For Project Fitzwilliam, O’Connor Sutton Cronin was named as worthy winner of a prestigious 2022 ACEI Engineering Excellence Award at a gala dinner in Dublin’s Shelbourne Hotel on Friday, September 2nd. Building Ireland caught up with structural engineer Ian Crehan to find out more about this landmark, award-winning project.
O’Connor Sutton Cronin’s status as one of Ireland’s leading multi-disciplinary engineering consultancies was cemented once more at this year’s ACEI Engineering Excellence Awards, where the practice walked away with another major award, striking gold in the Structures (Large) category for their invaluable contribution to the magnificent Project Fitzwilliam.
Delivered on behalf of ESB and designed as a collaboration between Grafton Architects and O’Mahony Pike, with structural design by O’Connor Sutton Cronin and constructed by PJ Hegarty & Sons as main contractor, this breath-taking project provides a unique redevelopment of almost an entire city block located within the historic Georgian core of Dublin.
ESB have been located on this site since 1927 and insisted upon remaining here. The building that constitutes their new HQ has been designed to be sympathetic to its setting but to display proudly its fabric in the form of exposed concrete finishes internally with a mixture of hand-laid brick and precast concrete externally.
From a conservation and heritage perspective, Fitzwilliam Street is considered one of the most important Georgian streets in Europe and, as a result, the project was of tremendous interest not only to the client and their staff but also to the entire design and construction teams and also of course the general public.
OCSC’s lead structural engineer on the project, Ian Crehan, was delighted to receive an ACEI Engineering Excellence Award for Project Fitzwilliam and is not slow is sharing the credit: “We were delighted to get this acknowledgement from our peers but the award is not just for ourselves but for the whole project team.
“A number of years of hard work went into realising the project and it was a collaborative team effort from start to completion. The award recognises the client, the architects, the main contractor, ourselves and everybody else involved in producing design and construction excellence to this standard and at this scale.”
Bounded by Fitzwilliam Street to the west, Mount Street Upper to the north, James Street East to the east and Baggot Street to the south, the development replaced the existing ESB offices on a brownfield, city centre site and required the careful demolition of existing structures adjacent to existing protected structures. In addition, the piling works, excavations and basement construction were undertaken abutting existing structures and also through existing basement structures.
The redevelopment of the site can be split into two distinct sections: the main portion of Project Fitzwilliam was divided into two blocks over a common basement, with No 27 Fitzwilliam (Block B) forming the new ESB Head Office and No 28 Fitzwilliam (Block A) a speculative office development. The project consists of a five- to seven-storey reinforced concrete structure over double, and part triple, level basement providing c.30,000m2 of office accommodation in an overall development of c.42,000m2.
Meanwhile, the second portion comprised the Block E structures, which have been renovated and restored for a mixture of residential and office use.
From initial consultations at the planning and earliest design stage, OCSC were involved in Project Fitzwilliam for more than a decade up until the final building was completed in December, 2021. “It’s not often you get to regenerate not only a whole city block but one that’s located on a historic site within the Georgian Mile,” reflects Ian.
“It goes without saying that this was a highly-important project to us and, from a personal point of view, it was both the most challenging and rewarding of my 17-year career to date. As a structural engineer, you get to leave your mark on the city and that comes with a great deal of responsibility. Especially with this being such a prominent building within such a historic streetscape, it was vitally important that we got it right.”
Essentially comprising the urban renewal and reimagining of a city block located within the heart of the historic Georgian Quarter of Dublin City, the project showcased industry-leading excellence from all of those who contributed. Coming up with solutions that are both structurally and visually pleasing is always challenging and, for Project Fitzwilliam, the goal was to have an end building that looked fresh and new but also merged seamlessly with its surroundings.
