland.  Also on the N22 project, the Sullane River bridge (structure 28) now holds the title for the longest two-span prestressed concrete bridge, each of the 14 48.75 beams were successfully lifted into place in February 2021.

In terms of design structure, 28 pushed more design boundaries than the 50m beams. Although the beam lengths are slightly shorter, the effects of the bridge being integral posed increased ratcheting challenges.

Bohill River bridge: Steel and concrete

The longest bridge on the scheme, this structure is a two-span composite steel girder, and manufactured by Tecade in Seville, Spain. It is integral at west abutment, continuous over the intermediate pier and non-integral at east abutment. It has a length of 121m with intermediate pier located such that the western span length is 40.5m and the eastern span is 80.5m long.

The superstructure consists of a pair of steel girders and reinforced concrete slab in a ladder deck arrangement. The structural depth is 4.0m excluding cross-falls and surfacing. Concrete bridge deck will comprise of participating permanent formwork (almost 200 precast RC omnia planks) and cast in-situ reinforced concrete slab. The overall deck thickness (participating formwork and insitu slab) will be 275mm.

The preparation works for this mammoth steel structure included a supporting pier comprising of 232 cubic metres of concrete (560 tonnes) along with 12km of steel reinforcing bars. The finished pier is 10.5m high and 18.9m wide at the top, curving down to a width of 8m at the base and is 2m deep. A total of 33 loads of concrete were delivered by Roadstone over the 12-hour pour. 

The concrete used had a compressive strength of 45 Newton, which is normal structural concrete. Additives to allow water reduction in the concrete mix and for improved workability were used in the concrete. In addition, 50% of the combat content was replaced with GGBS (Ground Granular Blast-furnace Slag) to regulate the rise in temperature of the concrete in the curing process, so as to mitigate against cracking. The use of GGBS also reduces the carbon content of the structure.

The steel push was completed in March 2021 on the S03 Bohill River bridge. Deemed as a critical infrastructure project, the N22 Macroom bypass was permitted to continue during lockdown restrictions under strict Covid-19 health and safety guidelines and is due for completion by the autumn of 2023. 

Video: Design and Construction of Bohill River Bridge