Caroline Cavanagh discusses aspects of the N22 Macroom bypass project, where the longest prestressed bridge beams in the UK or Ireland have been installed. Part I outlines 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.
The N22 Ballyvourney to Macroom project in Co Cork includes the construction of 22km of dual carriageway. The road is being constructed through challenging terrain which varies through the development from hilly remote land with rock outcrops at the western end, to low-lying pasture lands to the east of Macroom and will cross a land-locked section of the Inniscarra reservoir to the southeast of Macroom.
This is one of the largest infrastructure projects under way in Ireland right now; it’s a design and build contract and it is envisaged that it will be completed by the autumn of 2023.
On-site construction
The list below outlines the construction sequence:
- Drive circular hollow section (CHS) steel bearing piles
- Construct pile caps
- Construct abutment walls
- Install seven no 50m W beams
- Diaphragm to underside of deck slab poured
- Finish construction of wingwalls and backfill behind abutments
- Complete structure finishes, installation of precast parapets
Bridge beam details
- Seven x W19 prestressed concrete bridge beams;
- 50m long;
- 2.5m high;
- 175-tonne lift.
Concrete mix designs used on the Laney River Bridge
Prior to delivery of the beams there were a number of in-situ reinforced concrete elements constructed on site by the CJV to accommodate the precast beams.
There were 29 separate mix designs approved by the designers on the project to accommodate the various elements and their use, each designed and tested specifically for the various conditions, ie, buried concrete or different exposure classes.
Concrete testing was carried out on site, each load was slump tested ensuring a compliant product was placed in situ. Samples were taken at the required frequency and tested for compressive strength.
- In-situ foundations C32/40
- Abutment elevations C35/45
- Abutment wingwalls C45/55 (50% GGBS)
- Precast prestressed beams C60/75
- Parapet edge beam C50/60 (50% GGBS)
- Insitu deck slab C34/45
The finish requirement of the concrete was dependent on the location of the different elements, below is a list of the specified finishes that were achieved:
- Exposed unformed surfaces excluding areas to receive
- Buried unformed surfaces U1
- Sprayed deck waterproofing U3
- Buried in-situ concrete surfaces F1
- Parapet edge beam F3/U3
- Precast beams F5
- Deck cantilever F4
- Precast elements F4
- Area of deck to receive waterproofing U4
- End supports – abutment wall PPF1*
Concrete cover, compressive strength and exposure class is outlined in the table below. Concrete with 50% GGBS was used in the wingwalls and parapets edge beams due to their exposure classification.
The construction team in collaboration with the design site representatives carefully inspected all elements of work with the client to ensure compliance with the table below:
Once piling was completed, the site team constructed the foundation bases on the piles which, in turn, holds the abutment walls. The bases, one on each river bank, contain 180m3 of concrete. Once cast, the team moved onto the abutment walls, these are 4m and 5m in height, 1.5m wide.
Owing to the size of these abutment walls, we designed a bespoke temporary works design for the walls and stop ends. Due to the necessity to have continuity of reinforcement, which would later tie into cantilever wingwalls and diaphragms following placement of the beams, a complex stop end and construction joint detail was introduced at a 45-degree angle to the line of the walls. The concrete pressures were calculated and rate of pour was carefully managed as the walls were poured, using a concrete pump and 150m3 of concrete.
Following installation of the 50m W beams, the construction team moved to cast the diaphragms from beam seating level to the underside of deck, some 2.5m high. These locked the precast and in-situ elements of the structure into one integral unit. Each diaphragm required 75m3 of concrete, with bespoke formwork constructed on site to follow the profile of the W beams.
Cantilever wingwalls with an overhang corbel detail were constructed on each of the four corners of the structure following on from the diaphragms, with a cantilever of more than 6m.
On large areas of exposed concrete, such as the abutment walls and wingwalls, a pattern profile finish was incorporated into the face of the walls, 282mm wide, 18mm deep at 600mm centres. This provided a more aesthetically pleasing appearance on large sections of exposed concrete.
The concrete deck was poured in March 2021 using a 60m concrete pump and more than 230m3 of concrete. Utilisation of the walkway installed on the external beams allowed the CJV to cast the full extent of the deck, containing more than 70T of reinforcement in one continuous slab.