“When you are dealing with that scale of streetscape, you want to try to merge it in with the original buildings either side of it, whilst also allowing it to stand out in its own right,” says Ian. “You need to get that balance right. The contractor sourced the handmade bricks from the continent and a lot of time and resources were invested into making sure that the team found a brick that everybody was happy with and that would be subtly different yet blend in with its surroundings.
“Window sizes is another consideration. In keeping with the character of Georgian buildings, larger windows were used on the first floor and these get gradually smaller as you go up through the levels. The architects – Grafton and O’Mahony Pike – worked very hard on getting that elevation correct and again this was key to the success of the project.”
The architectural and structural design along Fitzwilliam Street sought to use brickwork as a primary load-bearing element, as opposed to a cladding element (as would most commonly be the case in modern construction) to complement the setting of the development.
To meet requirements of disproportionate collapse without the requirement for vertical ties up through the walls, the below ground level sections of the wall were cast in-situ with the walls achieving a special finish internally.
The project includes a number of novel uses of concrete in the form of vierendeel trusses. The first of these are used to provide a two-storey high, 18m wide structural block with a clear span of 15m over a triple height/part four storey height space underneath. This block consists of two vierendeel trusses formed in the outer walls of the space with downstand beams at third points across the width of the block to limit the load onto the vierendeels.
The dimensions of the horizontal and vertical elements of the trusses are co-ordinated with the precast façade elements, with the edges of the concrete offset from the façade elements by a consistent dimension to allow for the window frames and finishes.
Meanwhile, the same engineering principles were adopted in two of the rear courtyards facing onto James Street East, but with the trusses acting as cantilever of up to 7m in this area. Again, the structure was designed as a double height vierendeel truss but with up to three storeys above this truss requiring transfer as well as the two level tied into the truss itself.
The superstructure consists of RC flat slab construction, with a typical slab thickness of 325mm at suspended slabs, wand a deeper slab thickness of 350mm at ground floor and roof levels to account for larger build-ups. The perimeter concrete elements behind the 3m module of precast façade are engaged with the structure and act as load-bearing elements throughout, leading to the adoption of transfer structure at lower levels to achieve the clear spans and building offsets in the architectural design. This structure, together with the Vierendeel elements, required a significant temporary works and formwork arrangement to be provided with extensive co-ordination between OCSC, main contractor and subcontractors and temporary works designers to ensure the works would be constructed in a safe and efficient manner.
Due to a number of factors, slipform construction was chosen as the most efficient means of delivering the RC cores. This method of construction involves a continuous 24/7 operation with fixing of reinforcement and placing of concrete continuing as the working platform slides up the core with each core taking between 5-10 days to complete depending on plan area.
From the start of the project, a high quality concrete surface finish was specified throughout the building and this was outlined in both the structural engineer and architect’s specifications. The building has been designed and constructed with exposed concrete elements throughout to slab soffits as well as vertical elements. The sections of floor slab supported on the Fitzwilliam Street masonry wall were constructed in a coffered or stepped slab soffit fashion to achieve the effect of a deeper slab at the walls and a shallower section over the windows. The main circulation stairs within Block A are of cast in-situ construction with the design carefully detailed and co-ordinated to align seamlessly with the façade elements. The lower flight of each stair is cranked for the necessary quarter landing with the half landing hung by means of a concrete downstand from the level above.
The project incorporates a number of sustainability measures such as GGBS used throughout the main structure with up to 70% replacement of cement in portions of the substructure and basement and 30% generally throughout the superstructure. The use of slipform technique for the cores – with the same platform adopted for use on all seven cores that were constructed with this technique – requires a fraction of the shuttering and formwork which would be required.
Project Fitzwilliam has been designed to a 100-year design life to provide a durable and long lasting building into the future.
O’Connor Sutton Cronin,
9 Prussia Street,
Dublin 7,
Tel: 01 868 2000
Fax: 01 868 2100
Email: [email protected]
Web: ocsc.ie
This article was published in Building Ireland Magazine, February 2023, Vol 9 No 2