The final element of construction was to place the precast parapet sections and cast the stitch section, which tied the precast parapets into the concrete deck. These parapets exhibit a curved appearance, forming a continuous and symmetrical circular curve over the clear span of the structure.
Buried concrete elements received two coats of epoxy resin waterproofing paint. Spray applied waterproofing membrane was applied to the bridge deck surface.
All exposed concrete surfaces were treated with hydrophobic pore liner to protect these from the elements and ensure the concrete was protected for its anticipated lifespan. After backfilling of both abutments, the structure was made live to site construction traffic in May 2021 enabling the earthworks to progress the project.
Adapting initial proposal of steel bridge to concrete structure
The design of the S26 Laney bridge required a highly complex design process to adapt the initial proposal of a steel bridge to an in-situ concrete structure incorporating the longest precast beams produced in Ireland or the UK.
Ground investigation and boreholes determined that the abutments would be supported on a piled foundation. A total of 88no CHS piles 323mm in diameter were driven into the bedrock, varying in depth to between nine and 12m below existing ground level, to achieve the required structural support for the weights of the superstructure.
The dead weight or the structure was considered, as well as the superimposed weight, which included road construction materials acting on the deck as a uniformly distributed load (pavement, barriers, sidewalks). Horizontal and vertical components of soil weight on each side of the structure, thermal actions, water level.
The proposed structure was designed for normal traffic load models LM1 & LM2 in accordance with IS EN 1991-2. As this structure will carry a main road, in accordance with IAN02, Load Model LM3 (SV100 and SV196 vehicles) was considered in the design in accordance with IS EN 1991-2 and its associated Irish National Annex.
The global model took into account constructions stages, including partial backfilling of abutment walls, simply supported condition of prestressed beams during deck pour and full frame structure. Cracked section properties were used for deck elements within the area of hogging zone over the supports.
Design details
The structure was designed by means of linear elastic methods. Two models were prepared for this analysis – a 2D frame model representing a 1m strip was initially used to analyse the proposed structure as well as space frame/3D grillage model and FEM model were then produced in order to take into account all stages of construction. With the first (2D) model, the load coming from the ratcheting effect could be estimated and shown in a clear manner.
Longest span prestressed concrete bridge Ireland and UK design.
Cross-section of the 155-tonne W19 beams.
Space frame model/combined FEM model. This second (3D) model was used to determine specific section forces for each beam, as part of the grillage. The global model took into account constructions stages, including partial backfilling of abutment walls, simply supported condition of prestressed beams during deck pour and full frame structure. Cracked section properties were used for deck elements within the area of hogging zone over the supports.
That 3D model, where the superstructure was modelled as a grillage, was used for the analysis of construction stages and to determine the effects of superimposed dead loads, live loads, temperature and earth pressure loads during service period. For the precast beam installation and deck slab pour, beam-only properties were used with beam self-weight and wet concrete.
The beam is considered as an open section under slab weight and as a box once the slab is hardened. The box torsional stiffness is considered in the model, meaning there is an uneven distribution of shear between both webs to be considered for the design of these beams. This also allows for the determination of the additional shear forces on the top/bottom flanges from torsion.
Consequently, the deck is designed as a multi-cellular box girder. This generates different shear forces on each web. This approach to take the torsional moment from the grillage model and determine the shear on each web and slab has been agreed and coordinated both with the CAT3 Checker and the supplier (Banagher) as it follows the recommendations from Eurocode I.S. EN 1992-2:2005, Design of concrete structures – concrete bridges.
The concrete strength class used in the N22 S26 was C60/75 with C45/55 at transfer. These 50m long beams were designed to Eurocode 2 had 255 tonnes of prestressing force compared to the 226 tonnes of prestressing force required to span 45m for the Limerick Southern Ring Road (LSRR) S06 beams 15 years previously, which was designed to British Standard 5400 with C57/70 concrete at a similar beam spacing.
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 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.
It would not have been possible to accommodate the extra prestress required by BS 5400 in the standard W19 beam section and this would have necessitated an expensive modification of the W19 cross-section, increasing its weight by 9%.
In Part II, we will examine the logistical planning to get the 50m beams to site; a number of other culvert structures that 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.
Author: Caroline Cavanagh